Global marine metagenomics reveals the functional diversity and ecological adaptations of diazotrophs across marine ecosystems

Biological nitrogen fixation by diazotrophs is vital for nitrogen supply and marine productivity. Despite extensive documentation of diverse diazotrophs, their functional diversity and adaptive strategies across ecosystems remain largely underexplored. Here, we analyzed 561 metagenomes from 16 distinct marine environments to characterize the distribution, functional diversity, and ecological adaptations of marine diazotrophs on a global scale. Our results reveal a high relative abundance of diazotrophs in extreme environments, such as hydrothermal vents and cold seeps, underscoring their roles in biogeochemical processes of these ecosystems. Analysis of 167 metagenome-assembled genomes representing 22 phyla revealed considerable metabolic versatility among diazotrophs, along with habitat-specific functional adaptations that facilitate their survival in diverse marine environments. For example, diazotrophs in the coastal area have adapted to utilize readily available energy sources such as starch and sucrose, while those in the open ocean have developed mechanisms to degrade refractory organic matter. In the chemosynthetic ecosystem, diazotrophs closely associate with methane and sulfur metabolism, demonstrating their capacity to thrive in extreme environments. Diazotrophs affiliated with Alphaproteobacteria and Desulfobulbia were found to be ubiquitous in aerobic and anaerobic environments, respectively. This study sheds light on the ecological significance and adaptive mechanisms of diazotrophs across diverse marine ecosystems. Diazotrophs (nitrogen-fixing microorganisms) display high relative abundances in extreme environments, such as hydrothermal vents and cold seeps, according to analyses of globally distributed metagenomes from multiple distinct marine environments.