Environmental heterogeneity ruling the action of ecological processes over the bacterial metacommunity assembly

Determining which processes take place in the spatial distribution of bacterioplankton metacommunities has been a central goal of microbial ecology. In freshwater ecosystems, selection has been spotted as the main driver shaping bacterial community. However, its relative importance compared with others processes (dispersal, drift, diversification) may depend on spatial heterogeneity and dispersal rates within a metacommunity. Still, a decrease in the role of selection is expected with increasing dispersal homogenization. Here, we investigate the main ecological processes modulating the bacterial assembly in contrasting scenarios of environmental heterogeneity. We carried out a spatiotemporal survey in the floodplain system of Parana River. The bacterioplankton metacommunity was studied by a step-forward analysis combining different statistical inferences based on the phylogenetic and taxa turnover as well as co-occurrence networks. We found that selection was the main process even at both extremes of environmental heterogeneity and homogeneity, challenging the general view that the strength of selection is weakened due to dispersal homogenization. The ecological processes acting on the community also determined the complexity and randomness of bacterial networks. The heterogeneous selection promoted greater network complexity increasing the {beta}-diversity, and bacterial associations were more random with the increasing influence of stochasticity. Finally, the spatiotemporal heterogeneity was an important factor determining the number and identity of keystone taxa in the system. Integrating all these empirical evidences we propose a new conceptual model that elucidates how the environmental heterogeneity guides the action of ecological processes shaping the bacterial metacommunity.