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Diatoms

Diatoms

Diatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although some form chains or simple colonies. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silicate. These walls show a wide diversity in form, some quite beautiful and ornate.

General biology

There are more than 200 genera of living diatoms, and it is estimated that there are approximately 100 000 extant species (Round & Crawford, 1990). Diatoms are a widespread group and can be found in the oceans, in freshwater, in soils and on damp surfaces. Most live pelagically in open water, although some live as surface films at the water-sediment interface, or even under damp atmospheric conditions. They are especially important in oceans, where they are estimated to contribute up to 45% of the total oceanic primary production (Mann, 1999). Diatoms belong to a large group called the heterokonts, including both autotrophs (e.g. golden algae, kelp) and heterotrophs (e.g. water moulds). Their chloroplasts are typical of heterokonts, with four membranes and containing pigments such as fucoxanthin. Individuals usually lack flagella, but they are present in gametes and have the usual heterokont structure, except they lack the hairs (mastigonemes) characteristic in other groups. Most diatom species are non-motile but some are capable of an oozing motion. As their relatively dense cell walls cause them to readily sink, planktonic forms in open water usually rely on turbulent mixing of the upper layers by the wind to keep them suspended in sunlit surface waters. Some species actively regulate their buoyancy to counter sinking. Diatoms cells are contained within a unique silicate (silicic acid) cell wall comprised of two separate valves (or shells). This cell wall is also called a frustule or test, and its two parts typically overlap one other like the two halves of a petri dish. In most species, when a diatom divides to produce two daughter cells, each cell keeps one of the two valves and grows a smaller valve within it. As a result, after each division cycle the average size of diatom cells in the population gets smaller. Once such cells reach a certain minimum size, rather than simply divide vegetatively, they reverse this decline by forming an auxospore. This expands in size to give rise to a much larger cell, which then returns to size-diminishing divisions. Auxospore production is almost always linked to meiosis and sexual reproduction.

Ecology

meiosis Planktonic forms in freshwater and marine environments typically exhibit a "bloom and bust" lifestyle. When conditions in the upper mixed layer (nutrients and light) are favourable (e.g. at the start of spring) their competitive edge (Furnas, 1990) allows them to quickly dominate phytoplankton communities ("bloom"). As such they are often classed as opportunistic r-strategists (i.e. those organisms whose ecology is defined by a high growth rate, r). When conditions turn unfavourable, usually upon depletion of nutrients, diatom cells typically increase in sinking rate and exit the upper mixed layer ("bust"). This sinking is induced by either a loss of buoyancy control, the synthesis of mucilage that sticks diatoms cells together, or the production of heavy resting spores. Sinking out of the upper mixed layer removes diatoms from conditions inimical to growth, including grazer populations and higher temperatures (which would otherwise increase cell metabolism). Cells reaching deeper water or the shallow seafloor can then rest until conditions become more favourable again. In the open ocean, many sinking cells are lost to the deep, but refuge populations can persist near the thermocline. Ultimately, diatom cells in these resting populations re-enter the upper mixed layer when vertical mixing entrains them. In most circumstances, this mixing also replenishes nutrients in the upper mixed layer, setting the scene for the next round of diatom blooms. In the open ocean (away from areas of continuous upwelling; see Dugdale & Wilkerson, 1998), this cycle of bloom, bust, then return to pre-bloom conditions typically occurs over an annual cycle, with diatoms only being prevalent during the spring and early summer. In some locations, however, an autumn bloom may occur, caused by the breakdown of summer stratification and the entrainment of nutrients while light levels are still sufficient for growth. Since vertical mixing is increasing, and light levels are falling as winter approaches, these blooms are smaller and shorter-lived than their spring equivalents. In the open ocean, the condition that typically causes diatom (spring) blooms to end is a lack of silicon. Unlike other nutrients, this is only a major requirement of diatoms so it is not regenerated in the plankton ecosystem as efficiently as, for instance, nitrogen or phosphorus nutrients. This can be seen in maps of surface nutrient concentrations - as nutrients decline along gradients, silicon is usually the first to be exhausted (followed normally by nitrogen then phosphorus). Because of this boom-and-bust lifestyle, diatoms are believed to play a disproportionately important role in the export of carbon from oceanic surface waters (Smetacek, 1985; Dugdale & Wilkerson, 1998; see also the biological pump). Significantly, they also play a key role in the regulation of the biogeochemical cycle of silicon in the modern ocean (Treguer et al., 1995; Yool & Tyrrell, 2003). silicon The use of silicon by diatoms is believed by many researchers to be the key to their ecological success. In a now classic study, Egge & Aksnes (1992) found that diatom dominance of mesocosm communities was directly related to the availability of silicate. Above concentrations of silicon of 2 m mol m^-3, diatoms typically comprised more than 70% of the phytoplankton community. Raven (1983) noted that, relative to organic cell walls, silica frustules require less energy to synthesize (approximately 8%), potentially a significant saving on the overall cell energy budget. Other researchers (Milligan & Morel, 2002) have suggested that the biogenic silica in diatom cell walls acts as an effective pH buffer, facilitating the conversion of bicarbonate to dissolved CO₂ (which is more readily assimilated). Notwithstanding the possible advantages conferred by silicon, diatoms typically have higher growth rates than other algae of a corresponding size (Furnas, 1990).

Evolutionary history

Heterokont chloroplasts appear to be derived from those of red algae, rather than directly from prokaryotes as occurs in plants. This suggests they had a more recent origin than many other algae. However, fossil evidence is scant, and it is really only with the evolution of the diatoms themselves that the heterokonts make a serious impression on the fossil record. The earliest known fossil diatoms date from the early Jurassic (~185 Ma; Kooistra & Medlin, 1996), although recent genetic (Kooistra & Medlin, 1996) and sedimentary (Schieber, Krinsley & Riciputi, 2000) evidence suggests an earlier origin. Medlin et al. (1997) suggest that their origin may be related to the end-Permian mass extinction (~250 Ma), after which many marine niches were opened. The gap between this event and the time that fossil diatoms first appear may indicate a period when diatoms were unsilicified and their evolution was cryptic (Raven & Waite, 2004). Since the advent of silicification, diatoms have made a significant impression on the fossil record, with major deposits of fossil diatoms found as far back as the early Cretaceous, and some rocks (diatomaceous earth, diatomite, kieselguhr) being composed almost entirely of them. Although the diatoms may have existed since the Triassic, the timing of their ascendancy and "take-over" of the silicon cycle is more recent. Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle (Siever, 1991; Kidder & Erwin, 2001; Grenne & Slack, 2003). Subsequently, the cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter feeders primarily on the continental shelves (Racki & Cordey, 2000). Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms. However, the precise timing of the "take-over" is unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the eviction of siliceous sponges from the shelves (Maldonado et al., 1999), suggests that this takeover began in the Cretaceous (146 Ma to 65 Ma), while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic (65 Ma to present). Nevertheless, regardless of the details of the "take-over" timing, it is clear that this most recent revolution has installed much tighter biological control over the biogeochemical cycle of silicon.

