Quarterdeck Volume 5, Number 2, Summer 1997
Coccolithophores may be small, but they know how to get attention
Rahilla C. A. Shatto and Niall C. Slowey
The invention of the electron microscope in the 1950s opened our eyes to a surreal marine world of tiny, bizarre life forms. Inhabitants of this diminutive seascape include the coccolithophores, single-celled marine plants with a role in the global environment as intriguing as their appearance. Light microscopes can barely discern the minute cells which are only about two to forty micrometers in diameter, so the advent of more sophisticated technology in recent decades allows scientists to study them in more detail than was possible previously.
Coccolithophores are one type of phytoplankton, organisms that live near the ocean surface where they can use abundant light for photosynthesis. Hundreds of coccolithophore species have now been described and research shows they thrive in a variety of environmental conditions. Some coccolithophores flourish in places too spartan for many other phytoplankton; in the ocean centers where nutrients are scarce or in the lower parts of surface waters where there is little light to drive photosynthesis.
A day in the life of a coccolithophore
A day in the life of a coccolithophore is dominated by the pursuit of its two critical requirements-nutrients and sunlight. Using the sun's energy coccolithophores and other phytoplankton photosynthesize glucose and other organic compounds from inorganic nutrients in the water column such as nitrate, phosphate, and dissolved carbon. The resulting plant material can serve as food for other marine organisms.
Coccolithophores differ from all other phytoplankton in that they surround themselves with calcium carbonate structures called coccoliths. A coccolithophore may have 10 to over 100 coccoliths, with average cells having roughly 10 to 20 of the distinctive plates. Coccolith shapes vary wildly among species, lending the coccolithophores a spectacular diversity that scientists interpret as adaptations to different environments in the ocean (See "Survival of the strangest").
Coccolithophores create coccoliths internally with an organelle especially adapted for that purpose, then push the plates outside the cell membrane where they become part of a rigid, protective armor. In the laboratory, some species produce and shed coccoliths almost continuously while others produce them only when necessary to build or repair their outer covering.
Much of what we know about these fascinating organisms comes from studies of the most common species, Emiliania huxleyi. Researchers once thought that E. huxleyi reproduced only through cell division. We now know from laboratory cultures that it has a much more complex life cycle with multiple stages. Emiliania huxleyi produces coccoliths in just one of the stages, and there is even evidence to suggest that in another stage it can reproduce sexually in addition to ordinary cell division.
Other species can have different life stages characterized by differently shaped coccoliths or even stages in which no coccoliths are produced. With further research we may find that cells distinguished as separate species really represent successive life stages of a single species.
All those coccoliths really add up!
Most coccolithophores' lives come to an end when they are eaten by zooplankton, but their indigestible coccoliths frequently pass undamaged into the predators' fecal pellets. A single fecal pellet can contain as many as 100,000 coccoliths. Fecal pellets sink quickly due to their comparatively large size, and they act as protective packages that deliver coccoliths to the seafloor. Free-floating coccoliths would never survive the slow process of sinking-they would be carried far from their origins by ocean currents and ultimately dissolve in deep waters undersaturated with calcium carbonate.
Instead, enough coccoliths arrive on the seafloor to contribute to layers of calcareous sediment tens to thousands of meters thick in some places. These sediments are eventually compacted and transformed into chalk and limestone, which may be exposed on the earth's surface millions of years later. For example, the white cliffs of Dover, England, are made largely of coccoliths.
It is important to know whether or not coccolith assemblages in seafloor sediment accurately reflect living populations in the overlying water column. If we understand how living creatures become part of the sediment record, and we know what environmental conditions each species requires to survive, then we can study ancient sediments to deduce oceanic and atmospheric conditions that prevailed when that sediment formed. Thus coccolithophores and other organisms could be a useful tool for reconstructing past climates and assessing current trends.
Scientists who study the fate of coccoliths in the ocean use sediment traps, instruments designed to collect and preserve sediment particles descending through the water column. At first this work seemed to suggest that thanks to the increased sinking rate supplied by fecal pellets, species preserved in sediment approximately represent living populations.
