In this project we will combine field and experimental work to gain insight in the processes that structure metacommunities of aquatic microbial organisms. We will sample bacterial communities along natural ecotones and use molecular fingerprinting techniques to document patterns of lineage sorting and mass effects. In microcosmos experiments in outdoor or laboratory set-ups, we will manipulate dispersal rates among clear environmental gradients to quantify the relative importance of lineage sorting, dispersal limitation, priority efffects and mass effects in determining lineage composition of bacterial communities. We will focus on specific environmental gradients: medium, substrate type, biofilm-plankton. Using an experimental approach, we will also quantify the impact of differential predation on habitat choice in bacteria across the biofilm-plankton ecotone, as well as the impact of evolution on community assembly. This project will contribute to the development of a model system for research on metacommunity processes in semi-natural communities and conditions.