PROCESSES that control carbon uptake by marine phytoplankton are important in the global carbon cycle11-3. Uptake of COitself may be limited by diffusion4. Bicarbonate uptake may be limited by zinc as HCO -transport appears to involve the zinc metallo-enzyme carbonic anhydrases5,6 and the concentration of inorganic zinc in seawater7is low enough to limit the growth of certain phytoplankton in culture8,9. Here we show that HCO-uptake by the marine diatom Thalassiosira weissflogii is modulated by the partial pressure of COand 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 hypothesis10, posits that Zn (Fe) may limit oceanic production and influence the global carbon cycle. Because of the large13C enrichment of HCO-over CO our results may be important for the interpretation of δ13C measurements in seawater and sediments.
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