Dominic Eriksson, Nicolas Gruber, Fabio Benedetti, Damiano Righetti, Lucas Paoli, Guillem Salazar, Shinichi Sunagawa, Meike Vogt

Marine diazotrophs, a highly specialized group of marine prokaryotes, convert atmospheric nitrogen gas into bioavailable forms of nitrogen and are thus critical to maintain the fertility of the ocean. However, little is known about the link between global-scale diazotroph diversity and marine N2 fixation rates. Here, we address this question by integrating more than 22’000 DNA sequencing and microscopy-based observations for 14 diazotroph species into species distribution models. We identify distinct biogeographic patterns for the major known taxa of diazotrophs, including colony- forming, unicellular, symbiotic, and non-cyanobacterial diazotrophs. Non-cyanobacterial diazotrophs show a higher annual mean number of presences in upwelling regions compared to their cyanobacterial counterparts. In addition, the identified biogeographic patterns reveal a strong latitudinal gradient in diazotroph species richness, which is highest in tropical and subtropical regions and declines towards the poles. Temperature and nutrient-related parameters rank as the most important predictors of the biogeography explaining up to 36% of the variance in the data for specific taxa. We find diazotroph richness to be positively correlated with independently estimated nitrogen fixation rates, suggesting efficient resource partitioning rather than competitive exclusion as the dominant driver of the observed biodiversity patterns. Our work reveals that important biodiversity-ecosystem functioning relationships associated with global biogeochemical cycling exist in the marine plankton, and suggests that global nitrogen fixation rates and diazotroph diversity are likely to increase in a warming ocean.