Effect of low dissolved oxygen concentration on planktonic foraminifera: results from laboratory culture experiments and implications for oceanic anoxic events

During Cretaceous oceanic anoxic events (OAEs), substantial turnover of planktonic foraminiferal species occurred, however, the direct effects of the dissolved oxygen (DO) concentration on planktonic foraminifera remain obscure. Althogh culture experiments conducted under controlled conditions can quantify the relationships between foraminiferal ecology and environmental parameters, experiments controlling DO have yet to be conducted because it is difficult to maintain a stable oxygen concentration. In this study, we cultured two subtropical-transitional planktonic foraminifer species (one symbiotic species, Orbulina universa, and one nonsymbiotic species, Globigerina bulloides) under six different DO conditions (between 10% and 100% saturation). In both species, the gametogenesis rate was more than 60% even at a DO of 10%, suggesting that at least 'dysoxic' conditions (~0.7 mg O₂ L^-1) could not have directly caused the extinction of planktonic foraminifera during OAEs. Planktonic foraminifera originated from benthic lineages, and this origin is one possible explanation for their high tolerance to extremely low DO levels. Although the number of days to gametogenesis did not differ significantly among treatments in either species, final shell weight increased with increasing DO, suggesting that fossil foraminiferal shell weight could vary with past DO conditions. Our results suggest that the extinction of many planktonic foraminiferal species during OAEs may have been due to anoxic or euxinic conditions in the euphotic zone. The occurrence of these conditions can be explained either by the oxygen minimum layer model or by the stagnant ocean model combined with elevated riverine P input.