Бір жасушалы балдырлардың бордың қалыптасуына жауапты болуы

Unicellular algae responsible for the formation of chalk

Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom Protista, according to Robert Whittaker's five kingdom system, or clade Hacrobia, according to a newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae). Coccolithophores are almost exclusively marine, are photosynthetic, and exist in large numbers throughout the sunlight zone of the ocean. Coccolithophores are the most productive calcifying organisms on the planet, covering themselves with a calcium carbonate shell called a coccosphere. However, the reasons they calcify remain elusive. One key function may be that the coccosphere offers protection against microzooplankton predation, which is one of the main causes of phytoplankton death in the ocean. Coccolithophores are ecologically important, and biogeochemically they play significant roles in the marine biological pump and the carbon cycle. Depending on habitat, they can produce up to 40 percent of the local marine primary production. Management strategies are being employed to prevent eutrophication related coccolithophore blooms, as these blooms lead to a decrease in nutrient flow to lower levels of the ocean. The most abundant species of coccolithophore, Emiliania huxleyi, belongs to the order Isochrysidales and family Noëlaerhabdaceae. This makes E. huxleyi an important part of the planktonic base of a large proportion of marine food webs. It is also the fastest growing coccolithophore in laboratory cultures. It is studied for the extensive blooms it forms in nutrient depleted waters after the reformation of the summer thermocline. and for its production of molecules known as alkenones that are commonly used by earth scientists as a means to estimate past sea surface temperatures.

The Great Calcite Belt of the Southern Ocean is a region of elevated summertime upper ocean calcite concentration derived from coccolithophores, despite the region being known for its diatom predominance. The overlap of two major phytoplankton groups, coccolithophores and diatoms, in the dynamic frontal systems characteristic of this region provides an ideal setting to study environmental
influences on the distribution of different species within these taxonomic groups. The Great Calcite Belt, defined as an elevated particulate inorganic carbon (PIC) feature occurring alongside seasonally elevated chlorophyll a in austral spring and summer in the Southern Ocean, plays an important role in climate fluctuations, accounting for over 60% of the Southern Ocean area (30–60° S). The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO2) alongside the North Atlantic and North Pacific oceans.

Recent studies show that climate change has direct and indirect impacts on Coccolithophore distribution and productivity. They will inevitably be affected by the increasing temperatures and thermal stratification of the top layer of the ocean, since these are prime controls on their ecology, although it is not clear whether global warming would result in net increase or decrease of coccolithophores. As they are calcifying organisms, it has been suggested that ocean acidification due to increasing carbon dioxide could severely affect coccolithophores.

Coccolithophores are one of the more abundant primary producers in the ocean. As such, they are a large contributor to the primary productivity of the tropical and subtropical oceans, however, exactly how much has yet to have been recorded.

The ratio between the concentrations of nitrogen, phosphorus and silicate in particular areas of the ocean dictates competitive dominance within phytoplankton communities. Each ratio essentially tips the odds in favor of either diatoms or other groups of phytoplankton, such as coccolithophores. A low silicate to nitrogen and phosphorus ratio allows coccolithophores to outcompete other phytoplankton species; however, when silicate to phosphorus to nitrogen ratios are high coccolithophores are outcompeted by diatoms. The increase in agricultural processes lead to eutrophication of waters and thus, coccolithophore blooms in these high nitrogen and phosphorus, low silicate environments. Therefore, the overall result of large blooms of coccolithophores is a decrease in water column productivity, rather than a contribution to global warming.

Their predators include the common predators of all phytoplankton including small fish, zooplankton, and shellfish larvae. Viruses specific to this species have been isolated from several locations worldwide and appear to play a major role in spring bloom dynamics.

No environmental evidence of coccolithophore toxicity has been reported, but they belong to the class Prymnesiophyceae which contain orders with toxic species. Toxic species have been found in the genera Prymnesium Massart and Chrysochromulina Lackey. Members of the genus Prymnesium have been found to produce haemolytic compounds, the agent responsible for toxicity. Some of these toxic species are responsible for large fish kills and can be accumulated in organisms such as shellfish; transferring it through the food chain. In laboratory tests for toxicity members of the oceanic coccolithophore genera Emiliania, Gephyrocapsa, Calcidiscus and Coccolithus were shown to be non toxic as were species of the coastal genus Hymenomonas, however several species of Pleurochrysis and Jomonlithus, both coastal genera were toxic to Artemia. Trade offs associated with these faster growth rates include a smaller cell radius and lower cell volume than other types of phytoplankton.

