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

Abstract

Abstract. In drylands, microbes that colonize rock surfaces have been linked to erosion because water scarcity excludes traditional weathering mechanisms. We studied the origin and role of rock biofilms in geomorphic processes of hard lime and dolomitic rocks that feature comparable weathering morphologies, although these two rock types originate from arid and hyperarid environments, respectively. We hypothesized that weathering patterns are fashioned by salt erosion and mediated by the rock biofilms that originate from the adjacent soil and dust. We used a combination of microbial and geological techniques to characterize rock morphologies and the origin and diversity of their biofilms. Amplicon sequencing of the SSU rRNA gene suggested that bacterial diversity is low and dominated by Proteobacteria and Actinobacteria. These phyla only formed laminar biofilms on rock surfaces that were exposed to the atmosphere and burrowed up to 6 mm beneath the surface, protected by sedimentary deposits. Unexpectedly, the microbial composition of the biofilms differed between the two rock types and was also distinct from the communities identified in the adjacent soil and settled dust, showing a habitat-specific filtering effect. Moreover, the rock bacterial communities were shown to secrete extracellular polymeric substances (EPSs) that form an evaporation barrier, reducing water loss rates by 65 %–75 %. The reduced water transport rates through the rock also limit salt transport and its crystallization in surface pores, which is thought to be the main force for weathering. Concomitantly, the biofilm layer stabilizes the rock surface via coating and protects the weathered front. Our hypothesis contradicts common models, which typically consider biofilms to be agents that promote weathering. In contrast, we propose that the microbial colonization of mineral surfaces acts to mitigate geomorphic processes in hot, arid environments.

Comment on bg-2020-467

“…Overall, the results point to a very deterministic successional course of BRCs development on rock surfaces. Moreover, the similarly in the microbial composition between rocks that were in contact with the ground to the those that were detached from the ground (chalk BRCs found in Shivta), indicate a major role to aeolian processes in determining the community composition of BRCs in deserts as was previously reported (Wieler et al, 2019).…”

“…4C). This demonstrates the ecological filtering effect of the rock surfaces, which imposes unique abiotic challenges for the inhabiting microbes, and infer that dust particles are the main potential source for the microbial communities (Wieler et al, 2019). We thus suggest that local-scale environmental parameters play a major role in shaping the microbial taxa that colonize fresh rock surfaces in arid regions.…”

Estimating the growth rate in desert biological rock crusts by integrating archaeological and geological records

“…Microscopic examinations of thin sections (30 µm thick) that were prepared from the limestone and chalk blocks showed that, as expected, the BRCs were restricted to the atmospherically-exposed parts of the rocks (Wieler et al, 2019). The BRCs were characterised by a hardpan-laminated structure composed of masses of micritic to microsparitic carbonate layers interbedded with microbial coatings covering the lime and chalk host-rocks.…”

Measurements and calculations of seasonal evaporation rate from bare sandstone surfaces: Implications for rock weathering

“…The methodology in this work can advance our understanding of the crust's effect on evaporation rate and the spatial distribution of water near the sandstone surfaces. Quantification of both effects is also critical for understanding the role of biota in weathering processes (Carter and Viles, 2005;Wieler et al, 2019).…”