Despite these advances, many open questions remain. Thus, we would like to deepen our understanding on the role of stieleriacines in community formation and test if they act as quorum sensing molecules. To address this, we use (microfluidic) time-lapse microscopy to study the spatiotemporal organization of biofilms made up of planctomycetes and Roseobacters in the presence and absence of stieleriacines. We will also specifically look at the morphology of Roseobacters as morphology can have an important impact on biofilm formation. Using the technology present in the CRC, we will also be able to follow the spatiotemporal distribution of stieleriacines in these communities. We will then introduce phototrophs that act as a substrate for biofilms into this community, to mimic the natural habitat and investigate how planctomycetal chemical mediators affect the host and competing Roseobacters. In addition, we will investigate planctomycetal chemical mediators present in other aquatic Planctomycete-phototroph communities. Therefore, we perform community analyses in two relevant habitats. Based on these analyses, environmentally relevant synthetic co-cultures of planctomycetes and Roseobacters with macroalgae, microalgae and cyanobacteria will be reconstructed. This allows us to study their mutual effect on growth and morphology of the other partner. Using this approach, we aim to uncover additional (planctomycetal) chemical mediators, for which production is triggered by different interaction partners. These interactions can be visualized on cellular and chemical mediator basis. The project relies on intense interactions with multiple CRC projects and their expertise such as chemical synthesis, structure elucidation of chemical mediators and transcriptomic analyses.