Contrasting calcification responses to ocean acidification between two reef foraminifers harboring different algal symbionts

Ocean acidification, which like global warming is an outcome of anthropogenic CO₂ emissions, severely impacts marine calcifying organisms, especially those living in coral reef ecosystems. However, knowledge about the responses of reef calcifiers to ocean acidification is quite limited, although coral responses are known to be generally negative. In a culture experiment with two algal symbiont-bearing, reef-dwelling foraminifers, Amphisorus kudakajimensis and Calcarina gaudichaudii, in seawater under five different pCO₂ conditions, 245, 375, 588, 763 and 907 μatm, maintained with a precise pCO₂-controlling technique, net calcification of A. kudakajimensis was reduced under higher pCO₂, whereas calcification of C. gaudichaudii generally increased with increased pCO₂. In another culture experiment conducted in seawater in which bicarbonate ion concentrations were varied under a constant carbonate ion concentration, calcification was not significantly different between treatments in Amphisorus hemprichii, a species closely related to A. kudakajimensis, or in C. gaudichaudii. From these results, we concluded that carbonate ion and CO ₂ were the carbonate species that most affected growth of Amphisorus and Calcarina, respectively. The opposite responses of these two foraminifer genera probably reflect different sensitivities to these carbonate species, which may be due to their different symbiotic algae.