Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium

The increases in atmospheric pCO₂ over the last century are accompanied by higher concentrations of CO₂(aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)‐fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO₂(aq). We examined the influence of pCO₂ [preindustrial∼250 ppmv (low), ambient∼400, future∼900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom‐forming cyanobacterium contributing substantial inputs of ‘new N’ to the oligotrophic subtropical and tropical oceans. High pCO₂ enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO₂ cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO₂ cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO₂. Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO₂ to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.