Effect of Euxinic Conditions on Planktic Foraminifers: Culture Experiments and Implications for Past and Future Environments

The oceanic redox state is an essential determinant for life on Earth, and anoxic events, periods when large parts of the oceans were anoxic or euxinic (free hydrogen sulfide (H2S) present), may have substantially affected the oceanic biosphere. We present the first culture data showing the effects of euxinic conditions on planktic foraminifers. We cultured six species of planktic foraminifers (n = 31) in three treatments: similar to 2 mg H2S/L, similar to 9 mg H2S/L, and a control, without H2S. Under the H2S treatments, planktic foraminifers did not survive for more than 48 hr, and gametogenesis percentages were very low (8% and 17%), while the time to gametogenesis was short (less than 1 day). However, some species (e.g., Neogloboquadrina dutertrei) survived episodic or temporary exposure to H2S for a relatively short time (i.e., <24 hr, such as a tidal cycle), indicating species-specific tolerance levels for H2S. The biological response of foraminifers to euxinia thus is fundamentally different from that of dysoxia (i.e., low dissolved oxygen; similar to 0.7 mg O-2/L or similar to 22 mu mol O-2/L), when they survive, add chambers, feed, and undergo gametogenesis. The presence/absence of planktic foraminifers in sediments thus may help to elucidate the redox state of the euphotic zone in the oceans of the past, with planktic foraminifers absent in sediments deposited when euphotic waters became euxinic.