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.