Classification

The classification of heterokonts is still unsettled, and they may be treated as a division (or phylum), kingdom, or something in-between. Accordingly, groups like the diatoms may be ranked anywhere from class (usually called Bacillariophyceae) to division (usually called Bacillariophyta), with corresponding changes in the ranks of their subgroups. Diatoms are traditionally divided into two orders: centric diatoms (Centrales), which are radially symmetric, and pennate diatoms, which are bilaterally symmetric (Pennales). The former are paraphyletic to the latter. A more recent classification is that of Round & Crawford (1990), who divide the diatoms into three classes: centric diatoms (Coscinodiscophyceae), pennate diatoms without a raphe (Fragilariophyceae), and pennate diatoms with a raphe (Bacillariophyceae). It is probable there will be further revisions as our understanding of their relationships increases. Round & Crawford (1990) and Hoek et al. (1995) provide comprehensive coverage of the diatoms.

Collection

Living diatoms are often found clinging in great numbers to filamentous algae, or forming gelatinous masses on various submerged plants. Cladophora is frequently covered with Cocconeis, an elliptically shaped diatom; Vaucheria is often covered with small forms. Diatoms frequently present as a brown, slippery coating on submerged stones and sticks, and may be seen to "stream" with river current. The surface mud of a pond, ditch, or lagoon will almost always yield some diatoms. They can be made to emerge from the mud by putting black paper around the jar and letting direct sunlight fall upon the surface of the water. The diatoms, within a day or less, will come to the top in a scum which can be easily isolated and secured. Since diatoms form an important part of the food of molluscs, tunicates, and fishes, the alimentary tracts of these animals often yield forms that are not easily secured in other ways. Marine diatoms can be collected by direct water sampling, though benthic forms can be secured by scraping barnacles, oyster shells, and other shells. The silicious shells of diatoms are among the most beautiful objects which can be examined with the microscope. To obtain perfectly clean mounts requires considerable time and patience, but once the material is cleaned, preparations may be made at any time with very little trouble. Note : Much of the text in this section (Collection) is from Methods in Plant Histology from the 1900s. Handle with care!

References

- Dugdale, R. C. and Wilkerson, F. P. (1998). Silicate regulation of new production in the equatorial Pacific upwelling. Nature 391, 270-273.
- Egge, J. K. and Aksnes, D. L. (1992). Silicate as regulating nutrient in phytoplankton competition. Mar. Ecol. Prog. Ser. 83, 281-289.
- Furnas, M. J. (1990). In situ growth rates of marine phytoplankton : Approaches to measurement, community and species growth rates. J. Plankton Res. 12, 1117-1151.
- Grenne, T. and Slack, J. F. (2003). Paleozoic and Mesozoic silic-rich seawater : evidence from hematitic chert (jasper) deposits. Geology 31, 319-322.
- Hoek, C. van den, Mann, D. G. and Jahns, H. M. (1995). Algae : An introduction to phycology, Cambridge University Press, UK.
- Kidder, D. L. and Erwin, D. H. (2001). Secular distribution of biogenic silica through the Phanerozoic : Comparison of silica-replaced fossils and bedded cherts at the series level. J. Geol. 109, 509-522.
- Kooistra, W. H. C. F. and Medlin, L. K. (1996). Evolution of the diatoms (Bacillariophyta) : IV. A reconstruction of their age from small subunit rRNA coding regions and the fossil record. Mol. Phylogenet. Evol. 6, 391-407.
- Maldonado, M., Carmona, M. C., Uriz, J. M. and Cruzado, A. (1999). Decline in Mesozoic reef-building sponges explained by silicate limitation. Nature 401, 785-788.
- Mann, D. G. (1999). The species concept in diatoms. Phycologia 38, 437-495.
- Medlin, L. K., Kooistra, W. H. C. F., Gersonde, R., Sims, P. A. and Wellbrock, U. (1997). Is the origin of the diatoms related to the end-Permian mass extinction? Nova Hedwegia 65, 1-11.
- Milligan, A. J. and Morel, F. M. M. (2002). A proton buffering role for silica in diatoms. Science 297, 1848-1850.
- Racki, G. and Cordey, F. (2000). Radiolarian palaeoecology and radiolarites : is the present the key to the past? Earth-Science Reviews 52, 83-120.
- Raven, J. A. (1983). The transport and function of silicon in plants. Biol. Rev. 58, 179-207.
- Raven, J. A. and Waite, A. M. (2004). The evolution of silicification in diatoms : inescapable sinking and sinking as escape? New Phytologist 162, 45-61.
- Round, F. E. and Crawford, R. M. (1990). The Diatoms. Biology and Morphology of the Genera, Cambridge University Press, UK.
- Schieber, J., Krinsley, D. and Riciputi, L. (2000). Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature 406, 981-985.
- Siever, R. (1991). Silica in the oceans : biological-geological interplay. In : Schneider, S. H., Boston, P. H. (eds.), Scientists On Gaia, The MIT Press, Cambridge MA, USA, pp. 287-295.
- Smetacek, V. S. (1985). Role of sinking in diatom life-history cycles : Ecological, evolutionary and geological significance. Mar. Biol. 84, 239-251.
- Treguer, P., Nelson, D. M., Van Bennekom, A. J., DeMaster, D. J., Leynaert, A. and Queguiner, B. (1995). The silica balance in the world ocean : A reestimate. Science 268, 375-379.
- Yool, A. and Tyrrell, T. (2003). Role of diatoms in regulating the ocean's silicon cycle. Global Biogeochemical Cycles 17, 1103, doi:10.1029/2002GB002018.

External links

- [http://www.microscopy-uk.net/mag/artfeb05/cbdiatoms.html Geometry and Pattern in Nature 3: The holes in radiolarian and diatom tests] Category:Biological oceanography Category:Planktology Category:Heterokonts Category:Algae

Eukaryote

|- | style = "background: pink; padding: 4px;" | Animalia - Animals |- | style = "background: lightblue; padding: 4px;" | Fungi |- | style = "background: lightgreen; padding: 4px;" | Plantae - Plants |- | style = "background: khaki; padding: 4px;" | Protista A eukaryote (also spelled eucaryote) is an organism with complex cells, in which the genetic material is organized into membrane-bound nuclei. Eukaryotes comprise animals, plants, and fungi—which are mostly multicellular—as well as various other groups that are collectively classified as protists (many of which are unicellular). In contrast, other organisms, such as bacteria, lack nuclei and other complex cell structures; such organisms are called prokaryotes. The eukaryotes share a common origin, and are often treated formally as a superkingdom, empire, or domain. The name comes from the Greek eus (meaning true) and karyon (meaning nut, referring to the cell nucleus).

