Reduced calcification of marine plankton in response to increased atmospheric CO2

Abstract

The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments¹. This is important in regulating marine carbon cycling and ocean–atmosphere CO₂ exchange². The present rise in atmospheric CO₂ levels³ causes significant changes in surface ocean pH and carbonate chemistry⁴. Such changes have been shown to slow down calcification in corals and coralline macroalgae^5,6, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO₂ concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica . This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO₂ levels. We suggest that the progressive increase in atmospheric CO₂ concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO₂ to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO₂ levels.