Local-scale variability in snow chemistry drives distinct microbial communities in Alpine seasonal snowpack

Abstract. Seasonal snowpack is a dynamic system that accumulates atmospheric particles, including dust and biological material such as microorganisms, from both local and long-distance sources. Here, we investigated the relationship between bacterial community composition and chemical properties of seasonal snowpack in mid-altitude valleys of the Swiss Alps. We characterised microbial community structure and diversity in surface snow and underlying bulk snow across three adjacent valleys (elevations 1,798–2,578 m.a.s.l.). Analysis of major ions and organic acids was used to identify key environmental factors influencing microbial community composition. While geographical location showed no clear influence on either the chemical composition or the bacterial community structure, we identified significant differences between snow layers, with surface snow showing higher diversity than bulk snow and exhibiting greater cross-site similarity. Surface snow contained twice as many bacterial genera in the core community as bulk snow, with nearly complete overlap. Total inorganic nitrogen and Ca²⁺ were key drivers of microbial community composition in the snowpack. Using Weighted Gene Co-expression Network Analysis (WGCNA), we identified modules of co-occurring bacterial taxa with distinct responses to these chemical gradients. Spore-forming genera (Neobacillus, Niallia, Sporosarcina) were associated with nitrogen-enriched communities, while Brevundimonas, Cryobacterium, and Polaromonas drove calcium-enriched communities. The distinct patterns in chemistry and microbial community structure observed within just 10–15 cm reflect differences in atmospheric sources of deposited precipitation, with additional effects from post-depositional processes. This environmental filtering decreases local diversity while selecting different species based on initial community composition and local conditions. Understanding these bacterial-physicochemical relationships offers insights into how mountain ecosystems adapt to climate-driven changes in snow cover duration and atmospheric conditions.