Structure

Eukaryotic cells are generally much larger than prokaryotes, typically a thousand times by volume. They have a variety of internal membranes and structures, called organelles, and a cytoskeleton composed of microtubules and microfilaments, which play an important role in defining the cell's organization. Eukaryotic DNA is divided into several bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. In addition to asexual cell division, most eukaryotes have some process of sexual reproduction via cell fusion, which is not found among prokaryotes. sexual reproduction

Internal membranes

Eukaryotic cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles or vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles. The nucleus is surrounded by a double membrane, with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form what is called the endoplasmic reticulum or ER, which is involved in protein transport. It includes the Rough ER where ribosomes are attached, and the proteins they synthesize enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the Smooth ER. In most eukaryotes, the proteins may be further modified in stacks of flattened vesicles, called Golgi bodies or dictyosomes. Vesicles may be specialized for various purposes. For instance, lysosomes contain enzymes that break down the contents of food vacuoles, and peroxisomes are used to break down peroxide which is toxic otherwise. Many protozoa have contractile vacuoles, which collect and expel excess water, and extrusomes, which expel material used to deflect predators or capture prey. In multicellular organisms, hormones are often produced in vesicles. In higher plants, most of a cell's volume is taken up by a central vacuole or tonoplast, which maintains its osmotic pressure.

Mitochondria and plastids

Mitochondria are organelles found in nearly all eukaryotes. They are surrounded by double membranes, the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. They contain their own DNA and are only formed by the fission of other mitochondria. They are now generally held to have developed from endosymbiotic prokaryotes, probably proteobacteria. The few protozoa that lack mitochondria have been found to contain mitochondrion-derived organelles, such as hydrogenosomes and mitosomes. Plants and various groups of algae also have plastids. Again, these have their own DNA and developed from endosymbiotes, in this case cyanobacteria. They usually take the form of chloroplasts, which like cyanobacteria contain chlorophyll and produce energy through photosynthesis. Others are involved in storing food. Although plastids likely had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion. Endosymbiotic origins have also been proposed for the nucleus and eukaryotic flagella, but this is not generally accepted, both from a lack of cytological evidence and difficulty in reconciling this with cellular reproduction.

Cytoskeletal structures

Many eukaryotes have slender motile projections, usually called flagella when long and cilia when short, that are variously involved in movement, feeding, and sensation. These are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a basal body, also called a kinetosome or centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs or mastigonemes, scales, connecting membranes and internal rods. Their interior is continuous with the cell's cytoplasm. Centrioles are often present even in cells and groups that do not have flagella. They generally occur in groups of one or two, called kinetids, that give rise to various microtubular roots. These form a primary component of the cytoskeletal structure, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles may also be associated in the formation of a spindle during nuclear division. Some protists have various other microtubule-supported organelles. These include the radiolaria and heliozoa, which produce axopodia used in flotation or to capture prey, and the haptophytes, which have a peculiar flagellum-like organelle called the haptonema.

Reproduction

Nuclear division is often coordinated with cell division. This generally takes place by mitosis, a process which allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes there is also a process of sexual reproduction, typically involving an alternation between haploid generations, where only one copy of each chromosome is present, and diploid generations, where two are present, occurring through nuclear fusion (syngamy) and meiosis. There is considerable variation in this pattern, however.

Origin and evolution

The origin of the eukaryotic cell was a milestone in the evolution of life because they became the ancestors of all multi-cellular organisms; of all animals, plants, and even complex single-celled organisms. Eukaryotes likely emerged from prokaryotic ancestry approximately 1.6 - 2.1 billion years ago (Knoll, 1992). Early fossils such as acritarchs are difficult to interpret. Forms that can be related to modern groups start appearing around 800 million years ago, and most fossil lines are known by the end of the Cambrian, around 500 million years ago. Genetic studies during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group, usually divided by the form of the cristae in their mitochondria. The few amitochondriate groups branched first on rRNA trees and so were considered basal, but this is now considered to be an error caused by long branch attraction. A new picture has been slow to develop. Most eukaryotes are now included in several supergroups: However, some protists are not closely related to any of these lines, and the relationships between the different supergroups remain almost entirely uncertain. In particular, there is dispute about where the root of the evolutionary tree belongs, and as a result what the earliest eukaryotes were like.

References

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- Category:Eukaryotes ko:진핵생물 ja:真核生物 th:ยูแคริโอต

Phytoplankton

Phytoplankton refers to the autotrophic component of the plankton that drifts in the water column. The name comes from the Greek terms, phyton or "plant" and

\pi\lambda\alpha\gamma\kappa\tau\nu

, meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high numbers, their presence may appear as discoloration of the water (the color of which may vary with the phytoplankton present). Phytoplankton, like plants, obtain energy through a process called photosynthesis, and so must live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, or lake. Through photosynthesis, phytoplankton (and terrestrial plants) are responsible for much of the oxygen present in the Earth's atmosphere. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and some freshwater food chains (though see also chemosynthesis). As a side note, one of the more remarkable food-chains in the ocean—remarkable because of the small number of links—is that of phytoplankton fed on by krill (a type of shrimp) fed on by baleen whales. While almost all phytoplankton species are obligate photoautotrophs, there are some that are mixotrophic and other, non-pigmented species that are actually heterotrophic (the latter are often viewed as zooplankton). Of these, the most well known are dinoflagellate genera such as Noctiluca and Dinophysis, that obtain organic carbon by ingesting other organisms or detrital material. In terms of numbers, the most important groups of phytoplankton include the diatoms, cyanobacteria and dinoflagellates, although many other groups of algae are represented. One group, the coccolithophorids, is responsible (in part) for the release significant amounts of dimethyl sulfide (DMS) into the atmosphere. DMS is converted to sulfate and these sulfate molecules act as cloud condensation nuclei, increasing general cloud cover.

Reference

- [http://saga.pmel.noaa.gov/review/dms_climate.html DMS and Climate] Category:Biological oceanography Category:Planktology

Colony (biology)

In biology, a colony (from Latin colonia) refers to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defences, the ability to attack bigger prey, etc. Some insects (ants and honey bees, for example) live only in colonies. The Portuguese Man o' War, is an example of a colony of four different polyps. A colony of single-celled organisms is known as a colonial organism. Colonial organisms were probably the first step towards multicellular organisms during evolution. The difference between a multicellular organism and a colonial organism is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular lifeform (e.g., liver cells) cannot. Volvox is an example for the border between these two states. For bacterial colony, it is defined as a cluster of microorganisms growing on the surface of or within a solid medium, usually cultured from a single cell

