Zinc and carbon co-limitation of marine phytoplankton

PROCESSES that control carbon uptake by marine phytoplankton are important in the global carbon cycle1 1–3 . Uptake of CO 2 itself may be limited by diffusion 4 . Bicarbonate uptake may be limited by zinc as HCO − 3 transport appears to involve the zinc metallo-enzyme carbonic anhydrases 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 HCO – 3 uptake by the marine diatom Thalassiosira weissflogii is modulated by the partial pressure of CO 2 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 13 C enrichment of HCO − 3 over CO 2 , our results may be important for the interpretation of δ 13 C measurements in seawater and sediments.