Coccolithophorid

Coccolithaceae

Temporal Range: Late Triassic - Present[1]

The coccolithophore Gephyrocapsa oceanica
Scientific classification
Domain: Eukaryota
Kingdom: Chromalveolata
Phylum: Haptophyta
Class: Coccolithophyceae
(Prymnesiophyceae) Rothmaler 1951
Order: Isochrysidales,
Coccolithales

Coccolithophores (also called coccolithophorids) are unicellular, eukaryotic phytoplankton (algae). They belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or Chromalveolata, according to the newer Thomas Cavalier-Smith Biological Classification system. Within the Chromalveolata, the coccolithophorids are in the phylum or division Haptophyta, class Coccolithophyceae.[2] Coccolithophorids are distinguished by special calcium carbonate plates (or scales) of uncertain function called coccoliths, which are also important microfossils. Coccolithophores are almost exclusively marine and are found in large numbers throughout the sunlight zone of the ocean.

The most abundant species of coccolithophore, Emiliana huxleyi, belongs to the order Isochrysidales and family Noëlaerhabdaceae.[2] It is found in temperate, subtropical, and tropical oceans.[3] 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.[4] It is studied for the extensive blooms it forms in nutrient depleted waters after the reformation of the summer thermocline.[5] and for its production of a group resistant alkenones commonly used by earth scientists as a means to estimate past sea surface temperatures.[6] Coccolithophores are of particular interest to those studying global climate change because as ocean acidity increases, their coccoliths may become even more important as a carbon sink.[7] Furthermore, 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.[8]

Structure

Coccolithophores are spherical cells about 5–100 micrometres across, enclosed by calcareous plates called coccoliths, which are about 2–25 micrometres across. Each cell contains two brown pigment which surrounds the nucleus.[9]

Exoskeleton (Coccosphere)

Each unicellular plankton is enclosed in its own collection of coccoliths, the calcified scales, which make up its exoskeleton or coccosphere.[10] The plankton make their own coccoliths, and while some keep the same layer throughout life, others continually shed their coccospheres throughout life.

Composition

The primary constituent of coccoliths is calcium carbonate or limestone. Calcium carbonate itself is easily penetrated by light, so the organisms’ photosynthetic activity is not compromised by encapsulation in a coccosphere.[11]

Formation

Coccoliths are produced by a biomineralization process known as coccolithogenesis.[9] Generally, calcification of coccoliths occurs in the presence of light, and these scales are produced much more during the exponential phase of growth than the stationary phase.[12] 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.[13] 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 new coccoliths are themselves coated by old coccoliths.[14] 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 calcite crystals. These crystals are thought to form at least partially outside the cell. Heterococcoliths occur only in the diploid state, have radial symmetry, and are composed of few crystals (<100) (See Figure 1). Although they are rare, combination coccospheres, which contain both holococcoliths and heterococcoliths, have been observed in the fossil record preserving coccolithophore life cycle transitions.12 Finally, the coccospheres of some species are highly modified with various appendages made of specialized coccoliths (see Figure 2).[15]


Function

While the exact function of the coccosphere is unclear, many potential functions have been proposed. In addition to protecting the phytoplankton from the predators, these exoskeletons may also confer an advantage in energy production, as coccolithogenesis seems highly coupled with photosynthesis. Their function may be as simple as providing a cell wall-like barrier to isolate intracellular chemistry from the marine environment.[16] More specific, defensive properties of coccoliths may include protection from osmotic changes, chemical or mechanical shock, and short-wavelength light.[17] It has also been proposed that the added weight of multiple layers of coccoliths simply 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.[18] Coccolith appendages have also been proposed to serve several functions, such as inhibiting grazing by zooplankton.[15]

Uses

Coccoliths are the main component of commercially available chalk and chalk-based rock formations such as the White Cliffs of Dover. Removed coccoliths also form a calcareous substance that covers up to 35% of the ocean floor and is kilometers thick.[13] Because of its abundance and geographic span, the coccoliths which make up the layers of this substance serve as valuable microfossils.

