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KM Shafi, PhD Candidate

Department of Biological Sciences, Michigan Technological University

Abstract:

Thermokarst lakes dominate the landscape of the North Slope of Alaska. These inland water bodies remain ice-covered for six to nine months of the year, creating three distinct vertical habitats: ice, water, and sediment. Microbial communities present in these habitats play critical roles in food web dynamics, nutrient cycling, and controlling greenhouse gas fluxes. A subset of specialized bacteria can even interact directly with ice by catalyzing ice formation or inhibiting ice growth. To assess controls on microbial communities in this ice-dominated landscape, we first asked whether the microbial community is structured more by vertical habitat (i.e., differences between ice, water, and sediment) or if it is overridden by horizontal (lake-to-lake) physicochemical variation. We then asked how bacteria from these lakes physically aid in ice formation. To address the first question, this study examined microbial community structure, diversity, and partitioning across three different habitats of three closely spaced (~ 4 to 8 km apart) but physicochemically distinct lakes on the North Slope of Alaska near the town of Utqiaġvik (Latitude: ~ 71.2906°N, Longitude: ~156.7886°W). We found that microbial communities were structured by vertical habitats rather than horizontal variation, and community composition data revealed habitat specialization where water communities were dominated by members of the orders Burkholderiales, Frankiales, and Methylococcales; ice communities by Burkholderiales, Micrococcales, Flavobacteriales and sediment communities by Bacteroidales, Anaerolineales, and Syntrophales. To address the second question, the study investigated biological ice-nucleation, which is the physical process by which stable ice crystals form from supercooled water, typically triggered by ice-nucleating particles (INPs) that lower the energy barrier for freezing. Biological ice nucleators can catalyze ice formation at relatively warm subzero temperatures compared to other nucleators, influencing snow and ice formation. Through cultivation, genomic analysis, and activity assays, we characterized bacterial ice-nucleators and examined their genetic and phenotypic adaptations. Notably, some strains show ice-nucleation activity not only in cells but also in cell-free supernatants, indicating extracellular or membrane-derived nucleators and underscoring the novelty and ecological significance of these Arctic ice nucleators in cryospheric microbial ecology. By integrating ecosystem-scale community analysis with targeted molecular-mechanism studies, we demonstrate that microbial communities in Arctic lakes are primarily structured by physical habitat filtering. The presence of ice-nucleating bacteria within these habitats provides a critical functional adaptation that enables microbial interactions with—and persistence in—ice-dominated ecosystems.