Expanding the Aiptasia coral model system to study host-bacteria interactions

The coral holobiont, comprised of the cnidarian animal host, its associated algal endosymbionts of the family Symbiodiniaceae, and other microbes (bacteria, fungi, viruses, etc.), is the foundation metaorganism of coral reefs. Despite the putative importance of associated microorganisms, elucidation of the specific functions bacteria contribute to the coral holobiont are still largely missing. The sea anemone Aiptasia (sensu Exaiptasia diphana) is regarded as a tractable model to study cnidarian-algal symbioses due to its ability to associate with similar Symbiodiniaceae strains as corals and surviving in symbiotic and aposymbiotic (algal-free) states. The motivation of this dissertation was to expand Aiptasia as a coral model to interrogate host-bacteria interactions. I firstly compared bacterial community composition of Aiptasia in symbiotic and aposymbiotic states and found them to be significantly different. I could also show that some identified bacterial taxa were similar to those found in corals. I further assessed the ectodermal surface topography of several cnidarians as putative bacterial habitats, using electron microscopy. I could show that Aiptasia and corals possess similar surface topographies which differ from other cnidarian models, such as Hydra. In addition, bacterial carrying capacity of Aiptasia polyps was estimated to be between 104 - 105 bacterial cells, roughly equating to the 106 bacterial cells/cm2 reported for corals. To assess the prospect of microbiome manipulation as a tool to study bacterial function and alter bacterial association, I first developed a method to generate bacteria-depleted/gnotobiotic Aiptasia and axenic Symbiodiniaceae cultures and subsequently conducted inoculation experiments. In a pilot experiment, I could show that endogenous and exogenous bacterial isolates could be detected after re-inoculation to gnotobiotic Aiptasia. Further, I conducted coral microbiome transplants to Aiptasia. I could show that some bacterial taxa from corals were detected and active 7 days after transplantation, indicating a certain degree of plasticity in the anemone’s bacterial associations. Overall, my work suggests that Aiptasia might be a suitable model to study coral-bacteria interactions. The thesis presents a foundation for further studies and sheds light on the difficulties associated with generating axenic cnidarian hosts and manipulating bacterial associations.