AUTHOR=Bathia Jay , Schröder Katja , Fraune Sebastian , Lachnit Tim , Rosenstiel Philip , Bosch Thomas C. G. 

TITLE=Symbiotic Algae of Hydra viridissima Play a Key Role in Maintaining Homeostatic Bacterial Colonization

JOURNAL=Frontiers in Microbiology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.869666

DOI=10.3389/fmicb.2022.869666

ISSN=1664-302X

ABSTRACT=<p>The freshwater polyp <italic>Hydra viridissima</italic> (<italic>H. viridissima</italic>) harbors endosymbiotic <italic>Chlorella</italic> algae in addition to a species-specific microbiome. The molecular basis of the symbiosis between <italic>Hydra</italic> and <italic>Chlorella</italic> 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 <italic>Hydra</italic>’s endodermal epithelium, with main focus on <italic>Legionella</italic> 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) <italic>H. viridissima</italic> is significantly different and dominated by <italic>Legionella spp. Hvir</italic> in aposymbiotic animals. Co-cultivation of these animals resulted in horizontal transmission of <italic>Legionella spp. Hvir</italic> 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 <italic>Legionella spp. Hvir</italic> and resulted in a decrease of the relative abundance of this bacterium. Prolonged co-cultivation ultimately resulted in the disappearance of the <italic>Legionella spp. Hvir</italic> bacterium from the <italic>Hydra</italic> tissue. Our observations suggest an important role of the photobiont in controlling an invasive species in a metacommunity and, thereby, shaping the microbiome.</p>