Coccolithophores, also called coccolithophorids, are single-celled organisms that belong to a group of tiny plants called phytoplankton. These organisms are self-feeding and live in the ocean. Scientists classify them in different ways: some place them in the kingdom Protista, while others group them under a newer system called Hacrobia. Within this system, they are part of the group Haptophyta, which includes the class Prymnesiophyceae. Coccolithophores are almost always found in the ocean, use sunlight to make food, and can also take in nutrients from their environment. They live in the upper part of the ocean where sunlight reaches.

Coccolithophores are the most productive organisms that create calcium carbonate shells. These shells, called coccospheres, are made of tiny plates called coccoliths. The coccoliths form inside the cell and fit together to cover the cell completely. The size of these plates depends on the size of the cell itself. Different species have coccoliths of varying sizes and shapes. While the way the coccoliths fit together helps protect the cell from tiny ocean animals that eat phytoplankton, scientists are still studying how exactly the coccoliths connect and how they are arranged.

Coccolithophores are important for the ocean’s health. They help move carbon from the surface of the ocean to deeper layers and play a key role in the ocean’s carbon cycle. In some areas, they can produce up to 40% of the food made by marine plants. Scientists study them because ocean acidification might make their shells even more important for storing carbon. Efforts are being made to control large blooms of coccolithophores, which can reduce the flow of nutrients to deeper ocean layers.

The most common coccolithophore, Emiliania huxleyi, is part of the group Isochrysidales and the family Noelaerhabdaceae. It lives in temperate, subtropical, and tropical oceans and is a key part of many marine food chains. It grows quickly in laboratory settings and forms large blooms in areas with low nutrients after summer water layers mix. Scientists also study E. huxleyi because it produces molecules called alkenones, which help researchers estimate past ocean temperatures.

Overview

Coccolithophores, also called coccolithophorids, are a group of about 200 types of phytoplankton. Scientists classify them in different ways. Some older systems place them in the Protista kingdom, while newer systems group them in the Hacrobia clade. Within Hacrobia, coccolithophores are part of the Haptophyta phylum and the Prymnesiophyceae (or Coccolithophyceae) class. These organisms are known for having special calcium carbonate plates called coccoliths. These plates form a spherical covering around the cell called a coccosphere. However, some species in the Prymnesiophyceae class do not have coccoliths, such as those in the Prymnesium genus.

Coccolithophores are single-celled phytoplankton that create small calcium carbonate scales, which form a spherical covering called a coccosphere. Many species can perform both photosynthesis and consume prey, a trait called mixotrophy.

Coccolithophores have been part of marine plankton communities since the Jurassic period. Today, they contribute about 1–10% of inorganic carbon fixation in the ocean’s surface and about 50% of pelagic calcium carbonate sediments. Their calcium carbonate shells increase the speed at which organic matter sinks into the deep ocean, helping move carbon from the surface to the deep sea. However, the process of forming coccoliths also reduces seawater alkalinity and releases carbon dioxide. This means coccolithophores influence the ocean’s ability to absorb atmospheric carbon dioxide and affect the efficiency of the biological carbon pump.

As of 2021, scientists do not fully understand why coccolithophores produce coccoliths or how this trait contributes to their success in marine ecosystems. The coccosphere may help protect them from predators or viruses. Viruses are a major cause of phytoplankton death, and recent studies suggest that calcification can affect interactions between coccolithophores and viruses. The main predators of marine phytoplankton are microzooplankton, such as ciliates and dinoflagellates, which consume about two-thirds of ocean primary production. These predators can strongly reduce coccolithophore populations. While calcification does not stop predation, the coccosphere may make it harder for predators to consume the organic material of coccolithophores. Heterotrophic protists can choose prey based on size, shape, or chemical signals, which may lead them to prefer other prey that lacks coccoliths.

Structure

Coccolithophores are round cells measuring 5–100 micrometres in size. These cells are surrounded by calcium-based plates called coccoliths, which range from 2–25 micrometres in size. Inside each cell are two brown-colored chloroplasts that enclose the nucleus.