Cellular Anatomy

Enclosed in each coccosphere is a single cell with membrane bound organelles. Two large chloroplasts with brown pigment are located on either side of the cell and surround the nucleus, mitochondria, golgi apparatus, endoplasmic reticulum, and other organelles. Each cell also has two flagellar structures, which are involved not only in motility, but also in mitosis and formation of the cytoskeleton.[19] In some species, a functional or vestigial haptonema is also present.[17] This structure, which is unique to haptophytes, coils and uncoils in response to environmental stimuli. Although poorly understood, it has been proposed to be involved in prey capture.[19] See Figure 3 for a detailed view of the cellular structure.


Ecology

Life History Strategy

The life cycle of coccolithophores is characterized by an alternation of diploid and haploid phases. They alternate from the haploid to diploid phase through syngamy and from diploid to haploid through meiosis. In contrast with most organisms with alternating life cycles, haptophytes occur equally in both phases without either of them being dominant. Furthermore, reproduction by meiosis is possible in both phases of the life cycle.[14] Both abiotic and biotic factors may affect the frequency with which each phase occurs.[20] See Figure 4 for a diagrammatic representation of this life cycle.

Figure 4.

Coccolithophores reproduce asexually through a process called binary fission. In this process the coccoliths from the parent are passed on to the daughter cells. There have been suggestions stating the possible presence of a sexual reproduction process due to the diploid stages of the coccolithophores, but this process has never been observed.[21] K or r- selected strategies of coccolithophores depend on their life cycle stage. When coccolithophores are diploid, they are r-selected. In this phase they tolerate a wider range of nutrient compositions. When they are haploid they are K- selected and are often more competitive in stable low nutrient environments.[21] Most coccolithophores are K strategist and are usually found on nutrient-poor surface waters. They are poor competitors when compared to other phytoplankton and thrive in habitats where other phytoplankton would not survive.[11] These two stages in the life cycle of coccolithophores occur seasonally, where more nutrition is available in warmer seasons and less is available in cooler seasons. This type of life cycle is known as a complex heteromorphic life cycle.[21]

Global Distribution

Coccolithophores presence can be felt in all corners of the ocean. Their distribution varies vertically by stratified layers in the ocean and geographically by different temporal zones.[22] While most modern coccolithophores can be located in their associated stratified oligotrophic conditions, the most abundant areas of coccolithophores where there is the highest species diversity are located in subtropical zones with a temperate climate.[23] While water temperature and the amount of light intensity entering the water’s surface are the more influential factors in determining where species are located, the ocean currents also can determine the location where certain species of coccolithophores are found.[24]

Although motility and colony formation vary according to the life cycle of different coccolithophore species, there is a general alternation between a motile, haploid phase, and a non-motile, colony-forming diploid phase. In both phases, the organism’s dispersal is largely due to ocean currents and circulation patterns.[13] Within the Pacific Ocean, approximately 90 species have been identified with six separate zones relating to different Pacific currents that contain unique groupings of different species of coccolithophores.[25] The highest diversity of coccolithophores in the Pacific Ocean was in an area of the ocean considered the Central North Zone which is an area between 30 oN and 5 oN, composed of the North Equatorial Current and the Equatorial Countercurrent. These two currents move in opposite directions, east and west, allowing for a strong mixing of waters and allowing a large variety of species to populate the area.[25] In the Atlantic Ocean, the most abundant species are E. huxleyi and Florisphaera profunda with smaller concentrations of the species Umbellosphaera irregularis, Umbellosphaera tenuis and different species of Gephyrocapsa.[25] Deep-dwelling coccolithophore species abundance is greatly affected by nutricline and thermocline depths. These coccolithophores increase in abundance when the nutricline and thermocline are deep and decrease when they are shallow.[26] The complete distribution of coccolithophores is currently not know and some regions, such as the Mediterranean and Indian Oceans, are not as well known as other locations in the Pacific and Atlantic Oceans. It is also very hard to explain distributions further than that of simplifications due to multiple constantly changing factors involving the ocean properties, such as coastal and equatorial upwelling, frontal systems, benthic environments, unique oceanic topography, and pockets of isolated high or low water temperatures.[15]

Table 1 and Table 2 show simplified specific species preferences based on geographic location and water depth respectively. These do not contain all coccolithophore species and are just a simplification of major, more well-known species.




