Calcification in Coccolithophoridae: Wasteful or Functional?

The biosynthesis of coccoliths in Emiliania huxleyi requires the formation of a very complex calcifying apparatus. A special vacuolar system, consisting of a coccolith vesicle in which the coccolith is formed and an associated meshwork of anastomosing canals (the reticular body), is probably derived from the Golgi apparatus. In it a complex acidic polysaccharide is produced that is assumed to play a regulatory role in calcification. Some 3 × 10⁶$\text{Ca}^{2+}$ions are pumped per second from the medium into the cytoplasm (a "vacuum" of free calcium) and from there into the coccolith vesicle. The coccolith must then be assembled through carefully controlled nucleation, growth and termination of CaCO₃-crystallization. Finally, the coccolith must be extruded and taken up in the extracellular coccosphere. In contrast to these apparent high cellular expenses of coccolith biosynthesis is the fact that overproduction of coccoliths is very common, the redundant coccoliths either being accumulated around the cells or simply shed into the medium. It is suggested that coccolith biosynthesis allows the cells to benefit from the concerted operation of photosynthesis and calcification while remaining afloat and inhabiting the open ocean.