Biological nitrogen fixation driven by methane anaerobic oxidation supports the complex biological communities in cold-seep habitat

Cold seeps represent a class of chemosynthetic ecosystems that are prevalent in deep-sea environments. Despite extensive research on the functional microbiology of cold seeps, it remains unclear how methane oxidation is coupled with the nitrogen and sulfur cycles in the reduced sediment habitats across various spatial scales. Here, we employed metagenomic sequencing to investigate the microbial communities within the Haima cold seep sediments, with particular attention to microorganisms involved in biogeochemical cycles at varying spatial scales. In the surface layer of regular sediment areas, the sulfur-oxidizing bacteria such as Chromatiates, PS1, SZUA-229, and GCF-002020875 were the most numerically abundant groups. These bacteria recycle sulfide and generate sulfate while facilitating nitrate removal to support the methane anaerobic oxidation in the subsurface layer. However, in the biogenous sediments of cold seep ecosystems, methane anaerobic oxidation and nitrogen fixation emerge as the predominant processes. The microbial coupling groups, including sulfate reducing bacteria C00003060 and ANME-1, utilize energy from methane anaerobic oxidation to complete the nitrogen fixation. These findings suggest that cold-seep diazotrophs can support complex biological communities by supplying fixed nitrogen. This study enhances our understanding of the functional microbial structure and metabolic capabilities within cold seep ecosystems.