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 COitself may be limited by diffusion⁴. Bicarbonate uptake may be limited by zinc as HCO ^-transport appears to involve the zinc metallo-enzyme carbonic anhydrases^5,6 and the concentration of inorganic zinc in seawater⁷is low enough to limit the growth of certain phytoplankton in culture^8,9. 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.