Diverse and sometimes odd calcium carbonate plates lend coccolithophores an eccentric quality and fascinate us with their stunning beauty. We do not know why these features originally developed or what purposes they serve, but researchers have suggested several possibilities.
Coccoliths may have originated as organic scales, perhaps as tools for protecting fragile cell membranes from sudden changes in the environment, excessive ultraviolet light, or physical trauma. For some coccolithophores, calcified plates may serve no other purpose. For others, coccoliths offer the advantage of versatility. They can be formed into shapes designed to do double duty-protect the cell membrane and cope with other challenges posed by the environment.
Coccoliths do not protect coccolithophores from being eaten, but the indigestible plates both armor cells and reduce their nutritional value, perhaps making coccolithophores a little less appetizing. Spine-like coccoliths increase cell diameter by as much as four times in the case of Rhabdosphaera clavigera, perhaps to further discourage predators.
Scientists also speculate that coccoliths trap water, creating a buffer zone in which the cell has greater control over water chemistry. For example, coccoliths might help the cell gather and store nutrients that will ultimately pass through the cell membrane. This may explain coccoliths that have two layers with an intervening space, or it might account for umbrella-like coccoliths which overlap to create a continuous outer sphere.
In a few species coccolith production might aid photosynthesis. The chemical reaction coccolithophores use to create calcium carbonate also releases carbon dioxide inside the cell where it can be consumed immediately. This could explain why a few species, such as Emiliania huxleyi, produce many more coccoliths than required to cover the cell. In laboratory cultures E. huxleyi frequently grows multiple layers of coccoliths and sheds the outer ones almost continually. Other coccolithophores produce just enough coccoliths for a single-layer cell covering, however, and probably do not depend on the carbon dioxide created in the process.
Specially adapted coccoliths might alter buoyancy for some species, helping cells maintain access to sunlight or nutrients. For example, heavy coccoliths could increase a cell's density, allowing the coccolith-ophore to sink through the water. Sinking gives the cell more reliable access to nutrients than staying in one place. If cells descend too much, however, they fall out of sunlit waters. Adaptations that accelerate sinking only help species in turbulent waters where cells can rely on being periodically swept toward the surface.
In contrast, species in calm waters may use coccoliths to reduce sinking rates. Spiny coccoliths with flaring ends perhaps evolved to increase cell diameter without significantly increasing weight. This might lower overall cell density and increase buoyancy, helping the cell stay near the surface where there is plenty of sun.
Coccoliths might also regulate light entering a coccolithophore cell. Too much ultraviolet light can be detrimental to phytoplankton, but the reflective properties of calcium carbonate may allow species to adapt to harsh light conditions high in the water column. Alternatively, some coccolith configurations could focus light into the cell, functioning as light gatherers. This adaptation would benefit species such as Florisphaera profunda, which live in deep waters where little sunlight penetrates.
