Coccoliths are microscopic structures found throughout the world’s oceans. These tiny, ornate creations, though invisible to the naked eye, play a significant role in marine environments.
The Tiny Builders of the Ocean
Coccoliths are individual plates or scales made of calcium carbonate. These structures typically measure between 2 and 25 micrometers across. They are produced by single-celled marine algae known as coccolithophores, a type of phytoplankton. Coccolithophores envelop themselves in these plates, forming a spherical shell called a coccosphere, which can be around 5-100 micrometers in diameter. These organisms are abundant, with estimates of 10,000 coccoliths per liter of seawater.
Why Coccoliths Are Made
Coccolithophores produce these calcium carbonate plates through a process called biomineralization. The exact reasons for this calcification are still under investigation, but several hypotheses exist. One prominent idea is that the coccosphere offers protection against microzooplankton predation. It has also been suggested that coccoliths help regulate buoyancy, scatter or focus light to optimize photosynthesis, or even serve as a way to remove excess calcium from the cell.
Their Role in Marine Ecosystems
Coccolithophores are primary producers, forming the base of many marine food webs by converting carbon dioxide into organic carbon through photosynthesis. This process supports a wide array of marine life, as zooplankton feed on coccolithophores, which are then consumed by larger animals like fish and whales. Beyond their role as food, coccolithophores influence ocean chemistry and the carbon cycle. When coccolithophores die, their coccoliths sink, transporting carbon to the deep ocean, a process known as the “biological pump” or “carbonate pump.” Large blooms of coccolithophores can also change the ocean’s appearance, turning vast areas a milky turquoise due to the light scattering properties of their abundant coccoliths.
Coccoliths and Earth’s Climate
The accumulation of coccoliths over millions of years has led to the formation of massive geological deposits. Chalk cliffs, like the famous White Cliffs of Dover, and extensive limestone beds are primarily composed of these tiny calcium carbonate remains. These formations act as long-term carbon sinks, locking away atmospheric carbon dioxide. The fossil record of coccoliths also provides insights into past ocean conditions and climate change.
Coccoliths are sensitive to current ocean acidification, a consequence of increased atmospheric carbon dioxide absorption by the ocean. This acidification makes it more difficult for coccolithophores to form their calcium carbonate shells, with studies showing a reduction in coccolith area and a weakening of their mechanical properties under increased acidity. Such impacts can affect the coccolithophores’ ability to grow and reproduce, potentially disrupting the marine food web by reducing their nutritional content and making them a lower-quality food source for consumers. Different coccolithophore species may respond differently to ocean acidification.