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

Abstract

In drylands, microbes that colonise rock surfaces were 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 though originating from arid and hyperarid environments, respectively. We hypothesised 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 characterise rocks morphologies and the origin and diversity of their biofilm. Amplicon sequencing of the SSU rRNA gene suggested that bacterial diversity is low and dominated by Proteobacteria and Actinobacteria. These phyla formed laminar biofilms only 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 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 crystallisation in surface pores, which is thought to be the main force for weathering. Concomitantly, the biofilm layer stabilises the rock surface via coating and protects the weathered front. Our hypothesis contradicts common models, which typically consider biofilms as weathering-promoting agents. In contrast, we propose the microbial colonisation of mineral surfaces acts to mitigate geomorphic processes in hot, arid environments.

Microscopic and biomolecular complementary approaches to characterize bioweathering processes at petroglyph sites from the Negev Desert, Israel

“…2). This is also in agreement with our previous study (Nir et al, 2019a;Nir et al, 2019b) and with additional studies from the Negev desert (Wieler et al, 2018;Langyona et al, 2018). This biocomplexity was also reflected in the electron microscopy study.…”

Living Lithic and Sublithic Bacterial Communities in Namibian Drylands

“…Microorganisms on rock surfaces, in cracks or in pores sometimes form biofilms that influence the breakdown of rocks [ 14 ]. Microbial-driven rock weathering has been observed around the world, for example in hot and cold deserts [ 15 , 16 , 17 ] or in semi-arid to temperate regions [ 18 , 19 ] and the importance of microbially-promoted mineral weathering for nutrient acquisition was reported [ 20 ]. Therefore, modern subaerial endolithic structures of extreme environments have the potential to provide insights into early principles of organo-mineral interactions and soil formation from hard rocks [ 6 ].…”

The tombstones at the Monumental Cemetery of Milano select for a specialized microbial community

“…Our results reflect the strong niche filtering of rocks and possibly the presence of a microbial seed bank. As previously demonstrated for biological rock crusts, soil and dust microbial communities can be very different from biofilm communities, and still sharing a subset of taxa that persist and form a subaerial biofilm (Wieler et al 2019). The ubiquity and the difference in abundance of the shared rare taxa identified (from very abundant in air and soil to low abundant in stone communities) suggest that they might constitute a microbial seed bank, a reservoir of rare taxa that activate and grow only under specific environmental conditions (Lennon and Jones 2011).…”

“…The interface between air and rocks is an ancient and ubiquitous habitat supporting subaerial microbial growth as biofilms (Costerton and Woodely 2003;Gorbushina and Broughton 2009). Subaerial biofilms play a central role in the ecology of alpine and desert environments, soil and rhizosphere (Dang and Lovell, 2009;Wieler et al 2019), but also in the biodeterioration of many outdoor monuments and pieces of art in stone, thus being a reason of concern for the conservation of our cultural heritage (Polo et al 2012;Schreerer et al 2009). The first microorganisms to colonize rocks are able to exploit the little moisture and compounds in the air, given the poor availability of water and organic material (Villa et al 2016).…”

“…Despite the importance of these communities for the ecology of rocks and the conservation of our cultural heritage, the origin of the microorganisms forming subaerial biofilms on rocks is still open for discussion. In arid and hyperarid regions, Wieler and colleagues reported that the taxonomic composition of biofilm communities is very different from the dust and the surrounding soil, suggesting that biofilms microbes are not simply deposited by dust, but they are actively responding to the rock selective pressure (Wieler et al 2019). To the best of our knowledge, no other studies explore the origin of microorganisms in rock subaerial biofilms and verify the possibility that these communities are formed by air-and soil-borne bacteria, but strongly shaped by the selective pressure of a life on rocks.…”