Methane is an important selection factor in microbial community assembly along the Greenland Ice Sheet margin

Ice sheet beds host metabolically active microbial communities and serve as sites of methane (CH 4 ) production, consumption, and mineralization. Microbially derived CH 4 is released from the western margin of the Greenland Ice Sheet (GrIS) as meltwater-dissolved CH 4 (CH 4(aq) ) at concentrations spanning five orders of magnitude (from ~0.4 nM to ~50,000 nM), indicating a strong gradient in its availability. Here, we test the significance of CH 4 as a determinant of microbial assemblage structure along the GrIS’s western margin by serving as an important carbon and energy source. We sampled microbial assemblages from glacial runoff spanning a ~2,000-km transect and characterized them using 16S rRNA amplicon sequencing. The assemblages were dominated by the genera Rhodoferax , Polaromonas , Methylotenera , and Crenothrix , the latter an important methanotrophic group. Distance-based redundancy analysis identified CH 4(aq) concentration as the strongest predictor of microbial assemblage structure. Differential abundance analysis revealed a coupling between organisms involved in complex organic matter degradation and CH 4 cycling, suggesting microbial interactions linking methanogenic and methanotrophic processes. Consistent with these patterns, phylogenetic bin-based null modeling indicated CH 4 is a key driver of community assembly, with higher CH 4(aq) concentration sites (i.e., >4.9 nM) exhibiting a stronger signal of homogeneous selection, whereas sites below this threshold were more strongly influenced by stochastic processes, particularly drift and dispersal limitation. Together, these results suggest that CH 4 exerts a strong deterministic influence on microbial assembly at the GrIS margins and highlight the role of these ecosystems in carbon cycling and CH 4 dynamics in a warming Arctic. IMPORTANCE Our findings provide new insight into the ecological role of CH 4 in the Greenland Ice Sheet ecosystem by linking its concentration gradient to the microbial community assembly process at the ice sheet margin and contribute to understanding carbon cycling in this rapidly warming ecosystem.