Silicate

In chemistry, a silicate is a compound consisting of silicon and oxygen (Si_xO_y), one or more metals, and possibly hydrogen. It is also used to denote the salts of silica or of one of the silicic acids. In common conditions, the most stable form is silicon dioxide, SiO₂, often called quartz, and similar species. This always has, in equilibrium, a minute amount of silicic acid, H₄SiO₄. Chemists consider quartz as insoluble, but it moves around at longer timescales. Also, in basic conditions, we find H₂SiO₄^2-. Silicate minerals are noted for their tetrahedral form. Sometimes the tetrahedra are joined in chains, double chains, sheets, and three-dimensional frameworks. They are subclassified into groups based on the degree of polymerization of the tetrahedra, such as nesosilicates, cyclosilicates, and so forth. In geology and astronomy, the term silicate is used to denote a type of rock that consists of silicon and oxygen (usually as SiO₂ or SiO₄), one or more metals, and possibly hydrogen. Such rocks range from granite to gabbro. Most of the Earth's crust is made up of silicate rocks, as are the crusts of other terrestrial planets. Mineralogically, silicate minerals are divided according to their molecular structure into the following groups:
- Olivine (single tetrahedron) - Nesosilicates
- Epidote (double tetrahedra) - Sorosilicates
- Tourmaline (rings of tetrahedra) - Cyclosilicates
- Pyroxene (single chain) - Inosilicates
- Amphibole (double chain) - Inosilicates
- Micas and clays (sheet) - Phyllosilicates
- Feldspars (framework) - Tectosilicates
- Quartz (SiO₂ framework) Silicate was also the name given to the bone-sucking monsters in the British horror movie Island of Terror (aka Night of the Silicates). These were silicon-based organisms created by cancer research gone wrong, which consumed the calcium phosphate in the bones of carbon-based lifeforms.

Species

In biology, a species is the basic unit of biodiversity. In scientific classification, a species is assigned a two-part name in Latin. The genus is listed first (and capitalized), followed by a specific epithet. For example, humans belong to the genus Homo, and are in the species Homo sapiens. The name of the species is the whole binomial not just the second term (the specific epithet). The binomial, and most other purely formal aspects of the biological codes of nomenclature, were formalized by Carolus Linnaeus in the 1700's and as a result are called the "Linnaean system". At that time, species were thought to represent independent acts of creation by God, and were therefore considered objectively real and immutable. Since the advent of the theory of evolution, the conception of species has undergone vast changes in biology, however no consensus on the definition of the word has yet been reached. The most commonly cited definition of "species" was first coined by Ernst Mayr. By this definition, called the biological species concept or isolation species concept, species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups". However, many other species concepts are also used (see other definitions of species below). The scientific name of a species is properly typeset in italics. When an unknown species is being referred to this may be done by using the abbreviation "sp." in the singular or "spp." in the plural in the place of the second part of the scientific name. Note that the word "specie" is not the singular of "species". It refers to coined money.

Definitions of species

The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist: ; Typological species : A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (ie: longer and shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, we now know that different phenotypes do not always constitute different species (e.g.: a 4-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies. ; Morphological species : A population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveals that genetically distinct populations may look very similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology. ; Biological / Isolation species : A set of actually or potentially interbreeding populations. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in instances of allopatric (geographically isolated) populations. The results of breeding experiments done in artificial conditions may or may not reflect what would happen if the same organisms encountered each other in the wild, making it difficult to gauge whether or not the results of such experiments are meaningful in reference to natural populations. ; Mate-recognition species : A group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually. Unlike the isolation species concept, it focuses specifically on pre-mating reproductive isolation. ; Phylogenetic / Evolutionary / Darwinian species : A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. ; Microspecies : Species that reproduce without meiosis or mitosis so that each generation is genetically identical to the previous generation. See also apomixis. In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition. For most vertebrates, this is the biological species concept, and to a lesser extent (or for different purposes) the phylogenetic species concept. Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete.

Importance in biological classification

The idea of species has a long history. It is one of the most important levels of classification, for several reasons:
- It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
- It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
- It is the only taxonomic level which has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them. After thousands of years of use, the concept remains central to biology and a host of related fields, and yet also remains at times ill-defined and controversial.

Implications of assignment of species status

The naming of a particular species should be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be confirmed or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or convivially, as lumping. Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. Many populations which were formerly regarded as separate species are now considered to be a single taxon, and many formerly grouped populations have been split. Any taxonomic level (species, genus, family, etc.) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound. From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspecies is used, while in botany the variety, subvariety, and form are used as well.

The isolation species concept in more detail

In general, for large, complex, organisms that reproduce sexually (such as mammals and birds), one of several variations on the isolation or biological species concept is employed. Often, the distinction between different species, even quite closely related ones, is simple. Horses (Equus caballus) and donkeys (Equus asinus) are easily told apart even without study or training, and yet are so closely related that they can interbreed after a fashion. Because the result, a mule or hinny, is not usually fertile, they are clearly separate species. But many cases are more difficult to decide. This is where the isolation species concept diverges from the evolutionary species concept. Both agree that a species is a lineage that maintains its integrity over time, that is diagnosably different to other lineages (else we could not recognise it), is reproductively isolated (else the lineage would merge into others, given the chance to do so), and has a working intra-species recognition system (without which it could not continue). In practice, both also agree that a species must have its own independent evolutionary history—otherwise the characteristics just mentioned would not apply. The species concepts differ in that the evolutionary species concept does not make predictions about the future of the population: it simply records that which is already known. In contrast, the isolation species concept refuses to assign the rank of species to populations that, in the best judgement of the researcher, would recombine with other populations if given the chance to do so.

