Spatial and Melt-Season Controls on the Downstream Contribution and Persistence of Glacier-Originating Bacteria Along...

Understanding the transport and fate of glacier-originating microorganisms along glacier-fed stream (GFS) gradients is critical for elucidating their downstream impacts. Yet, how glacier-originating microbes contribute to downstream communities across the GFS continuum and over the melt season remains poorly understood. Using 16S rRNA gene sequencing, we integrated spatial and temporal sampling to characterise bacterial communities along the hydrologic continuum of the Longxiazailongba (LXZLB) Glacier in the Tanggula Mountains, spanning supraglacial, glacier terminal and proglacial streams. Alphaproteobacteria, Gammaproteobacteria, Bacteroidia and Actinobacteria dominated communities. Bacterial diversity varied along the continuum, with higher alpha diversity in proglacial than in supraglacial and glacier terminal waters, driven by environmental variables including water temperature, pH and conductivity. Glacier-originating bacteria accounted for a substantial proportion of downstream communities but declined in relative contribution with distance from the glacier. Melt-season variability modulated the downstream representation of glacier-originating ASVs, reflecting changes in meltwater-driven hydrologic connectivity and transport intensity. Taxa exhibiting distinct downstream distributions responded differently to environmental gradients, leading to shifts in community structure along the stream. These findings indicate that spatial environmental gradients and melt-season hydrologic variability jointly shape the downstream contribution and persistence of glacier-originating bacteria, with implications for how continued glacier retreat may restructure microbial communities downstream.