Elucidating the Microbial Resuscitation Cascade in Biological Soil Crusts

Biological soil crusts (biocrusts) are photosynthetic mats formed through an association of prokaryotic and eukaryotic microorganisms with soil particles. Biocrusts can be found in virtually any terrestrial ecosystems where vascular plant coverage is limited by abiotic conditions, such as drylands. As the largest terrestrial biome, drylands cover around 40% of the land surface and are therefore the primary habitat for most biocrusts. In these ecosystems water availability is limiting and the crusts are dormant most of the year. Short and sporadic pulses of rain lead to a burst of microbial activity in the crusts followed by a return to dormancy upon dry out. A large body of literature exists on bioctusts going back at least to the beginning of the 20th century, classifying their various forms, habitats, compositions and functions. Yet while the photosynthetic and fungal members of these lifeforms received much attention, all other bacterial and archaeal inhabitants of the crust were only sporadically mentioned and have rarely been the focus of research. In this work we studied the dynamics of the active bacterial community in two biocrusts from an arid and a hyperarid region in the Negev Desert, Israel, under light–oxic and dark–anoxic incubation conditions after simulated rainfall. We used 18O-H2O for hydrating the crusts and analysed the bacterial community using an RNA-Stable Isotope Probing approach coupled with 454-pyrosequencing. Our experimental set-up allowed us to reliably mimic the unique conditions distinguishing the top and lower part of a biocrust. Two distinct bacterial communities developed under each incubation condition. The active anaerobic communities were initially dominated by members of the order Bacillales which were later replaced by Clostridiales. The aerobic communities on the other hand were dominated by Sphingobacteriales and several Alphaproteobacteria (Rhizobiales, Rhodobacterales, Rhodospirillales and Rubrobacteriales). Actinomycetales were the dominant bacterial order in the dry crusts but quickly collapsed and accounted for less than 1% by the end of the incubation. Our study shows that biocrusts host a diverse community whose members display complex interactions as they resuscitate from dormancy.