Functions of coccolith formation1

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 HCO,hut that CO, 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 transmcmbranc 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. Coccolith formation and algal C Sikes et al. 1980). However, the enhancement of photosynthesis is not a sufficient explanation of coccolith formation because such a function would not require the retention of CaCO, at the cell surface in elaborate crystalline form. Moreover the presence of the coccosphere makes cells which are otherwise relatively buoyant sink from the photic zone on which they depend (Eppley et al. 1967; Sikes and Wilbur 1980). Consideration of other functions is therefore warranted. Other possible roles for coccolith formation relate to an analogy between the coccosphere and a cell wal1. Although coccolithophorids have no cell walls as such and are sometimes listed as examples of wall-less plants (Fogg 1975), the coccosphere might function as a cytoskeletal structure surrounding the cell (Outka and Williams 1971; Isenberg and Lavine 1973). In fact, coccolith formation is in some ways similar to wall formation l This work was supported in part by NSF grant PCM 78-22242. Funding for mass spcctrometry was granted by the Duke University Research Council. 2 Present address: Dept. Biol. Sci., Univ. South Alabama, Mobile 36688. and has been discussed as a specialized form of the process (Morrh and Mollenhauer 1976). If the coccosphere acts as a cell wall, there are several roles it may play. For example, some evidence suggests that the cell wall is necessary for maintenance of the inside-negative potential difference (PD) across the membrane (Racusen et al. 1977; Kinnersley et al. 1978; Drake et al. 1978). With cell walls removed enzymatically or during plasmolysis of intact cells, the PD can change from less than 100 mV to slightly positive values. Clearly, this could affect ion movement. In the case of coccolithophorids, passive calcium uptake might be favored in calcified cells if their PD is highly insidenegative. Algal cell walls may offer protection against herbivores (Porter 1977; Pentecost 1980). A protected cell can pass through the gut of a zooplankter undigested and viable, or it may be rejected before ingestion because of its shape or taste. Cell walls also play a role in volume regulation, required because of the presence in cells of impermeant macromolecules. Without walls or regulatory ion pumps such as the outwardly directed Na pump of animal cells, algal cells would swell and perhaps lyse from osmotic stress (Gutknecht et al. 1978), as during changes in the salinity of estuarics. Our purpose here was to investigate possible functions of coccolith formation.