Understanding stochastic and deterministic assembly processes in microbial communities along temporal, spatial and environmental scales.

Identifying the main drivers of community assembly remains an open fundamental question in ecology. Dispersal processes introduce randomness in community composition while selection for particular environments creates predictable assemblages. However, the interaction of selection and dispersal processes is still poorly understood. Here, we address this question in bacterial and microeukaryotic communities inhabiting a highly dynamic system of ephemeral (hyper)-saline lakes. We showed that the combination of beta-diversity decomposition methods and a temporal approach based on colonization and extinction dynamics yield new insights into the relative effect of selection and dispersal along environmental gradients. Selective pressure and dispersal-related processes simultaneously shape each local community with variable strength and effect. The dominance of selection vs dispersal shifted from stochastic to deterministic assembly as salinity increased along the gradient. This transition had also an impact on lakes temporal dynamics as community turnover decreased at high salinities because both colonization and extinction rates slowed down. Only microeukaryotic richness decreased along the gradient due to lower effective colonization at higher salinities, suggesting that the net effect of selection and dispersal is determined by both environmental conditions and the idiosyncrasy of the different microbial ecologies. Our results emphasized the use of temporal approaches in combination with standard statistical methods for a better understanding of the dynamic processes underlying community assembly.