Giant DNA containing viruses are known to lytically infect coccolithophores, particularly E. huxleyi. These viruses, known as E. huxleyi viruses (EhVs), appear to infect the coccosphere coated diploid phase of the life cycle almost exclusively. It has been proposed that as the haploid organism is not infected and therefore not affected by the virus, the co evolutionary "arms race" between coccolithophores and these viruses does not follow the classic Red Queen evolutionary framework, but instead a "Cheshire Cat" ecological dynamic. More recent work has suggested that viral synthesis of sphingolipids and induction of programmed cell death provides a more direct link to study a Red Queen like coevolutionary arms race at least between the coccolithoviruses and diploid organism. The oldest known coccolithophores are known from the Late Triassic, around the Norian Rhaetian boundary. Diversity steadily increased over the course of the Mesozoic, reaching its apex during the Late Cretaceous. However, there was a sharp drop during the Cretaceous Paleogene extinction event, when more than 90% of coccolithophore species became extinct. Coccoliths reached another, lower apex of diversity during the Paleocene Eocene thermal maximum, but have subsequently declined since the Oligocene due to decreasing global temperatures, with species that produced large and heavily calcified coccoliths most heavily affected. The coccoliths are created inside the coccolithophore cell and while some species maintain a single layer throughout life only producing new coccoliths as the cell grows, others continually produce and shed coccoliths.

The primary constituent of coccoliths is calcium carbonate, or chalk. Calcium carbonate is transparent, so the organisms' photosynthetic activity is not compromised by encapsulation in a coccosphere.

Coccoliths are produced by a biomineralization process known as coccolithogenesis. Although not yet entirely understood, the biomineralization process is tightly regulated by calcium signaling. Calcite formation begins in the golgi complex where protein templates nucleate the formation of CaCO3 crystals and complex acidic polysaccharides control the shape and growth of these crystals. As each scale is produced, it is exported in a Golgi derived vesicle and added to the inner surface of the coccosphere. This means that the most recently produced coccoliths may lie beneath older coccoliths. Depending upon the phytoplankton's stage in the life cycle, two different types of coccoliths may be formed. Holococcoliths are produced only in the haploid phase, lack radial symmetry, and are composed of anywhere from hundreds to thousands of similar minute (ca 0.1 μm) rhombic calcite crystals. These crystals are thought to form at least partially outside the cell. Heterococcoliths occur only in the diploid phase, have radial symmetry, and are composed of relatively few complex crystal units (fewer than 100). Although they are rare, combination coccospheres, which contain both holococcoliths and heterococcoliths, have been observed in the plankton recording coccolithophore life cycle transitions. Finally, the coccospheres of some species are highly modified with various appendages made of specialized coccoliths. <! Deleted image removed: [[File:Figure 1. jpg|frame|alt=Figure 1|Figure 1: Main types of coccoliths produced. Adapted from Figure 1 of Figure 1 of (Young, et al., 2003) In addition, these exoskeletons may confer an advantage in energy production, as coccolithogenesis seems highly coupled with photosynthesis. Organic precipitation of calcium carbonate from bicarbonate solution produces free carbon dioxide directly within the cellular body of the alga, this additional source of gas is then available to the Coccolithophore for photosynthesis. It has been suggested that they may provide a cell wall like barrier to isolate intracellular chemistry from the marine environment. More specific, defensive properties of coccoliths may include protection from osmotic changes, chemical or mechanical shock, and short wavelength light. It has also been proposed that the added weight of multiple layers of coccoliths allows the organism to sink to lower, more nutrient rich layers of the water and conversely, that coccoliths add buoyancy, stopping the cell from sinking to dangerous depths. Coccolith appendages have also been proposed to serve several functions, such as inhibiting grazing by zooplankton. The diagram on the right shows the energetic costs of coccolithophore calcification:

(A) Transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca2+ and HCO3− (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca2+ through the cytoplasm to the CV is the dominant cost associated with calcification. Adapted cells would have to activate proton channels more frequently, adjust their membrane potential, and/or lower their internal pH. Reduced intra cellular pH would severely affect the entire cellular machinery and require other processes (e. g. photosynthesis) to co adapt in order to keep H+ efflux alive. The obligatory H+ efflux associated with calcification may therefore pose a fundamental constraint on adaptation which may potentially explain why "calcification crisis" were possible during long lasting (thousands of years) CO2 perturbation events even though evolutionary adaption to changing carbonate chemistry conditions is possible within one year. with generally inefficient CO2 fixing RuBisCO enzymes may even profit from chemical changes since photosynthetic carbon fixation as their source of structural elements in the form of cellulose should be facilitated by the ocean acidification associated CO2 fertilization. Under the assumption that any form of shell/exoskeleton protects phytoplankton against predation

Coccolith fossils are prominent and valuable calcareous microfossils. They are the largest global source of biogenic calcium carbonate, and significantly contribute to the global carbon cycle. They are the main constituent of chalk deposits such as the white cliffs of Dover. Of particular interest are fossils dating back to the Palaeocene Eocene Thermal Maximum 55 million years ago. This period is thought to correspond most directly to the current levels of in the ocean. Finally, field evidence of coccolithophore fossils in rock were used to show that the deep sea fossil record bears a rock record bias similar to the one that is widely accepted to affect the land based fossil record.

The coccolithophorids help in regulating the temperature of the oceans. They thrive in warm seas and release dimethyl sulfide (DMS) into the air whose nuclei help to produce thicker clouds to block the sun. When the oceans cool, the number of coccolithophorids decrease and the amount of clouds also decrease. When there are fewer clouds blocking the sun, the temperature also rises. This, therefore, maintains the balance and equilibrium of nature.