The isolation question

There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Secondly, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.
- The proposed species are sympatric—they occupy the same habitat. Observation of many species over the years has failed to establish even a single instance of two diagnostically different populations that exist in sympatry and have then merged to form one united population. Without reproductive isolation, population differences cannot develop, and given reproductive isolation, gene flow between the populations cannot merge the differences. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species.
- The proposed species are allopatric—they occupy different geographical areas. Obviously, it is not possible to observe reproductive isolation in allopatric groups directly. Often it is not possible to achieve certainty by experimental means either: even if the two proposed species interbreed in captivity, this does not demonstrate that they would freely interbreed in the wild, nor does it always provide much information about the evolutionary fitness of hybrid individuals. A certain amount can be inferred from other experimental methods: for example, do the members of population A respond appropriately to playback of the recorded mating calls of population B? Sometimes, experiments can provide firm answers. For example, there are seven pairs of apparently almost identical marine snapping shrimp (Altheus) populations on either side of the Isthmus of Panama, which did not exist until about 3 million years ago. Until then, it is assumed, they were members of the same seven species. But when males and females from opposite sides of the isthmus are placed together, they fight instead of mating. Even if the isthmus were to sink under the waves again, the populations would remain genetically isolated: therefore they are now different species. In many cases, however, neither observation nor experiment can produce certain answers, and the determination of species rank must be made on a 'best guess' basis from a general knowledge of other related organisms.
- The proposed species are parapatric—they have breeding ranges that abut but do not overlap. This is fairly rare, particularly in temperate regions. The dividing line is often a sudden change in habitat (an ecotone) like the edge of a forest or the snow line on a mountain, but can sometimes be remarkably trivial. The parapatry itself indicates that the two populations occupy such similar ecological roles that they cannot coexist in the same area. Because they do not crossbreed, it is safe to assume that there is a mechanism, often behavioral, that is preventing gene flow between the populations, and that therefore they should be classified as separate species.
- There is a hybrid zone where the two populations mix. Typically, the hybrid zone will include representatives of one or both of the 'pure' populations, plus first-generation and back-crossing hybrids. The strength of the barrier to genetic transmission between the two pure groups can be assessed by the width of the hybrid zone relative to the typical dispersal distance of the organisms in question. The dispersal distance of oaks, for example, is the distance that a bird or squirrel can be expected to carry an acorn; the dispersal distance of Numbats is about 15 kilometres, as this is as far as young Numbats will normally travel in search of vacant territory to occupy after leaving the nest. The narrower the hybrid zone relative to the dispersal distance, the less gene flow there is between the population groups, and the more likely it is that they will continue on separate evolutionary paths. Nevertheless, it can be very difficult to predict the future course of a hybrid zone; the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial.
- The variation in the population is clinal; at either extreme of the population's geographic distribution, typical individuals are clearly different, but the transition between them is seamless and gradual. For example, the Koalas of northern Australia are clearly smaller and lighter in colour than those of the south, but there is no particular dividing line: the further south an individual Koala is found, the larger and darker it is likely to be; Koalas in intermediate regions are intermediate in weight and colour. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) Australian Magpies do not blend from one type to another: northern populations have black backs, southern populations white backs, and there is an extensive hybrid zone where both 'pure' types are common, as are crossbreeds. The variation in Koalas is clinal (a smooth transition from north to south, with populations in any given small area having a uniform appearance), but the variation in magpies is not clinal. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either. The gene flow between northern and southern magpie populations is judged to be sufficiently restricted to justify terming them subspecies (not full species); but the seamless way that local Koala populations blend one into another shows that there is substantial gene flow between north and south. As a result, experts tend to reject even subspecies rank in this case.

The difference question

Obviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends". In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case. More typically, one or other of the following requirements must be met:
- It is possible to reliably measure a quantitative difference between the two groups that does not overlap. A population has, for example, thicker fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to populations, not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies within a population and the more it varies between populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.
- It is possible to distinguish a qualitative difference between the populations; a feature that does not vary continuously but is either entirely present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence. Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit. When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two).

Historical development of the species concept

In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. Aristotle used the words genus and species to mean generic and specific categories. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. To the modern mind, many of the schemes delineated are whimsical at best, such as those that determined consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants). In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At the time, it was still widely believed that there is no organic connection between species, no matter how similar they appear; every species was individually created by God, a view today called creationism. This approach also suggested a type of idealism: the notion that each species exists as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect. By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. As such, the new emphasis was on determining how a species could change over time. Jean-Baptiste Lamarck suggested that an organism could pass on an acquired trait to its offspring, i.e., the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). Lamarck's most important insight may have been that species can be extraordinarily fluid; his 1809 Zoological Philosophy contained one of the first logical refutations of creationism. With the acceptance of the work of Charles Darwin in the 1860s, Lamarck's view of evolution was quickly eclipsed. It was not until the late 20th century that his work began to be reexamined, and took its place as a fundamental stepping stone to the modern theory of adaptive mutation. Lamarck's long-discarded ideas of the goal-oriented evolution of species, also known the teleological process, have also received renewed attention, particularly by proponents of artificial selection. Charles Darwin and Alfred Wallace provided what scientists now consider the most powerful and compelling theory of evolution. Basically, Darwin argued that it is populations that evolve, not individuals. His argument relies on a radical shift in perspective from Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive will be eliminated. It should be emphasized that whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. In this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory are genes. The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species. Although the current scientific understanding of species suggests there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but nonetheless has some problems, however. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this notion of species. In recent years we have witnessed the drastic reduction in the size of breeding populations and the geographical range of many physically large mammals. In earlier times it was assumed that every species existed in at least a few thousand living individuals, except very rare relic, isolated groups. In the present, many well know mammal & bird species are so stressed by habitat loss, and other effects of the modern world, that only a very few breeding males may contribute the genetic material to a small number of breeding females. In these highly stressed conditions, the likelihood of change is very much greater. Mammals may become smaller, have darker fur, more stripes, more cautious behavior, even over time learn to co-exist with the human world. Very likely, evolution is radically accelerated, and we are only beginning to notice it. Species in transition before our eyes. It is possible that this severe stress is essential to the creation of new species, and may have been a prime factor throughout biological history, from other population reducing influences. Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree. For example, in many amphibians, most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes which are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century and coming into full swing with Charles Sibley's ground-breaking DNA-DNA hybridisation studies in the 1970s. The results of the technique caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. Instances of hybridization can result in misleading molecular data, the Pomarine Skua - Great Skua phenomenon being a famous example. Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. The hybridization technique is however no longer considered a good technique and more reliable computational techniques for sequence comparison are now used for. Molecular taxonomy does not directly measure the evolutionary processes, but rather the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself.

External links

- http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
- [http://www.sciencedaily.com/releases/2003/12/031231082553.htm 2003-12-31, ScienceDaily: Working On The 'Porsche Of Its Time': New Model For Species Determination Offered] Quote: "...two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone. "This is a lot less variation than I'd expected", said Novak..."
- [http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."
- [http://www.sciencedaily.com/releases/2004/01/040109064407.htm 2004-01-09 ScienceDaily: Mayo Researchers Observe Genetic Fusion Of Human, Animal Cells; May Help Explain Origin Of AIDS] Quote: "...The researchers have discovered conditions in which pig cells and human cells can fuse together in the body to yield hybrid cells that contain genetic material from both species... "What we found was completely unexpected", says Jeffrey Platt, M.D."
- [http://www.sciencedaily.com/releases/2000/09/000913211733.htm 2000-09-18, ScienceDaily: Scientists Unravel Ancient Evolutionary History Of Photosynthesis] Quote: "...gene-swapping was common among ancient bacteria early in evolution..."
- [http://plato.stanford.edu/entries/species/ Stanford Encyclopedia of Philosophy entry]
- [http://www.barcodinglife.org/ Barcoding of species] rank22 rank22 ms:Spesies ja:種 (生物) th:สปีชีส์

