Determinism and stochasticity drive microbial community assembly and microbial interactions in calcareous glacier forefields

Calcareous glacier forefields challenge prevailing ecological frameworks on microbial biodiversity and community assembly due to their unique bedrock. Early stages of soil development in these environments are notorious for their high turnover rates, demanding a high degree of replication for obtaining conclusive data. However, studies across different calcareous glaciers are still missing. Here, we robustly investigated both bacterial and fungal diversity, association networks, and assembly processes in four calcareous glacier forefields of the Alps, focusing on the earliest soil developmental stages (<25 years) early in the snow-free season. We found a diverse community of bacteria and fungi, potentially involved in P and N nutrient cycling. A core microbiome existing across all four locations suggests that certain microbes might be more successful colonizers of these ecosystems than others. Nearest taxon index revealed phylogenetically clustered microbial communities. These findings suggest that the distribution and colonization of some microbes were influenced by selective forces such as geography and climate during the early stages of soil development in calcareous glaciers. Interestingly, there were no common bacterial-fungal associations across the four locations, indicating that this habitat does not select for specific bacterial-fungal associations and that associations were driven by neutral processes. We discuss microbial communities and their interactions in these special calcareous glacier forefield habitats. Moreover, we present innovative approaches for studying microbial assembly that address both deterministic, intrinsic drivers, like specific microbial traits, and stochastic, extrinsic drivers, such as the opportunistic behavior of microbes.