Anthropogenic carbon dioxide (CO2) emissions are increasing at an unprecedented rate with clear environmental and social consequences. An understanding of natural feedback mechanisms under modern warming is important to construct precise and accurate predictions of future climate. The oceans are strongly affected by changes in atmospheric CO2; however, they also contribute to important climate regulating feedback mechanisms. Among marine biota, coccolithophores, a group of calcifying marine phytoplankton, play a key role in the ocean carbon cycle at various timescales providing pivotal information on the oceans’ capacity towards CO2buffering. This study compared changes to the coccolithophore assemblage during Marine Isotope Stage 5 (130-70 kyr), a geological interval considered as an analogue for modern warming, with the results of monospecific CO2-controlled culture experiments of Helicosphaera carteri. Community composition and the contribution of coccolithophores to carbon export were quantified from the nannofossil assemblage at Site U1501 of the International Ocean Discovery Program (IODP) Expedition 368 to the South China Sea. Growth rate and size were measured for H. carteri cultured at 425 ppm CO2, a level consistent with the best-case scenario predicted by the Intergovernmental Panel on Climate Change (IPCC). The contribution of coccolithophores to carbon export increased during MIS 5e; however, a decrease in the potential for carbon export of the species H. carteri was observed under 425 ppm CO2. The results suggest the tendency of coccolithophores towards climate regulation through carbon sequestration may reach a critical threshold under anthropogenic climate change.
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