The potential contribution of microbial communities to carbon fixation and nitrogen cycle in the Eastern Indian Ocean.

This study investigated the diversity and metabolic potential of microbial communities in the Eastern Indian Ocean (EIO) through 16S rDNA gene sequencing and metagenomics analyses. Water samples were collected from the surface waters (5 m depth) and 150 m depth layer in the EIO between March 20th and June 6th, 2019. This study reveals microbial-driven biogeochemical dynamics in the oligotrophic Eastern Indian Ocean, where vertically stratified communities (Cyanobacteria/Proteobacteria-dominated surface vs. diversified Proteobacteria at 150 m) and latitudinal diversity gradients reflect nutrient limitations. Metagenomics identified four carbon fixation strategies: the Calvin cycle dominated epipelagic CO2 assimilation, while the 3-hydroxypropionate bicycle showed elevated surface activity, alongside reductive citrate and Wood-Ljungdahl pathways involving novel Actinobacteria. Nitrogen cycling exhibited spatial heterogeneity: nifH-dominated nitrogen fixation in the surface waters, prevalent narGHI nitrate reduction, and divergent nirS/nirK/nosZ distributions tied to nutrient gradients. Proteobacteria and Actinobacteria were key nitrogen fixers, with novel Actinobacteriota diazotrophs expanding known diversity. Elevated nosZ abundance in the Bay of Bengal underscored regional nitrous oxide consumption hotspots. These findings underscore microbial mediation of carbon-nitrogen fluxes in oligotrophic systems, providing genomic insights into ecosystem responses to climate-driven ocean changes.