Habitability of Polygonal Soils in the Hyper‐Arid Atacama Desert After a Simulated Rain Experiment

In the hyper‐arid Atacama Desert, microbial life thrives near its “dry limit” in scarcely distributed habitats. Fracture networks of salt‐poor sand wedges outlining salt‐cemented polygons on alluvial surfaces in the Yungay region (Chile) represent potential microbial habitats. The degree of soil habitability at the surface (0–5 cm depth) and subsurface (10–15 cm depth) of a polygon and adjacent sand wedge was assessed before and up to 42 days after a 20 mm simulated rain experiment through the abundance of phospholipid fatty acids (PLFAs). Mineralogical composition, salinity, pH, electrical and thermal conductivity, water content, and water activity were analyzed for their relevance to habitability. After wetting, the PLFA content exclusively increased steadily with time in the polygon subsurface indicating the growth of an indigenous bacterial community. This increased habitability is presumably related to the soil's ability to retain water for at least 6 weeks at this depth. The lack of a continuous growth signal at the surface is likely due to rapid desiccation. In the sand wedge subsurface, the increase in PLFA content is not continuous despite the water activity being >0.9. The reason for this remains unclear but indicates that not only water availability is relevant for habitability but also the here described soil heterogeneities might impact the detection of the microbial response. Yet, the increasing PLFA trend in the polygon subsurface emphasized its relevance as a saline microbial habitat in an otherwise hostile environment, which could have implications for the assessment of soil habitability on Mars. Plain Language Summary The limited water availability in the Atacama Desert challenges microbes which often occupy habitats that provide more water or more protected conditions than their surroundings. The extreme dryness led to fracture networks in the soils consisting of salt‐poor sand wedges outlining salt‐rich polygons representing major patterns of soil heterogeneity, and thus potential microbial habitats. A 20 mm rain was simulated to investigate their habitability (ability of the soil to support microbial activity), based on the content of phospholipid fatty acids (PLFAs). Mineral composition, pH, electrical and thermal conductivity, water activity and content were analyzed for their relevance to habitability. After moistening, continuously increasing PLFA abundances were monitored exclusively in the polygon subsurface, indicating enhanced