Ocean

:For other uses see Ocean (disambiguation) Ocean (disambiguation)] Ocean (from Okeanos, Greek for river, the ancient Greeks noticed that a strong current flowed off Gibraltar, and assumed it was a great river); covers almost three quarters (71%) of the surface of the Earth, and nearly half of the world's marine waters are over 3000 m deep. This global, interconnected body of salt water, called the World Ocean, is divided by the continents and archipelagos into the following four bodies, from the largest to the smallest: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean, and, according to some authorities such as International Hydrographic Organization(IHO), a fifth ocean, the Southern Ocean. Some geographers and some governments but not the US, recognize the IHO as defining official water body names and boundaries. (The US authority is the United States Board on Geographic Names.) The IHO officially sanctioned the Southern Ocean name only in 2000, but its definition by a line of latitude (with IHO members widely disputing which line of latitude) has left its acceptance as a fifth ocean open to question. The National Geographic Society and some other leading geographers and cartographers continue to use "South Pacific", "South Atlantic", and "Indian" Ocean for the waters around Antarctica. A few Oceanographers recognize only four oceans also, treating the Arctic Ocean (or the Arctic Sea) as a part of the Atlantic Ocean. Smaller regions of the oceans are called seas, gulfs, straits and other names. Geologically, an ocean is an area of oceanic crust covered by water. Oceanic crust is the thin layer of solidified volcanic basalt that covers the Earth's mantle where there are no continents. From this point of view, there are three "oceans" today: the World Ocean, and the Black and Caspian Seas that were formed by the collision of Cimmeria with Laurasia. The Mediterranean Sea is very nearly its own "ocean", being connected to the World Ocean through the Strait of Gibraltar, and indeed several times over the last few million years movement of the African Continent has closed the strait off entirely, making the Mediterranean a fourth "ocean". (The Black Sea is connected to the Mediterranean through the Bosporus, but this is in effect a natural canal, cut through continental rock some 7000 years ago, rather than a piece of oceanic sea floor like the Strait of Gibraltar.) The area of the World Ocean is 361 million km², its volume is 1370 million km³, and its average depth is 3790 m. Nearly half of the world's marine waters are over 3000 m deep. This does not include seas not connected to the World Ocean, such as the Caspian Sea. The total mass of the hydrosphere is about 1.4 × 10²¹ kg, ca. 0.023 % of the Earth's total mass. See sea water for a detailed discussion of ocean water composition, most notably its salinity.

Origins

The Oceans of the world most likely originated by comets striking the Earth.

Exploration

salinity Travel on the surface of the ocean through the use of boats dates back to prehistoric times, but only in modern times has extensive underwater travel become possible. The deepest point in the ocean is the Mariana Trench located in the Pacific Ocean near the Northern Mariana Islands. It has a maximum depth of 10,923 m (35,838 ft) [http://www.rain.org/ocean/ocean-studies-challenger-deep-mariana-trench.html]. It was fully surveyed in 1951 by the British naval vessel, "Challenger II" which gave its name to the deepest part of the trench, the "Challenger Deep". Much of the bottom of the world's oceans is unexplored and unmapped. A global image of many underwater features larger than 10 km was created in 1995 based on gravitational distortions of the nearby sea surface.

Climate

One of the most dramatic forms of weather occurs over the oceans: tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms). Ocean currents greatly affect Earth's climate by transferring warm or cold air and precipitation to coastal regions, where they may be carried inland by winds. The Antarctic Circumpolar Current encircles that continent, influencing the area's climate and connecting currents in several oceans.

Ecology

The oceans are home to the majority of plant and animal life on Earth. These lifeforms include:
- fish
- cetacea such as whales, dolphins and porpoises,
- cephalopods such as the octopus
- crustaceans such as lobsters and shrimp
- marine worms
- plankton
- krill

Economy

The oceans are essential to transportation: a huge portion of the world's goods are moved by ship between the world's seaports. Important ship canals include the Saint Lawrence Seaway, Panama Canal, and Suez Canal.

Ancient oceans

Continental drift has reconfigured the Earth's oceans, joining and splitting ancient oceans to form the current oceans. Ancient oceans include:
- Panthalassa, the vast world ocean that surrounded the Pangaea supercontinent.
- Tethys Ocean, the ocean between the ancient continents of Gondwana and Laurasia.
- Iapetus Ocean, the southern hemisphere ocean between Baltica and Avalonia.

Ocean rowing

Extraterrestrial oceans

Earth is the only known planet with liquid water on its surface, and is certainly the only such in our own solar system. However, liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Europa, and, with less certainty, its fellows Callisto and Ganymede. Other icy moons may have once had internal oceans that have now frozen, such as Triton. The planets Uranus and Neptune may also possess large oceans of liquid water under their thick atmospheres, though their internal structure is not well understood at this time. There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did; recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known. Liquid hydrocarbons are thought to be present on the surface of Titan, though it may be more accurate to describe them as "lakes" rather than an "ocean". The distribution of these liquid regions will hopefully be better known after the full analysis of data from the Huygens probe of the Cassini-Huygens space mission, which dropped onto Titan's surface in January 2005. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.

Oceans in film

- In the movie Muppet Treasure Island, a non-specific ocean is featured, and referred to as the "Big Blue Wet Thing". Oceans have also been featured in many other movies such as Free Willy. To list more, click edit beside "Oceans on Film"

External links

- [http://www.oceanexplorer.noaa.gov/ Ocean Explorer] - An educational and reference resource from NOAA
- [http://news.bbc.co.uk/2/hi/science/nature/4033555.stm Science taps into ocean secrets]
- [http://www.palomar.edu/oceanography/salty_ocean.htm Why is the ocean salty?]
- [http://ioc.unesco.org/oceanteacher/resourcekit/M3/Formats/Geography/OceansSeas.htm Official IHO boundaries of Oceans and Seas]
- [http://www.thehydrogenexpedition.com The Hydrogen Expedition] The first circumnavigation of the globe in a hydrogen fuel cell powered boat
- [http://www.coreocean.org Coreocean]
- [http://www.nopp.org/ NOPP - The National Oceanographic Partnership Program] Category:Bodies of water Category:Oceanography zh-min-nan:Hái-iûⁿ ko:대양 ms:Lautan ja:大洋 simple:Ocean th:มหาสมุทร

Freshwater

:For the village on the Isle of Wight, see Freshwater, Isle of Wight. Fresh water (also freshwater or fresh-water) is water that contains only minimal quantities of dissolved salts, especially sodium chloride, thus distinguishing it from sea water or brackish water. All freshwater ultimately comes from precipitation of atmospheric water vapor, reaching inland lakes, rivers, and groundwater bodies directly, or after melting of snow or ice (see hydrologic cycle). Access to fresh water is a critical issue for the survival of many species, including humans, especially in desert or otherwise arid areas. See water resources. Even on a ship or island in the ocean, there can be a "water shortage", which means a shortage of fresh water. Seawater is undrinkable directly. For fish, it strongly matters how much dissolved sodium chloride the water they live in has. Most species cannot live in both fresh and salt water, though some species move between the two. Salt water fish have access to an abundance of salt, and try to get as much salt out of their body as possible, while trying to keep the water. Fresh water fish do the opposite: they have too much water, and too little salt. simple:Fresh water

Soil

Soil is unconsolidated rock particles mixed with organic matter from plant decay. Soil is vital to all life on Earth because it supports the growth of plants, which supply food and oxygen and absorbs carbon dioxide and nitrogen. Soil serves as a habitat for animal life from microorganisms to small animals.

