Analyses of dryland biological soil crusts highlight lichens as an important regulator of microbial communities

Biological soil crusts (BSCs) provide important ecosystem services in dryland regions, including erosion control and contribution to nitrogen and CO2 fixation. As soil microorganisms are still rarely studied within the context of biodiversity planning, we describe, as a contribution to the Soil Crust International project, an approach that addresses this gap in biodiversity assessments. The purpose of the present study was a characterization of prokaryotic communities of BSCs formed by two species of lichenized fungi, Psora decipiens and Toninia sedifolia, in relation to surrounding BSCs and the below-crust soil layer from Tabernas basin (Almeria, Spain). Microbial community profiles were determined using 454 pyrosequencing targeting the V4 hypervariable region of the bacterial and archaeal 16S rRNA gene. The majority of the 65,497 sequences obtained belonged to Proteobacteria (mainly Alphaproteobacteria), Actinobacteria, Bacteroidetes and Cyanobacteria. Cyanobacteria were more abundant at the soil surface but rare in below-crust soils, whilst below-crust soils harbored significantly more Acidobacteria, Verrucomicrobia, Gemmatimonadetes, Planctomycetes and Armatimonadetes. Additionally, terricolous lichens were investigated using fluorescence in situ hybridization in conjunction with confocal laser scanning microscopy, the objective being to illustrate bacterial niches in BSC-forming lichens. Bacteria were mainly present at the upper cortex of the squamules and attachment organs. Our findings indicate that the composition of soil prokaryotes varies at a small scale not only in adjacent soil layers but also in BSC-forming lichen species. Furthermore, bacteria were shown to be attached to fungal structures, probably representing a case of fungal-bacterial interaction.