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 mixotrophic, 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.[1]
Coccolithophores are ecologically important, and biogeochemically they play significant roles in the marine biological pump and the carbon cycle.[2][1] Depending on habitat, they can produce up to 40 percent of the local marine primary production.[3] They 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.[4] 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.[5]
The most abundant species of coccolithophore, Emiliania huxleyi, belongs to the order Isochrysidales and family Noëlaerhabdaceae.[6] It is found in temperate, subtropical, and tropical oceans.[7] 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.[8] It is studied for the extensive blooms it forms in nutrient depleted waters after the reformation of the summer thermocline.[9][10] 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.[11]
Coccolithophores (or coccolithophorids, from the adjective[12]) form a group of about 200 phytoplankton species.[13] They belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae).[6] Coccolithophores are distinguished by special calcium carbonate plates (or scales) of uncertain function called coccoliths, which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths (e.g. in genus Prymnesium), so not every member of Prymnesiophyceae is a coccolithophore.[14]
Coccolithophores are single-celled phytoplankton that produce small calcium carbonate (CaCO3) scales (coccoliths) which cover the cell surface in the form of a spherical coating, called a coccosphere. Many species are also mixotrophs, and are able to photosynthesise as well as ingest prey.[15]
Coccolithophores have been an integral part of marine plankton communities since the Jurassic.[16][17] Today, coccolithophores contribute ~1–10% to inorganic carbon fixation (calcification) to total carbon fixation (calcification plus photosynthesis) in the surface ocean[18] and ~50% to pelagic CaCO3 sediments.[19] Their calcareous shell increases the sinking velocity of photosynthetically fixed CO2 into the deep ocean by ballasting organic matter.[20][21] At the same time, the biogenic precipitation of calcium carbonate during coccolith formation reduces the total alkalinity of seawater and releases CO2.[22][23] Thus, coccolithophores play an important role in the marine carbon cycle by influencing the efficiency of the biological carbon pump and the oceanic uptake of atmospheric CO2.[1]
As of 2021, it is not known why coccolithophores calcify and how their ability to produce coccoliths is associated with their ecological success.[24][25][26][27][28] The most plausible benefit of having a coccosphere seems to be a protection against predators or viruses.[29][27] Viral infection is an important cause of phytoplankton death in the oceans,[30] and it has recently been shown that calcification can influence the interaction between a coccolithophore and its virus.[31][32] The major predators of marine phytoplankton are microzooplankton like ciliates and dinoflagellates. These are estimated to consume about two-thirds of the primary production in the ocean[33] and microzooplankton can exert a strong grazing pressure on coccolithophore populations.[34] Although calcification does not prevent predation, it has been argued that the coccosphere reduces the grazing efficiency by making it more difficult for the predator to utilise the organic content of coccolithophores.[35] Heterotrophic protists are able to selectively choose prey on the basis of its size or shape and through chemical signals[36][37] and may thus favor other prey that is available and not protected by coccoliths.[1]
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 chloroplasts which surround the nucleus.[40]
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.[41] In some species, a functional or vestigial haptonema is also present.[42] 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.[41]
The complex life cycle of coccolithophores is known as a haplodiplontic life cycle, and is characterized by an alternation of both asexual and sexual phases. The asexual phase is known as the haploid phase, while the sexual phase is known as the diploid phase. During the haploid phase, coccolithophores produce haploid cells through mitosis. These haploid cells can then divide further through mitosis or undergo sexual reproduction with other haploid cells. The resulting diploid cell goes through meiosis to produce haploid cells again, starting the cycle over. With coccolithophores, asexual reproduction by mitosis is possible in both phases of the life cycle, which is a contrast with most other organisms that have alternating life cycles.[43] Both abiotic and biotic factors may affect the frequency with which each phase occurs.[44]
Coccolithophores reproduce asexually through binary fission. In this process the coccoliths from the parent cell are divided between the two 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.[45]
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.[45] 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.[46] 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.[45]
Coccolithophores occur throughout the world's oceans. Their distribution varies vertically by stratified layers in the ocean and geographically by different temporal zones.[47] 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.[48] 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.[49]
Although motility and colony formation vary according to the life cycle of different coccolithophore species, there is often alternation between a motile, haploid phase, and a non-motile diploid phase. In both phases, the organism's dispersal is largely due to ocean currents and circulation patterns.[50]
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.[51] 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.[51]
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.[51] 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.[52]
The complete distribution of coccolithophores is currently not known and some regions, such as the Indian Ocean, are not as well studied as other locations in the Pacific and Atlantic Oceans. It is also very hard to explain distributions due to multiple constantly changing factors involving the ocean's properties, such as coastal and equatorial upwelling, frontal systems, benthic environments, unique oceanic topography, and pockets of isolated high or low water temperatures.[54]
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.[48]
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.[57]
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,[58] plays an important role in climate fluctuations,[59][60] accounting for over 60% of the Southern Ocean area (30–60° S).[61] The region between 30° and 50° S has the highest uptake of anthropogenic carbon dioxide (CO2) alongside the North Atlantic and North Pacific oceans.[62]
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.