Phytoplankton play a key role in determining the partitioning of CO2 between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw-down of atmospheric CO2 and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) assimilation ratios modify the oceanic CO2 storage capacity. Here we show that increased atmospheric CO2 concentration enhances CO2 fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO2 led to further increases in the particulate C:N and C:P ratios. Enhanced CO2 uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO2, could increase carbon storage in the nitrogen-limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO2 levels.