The upper photic zone is low in nutrient concentration, high in light intensity and penetration, and usually higher in temperature. The lower photic zone is high in nutrient concentration, low in light intensity and penetration and relatively cool. The middle photic zone is an area that contains the same values in between that of the lower and upper photic zones.[23]

Effect of Global Climate Change on Distribution and Abundance

Recent studies show that climate change has direct and indirect impacts on Coccolithophore distribution and productivity. Coccolithophores are directly impacted by the acidification of the surface of the ocean due to CO2 changes in the environment. They are indirectly impacted by the increasing temperatures and thermal stratification of the top layer of the ocean. Increasing carbon dioxide could severely alter photosynthesis and calcification of coccolithophores.[26]

Role in the Food Web

Coccolithophores are the most 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.[27]

Dependence on Nutrients

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 more opportunistic species 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.[8]

Impact on Water Column Productivity

The calcite in calcium carbonate allows coccoliths to scatter more light than they absorb. This has two important consequences: 1) Surface waters become brighter, meaning they have a higher albedo, and 2) there is induced photoinhibition, meaning deeper waters become darker. Hence, a high concentration of coccoliths leads to a simultaneous increase in surface water temperature and decrease in the temperature of deeper waters. This results in more stratification in the water column and a decrease in the vertical mixing of nutrients. However, a recent study estimated that the overall effect of coccolithophores on the increased in radiative forcing of the ocean is less than that from anthopogenic factors.[28] Therefore, the overall result of large blooms of coccolithophores is a decrease in water column productivity, rather than a contribution to global warming.

Predator-Prey Interactions

Their predators include the common predators of all phytoplankton including small fish, zooplankton, and shellfish such as the shrimp, Artemic salinaare.[11][29] Viruses specific to this species have been isolated from several location worldwide and appear to play a major role in spring bloom dynamics. A recent expedition, led by Dr. Assaf Vardi from the Weizmann Institute of Science in Israel has recently tracked E. hux blooms between the Azores and Greenland to study the dynamic nature of these virus-host interactions.

Toxicity

Coccolithophores 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 genera 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.[29]

Community Interactions

Coccolithophorids can be found as either single, free-floating haploid cells, colonies of haploid cells, single diploid cells, and even filamentous and mucous-bound aggregations of diploid cells.[22]

Competition

Most phytoplankton need sunlight and nutrients from the ocean to survive, so they thrive on the surface of areas with large swells of nutrient rich water rising from the lower levels of the ocean. Most coccolithophores, only require sunlight for energy production and have a higher ratio of nitrate uptake over ammonium uptake (nitrogen is required for growth and can be used directly from nitrate but not ammonium). Because of this they thrive in still, nutrient-poor environments where other phytoplankton are starving.[30] Trade-offs associated with these faster growth rates, however, include a smaller cell radius and lower cell volume than other types of phytoplankton.

Viral Infection and Coevolution

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.[31] 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.[20]

Importance in Global Climate Change

Impact on the Carbon Cycle

Coccolithophores have both long and short term effects on the carbon cycle. The production of coccoliths requires the uptake of dissolved inorganic carbon and calcium. Calcium carbonate and carbon dioxide are produced from calcium and bicarbonate by the following chemical reaction: Ca2+ + 2HCO3-- < -- > CaCO3 + CO2 + H2O.[32] Because coccolithophores are photosynthetic organisms, they are able to use some of the CO2 released in the calcification reaction for photosynthesis.[33]

       	CO2 + H2O < -- > CH2O + O2

However, the production of calcium carbonate drives surface alkalinity down, and in conditions of low alkalinity the CO2 is instead released back into the atmosphere.[34] As a result of this, researchers have postulated that large blooms of coccolithophores may contribute to global warming in the short term.[35] A more widely accepted idea, however, is that over the long term coccolithophores contribute to an overall decrease in atmospheric CO2 concentrations. During calcification two carbon atoms are taken up and one of them becomes trapped as calcium carbonate. This calcium carbonate sinks to the bottom of the ocean in the form of coccoliths and becomes part of sediment; thus, coccolithophores provide a sink for emitted carbon, mediating the effects of greenhouse gas emissions.[35]

