| Over the past thirty years, coral reefs have experienced extensive degradation worldwide. One etiology for this global degradation is a syndrome known as coral bleaching, a process whereby corals lose their algal symbionts or the symbiont's photosynthetic pigments degrade. Coral bleaching can be induced by diverse factors including high temperature, intense light or high-energy light, cold temperature, salinity changes and bacterial infection. There are very few models of coral bleaching that incorporate cellular and molecular mechanisms that have been proposed or tested. The overall objective of my dissertation is to elucidate the major cellular aspects of bleaching in coral as a result of high temperature, high light, the compound exposure of both heat stress and light stress, prolonged darkness, and hypo-salinity. The primary thesis of my dissertation is that heat stress and light stress initiate two distinct cellular mechanisms that result in coral bleaching, and that regardless of the initiating mechanisms, these two processes ultimately culminate into a single coordinated pathway whereby the coral host (animal component of the symbiosis) digests the zooxanthellae through an autophagic-related process that I term symbiophagy. Heat stress induces symbiophagy through a non-oxidative-stress regulatory switch. Light stress, a combined heat/light stress, or any stress that imposes an oxidative stress on the coral coral host (e.g., hypo-salinity exposure), probably induces symbiophagy through an oxidation-mediated signal cascade. Bleaching from prolonged darkness is a mechanism distinct from the processes observed from a light-based stress. A second thesis of my dissertation is that there is a defined sequence of events for all the processes of bleaching. Light stress induces significant accumulation of oxidative damage products, dispersion of thylakoid membrane structure, and a collapse of integrity of the chloroplast membrane. Heat stress induces a phase separation and condensation of the thylakoid membrane through a thermodynamic process, and not an oxidative stress process. Heat and light stress are not the only environmentally-relevant stressors known to induce population-level bleaching: hypo-salinity has been associated with large bleaching events. My work demonstrates that a brief hypo-salinity exposure induces an osmotic shock and subsequent oxidative stress, to both the host and the zooxanthellae. |
