Coral fluorescence and symbiosis: Photoacclimation, thermal shock, life history changes, and implications for reef monitoring

Coral reefs ecosystems are diverse, productive and globally threatened. Corals have endosymbiotic dinoflagellates, which provide important nutrition. This symbiosis depends on a precarious balance of sunlight captured and processed through photochemistry that can be easily disrupted by environmental stressors including changes in irradiance and temperature. Photoprotective mechanisms are essential to maintaining the symbiosis equilibrium. Corals produce fluorescent proteins, homologous to the popular green fluorescent protein (GFP), that have the potential to be used in photoprotection because of their inherent absorption and emission properties. This dissertation investigated the responses of corals and their dinoflagellate symbionts during photoacclimation, thermal shock and life history stages with particular focus on the effects on fluorescent proteins. In carefully controlled experiments on Acropora yongei, GFP abundance was positively correlated with light intensity and modulated the coral cell internal light environment. A. yongei thermal shock experiments caused the active degradation of GFP, possibly suggesting that GFP provided some role during stress. Cold shock caused more immediate pressure on the corals, but the heat shock was ultimately more deleterious. Life history stages of Seriatopora hystrix expressed different colors of fluorescent proteins, which shifted accordingly to changes in the light environment. Brooded larvae from S. hystrix exhibited a wide range of physiological characteristics, but there were subtle influences of parental environment on larval settlement. A novel methodology using fluorescence to enhance detection of coral recruits was developed and used to characterize the distribution, abundance and microhabitat of small juvenile corals (≤5 mm) on a healthy coral reef, providing a baseline for coral recruitment and re-evaluating the coral demographics of healthy coral reef. The responses of fluorescent proteins with photoacclimation and thermal shock and the life history patterns of fluorescence support a photoprotective function of fluorescent proteins in corals, which do not exclude other possible functions. This dissertation provides evidence that coral fluorescence could be used as an early indicator of coral stress and as a tool to monitor coral recruits. Because multiple stressors threaten coral reefs, understanding the coral symbiosis physiology and having nondestructive tools to monitor coral health will be critical for the conservation and management of reefs.