Evolutionary Responses to Ocean Acidification

Research also suggests that increasing concentrations of CO2 in the atmosphere may affect the calcification machinery of coccolithophores. Hence, oceanic conditions may affect not only immediate events such as increases in population or coccolith production, they may induce divergent evolution of coccolithophore species over longer periods of time. For example, in 2011, it was discovered that coccolithophores use H+ ion channels in to constantly pump H+ ions out of the cell during coccolith production. This allows them to avoid acidosis, as coccolith production would produce a toxic excess of H+ ions if left unchecked. When the function of these channels is disrupted, the coccolithophores stop the calcification process to avoid acidosis.[36] Low ocean alkalinity, therefore, places selective pressure on coccolithophore ion channels as their function depends upon particular gradients between the inside of the cell and the outer environment. The existence of this feedback loop makes coccolithophores (and other ocean calcifiers) particularly vulnerable to ocean acidification.[37]

Impact on Nanofossil Record

Coccolith fossils are prominent and valuable calcareous micro- and nannofossils (see Micropaleontology). Although finding intact single-celled phytoplankton in sediment that is millions of years old is difficult, scientists can use a variety of techniques, such as scanning electron microscopy to decipher the composition of coccolithophores of the past.[38] 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 CO2 in the ocean.[39] In 2008, field evidence indicating an increase in in calcification of newly formed ocean sediments containing coccolithophores bolstered the first ever experimental data showing that an increase in ocean CO2 concentration results in an increase in calcification of these organisms.[25] 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.[40]

Biological Control Options

Coccolithophore populations can be monitored in many ways, with one of the most direct ways being the measurement of bio-optics. Monitoring of these populations is important because of the significance of coccolithophores as a carbon sink and their effects on vertical nutrient flow in the ocean.

Monitoring Ocean Acidification

Decreasing coccolith mass is related to both the increasing concentrations of CO2 and decreasing concentrations of CO3 in the world’s oceans. This lower calcification is assumed to put coccolithophores at ecological disadvantage. Some species like Calcidiscus leptoporus, however, are not affected in this way, while the most abundant coccolithophore species, E. huxleyi is.[36] Also, highly calcified coccolithophorids have been found in conditions of low CO−3 contrary to predictions.[7] Understanding the effects of increasing ocean acidification on coccolithophore species is absolutely essential to predicting the future chemical composition of the ocean, particularly its carbonate chemistry. Viable conservation and management measures will come from future research in this area. Groups like the European-based CALMARO[41] are monitoring the responses of coccolithophore populations to varying pH’s and working to determine environmentally sound measures of control.

Preventing Eutrophication

Many efforts are being taken to prevent coccolithophore blooms due to eutrophication. The Baltic Marine Environment Protection Commission was established to monitor and collect data to help prevent future eutrophication in the Baltic Sea in 1974. The Baltic Monitoring Program has been developed to collect data and assess trends in the Baltic Sea.[8] Developments have been made in the aspect of monitoring algal blooms through satellite remote sensing. Devices such as multiple-spectral sensors and hyperspectral instruments are currently used as satellite remote sensing devices.[42]

See also

References

External links

  • Cocco Express — Coccolithophorids Expressed Sequence Tags (EST) & Microarray Database
  • University of California, Berkeley. Museum of Paleontology: "Introduction to the Prymnesiophyta".
  • The Paleontology Portal: Calcareous Nanoplankton
  • What is a Coccolithophore?
  • Home Page
  • BOOM — Biodiversity of Open Ocean Microcalcifiers
  • INA — International Nannoplankton Association
  • Nannotax – illustrated guide to coccolithophores and other nannofossils.
  • RadioLab – podcast on coccolithophores
de:Haptophyta
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