Fine-scale landscape heterogeneity drives microbial community structure at Robinson Ridge, East Antarctica.

Life at Robinson Ridge, located in the Windmill Islands region of East Antarctica, is susceptible to a changing climate. At this site, responses of the vegetation communities and moss-beds have been well researched, but corresponding information for microbial counterparts is still lacking. To bridge this knowledge gap, we established baseline data for monitoring the environmental drivers shaping the soil microbial community on the local 'hillslope' scale. Using triplicate 300-m long transects encompassing a hillslope with wind-exposed arid soils near the top, and snowmelt-sustained-moss beds at the bottom, we assessed the fine-scale heterogeneity of the soil environmental and microbial properties. Moist, low-lying, and vegetated soils exhibited higher soil fertility and unique biodiversity, with taxa adapted to thrive in moist conditions (i.e., Tardigrada, Phragmoplastophyta, Chloroflexi) and those that have previously demonstrated strong specificity for moss species (i.e., Fibrobacterota, Mucoromycota and Cyanobacteria) dominating. In contrast, elevated soils with limited moisture and nutrients were dominated by metabolically diverse phyla like Actinobacteriota and Ascomycota. Significant differences in microbial communities were observed at both hillslope (50-300 m) and fine spatial scales, as small as 0.1 m. Vertical heterogeneity was observed with higher abundances of Cyanobacteria and micro-algae in surfaces compared to subsoil, potentially indicating early biocrust formation. Stochastic and deterministic processes governing phylogenetic assembly were linked to soil positional groups and microbial domains rather than soil depth. Gradient Forest modeling identified critical environmental thresholds, such as ammonia, manganese, and sulphur, responsible for drastic community changes following level alterations. This reinforces the existence of strong niche preferences and distinct distribution patterns within the local microbial communities. This study highlights the need for finer-scale investigations considering site topography to better understand the relationship between environmental drivers and local microbiota. Ultimately, these insights enable us to understand environmental drivers and predict Antarctic ecosystem responses, helping safeguard this fragile environment.