Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions

Dissolution of anthropogenic CO₂ increases the partial pressure of CO₂ (pCO₂) and decreases the pH of seawater. The rate of Fe uptake by the dominant N₂-fixing cyanobacterium Trichodesmium declines as pH decreases in metal-buffered medium. The slower Fe-uptake rate at low pH results from changes in Fe chemistry and not from a physiological response of the organism. Contrary to previous observations in nutrient-replete media, increasing pCO₂/decreasing pH causes a decrease in the rates of N₂ fixation and growth in Trichodesmium under low-Fe conditions. This result was obtained even though the bioavailability of Fe was maintained at a constant level by increasing the total Fe concentration at low pH. Short-term experiments in which pCO₂ and pH were varied independently showed that the decrease in N₂ fixation is caused by decreasing pH rather than by increasing pCO₂ and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N₂ fixation efficiency at low pH, Trichodesmium synthesizes additional nitrogenase. This increase comes partly at the cost of down-regulation of Fe-containing photosynthetic proteins. Our results show that although increasing pCO₂ often is beneficial to photosynthetic marine organisms, the concurrent decreasing pH can affect primary producers negatively. Such negative effects can occur both through chemical mechanisms, such as the bioavailability of key nutrients like Fe, and through biological mechanisms, as shown by the decrease in N₂ fixation in Fe-limited Trichodesmium.