Bacterial exposure impacts host health of the sea anemone Exaiptasia pallida

Coral reefs are threatened by rising ocean temperatures associated with climate change, leading to a breakdown of the cnidarian-dinoflagellate symbiosis and growth of pathogenic bacteria. The host response to diverse bacteria can vary from species to species. In this study, the sea anemone Exaiptasia diaphana (commonly referred to as Aiptasia), a prominent laboratory model for coral-symbiosis research, was used to investigate host responses to three key bacterial species associated with both Aiptasia and corals. Ruegeria mobilis, Vibrio alginolyticus, and Alteromonas macleodii were individually inoculated into the seawater of both symbiotic (with dinoflagellates) and aposymbiotic (without dinoflagellates) hosts to determine their effects on host health. Each bacterium was hypothesized to have a different effect on host health-- beneficial, pathogenic, or no effect. Various densities of each bacterium were used to assess impacts on host morphology and survival over 14 days at 27{degrees}C. Host biomass, protein content, and dinoflagellate abundance (to assess bacterial effects on host bleaching) were examined over 7 days at 27{degrees}C in response to an inoculation density of 8x108 cells ml-1 of each bacterium. When compared to un-inoculated Aiptasia, V. alginolyticus reduced host survival of symbiotic and aposymbiotic hosts, and biomass by 27% and 54% in symbiotic and aposymbiotic Aiptasia, respectively. V. alginolyticus also diminished dinoflagellate abundance by 20% in symbiotic Aiptasia. In contrast, R. mobilis enhanced survival of symbiotic Aiptasia, biomass by 9%, protein content by 50%, and dinoflagellate abundance by 60%, but had no impacts on aposymbiotic Aiptasia. A. macleodii had no major effects on host health at all. As a result, V. alginolyticus generally has negative impacts on host health, while R. mobilis appears to be a beneficial bacterium. Moreover, host susceptibility to bacteria is dependent on whether animals have their dinoflagellates to begin with and, therefore, bleached cnidarians may experience more drastic responses to pathogens like V. alginolyticus, but no benefits from a potential probiotic like R. mobilis. ImportanceIdentification of specific beneficial microbes is a major contribution to conservation techniques, such as microbiome manipulation, that aim to assist cnidarians in adapting to climate change. However, to maintain these conservation efforts in the long term, moving forward, we must prioritize foundational questions that examine dinoflagellate-bacterial interactions and bacterial-colonization dynamics within the cnidarian host.