Differing responses of marine N2 fixers to warming and consequences for future diazotroph community structure

The globally distributed colonial cyanobacterium Trichodesmium and unicellular diazotrophs including Crocosphaera together carry out the majority of marine biological nitrogen (N-2) fixation. Future sea surface warming is predicted to influence their abundance and distribution, but temperature reaction norms have been determined for very few representatives of each genus. We compared thermal responses within and between the 2 genera Trichodesmium and Crocosphaera by measuring reaction norms for growth, N-2 fixation, carbon fixation, and elemental ratios in 7 strains from a global culture collection. Temperature reaction norms of Trichodesmium and Crocosphaera were remarkably similar for all isolates within each genus, regardless of their geographic origin. Thermal limits of Trichodesmium and Crocosphaera ranged from 18 to 32 degrees C and 24 to 32 degrees C, and optimum growth temperatures were similar to 26 and similar to 30 degrees C, respectively. The highest cellular ratios of nitrogen to phosphorus and carbon to nitrogen were found at optimum growth temperatures, and the lowest ratios near their thermal limits. In a mixed competition experiment, Trichodesmium growth rates were similar to 25% higher than those of Crocosphaera at 24 degrees C, while those of Crocosphaera were similar to 50% higher at 28 degrees C. Comparison of these results to current and projected seasonal temperature regimes in the subtropical Atlantic and Pacific Oceans suggests that predicted warmer temperatures may favor Crocosphaera over Trichodesmium, but that both genera may be excluded where future temperatures consistently exceed 32 degrees C. Sea surface warming could profoundly alter the community structure and stoichiometry of marine N-2-fixing cyanobacteria, thus fundamentally changing the biogeochemical cycling of this globally significant source of new nitrogen.