Symbiosis and Immunity at the Tri-kingdom Interface

Abstract

The rise in global sea-surface temperatures drastically and adversely impacts coral reefs worldwide through coral bleaching, diseases, and subsequent coral mortality. Corals become bleached by the expulsion of endo-symbiotic dinoflagellates, which reduces host pigmentation. Coral bleaching can also lead to enhanced disease prevalence immediately after thermal-stress and during the associated re-uptake of symbiotic dinoflagellates. This aspect has yet to be thoroughly investigated. A model system is necessary to study the relationships between symbiosis and disease that has sufficient flexibility to include a variety of host symbionts as well as pathogens. For this dissertation project, Exaiptasia pallida was investigated as a model system to evaluate the relationship between hosts, their symbionts, and pathogens. This system tested representative species of symbiotic dinoflagellates and the pathogen Serratia marcescens. This study showed that the pathogen S. marcescens maintained viable culture densities in a closed saltwater system, but decreased when co-cultured with symbiotic dinoflagellates. There was no sign of antimicrobial activity, suggesting symbiotic dinoflagellates are able to outcompete S. marcescens for resources. The effect of symbiotic state on disease resistance was investigated in a 24- well plate assay by exposing the model host E. pallida, in either a stable symbiotic or aposymbiotic state, to the pathogen S. marcescens. There were differences in host mortality between 108 CFU/ml and 107 CFU/ml concentrations of pathogen, however there were no differences in host mortality based on symbiotic state. Since being in a stable bleached or unbleached state had no effect on host mortality, an increase in disease prevalence might be a consequence of thermal stress or transition to a symbiotic state. To investigate the effect of transitions between symbiotic state on disease susceptibility, bleached E. pallida were exposed to the symbiotic dinoflagellate Breviolum minutum from 6 hours to 14 days. Disease resistance increased at early time points, decreased at 7 days, and increased at 14 days. Other species of symbiotic dinoflagellates, Symbiodinium microadriaticum, Cladocopium goreaui, and Durusdinium trenchii, were introduced to aposymbiotic hosts for either 7 or 14 days. At 7 days, disease resistance significantly increased for the D. trenchii treatments. At 14 days, S. microadriaticum and B. minutum treatments were more disease resistant than symbiotic or aposymbiotic controls, and treatments with C. goreaui and D. trenchii were even more disease resistant. Yet in natural systems E. pallida associates with S. microadriaticum and B. minutum, whereas it is not known to associate with C. goreaui or D. trenchii. Rejecting incompatible symbionts might prime the innate-immune system. We hypothesize that symbiotic compatibility confers no disease resistance to the host, whereas symbiotic incompatibility elicits an immune response by the host that confers temporary disease resistance. An unidentified bacterium appeared in some of the pathogen control experiments. The number of culturable unidentified microbes increased with exposure to S. marcescens, whereas the density of culturable S. marcescens decreased. Aposymbiotic anemones were inoculated at 108 CFU/ml of the unknown, and produced mortality in excess of S. marcescens exposure, with complete sample mortality within 4 days. When E. pallida was exposed to E. coli HB101 at these concentrations there was no mortality, supporting the possibility that specific interactions exist between the disease causing bacteria and the host, rather than a general increase in bioload. The severity of disease state appears mediated by host exposure to symbiotic dinoflagellates, which varies across time and by symbiont species. On coral reefs, it is probable that the appearance of certain disease states might be induced by foreign bacteria that disrupt the host’s microbial community. These interactions are complex, however it should be possible to use E. pallida in future efforts to understand key factors responsible for coral disease.