Date of Award

2021

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Specialization

Cell & Microbiology

Department

Cell & Molecular Biology

First Advisor

David R. Nelson

Abstract

Astrangia poculata is a facultatively symbiotic temperate coral that is being explored as a model system for studying the physiology and ecology of cnidarian-microbe symbiosis. Vibrio coralliilyticus is a known causative agent of a class of coral diseases called “white syndromes” that result in bleaching in tropical coral species. It is an effective pathogen due to a wide array of virulence factors including two Type 6 Secretion Systems (T6SS1 and T6SS2). In this study, we investigated the pathogenic potential of V. coralliilyticus RE22Sm in A. poculata and in cultures of its endosymbiont, Breviolum psygmophilum. To independently gauge the antagonistic effects of each of the two T6SSs, allelic exchange mutants of the hcp genes were utilized. In the A. poculata challenge, V. coralliilyticus RE22Sm caused tissue lysis in coral samples. Both aposymbiotic and symbiotic corals were susceptible to infection, and aposymbiotic corals displayed tissue lysis faster than symbiotic corals. Mutation of the T6SS1 hcp1 gene resulted in the greatest attenuation of virulence in the coral system. Coral survival increased from 12% in the wild-type challenged samples, to 60% for those challenged with Δhcp1. Virulence was also attenuated in corals challenged with Δhcp2, with 30% survival. Similarly, B. psygmophilum challenged with the Δhcp1 strain had a 20% increase in both cell survival and chlorophyll a content, compared to cultures exposed to wild type V. coralliilyticus RE22Sm. An hcp1 hcp2 double mutant resulted in minor attenuation of virulence in both coral and endosymbiont trials. Revertant strains with restored wild-type copies of the hcp genes displayed comparable virulence to wild-type V. coralliilyticus RE22Sm. These results suggest that Type 6 Secretion is a major component of pathogenesis against the temperate coral A. poculata and B. psygmophilum. Heightened susceptibility of aposymbiotic coral samples to bacterial challenge is consistent with literature that suggests symbiotic A. poculata is more effective than aposymbiotic colonies at mitigating of environmental stress. The data are consistent with bacterial challenges in an oyster larval system, which indicate that T6SS1 is primarily involved in eukaryotic antagonism.

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