Each coccosphere contains one cell with organelles enclosed by membranes. Two large chloroplasts with a brown color are positioned on opposite sides of the cell and surround the nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, and other organelles. Each cell also has two flagellar structures that help the cell move, and also assist with cell division and forming the cell’s structure. In some species, a structure called a haptonema may be present. This structure, found only in haptophytes, changes shape in response to environmental changes. Although scientists do not fully understand its function, it is thought to help the cell capture food.

Ecology

Coccolithophores have a life cycle that changes between two stages. One stage is called the haploid phase, where they reproduce without sex. The other stage is called the diploid phase, where they reproduce with sex. During the haploid phase, they make new cells through a process called mitosis. These cells can grow more or join with other haploid cells to make diploid cells. The diploid cells then split into haploid cells again through meiosis, restarting the cycle. Unlike many other organisms, coccolithophores can reproduce without sex in both stages of their life cycle. The frequency of each stage can be influenced by both non-living (abiotic) and living (biotic) factors.

Coccolithophores reproduce without sex by splitting into two new cells, a process called binary fission. During this, the hard plates called coccoliths from the parent cell are shared between the two new cells. Some scientists think sexual reproduction might happen because of the diploid stage, but this has never been seen.

The way coccolithophores reproduce depends on their life cycle stage. When they are diploid, they are r-selected, meaning they can survive in many different environments with varying nutrients. When they are haploid, they are K-selected, meaning they are better at surviving in stable, low-nutrient environments. Most coccolithophores are K-selected and live in nutrient-poor surface waters. They are not as strong competitors as other phytoplankton but can survive in places where other phytoplankton cannot. These life stages change with the seasons, as more nutrients are available in warmer months and fewer in cooler months. This life cycle is called a complex heteromorphic life cycle.

Coccolithophores are found in oceans worldwide. Their locations vary based on ocean layers and geographic regions. Many live in nutrient-poor, stratified waters, but the most diverse areas are in subtropical zones with temperate climates. Water temperature and light intensity are the main factors affecting where they live, but ocean currents also influence their distribution.

Although movement and group formation differ among species, most coccolithophores alternate between a moving, haploid phase and a non-moving, diploid phase. In both phases, their spread is mainly caused by ocean currents and water movement.

In the Pacific Ocean, about 90 species have been found. These species are grouped into six zones based on different ocean currents. The area with the most diversity is the Central North Zone, located between 30°N and 5°N. This region is where the North Equatorial Current and the Equatorial Countercurrent flow in opposite directions, mixing waters and supporting a wide variety of species.

In the Atlantic Ocean, the most common species are Emiliania huxleyi and Florisphaera profunda, with smaller numbers of Umbellosphaera irregularis, Umbellosphaera tenuis, and some Gephyrocapsa species. The number of deep-dwelling coccolithophores depends on the depth of nutrient and temperature layers in the ocean. These species are more common when these layers are deep and less common when they are shallow.

The full distribution of coccolithophores is not fully known. Some areas, like the Indian Ocean, are not as well studied as the Pacific and Atlantic Oceans. Their distribution is difficult to explain because of many changing ocean conditions, such as coastal and equatorial upwelling, ocean fronts, seabed environments, unique underwater shapes, and isolated areas with high or low water temperatures.

The upper part of the ocean, called the photic zone, has low nutrients, high light, and warm temperatures. The lower part of the photic zone has high nutrients, low light, and cooler temperatures. The middle part has conditions between the upper and lower zones.

The Great Calcite Belt in the Southern Ocean is an area with high levels of calcium carbonate from coccolithophores, even though diatoms are more common there. The overlap of coccolithophores and diatoms in this region, which has strong ocean fronts, makes it a good place to study how environmental factors affect different phytoplankton groups.