Soil components

Soils vary widely in composition and structure from place to place. Soils are formed through the weathering of rock and the breakdown of organic matter. Weathering is the action of wind, rain, ice, sunlight and biological processes on rocks, which breaks them down into small particles. The proportions of minerals and organic matter determine the structure and other characteristics of a particular soil. Soils can be divided into two general layers or strata: topsoil, the topmost layer, where most plant roots, microorganisms, and other animal life are located, and subsoil, which is deeper and often more dense and less rich in organic matter. Water and air are also components of most soils. Air, trapped in spaces between soil particles, and water, trapped in spaces and on the surface of particles, comprises about half of the soil by volume. Both are important to plant growth and other life in the soil profile of a particular ecosystem. The rock and mineral content of soil is categorized according to particle size as sand (coarsest), silt or clay (finest); the ratio of these particles to a great degree determines the soil classification and characteristics. Former soils which become buried below the effects of organisms are called paleosols. Soil develops naturally over time through the action of plants, animals, and weathering. Soil is also affected by human habitation. People can alter soil to make it more suitable for plant growth through the addition of organic material and natural or synthetic fertilizer, and by improving its drainage or water-retaining capacity. Human actions also can degrade soil through the depletion of nutrients, pollution, contamination, and compaction, and by increasing the rate of erosion, which is the relocation of soil through the movement of water or wind.

Natural soil development

An example of soil development from bare rock occurs on recent lava flows in warm regions under heavy and very frequent rainfall. In such climates plants become established very quickly on basaltic lava, even though there is very little organic material. The plants are supported by the porous rock becoming filled with nutrient bearing water, for example carrying dissolved bird droppings or guano. The developing plant roots themselves gradually break up the porous lava and organic matter soon accumulates but, even before it does, the predominantly porous broken lava in which the plant roots grow can be considered soil.

Chemical processes in soils

Weathering releases ions such as Potassium (K⁺) and Magnesium (Mg²⁺) into the soil solution. Some of these elements (as ions) are taken up by plants, but the majority not left in solution are absorbed through ion exchange by clays such as montmorillonite. When the level of ions is low in the soil an equilibrium process forces ions back into solution, where they can be used by plants. However if acid is introduced into soil, e.g. by acid rain, hydrogen ions bind in preference to clays, forcing ions out where they can be washed away during rain. Acidity also encourages the weathering of clays, releasing toxic aluminium ions (of which clays are composed) into the solution. To stop this occurring, farmers may apply alkaline materials such as slaked lime. Although the elements nitrogen, potassium and phosphorus, which are necessary for plant growth, may be abundant in soil, only a fraction of these elements may be in a chemical form which plants can use. In processes such as nitrification and mineralisation, bacteria and other organisms convert unusable forms (such as NH₄⁺) to usable forms (such as NO₃^-). The raw products are initially present as gases in the atmosphere. Processes such as the nitrogen cycle and carbon cycle continually exchange nutrients between the soil and atmosphere. The organic component of soils originate in plant debris (such as fallen leaves), animal excreta, and other decomposing organic materials. These materials, when broken down, form humus, a dark, nutrient-rich material. Chemically, humus is composed of very large molecules including esters of carboxylic acid, phenolic compounds, and derivatives of benzene. Organic material in soil provides nutrients necessary for plant growth. Organic material also contributes to water retention, drainage ability, and oxygenation of soil. If oxygen enters a wet soil, because of lowered ground water table, organic matter in the soil will be broken down further by oxidation, which can lead to subsidence. An example of this can be seen in soils in the Everglades region of Florida, which have been drained by canals for agriculture, primarily sugar production. Originally very high in organic content, oxygenation and compaction have led to breakdown of the soil structure and nutrient content, and degradation of the soil's ability to support continued high crop yields.

Biological processes in soil

Wetland soil processes

The diffusion of dissolved oxygen in saturated soils is slower than in unsaturated soils. Wetland (also referred to as hydric) soils form due to soil microbial cellular respiration in excess of soil oxygen supply, resulting in oxygen depletion. Anaerobic soil chemistry results, which creates a reducing environment. This eliminates plants and creatures not adapted for life in saturated soil conditions.

Biological soil crusts

Biological soil crusts are formed by living organisms and their by-products, creating a surface crust of soil particles bound together by organic materials.

References

- Soil Survey Staff. (1975) Soil Taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office. Washington, DC.
- Soil Survey Division Staff. (1993) Soil survey manual. Soil Conservation Service. U.S. Department of Agriculture Handbook 18.
- Logan, W. B., Dirt: The ecstatic skin of the earth. 1995 ISBN 1573220043
- Faulkner, William. Plowman's Folly. New York, Grosset & Dunlap. 1943. ISBN 0933280513
- Jenny, Hans, Factors of Soil Formation: A System of Quantitative Pedology 1941
- [http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec1/Lec1.html Why Study Soils?]
- [http://www.hort.purdue.edu/newcrop/tropical/lecture_06/chapter_12l_R.html Soil notes]
- [http://www.home2garden.org/soil.html Soil articles]

Pelagic

The pelagic zone is the part of the open sea or ocean comprising the water column, i.e. all of the sea other than that near the coast or the sea floor. In contrast, the demersal zone comprises the water that is near to (and thus is significantly affected by) the coast or the sea floor. The name is derived from the Greek πέλαγος (pélagos), which is roughly translated as "sea" but is more accurately translated as "open sea". The pelagic zone is further divided into a number of sub-zones, based on their different ecological characteristics (which is roughly a function of depth):
- Epipelagic (from the surface down to around 200 metres) - the illuminated surface zone where there is enough light for photosynthesis, and thus plants and animals are largely concentrated in this zone. Here one will typically encounter fish such as tuna and many sharks.
- Mesopelagic (from 200 metres down to around 1000 metres) - the twilight zone. Although some light penetrates this deep, it is insufficient for photosynthesis. The name stems from Greek μέσον, middle.
- Bathypelagic (from 1000 metres down to around 4000 metres) - by this depth the ocean is almost entirely dark (with only the occasional bioluminescent organism). There are no living plants, and most animals survive by consuming the snow of detritus falling from the zones above, or (like the marine hatchetfish) by preying upon others. Giant squid live at this depth, and here they are hunted by deep-diving sperm whales. From Greek βάθος (vathos), depth, and βαθύς (vathys), deep.
- Abyssopelagic (from 4000 metres down to above the ocean floor) - no light whatsoever penetrates to this depth, and most creatures are blind and colourless. The name is derived from the greek άβυσσος (ábyssos), abyss, meaning bottomless (a holdover from the times when the deep ocean was believed to be bottomless).
- Hadopelagic (the deep water in ocean trenches) - the name is derived from Hades, the classical greek underworld. This zone is 90% unknown, only very few species live here. The epipelagic and (arguably) the mesopelagic zones together comprise the open ocean's photic zone. The remaining (lower) zones comprise the open ocean's aphotic zone. The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some marine biologists elide them into a single zone or consider the latter two to be the same. Some define the hadopelagic as waters below 6000 meters, whether in a trench or not.

