Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium

Diazotrophic (N 2 ‐fixing) cyanobacteria provide the biological source of new nitrogen for large parts of the ocean. However, little is known about their sensitivity to global change. Here we show that the single most important nitrogen fixer in today's ocean, Trichodesmium , is strongly affected by changes in CO 2 concentrations. Cell division rate doubled with rising CO 2 (glacial to projected year 2100 levels) prompting lower carbon, nitrogen and phosphorus cellular contents, and reduced cell dimensions. N 2 fixation rates per unit of phosphorus utilization as well as C:P and N:P ratios more than doubled at high CO 2 , with no change in C:N ratios. This could enhance the productivity of N‐limited oligotrophic oceans, drive some of these areas into P limitation, and increase biological carbon sequestration in the ocean. The observed CO 2 sensitivity of Trichodesmium could thereby provide a strong negative feedback to atmospheric CO 2 increase.