Resilience of aggregated microbial communities subjected to drought--small-scale studies.

The response of microbial communities to disturbance is a major concern for microbial ecologists since potential modifications in their composition and functioning may affect ecosystems to a larger extent. Microbial ecosystems may be resistant (not affected) or may present engineering (return to initial state) or ecological resilience. In the latter case, when the disturbance is released, the ecosystem evolves towards a new equilibrium state. The aim of this study was to determine if variations in the magnitude of a disturbance could induce either engineering or ecological resilience. We used phototrophic biofilms grown in mesocosms as a model of microbial ecosystem and increasing drought duration (1-8 weeks) as a range of disturbances. Biofilm composition (algal and prokaryotic), photosynthetic activity (PhytoPAM), and potential functional diversity (Biolog) were determined at the end of dry phase and after a 2-week rewetting phase in individual aquaria. We only observed an ecological resilience of the biofilm, with a resistance of phototrophic component for the weakest disturbance. After rewetting, the biofilm could fulfill the same functions, but its species composition was highly modified. We observed a shift from cyanobacteria dominance towards diatom dominance. The disturbance caused a transition towards a new steady state of the biofilm. We also observed a positive effect of stress duration on biofilm productivity after resilience.