The origin and role of biological rock crusts in rocky desert weathering
<p>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 morpho...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-03-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/16/1133/2019/bg-16-1133-2019.pdf |
| Summary: | <p>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 <i>SSU rRNA</i> 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.</p> |
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| ISSN: | 1726-4170 1726-4189 |