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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2012-11, Vol.109 (45), p.E3094-E3100
• Main Authors: Shi, Dalin, Kranz, Sven A, Kim, Ja-Myung, Morel, François M. M
• Format: Article
• Language: English
• Subjects: acidification; Acidity; Acids - metabolism; adverse effects; Atmospheric pressure; Bacteria; Bacterial Proteins - metabolism; bioavailability; Biological Sciences; Carbon - metabolism; Carbon Cycle - drug effects; Carbon dioxide; Carbon Dioxide - metabolism; Chlorophyll - metabolism; Cyanobacteria - cytology; Cyanobacteria - enzymology; Cyanobacteria - growth & development; Cyanobacteria - metabolism; Hydrogen - metabolism; Hydrogen-Ion Concentration - drug effects; Iron; Iron - metabolism; Iron - pharmacology; Nitrogen; Nitrogen - metabolism; nitrogen fixation; Nitrogen Fixation - drug effects; nitrogen-fixing bacteria; nitrogenase; Nitrogenase - metabolism; nutrients; Oceans and Seas; Partial Pressure; Particulate Matter - metabolism; Photosynthesis - drug effects; Photosystem I Protein Complex - metabolism; Photosystem II Protein Complex - metabolism; physiological response; PNAS Plus; proteins; Seawater; Trichodesmium; trophic relationships
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1216012109

Dissolution of anthropogenic CO ₂ increases the partial pressure of CO ₂ (p CO ₂) and decreases the pH of seawater. The rate of Fe uptake by the dominant N ₂-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 p CO ₂/decreasing pH causes a decrease in the rates of N ₂ 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 p CO ₂ and pH were varied independently showed that the decrease in N ₂ fixation is caused by decreasing pH rather than by increasing p CO ₂ and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N ₂ 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 p CO ₂ 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 N ₂ fixation in Fe-limited Trichodesmium .