More research is showing that the relationship is not so simple. Only a fraction of all coccoliths produced in surface waters survive the long journey to the ocean floor. Fecal pellets do deliver millions of them to the bottom, but if predators prefer to eat certain coccolithophore species over others then the coccoliths they preserve show only part of the picture. Coccoliths are still somewhat vulnerable to decay and the variable processes of sediment formation on the seabed. Coccoliths' diverse shapes and sizes result in destruction of some types and preservation of others as they enounter different conditions in and near the sediment. Furthermore, the hundreds of species of coccolithophores probably produce coccoliths at varying rates and for different reasons, making it extremely difficult to estimate the number of living plants represented by a quantity of coccoliths in sediment. Much more research is needed to thoroughly understand how well the sediment record of coccolithophores represents living assemblages in the overlying water column.
Coccolithophores' roles in global climate
Coccolithophores do not just record environmental conditions, but they might have a significant role in regulating them.
Looking at the sea surface you might think that the ocean and atmosphere are independent entities. In fact, they are two parts of a single system, constantly exchanging chemicals with one another. The upper layer of the ocean, where phytoplankton live, alternately absorbs or releases the potent greenhouse gas carbon dioxide depending on local conditions.
Phytoplankton consume dissolved carbon dioxide in photosynthesis and they use carbon in other chemical forms to build the parts of their cells. Much of the carbon they use ultimately finds its way to deep water or the seafloor where it remains effectively hidden, perhaps for millenia. As phytoplankton deplete dissolved carbon in the water, the ocean absorbs more carbon dioxide from the air. Thus, phytoplankton play an important role in helping to pull carbon dioxide out of the atmosphere and into the ocean.
Coccolithophores do all this and more. They not only consume carbon dioxide in photosynthesis, but they also remove huge quantities of bicarbonate ions (HCO<sub>3</sub><sup>-</sup>) from seawater and use them to create their calcium carbonate plates. Some scientists believe coccolithophores sequester much more carbon in the ocean than ordinary plankton.
In addition to their role in the carbon cycle, coccolithophores produce a chemical that plays a part in cloud formation, dimethyl sulfide (DMS). Coccolithophores produce most of the DMS that comes from the ocean. Gaseous DMS is released when the cells die and then undergoes chemical transformations in the ocean and atmosphere until it is transformed into airborne sulfate particles. The particles are called condensation nuclei because they provide a surface on which water vapor in the air condenses to form clouds. In certain parts of the ocean where cloud condensation nuclei from other sources are not plentiful, DMS from coccolithophores probably plays a role in cloud formation. A considerable amount of current research concerns gauging the importance of DMS from coccolithophores.
The mysterious fairy glow
One coccolithophore species in particular holds a prominent place in the minds of climate scientists. Emiliania huxleyi is the most common species worldwide, but it does especially well in cool waters where nutrients are plentiful such as in the fjords of Norway.
Under optimal conditions E. huxleyi populations explode and form enormous blooms that can cover more than 100,000 square kilometers of ocean surface. Free coccoliths shed from the living cells scatter sunlight causing the water to take on a milky turquoise color once called "fairy glow" or "white waters" by seafarers.
Today the enormous blooms are easily detected by satellite due to the amount of light they scatter back into space. Emiliania huxleyi blooms can contain billions of cells per liter and generate tens of thousands of metric tons of calcium carbonate in the upper layer of a bloom alone.
About 1.4 million square kilometers of ocean host coccolithophore blooms each year, raising the possibility not only for local but also global environmental effects. Coccolithophore blooms can triple the amount of light reflected into space by the surface waters they occupy, reducing the amount of light that penetrates the upper layers of the ocean. The water beneath blooms becomes darker than usual, possibly disrupting other organisms that require light for photosynthesis. Scientists do not know what long-term effects, if any, these blooms might have on the water column.
Emiliania huxleyi blooms have also been described as giant chemical factories. Billions of cells consume dissolved carbon dioxide, nitrate, and phosphate while producing oxygen, ammonia, dimethyl sulfide, and other dissolved organic compounds. Blooming E. huxleyi incorporate huge quantities of carbon into their coccoliths, much of which is ultimately transported to deep waters and buried in the ocean floor.
The myriad roles coccolithophores play in our global environment remain largely unknown to us. Continuing research will yield more fascinating information about these mysterious ocean plants and how they have developed their unusual features and abilities.
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Last updated September 1, 1997