Trace metal limitation and its role in oxidative stress of coral algal symbionts; implications for thermally induced coral bleaching events.

Coral bleaching, the process in which corals expel their photosynthetic symbionts (Genus Symbiodinium), is caused by high temperature and irradiance stress. The synergistic effect of each stressor is the generation of damaging reactive oxygen and nitrogen species (ROS and RNS), which are normally mitigated physiologically by antioxidant enzyme activity. Antioxidant enzymes require iron, copper, manganese and zinc in their structure and the limited nature of these trace metals in reef environments may enhance ROS production under elevated temperature and irradiance. The hypothesis tested within this thesis was that a limited availability of the trace metals Fe, Mn, Cu and Zn would cause increases in intracellular ROS and RNS load of the symbiotic dinoflagellates of corals isolated from the Great Barrier Reef (GBR), Australia, under conditions of temperature and irradiance stress. Pulse amplitude fluorometry, flow cytometery and fluorescent ROS and RNS probes were utilized to measure photochemical efficiency (charge separation at PSII) and intracellular ROS and RNS levels. Trace metal limitation experiments on Stylophora pistillata collected from various GBR locations showed that Fe limitation under elevated temperature directly reduced PSII photochemical efficiency and increased protective pigments of symbiotic dinoflagellates in hospite. Increases in intracellular ROS were not detected, however, and decreases or an absence of change in intracellular ROS occurred under Fe limited conditions. Culture experiments on Symbiodinium Strains were then employed to understand this phenomenon and cells were grown under Fe-replete and deplete conditions in continuous culture and subject to heat and irradiance stress. Enhanced intracellular ROS and RNS were detected within Symbiodinium Strains 831 and 2432 in culture under Fe-deplete conditions at high temperature and irradiance. This thesis illustrates the intricate role of Fe in Symbiodinium photochemistry and oxidative stress in culture and in hospite, providing insight to the physiology behind coral bleaching