Stochasticity causes high β-diversity and functional divergence of bacterial assemblages in closed systems.

Stochasticity is a major cause of compositional β-diversity in communities that develop at similar environmental conditions. Such communities may exhibit functional similarity due to sympatric taxa with equivalent metabolic capacities in the source assemblage. However, the redundancy of individual physiological traits may differ in the original source community, which in turn might lead to more or less pronounced variability of individual functions among newly formed communities. We analyzed the degree of stochasticity during the primary assembly of bacterial communities originating from the same source and grown at identical conditions. We tested the links between community composition and functioning in parallel microcosms containing glucose and its dimer cellobiose. Bacteria from prefiltered lake water were diluted in artificial lake water and grown to stationary phase. The resulting assemblages exhibited high compositional variability of taxa that were rare in the source communities. Simulations showed that the observed richness and incidence-based β-diversity could be readily reproduced by dispersal limitation or low dispersal rates associated with ecological drift of the colonizers. Null model analysis supported an important influence of stochasticity and a synergy between dispersal limitation and both heterogeneous and homogeneous selection when performed on abundance-based data. Moreover, the communities functionally differed and the magnitude of functional variability depended on the substrate, as more communities consumed glucose compared to cellobiose. In general, there was no relationship between community structure and growth kinetics or substrate consumption. Thus, both, structural and functional variability may be a consequence of stochastic processes during initial colonization in closed microbial communities.