Taxon-specific response of marine nitrogen fixers to elevated carbon dioxide concentrations

Much of the bioavailable nitrogen that supports open ocean food webs and biogeochemical cycles is fixed from the atmosphere by marine cyanobacteria of the genera Trichodesmium and Crocosphaera. In previous experiments carried out with a limited set of cyanobacterial isolates, rates of cyanobacterial nitrogen fixation were shown to increase with carbon dioxide concentrations. Here, we report results from a series of laboratory experiments in which we grew seven strains of Trichodesmium and Crocosphaera from the Atlantic and Pacific oceans under a wide range of carbon dioxide concentrations, and monitored rates of nitrogen fixation and growth. We document large, strain-specific differences in the relationship between nitrogen fixation and carbon dioxide concentration, suggesting that individual strains within each genus are adapted to grow and fix nitrogen at different concentrations of carbon dioxide. We apply kinetic constants from the individual carbon dioxide response curves to an illustrative biogeochemical model of the ocean in 2100, which suggests that strains adapted to high carbon dioxide concentrations could potentially be favoured in a future acidified ocean. We suggest that surface ocean carbon dioxide concentrations could constitute a previously unrecognized selective force that shapes the community composition and diversity of nitrogen-fixing cyanobacteria.