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. In drylands, microbes that colonize rock surfaces have been linked to erosionbecause water scarcity excludes traditional weathering mechanisms. Westudied the origin and role of rock biofilms in geomorphic processes of hardlime and dolomitic rocks that feature comparable weathering morphologies,although these two rock types originate from arid and hyperarid environments, respectively. Wehypothesized that weathering patterns are fashioned by salt erosion andmediated by the rock biofilms that originate from the adjacent soil anddust. We used a combination of microbial and geological techniques tocharacterize rock morphologies and the origin and diversity of theirbiofilms. Amplicon sequencing of the SSU rRNA gene suggested that bacterialdiversity is low and dominated by Proteobacteria and Actinobacteria. Thesephyla only formed laminar biofilms on rock surfaces that were exposed to theatmosphere and burrowed up to 6 mm beneath the surface, protected bysedimentary deposits. Unexpectedly, the microbial composition of thebiofilms differed between the two rock types and was also distinct from thecommunities identified in the adjacent soil and settled dust, showing ahabitat-specific filtering effect. Moreover, the rock bacterial communitieswere shown to secrete extracellular polymeric substances (EPSs) that form anevaporation barrier, reducing water loss rates by 65 %–75 %. The reducedwater transport rates through the rock also limit salt transport and itscrystallization in surface pores, which is thought to be the main force forweathering. Concomitantly, the biofilm layer stabilizes the rock surface viacoating and protects the weathered front. Our hypothesis contradicts commonmodels, which typically consider biofilms to be agents that promote weathering. Incontrast, we propose that the microbial colonization of mineral surfaces acts tomitigate geomorphic processes in hot, arid environments.

The origin and role of biological rock crusts in rocky desert weathering