Baltic Sea microbial cohorts exhibit catabolic specialization and anabolic interdependencies across environmental gradients

Microbial communities are structured by environmental gradients and metabolic interactions, making the study of co-occurring populations essential for understanding emergent ecological properties. Here, we investigated microbial cohorts, recurring groups of co-occurring microorganisms shaped by shared environmental preferences and potential biotic interactions, across the Baltic Sea, a well-characterized region where microbial community composition and metabolic functions vary strongly across salinity, temperature, and oxygen gradients. We used a genomic catalog consisting of 701 species-representative genomes to recruit reads from 112 metagenomes and infer cohort structure, environmental distributions, and metabolic potential. We identified nine microbial cohorts that showed strong associations with environmental gradients, indicating deterministic assembly. Cohorts differed markedly in their genomic and functional characteristics. The most abundant and prevalent cohorts were dominated by taxa with smaller, streamlined genomes, whereas the low-oxygen cohort contained larger genomes and contributed disproportionately to nitrogen and sulfur transformations. Within cohorts, different biosynthetic pathways were consistently associated with different taxonomic groups. Complete amino acid biosynthesis pathways were common among individual genomes, whereas complete B-vitamin biosynthesis pathways were rare. The six biosynthetic pathways with the lowest completeness were consistently associated with different taxonomic groups across all cohorts, with individual genomes encoding only a subset of these pathways, consistent with widespread metabolic interdependencies. Together, these results show that Baltic Sea microbial cohorts are ecologically structured assemblages whose genomic repertoires reflect catabolic specialization and anabolic interdependencies.