This research was sponsored by the EPSRC. T.W.F. first suggested the electrochemical deoxidation of titanium metal, and G.Z.C. observed that it was possible to reduce thick layers of oxide on titanium metal using molten salt electrochemistry. D.J.F. proposed the experiment, which was conducted by G.Z.C., on the reduction of solid titanium dioxide pellets. M. S. P. Shaffer took the original SEM image of Fig. 4a. Correspondence and requests for materials should be addressed to D. J. F. (e-mail: djf2@hermes.cam.ac.uk). 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 sediments, regulating marine carbon cycling and ocean-atmosphere CO2 exchange. The rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry, slowing down calcification in corals and coralline macroalgae. However, most marine calcification occurs in planktonic organisms. This study reports reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, *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. The progressive increase in atmospheric CO2 concentrations may slow down the production of calcium carbonate in the surface ocean, potentially acting as a negative feedback on atmospheric CO2 levels. The response of organic and inorganic carbon production to CO2 concentration in laboratory-cultured coccolithophorids was examined under controlled laboratory conditions. The carbonate system of the growth medium was manipulated to cover a range from pre-industrial CO2 levels (280 p.p.m.v.) to approximately triple pre-industrial values (about 750 p.p.m.v.). Over this range, *E. huxleyi* and *G. oceanica* experienced a slight increase in photosynthetic carbon fixation and a comparatively larger decrease in the rate of calcification. The ratio of calcite to organic matter production (calcite/POC) for the two species decreased significantly. Scanning electron microscopy indicated that malformed coccoliths and incomplete coccospheres increased in relative numbers with increasing CO2 concentrations. Laboratory results are consistent with CO2-related responses of natural plankton assemblages collected in the subarctic north Pacific. After incubation at pCO2 levels of about 250 p.p.m.v. and about 800 p.p.m.v., the rate of calcification wasThis research was sponsored by the EPSRC. T.W.F. first suggested the electrochemical deoxidation of titanium metal, and G.Z.C. observed that it was possible to reduce thick layers of oxide on titanium metal using molten salt electrochemistry. D.J.F. proposed the experiment, which was conducted by G.Z.C., on the reduction of solid titanium dioxide pellets. M. S. P. Shaffer took the original SEM image of Fig. 4a. Correspondence and requests for materials should be addressed to D. J. F. (e-mail: djf2@hermes.cam.ac.uk). 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 sediments, regulating marine carbon cycling and ocean-atmosphere CO2 exchange. The rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry, slowing down calcification in corals and coralline macroalgae. However, most marine calcification occurs in planktonic organisms. This study reports reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, *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. The progressive increase in atmospheric CO2 concentrations may slow down the production of calcium carbonate in the surface ocean, potentially acting as a negative feedback on atmospheric CO2 levels. The response of organic and inorganic carbon production to CO2 concentration in laboratory-cultured coccolithophorids was examined under controlled laboratory conditions. The carbonate system of the growth medium was manipulated to cover a range from pre-industrial CO2 levels (280 p.p.m.v.) to approximately triple pre-industrial values (about 750 p.p.m.v.). Over this range, *E. huxleyi* and *G. oceanica* experienced a slight increase in photosynthetic carbon fixation and a comparatively larger decrease in the rate of calcification. The ratio of calcite to organic matter production (calcite/POC) for the two species decreased significantly. Scanning electron microscopy indicated that malformed coccoliths and incomplete coccospheres increased in relative numbers with increasing CO2 concentrations. Laboratory results are consistent with CO2-related responses of natural plankton assemblages collected in the subarctic north Pacific. After incubation at pCO2 levels of about 250 p.p.m.v. and about 800 p.p.m.v., the rate of calcification was