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Coccolithophore viewed throug microscope with false colour. Source: Neon/ja/Richard Bartz


caption Scientists using a fine net to collect Emiliania huxleyi, a species of coccolithophore. Note the color of water containing a coccolithophore bloom. Source: Patrick Holligan

Like any other type of phytoplankton, coccolithophores are one-celled marine plants that live in large numbers throughout the upper layers of the ocean. Unlike any other plant in the ocean, coccolithophores surround themselves with a microscopic plating made of limestone (calcite). These scales, known as coccoliths, are shaped like hubcaps and are only three one-thousandths of a millimeter in diameter. What coccoliths lack in size they make up in volume. At any one time a single coccolithophore is attached to or surrounded by at least thirty scales. Additional coccoliths are dumped into the water when the coccolithophores multiply asexually, die or simply make too many scales. In areas with trillions of coccolithophores, the waters will turn an opaque turquoise from the dense cloud of coccoliths. Scientists estimate that the organisms dump more than 1.5 million tons (1.4 billion kilograms) of calcite a year, making them the leading calcite producers in the ocean.

Ecology and habitat

caption Large numbers of detached coccliths gathered by sieving the filtering the ocean with a fine mesh during a bloom. Micrograph: Jeremy Young

Most phytoplankton need both sunlight and nutrients from deep in the ocean. The ideal place for them is on the surface of the ocean in an area where plenty of cooler, nutrient-carrying water is upwelling from below. In contrast, the coccolithophores prefer to live on the surface in still, nutrient-poor water in mild temperatures.

Coccolithophores do not compete well with other phytoplankton. Yet unlike their cousins, coccolithophores do not need a constant influx of fresh food to live. They often thrive in areas where their competitors are starving. Typically, once they are in a region, they dominate and become more than 90 percent of the phytoplankton in the area.

Coccolithophores live mostly in subpolar regions. Some other places where blooms occur regularly are the northern coast of Australia and the waters surrounding Iceland. In the past two years, large blooms of coccolithophores have covered areas of the Bering Sea. This surprises many scientists since the Bering Sea is normally a nutrient-rich body of water.

Coccolithophores are not normally harmful to other marine life in the ocean. The nutrient-poor conditions that allow the coccolithophores to exist will often kill off much of the larger phytoplankton. Many of the smaller fish and zooplankton that eat normal phytoplankton also feast on the coccolithophores. In nutrient-poor areas where other phytoplankton are scarce, the coccolithophores are a welcome source of nutrition.

Relation to greenhouse gases

caption Image taken from space, showing a coccolithophore bloom south of Iceland. Ocean currents and eddies can be seen in the swirls of aquamarine water. (Image courtesy Norman Kuring, SeaWiFS project)

In the long term, the plants appear to be beneficial for the environment. Coccolithophores make their coccoliths out of one part carbon, one part calcium and three parts oxygen (producing calcium carbonate, CaCO3). So each time a molecule of coccolith is made, one less carbon atom is available to roam freely in the world to form greenhouse gases and contribute to global warming. Three hundred twenty pounds of carbon go into every ton of coccoliths produced. All of this material sinks harmlessly to the bottom of the ocean to form sediment.

The coccolithophores' short-term effect on the environment is somewhat more complex. This effect again has to do with the formation of their coccoliths and the chemical reaction involved in the process. The chemical reaction that makes the coccolith also generates a carbon dioxide molecule, a potent greenhouse gas, from the oxygen and carbon already in the ocean. While much of the gas is sucked back in by the coccoliths (all plants take in carbon dioxide for food) some of it escapes into the atmosphere and immediately becomes part of the greenhouse gas problem. Scientists are concerned in the short term that greenhouse gases will cause the upper layers of the ocean to become more temperate and stagnant. This would increase the number of coccoliths in the world, which would produce more greenhouse gas.

The coccolithophores also affect the global climate in the short term by increasing the oceans' albedo. Albedo is the fraction of sunlight an object reflects--higher albedo values indicate more reflected light. Coccolithophore blooms reflect nearly all the visible light that hits them. Since most of this light is being reflected, less of it is being absorbed by the ocean and stored as heat.


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