The Great Calcite Belt is a region with high levels of calcium carbonate and chlorophyll in the Southern Ocean during spring and summer. It covers more than 60% of the Southern Ocean (30–60°S) and is a major area for absorbing human-made carbon dioxide, along with the North Atlantic and North Pacific Oceans.

Recent studies show that climate change affects where coccolithophores live and how productive they are. Rising ocean temperatures and stronger layers of warm water may change their populations, but it is unclear if global warming will increase or decrease their numbers. Since they are calcifying organisms, ocean acidification from increased carbon dioxide could harm them. However, recent increases in carbon dioxide have led to more coccolithophores.

Coccolithophores are among the most common primary producers in the ocean. They contribute significantly to the productivity of tropical and subtropical oceans, though the exact amount is still being studied.

The balance of nitrogen, phosphorus, and silicate in ocean areas affects which phytoplankton species dominate. When silicate levels are low compared to nitrogen and phosphorus, coccolithophores outcompete diatoms. When silicate levels are high, diatoms dominate. Increased farming has led to more nutrients in some areas, causing coccolithophore blooms in places with high nitrogen and phosphorus but low silicate.

Calcium carbonate in coccoliths reflects more light than it absorbs. This has

Evolution and diversity

Coccolithophores are part of the group Haptophyta, which is closely related to another group called Centrohelida. Both groups belong to a larger category known as Haptista. The oldest known coccolithophores appeared during the Late Triassic period, near the boundary between the Norian and Rhaetian stages. Their variety increased gradually throughout the Mesozoic era, reaching the highest level during the Late Cretaceous. However, a major extinction event at the end of the Cretaceous caused more than 90% of coccolithophore species to disappear. Their diversity rose again during the Paleocene-Eocene thermal maximum but declined afterward, especially during the Oligocene, as global temperatures dropped. Species that produced large, heavily calcified coccoliths were most affected by this decline.

Coccolithophore shells

• Exoskeleton: coccospheres and coccoliths

Each coccolithophore has a protective shell called a coccosphere, made of tiny, hard structures called coccoliths. These coccoliths are made of calcium carbonate, a type of chalk. Some species grow a single layer of coccoliths that expands as the cell grows, while others continuously make and shed new coccoliths.

Coccoliths are formed through a process called coccolithogenesis, which happens inside the cell. This process often occurs in the presence of light and is more active during the fastest-growing stage of the organism’s life. Calcium signaling, a type of chemical communication, helps control how coccoliths are made. The process starts in the Golgi complex, where proteins and other molecules help form calcium carbonate crystals. These crystals are then packaged into small vesicles and added to the inside of the coccosphere. This means newer coccoliths may be covered by older ones.

There are two main types of coccoliths: holococcoliths and heterococcoliths. Holococcoliths are made during the haploid stage of the life cycle and lack symmetry. They are made of many small rhombic crystals. Heterococcoliths are made during the diploid stage and have radial symmetry. They are made of fewer, more complex crystal units. Some coccospheres have both types, which may indicate changes in the organism’s life cycle. Some species also have special appendages made of coccoliths.

The exact purpose of the coccosphere is not fully understood, but several functions have been suggested. It may protect the organism from predators, help balance the pH of the water, and support energy production. During photosynthesis, carbon dioxide is removed from the water, making it more basic. During calcification, carbon dioxide is also used, but this process makes the water more acidic. These two processes may balance each other’s effects on pH. Coccoliths may also help the organism absorb more light for photosynthesis and provide a barrier between the cell and the ocean. They might also help the organism float or sink depending on the layers of coccoliths. Appendages made of coccoliths may also help prevent grazing by other organisms.

Coccoliths are the main component of the Chalk, a rock formation from the Late Cretaceous period found in southern England, including the White Cliffs of Dover. Today, coccoliths are a major part of calcareous oozes that cover large areas of the ocean floor. These layers are valuable as microfossils for studying Earth’s history.

Calcification, the process of making calcium carbonate, is important in the marine carbon cycle. Coccolithophores are a major group of plankton that produce calcium carbonate in the ocean.

