Symbiotic Algae of Play a Key Role in Maintaining Homeostatic Bacterial Colonization.

The freshwater polyp () harbors endosymbiotic algae in addition to a species-specific microbiome. The molecular basis of the symbiosis between and has been characterized to be metabolic in nature. Here, we studied the interaction between the extracellularly located microbiota and the algal photobiont, which resides in 's endodermal epithelium, with main focus on bacterium. We aimed at evaluating the influence of the symbiotic algae on microbial colonization and in shaping the host microbiome. We report that the microbiome composition of symbiotic and aposymbiotic (algae free) is significantly different and dominated by in aposymbiotic animals. Co-cultivation of these animals resulted in horizontal transmission of bacteria from aposymbiotic to symbiotic animals. Acquisition of this bacterium increased the release of algae into ambient water. From there, algae could subsequently be taken up again by the aposymbiotic animals. The presence of algal symbionts had negative impact on and resulted in a decrease of the relative abundance of this bacterium. Prolonged co-cultivation ultimately resulted in the disappearance of the bacterium from the tissue. Our observations suggest an important role of the photobiont in controlling an invasive species in a metacommunity and, thereby, shaping the microbiome.