Reduced calcification of marine plankton in response to increased atmospheric CO2

The formation of calcareous skeletons by marine planktonic organisms and th eir subsequent sinking to depth generates a continuous rain of calcium carb onate to the deep ocean and underlying sediments(1). This is important in r egulating marine carbon cycling and ocean-atmosphere CO2 exchange(2). The p resent rise in atmospheric CO2 levels(3) causes significant changes in surf ace ocean pH and carbonate chemistry(4). Such changes have been shown to sl ow down calcification in corals and coralline macroalgae(5,6), but the majo rity 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 cocc olithophorids Emiliania huxleyi and Gephyrocapsa oceanica. This was accompa nied by an increased proportion of malformed coccoliths and incomplete cocc ospheres. Diminished calcification led to a reduction in the ratio of calci te precipitation to organic matter production. Similar results were obtaine d in incubations of natural plankton assemblages from the north Pacific oce an when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow d own the production of calcium carbonate in the surface ocean. As the proces s of calcification releases CO2 to the atmosphere, the response observed he re could potentially act as a negative feedback on atmospheric CO2 levels.