The energy costs and benefits of calcification are shown in diagrams. Benefits include faster photosynthesis, protection from light damage, and defense against predators. However, ocean acidification may make calcification more difficult. As the ocean becomes more acidic, it becomes harder for coccolithophores to release protons needed for calcification. This could limit their ability to adapt to changing ocean conditions.

Other plankton, like diatoms and dinoflagellates, do not face the same challenges with calcification. Diatoms have silica-based shells that are easier to make, while dinoflagellates may benefit from ocean acidification because it increases carbon dioxide levels, which helps them build their cellulose-based shells. In contrast, coccolithophores may need more energy to build their calcium carbonate shells, especially in acidic conditions.

Importance in global climate change

Coccolithophores have both short-term and long-term effects on the carbon cycle. To create coccoliths, these organisms take in dissolved inorganic carbon and calcium. A chemical reaction then produces calcium carbonate and carbon dioxide from calcium and bicarbonate.

Because coccolithophores are photosynthetic, they can use some of the carbon dioxide released during calcification for photosynthesis. However, producing calcium carbonate lowers surface alkalinity. In conditions of low alkalinity, carbon dioxide is released back into the atmosphere. This has led researchers to suggest that large blooms of coccolithophores may temporarily increase global warming. Over time, however, coccolithophores are believed to reduce atmospheric carbon dioxide levels. During calcification, two carbon atoms are absorbed, and one becomes trapped as calcium carbonate. This calcium carbonate sinks to the ocean floor as coccoliths and becomes part of sediment, acting as a long-term storage for carbon and helping to reduce greenhouse gas emissions.

Research indicates that ocean acidification, caused by rising carbon dioxide levels, may affect the calcification process of coccolithophores. This could influence immediate changes, such as population growth or coccolith production, and may also drive evolutionary adaptations over time. For example, coccolithophores use hydrogen ion channels to remove excess hydrogen ions from their cells during coccolith production. This prevents acidosis, which would occur if hydrogen ions built up. If these channels are disrupted, coccolithophores stop calcification to avoid acidosis, creating a feedback loop. Low ocean alkalinity weakens ion channel function, increasing evolutionary pressure on coccolithophores and making them vulnerable to ocean acidification. In 2008, evidence from ocean sediments showed that increased carbon dioxide levels correlate with higher calcification rates in coccolithophores. Decreasing coccolith mass is linked to rising carbon dioxide levels and falling carbonate ion levels in the ocean. This reduced calcification may place coccolithophores at an ecological disadvantage. Some species, like Calcidiscus leptoporus, are unaffected, while others, such as Emiliania huxleyi, show mixed results. Additionally, highly calcified coccolithophorids have been found in low calcium carbonate saturation conditions, contrary to predictions. Understanding how ocean acidification affects these organisms is essential for predicting future ocean chemistry. Future research will help develop conservation strategies. Groups like the European-based CALMARO monitor coccolithophore responses to varying pH levels and work to identify sustainable management practices.

• Gephyrocapsa oceanica (scale bar: 1 μm)
• Rhabdosphaera clavigera
• Discosphaera tubifera

Coccolith fossils are important and valuable sources of biogenic calcium carbonate. They significantly contribute to the global carbon cycle and are the main component of chalk deposits, such as the white cliffs of Dover. Fossils from the Palaeocene-Eocene Thermal Maximum, 55 million years ago, are of particular interest because this period is believed to reflect current ocean carbon dioxide levels. Field evidence of coccolithophore fossils in rock layers has shown that the deep-sea fossil record has a bias similar to the one observed in land-based fossil records.

Coccolithophorids help regulate ocean temperatures. They thrive in warm waters and release dimethyl sulfide (DMS) into the air. DMS helps form cloud nuclei, which create thicker clouds that block sunlight. When ocean temperatures drop, coccolithophorid populations decline, reducing cloud cover. With fewer clouds to block sunlight, ocean temperatures rise, maintaining a natural balance and equilibrium.