Coral Bleaching: Photosynthetic Impacts on Symbiotic Dinoflagellates

xxxviiiABSTRACT Global climate change is leading to the rise of ocean temperatures and is triggering masscoral bleaching events on reefs around the world. This involves the expulsion of thesymbiotic dinoflagellate algae, known as zooxanthellae, from the coral host. Coralbleaching is believed to occur as a result of damage to the photosynthetic apparatus ofthese symbionts, although the specific site of initial impact is yet to be conclusivelyresolved. This thesis examined a number of sites within the light reactions ofphotosynthesis and evaluated the efficiency of photoprotective heat dissipating pathways.Upon expulsion, the capacity for long-term survivorship of expelled zooxanthellae in thewater column was also assessed. A reduction in photosystem II (PSII) photochemical efficiency during exposure toelevated temperature and high light (bleaching conditions) was found to be highlydependent upon the increase in abundance of QB non-reducing PSII centres (inactive PSIIcentres), indicating damage to the site of the secondary electron acceptor,QB, resulting ina limited capacity for its reduction. Therefore, this reduced the rate of the reoxidation ofthe primary electron acceptor, QA. Fast induction curve (FIC) analysis of the rise fromminimum fluorescence to maximum fluorescence revealed a lower amplitude in the J stepalong this curve, which was consistent with a reduction in the rate of QAreoxidation.This photoinhibition of PSII was found to occur once the effectiveness of excess energydissipation through energy-dependent quenching and state-transition quenching wasexceeded, suggesting that these mechanisms were incapable of preventing photodamage.