This study investigates the impact of increased atmospheric CO₂ on marine plankton calcification. The research shows that two dominant marine calcifying phytoplankton species, Emiliania huxleyi and Gephyrocapsa oceanica, exhibit reduced calcite production under elevated CO₂ conditions. This reduction is accompanied by an increase in malformed coccoliths and incomplete coccospheres, and a decrease in the ratio of calcite precipitation to organic matter production. Similar results were observed in natural plankton assemblages from the North Pacific when exposed to experimentally elevated CO₂ levels. The findings suggest that rising atmospheric CO₂ levels may slow down calcium carbonate production in the surface ocean. Since calcification releases CO₂ to the atmosphere, this response could act as a negative feedback on atmospheric CO₂ levels. The study also shows that the response of these two species to CO₂-related changes in seawater carbonate chemistry was examined under controlled laboratory conditions. The carbonate system of the growth medium was manipulated by adding acid or base 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 decreased significantly. These results are consistent with CO₂-related responses of natural plankton assemblages collected in the subarctic North Pacific. The rate of calcification was reduced by 36% to 83% in high-CO₂ treatments compared to low-CO₂ treatments in four independent experiments. The observed decrease in calcification with increasing pCO₂, if representative of biogenic calcification in the world's ocean, has significant implications for the marine carbon cycle. Owing to its effect on carbonate system equilibria, calcification is a source of CO₂ to the surrounding water. The buffer capacity of seawater determines the amount of CO₂ released during calcification. Theoretical calculations suggest that the buffer state of pre-industrial seawater resulted in 0.63 moles of CO₂ released per mole of CaCO₃ precipitated. Following the predictions of future atmospheric CO₂ rise, this value will increase to 0.79 in 2100. At constant global ocean calcification, this results in an additional source of CO₂ to the atmosphere. In the case of reduced calcification, this positive feedback is reversed. Model calculations suggest an additional storage capacity of the surface ocean for CO₂ between 6.2 Gt C and 32.3 Gt C for the period of 1950 to 2100. The results indicate that the ratio of calcite to organic matter production in cultured coccolithophorids and in oceanThis study investigates the impact of increased atmospheric CO₂ on marine plankton calcification. The research shows that two dominant marine calcifying phytoplankton species, Emiliania huxleyi and Gephyrocapsa oceanica, exhibit reduced calcite production under elevated CO₂ conditions. This reduction is accompanied by an increase in malformed coccoliths and incomplete coccospheres, and a decrease in the ratio of calcite precipitation to organic matter production. Similar results were observed in natural plankton assemblages from the North Pacific when exposed to experimentally elevated CO₂ levels. The findings suggest that rising atmospheric CO₂ levels may slow down calcium carbonate production in the surface ocean. Since calcification releases CO₂ to the atmosphere, this response could act as a negative feedback on atmospheric CO₂ levels. The study also shows that the response of these two species to CO₂-related changes in seawater carbonate chemistry was examined under controlled laboratory conditions. The carbonate system of the growth medium was manipulated by adding acid or base 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 decreased significantly. These results are consistent with CO₂-related responses of natural plankton assemblages collected in the subarctic North Pacific. The rate of calcification was reduced by 36% to 83% in high-CO₂ treatments compared to low-CO₂ treatments in four independent experiments. The observed decrease in calcification with increasing pCO₂, if representative of biogenic calcification in the world's ocean, has significant implications for the marine carbon cycle. Owing to its effect on carbonate system equilibria, calcification is a source of CO₂ to the surrounding water. The buffer capacity of seawater determines the amount of CO₂ released during calcification. Theoretical calculations suggest that the buffer state of pre-industrial seawater resulted in 0.63 moles of CO₂ released per mole of CaCO₃ precipitated. Following the predictions of future atmospheric CO₂ rise, this value will increase to 0.79 in 2100. At constant global ocean calcification, this results in an additional source of CO₂ to the atmosphere. In the case of reduced calcification, this positive feedback is reversed. Model calculations suggest an additional storage capacity of the surface ocean for CO₂ between 6.2 Gt C and 32.3 Gt C for the period of 1950 to 2100. The results indicate that the ratio of calcite to organic matter production in cultured coccolithophorids and in ocean