Elevated CO₂ enhances nitrogen fixation and growth in the marine cyanobacterium <i>Trichodesmium</i>

In oceanic regions, where light and nutrients such as P and Fe are not limiting, it is expected the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium and other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.

Abstract

Abstract The increases in atmospheric p CO 2 over the last century are accompanied by higher concentrations of CO 2 (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 CO 2 (aq). We examined the influence of p CO 2 [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 p CO 2 enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low p CO 2 cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high p CO 2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient p CO 2 . Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO 2 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.