Microbial dynamics in an alpine glacier forefield: Environmental drivers and ecological implications

The retreat of glaciers and ice sheets since the end of the Little Ice Age has created proglacial margins, where environmental conditions are initially too harsh for the establishment of higher organisms such as vascular plants. Less sensitive to disturbances and to the lack of resources commonly observed immediately after soil exposure, microbes (e.g. Bacteria, Archaea), however, rapidly colonise these environments. In doing so, they drastically alter soil biogeochemistry and properties. Microbes thus act as key ecosystem engineers, facilitating the development of macroorganisms in proglacial margins. The recent advancement of DNA extraction and metabarcoding along global glacier chronosequences has revealed significant microbial diversity in these ecosystems, with contrasting ecological trajectories over time. This can be attributed to their high sensitivity to changing environmental conditions, alongside time since deglaciation, suggesting a need for multidisciplinary studies to constrain the development of these pioneers. In our study, we evaluate the role of soil microbes as actors in ecosystem development following deglaciation. By collecting soil samples along a high-elevation site in the Southwestern Swiss Alps, we investigated the relationships between microbial community composition and soil biogeochemistry, local geomorphology, and seasonality. With this approach, we constrained the shifts in community composition as time since deglaciation increased, finding that soil biogeochemistry, texture and seasonality are the main drivers of these changes. We demonstrate that microbes are also subject to ecological succession and that environmental factors are essential to explain their ecology. In the context of new ecosystem emergence following accelerated glacier retreat in the 21^stcentury, our results underscore the importance of understanding microbial ecology to comprehend the future of these new landscapes.