Enrichment of convergent metabolic functions in microbial communities through imposed and emergent environmental niches

Microbial community composition is determined by numerous factors, including environmental filtering, biotic interactions, and stochasticity. Disentangling the relative contribution of these distinct processes is a fundamental problem for microbial ecology. Towards addressing it, here we used controlled experiments on replicate freshwater communities that vary individual factors, and used contig-based metagenomics methods to capture functional composition independently from uneven genome recovery across taxa. After repeated sub-culturing in minimal media lacking organic carbon but containing nitrate and vitamins, communities with distinct initial compositions consistently converged toward similar taxonomic structures and metabolic functions. These included oxygenic photosynthesis and nitrate metabolism, consistent with the imposed growth regime, and also showed reproducible enrichment of anoxygenic photosynthesis, vitamin biosynthesis and degradation pathways. These patterns indicate strong environmental filtering during assembly while also revealing a consistent role for emergent environments arising from microbial activity and metabolic interactions. To more directly test specific environmental drivers, we initiated replicate cultures from a single community and propagated them in the original medium and in variants lacking vitamins or both vitamins and nitrates. Only nitrate removal produced a distinct and statistically significant shift in both composition and function, with nitrate-free communities enriched for cyanobacterial nitrogen fixation and supporting metabolic functions in heterotrophs. Together, these results support a hierarchy of environmental filters determining community outcomes and provide a quantitative framework for predicting and steering community function through rational environmental design.