Functions of coccolith formation1

Abstract

Measurements of discrimination in fixation of stable carbon isotopes by two species of coccolithophorids support the concept that coccolith formation depends on a supply of HCO₃⁻ but that CO₂ is the principal substrate of photosynthesis. This in turn suggests that coccolith formation and photosynthesis are linked through their complementary influence on the internal equilibrium of dissolved inorganic carbon. Measurements of osmotic responses and transmembrane potential differences of calcified and noncalcified cells showed that although coccolithophorids behaved as if they were walled cells, the coccoliths themselves did not always contribute to this property. Also coccoliths did not protect cells from predation by marine copepods, which fed equally on calcified and noncalcified cells. The presence of a coccosphere did, however, seem to impart a greater tolerance to low salinity, allowing calcified cells to grow when noncalcified cells did not.