Note A model of carbon isotopic fractionation and active carbon uptake in phytoplankton

The carbon isotopic fractionation of phytoplankton photosynthesis ( p) has been interpreted by previous authors as inconsistent with active bicarbonate uptake. This interpretation contradicts the results of numerous physiological studies demonstrating signi cant active bicarbonate uptake in phytoplankton. Using a simple model of cellular regulation of carbon acquisition we show that an upward curvature of p as a function of the ratio of growth rate to carbon dioxide concentration does not exclude active bicarbonate uptake. Our model describes adequately published carbon isotope data for cyanobacteria, diatoms and coccolithophores consistent with active bicarbonate uptake.