Do Coccolith B/Ca Ratios Elucidate the Response of the Smallest Calcifiers to Ocean Acidification?

Coccolithophorid algae are microscopic but prolific calcifiers in modern and ancient oceans. Different species and strains have exhibited diverse calcification responses to laboratory ocean acidification experiments. This hampers our ability to predict future alteration of marine biogeochemical cycles. We used SIMS ion probe to measure B/Ca ratios of coccoliths from three different strains of Emiliania huxleyi and one strain of Coccolithus pelagicus braarudi under different pH conditions to ascertain if B/Ca in fossil coccoliths might be an indicator of calcification stress to past events. B/Ca in abiogenic calcites increases at higher pH because of the preferential incorporation of borate ion into the calcite lattice, relative to boric acid which is the dominant species of B at lower pH. We find, however, that the behavior of B/Ca in coccoliths differs substantially from that of abiogenic calcites. First, B/Ca ratios of coccoliths are generally lower than those of abiogenic calcites precipitated in a comparable pH range, suggesting that the transport of ions into the cell reduces the ratio of B to bicarbonate in the calcifying vesicle compared to seawater. The slowest growing strain of E. huxleyi and one strain of C. braarudi exhibited low B/Ca ratios (<10 μmol/mol) which were constant as a function of culture pH; the calcite/cell of this E. huxleyi strain decreased with decreasing pH whereas that of the C. braarudi was constant. Two other more rapidly growing strains of E. huxleyi exhibited a large range in B/Ca ratio (55 to 25 μmol/mol), inversely correlated with pH which is opposite to the relationship observed in abiogenic calcites. Calcite/cell in both of these strains was constant or increased slightly with decreasing pH. B/Ca ratios therefore do not show a clear relationship with calcification stress. The variation in B/Ca ratios is most plausibly explained by changes in transport of B into the cell. B intake may be controlled by passive boric acid uptake through lipid membranes, which may increase at low pH. The potentially compensating role of active borate transporters remains to be evaluated and may explain the constant B/Ca ratios in other strains. The importance of variable B uptake means that B/Ca ratios cannot be used to evaluate the homeostasis or disruption of pH in the calcification vesicle.