Depth-stratified boreal peatland microbiomes reveal recurring community organization and methane-cycling potential

Microbial communities in peatlands vary with depth, but most studies have been limited in spatial extent or replication, constraining assessment of general community patterns. We characterized microbial community structure and metabolic potential across replicated depth profiles in seven boreal peatlands in Sweden. Combining 16S rRNA gene sequencing and shotgun metagenomics, we identified recurring vertical stratification across sites. Microbial richness declined with depth, and core taxa including Rice Cluster II, Terriglobales, Subgroup 2, Roseiarcus, Methylocystis, and Candidatus Solibacter were shared across locations. Functional gene profiles revealed depth-related reorganization of carbon-cycling potential, with hydrolytic and fermentative functions relatively more abundant near the surface and acetogenic and methanogenic pathways more prominent in deeper anoxic horizons. Gene-centric analyses indicated an important contribution of Rice Cluster II-related methanogens to hydrogenotrophic methanogenesis, while co-occurrence networks identified a depth-conserved syntrophic module centered on this lineage. Environmental variables showed depth-specific associations with microbial communities. Despite geographic separation and site-specific variation among peatlands, recurring depth-resolved microbial assemblages and functional profiles were observed, suggesting that common environmental constraints associated with peat development contribute to broad patterns of microbial community organization and carbon-cycling potential.