Nitrogen fixation rates increase with diazotroph richness in the global ocean.

Marine nitrogen fixation is a key process to support and maintain the ocean's primary production, yet our knowledge of the distribution and diversity of the diazotrophic microbes that are capable of fixing nitrogen is very limited. Here, integrating microscopic and metagenomic data, we determine the biogeography and richness of the main diazotrophic taxa across the global ocean. Analyzing 22,000 records and 15 species, we deduce a latitudinal gradient in diazotroph richness, with higher richness to the tropics driven by temperature and nutrient levels. Cyanobacteria dominate in nutrient-poor gyres, while non-cyanobacterial diazotrophs thrive in nutrient-rich zones. Across the global ocean, diazotroph richness is found to correlate positively with nitrogen fixation rates, suggesting a positive biodiversity-ecosystem function relationship. While this relationship is robust to spatial autocorrelation and confounding environmental drivers, spatial dependence in the global datasets and potential unmeasured covariates may influence local-scale inferences. The findings suggest that positive biodiversity-ecosystem functioning relationships with implications for global biogeochemical cycling exist in marine plankton.