External links

- [http://oceanlink.island.net/oinfo/deepsea/deepsea.html The Deep Sea pages at Oceanlink]
- [http://maritime.haifa.ac.il/departm/lessons/ocean/lect25.htm University of Haifa's pages on deep sea oceanography] Category:Fisheries science Category:Oceanography

Primary production

Primary production is the production of biological organic compounds from inorganic materials through photosynthesis or chemosynthesis. Organisms that can create biomass in this manner (notably plants) are known as primary producers, and form the basis of the food chain. Primary production in oceanography and limnology refers to the production of biomass by phytoplankton in aquatic environments utilizing energy from sunlight. The small algal cells are then consumed by - and provide organic building blocks and energy for - small animals such as ciliates, copepods, and krill. Primary production is the basis for all life in the open oceans. Primary production is a flux that can be measured during a unit of time by 3 techniques: variations of oxygen concentrations in a bottle (technique used from 1927), incorporation of Carbon 14 (the ¹⁴C atom replaces the natural ¹²C in a molecule of sodium bicarbonate; Steeman-Nielsen technique of 1952) and modulated in vivo fluorescence (a more recent technique that is under development at present). However, the most common technique is the ¹⁴C technique using different incubator (under natural or artificial light) and different unit of time (from hours to day). According to the incubation time, net or growth primary production can be estimated. Net primary production will be estimated under short incubation time (1 hour), because loss (by respiration and organic material excretion) of ¹⁴C incorporated during incubation will be very limited. However, loss processes will be greater using long incubation time. Loss processes can range between 10 and 60% of incorporated ¹⁴C according to environmental conditions and species. So primary production is a complex process and rate estimations in a natural marine system of carbon flux must be considered cautiously. Category:Ecology Category:Photosynthesis ja:基礎生産

Heterokont

Colored groups
Chrysophyceae (golden algae)
Synurophyceae
Actinochrysophyceae (axodines)
Pelagophyceae
Phaeothamniophyceae
Bacillariophyceae (diatoms)
Raphidophyceae
Eustigmatophyceae
Xanthophyceae (yellow-green algae)
Phaeophyceae (brown algae)

Colorless groups
Oomycetes (water moulds)
Hypochytridiomycetes
Bicosoecea
Labyrinthulomycetes (slime nets)
Opalinea
Proteromonadea The heterokonts or stramenopiles are a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton. However some are colorless, most notably the parasitic water moulds, which superficially resemble fungi. The exact circumscription and treatment of the group varies considerably.

Chloroplasts

Heterokont algae have chloroplasts surrounded by four membranes, the last of which is continuous with the endoplasmic reticulum. These suggest that they were derived from a symbiotic eukaryote, presumably a red alga. The chloroplasts characteristically contain chlorophyll a and c, and usually the accessory pigment fucoxanthin, giving them a golden-brown or brownish-green color. Most basal heterokonts are colorless, suggesting they branched off before the appearance of chloroplasts within the group. However, fucoxanthin-containing chloroplasts are also found among the haptophytes, and there is some evidence the two groups share a common origin, and possibly the cryptomonads as well. In that case the ancestral heterokont was an alga, and all colorless groups arose through chloroplast loss.

Motile cells

Many heterokonts are unicellular flagellates, and most others produce flagellate cells at some point in their life-cycle, for instance as gametes or zoospores. The name heterokont refers to the characteristic form of these cells, which typically have two unequal flagella. The anterior or tinsel flagellum is covered with lateral bristles or mastigonemes, while the other flagellum is whiplash, smooth and usually shorter, or sometimes reduced to a basal body. The flagella are inserted subapically or laterally, and are usually supported by four microtubule roots in a distinctive pattern. Mastigonemes are manufactured from glycoproteins in the cell's endoplasmic reticulum before being transported to its surface. When the tinsel flagellum moves, these create a backwards current, pulling the cell through the water or bringing in food. The mastigonemes have a peculiar tripartite structure, which may be taken as the defining characteristic of the group, thereby including a few protists that do not produce cells with the typical heterokont form. They have been lost in a few lines, most notably the diatoms.

Classification

As noted above, classification varies considerably. Originally the heterokont algae were treated as two divisions, first within the kingdom Plantae and later the Protista: Division Chrysophyta
Class Chrysophyceae (golden algae)
Class Bacillariophyceae (diatoms)
Division Phaeophyta (brown algae) In this scheme, however, the Chrysophyceae are paraphyletic to both other groups. As a result, various members have been given their own classes and often divisions. Recent systems often treat these as classes within a single division, called the Heterokontophyta, Chromophyta or Ochrophyta. This is not universal, however - for instance Round et al. treat the diatoms as a division. The discovery that water molds and hypochytrids are related to these algae, rather than fungi as previously thought, has led many authors to include them among the heterokonts. Should it turn out that they evolved from colored ancestors, the group would be paraphyletic in their absence. Once again, however, usage varies. Patterson named this extended group the stramenopiles, characterized by the presence of tripartite mastigonemes, mitochondria with tubular cristae, and open mitosis. He used the stramenopiles as a prototype for a classification without Linnaean ranks. Their composition has been essentially stable, but their use within ranked systems varies. Cavalier-Smith treats the heterokonts as identical in composition with the stramenopiles; this is the definition followed here. He has proposed placing them in a separate kingdom Chromista, together with the haptophytes and cryptomonads. This is one of the most common revisions to the five-kingdom system, but has not been generally adopted, partly because some biologists doubt their monophyly. A few treat the Chromista as identical in composition with the heterokonts, or list them as a kingdom Stramenopila.

External links

- [http://tolweb.org/tree?group=Stramenopiles&contgroup=Eukaryotes Tree of Life Web Project: Stramenopiles]
- [http://www.personal.psu.edu/users/j/s/jsf165/Bio110.html Jacob Feldman: Stramenopila] Category:Protista Category:Algae
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Autotroph

An autotroph (from the Greek autos = self and trophe = nutrition) is an organism that produces

Diatoms

General biology

Ecology

Evolutionary history

Classification

Collection

References

See Also

External links

Eukaryote

Structure

Internal membranes

Mitochondria and plastids

Cytoskeletal structures

Reproduction

Origin and evolution

References

Phytoplankton

See also

Reference

Colony (biology)

See also

Silicate

See also

Species

Definitions of species

Importance in biological classification

Implications of assignment of species status

The isolation species concept in more detail

The isolation question

The difference question

Historical development of the species concept

See also

External links

Ocean

Origins

Exploration

Climate

Ecology

Economy

Ancient oceans

Ocean rowing

Extraterrestrial oceans

Oceans in film

See also

External links

Freshwater

Soil

Soil components

Natural soil development

Chemical processes in soils

Biological processes in soil

Wetland soil processes

Biological soil crusts

References

See also:

Pelagic

See also

External links

Primary production

Heterokont

Chloroplasts

Motile cells

Classification

External links

Autotroph