PROCESSES that control carbon uptake by marine phytoplankton are important in the global carbon cycle(1-3). Uptake of CO2 itself may be limited by diffusion(4). Bicarbonate uptake may be limited by zinc as HCO3- transport appears to involve the zinc metalloenzyme carbonic anhydrase(5,6) and the concentration of inorganic zinc in seawater(7) is low enough to limit the growth of certain phytoplankton in culture(8,9). Here we show that HCO3- uptake by the marine diatom Thalassiosira weissflogii is modulated by the partial pressure of CO2 and by the concentration of inorganic Zn (for which Cd and Co may substitute in carbonic anhydrase). This result leads naturally to a ’zinc hypothesis’ which, like the standing ’iron hypothesis’(10), posits that Zn (Fe) may limit oceanic production and influence the global carbon cycle. Because of the large C-13 enrichment of HCO3- over CO2, our results may be important for the interpretation of delta(13)C measurements in seawater and sediments.