The photophysiology of symbiotic dinoflagellates (Symbiodinium) under varying light and thermal conditions and the implications for coral bleaching

Date
2006
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University of Delaware
Abstract
Coral reefs are complex ecosystems full of extraordinary biodiversity. Reef building corals are reliant upon a symbiosis with intracellular dinoflagellates of the genus Symbiodinium. A considerable threat to coral reefs is the breakdown of this symbiosis, known as bleaching. Coral bleaching is characterized by the ghostly white tissue appearance resulting from loss of algal density. Over the past 20 years, there has been an increase in the frequency and magnitude of worldwide bleaching, which is strongly correlated with elevated seawater temperature, and exacerbated by increased irradiance. Bleaching has been linked to photosynthetic stress within the algae. ☐ The capacity for photoacclimation to light at 100 or 600 μmol photons•m-2•s-1 and the subsequent response to thermal stress was examined in four genetically distinct cultures of Symbiodinium (ITS2: A1, A1.1, B1, and F2). Cultures were maintained at the above light levels for at least one month prior to thermal investigation. Temperatures were ramped from the control temperature 26°C up to 32°C over 72hrs, where they remained for up to 10 days. Cultures were then monitored with chlorophyll fluorescence and protein techniques to monitor changes in photosynthesis. ☐ While all algal types showed a reduction in chlorophyll a content, there was a differential response in cellular growth rate and photosystem II (PSII) activity between cultures. When maintained at 32° C for up to ten days, significant variation in the susceptibility to thermal stress was observed in the rate of loss in PSII activity and electron transport, PSII reaction center degradation, and cellular growth rates. The order of thermal tolerance did not change between the two light levels; however high light exacerbated the effects. Phylotypes F2 and A1 showed a high degree of thermal tolerance, yet the cellular responses to light and temperature were markedly different. ☐ The use of chlorophyll fluorescence is a common method used to evaluate photosynthesis; however, only recently has research begun to evaluate the accuracy of this technique at estimating true photosynthetic activity in Symbiodinium. Both chlorophyll fluorescence and oxygen evolution were monitored simultaneously in two different cultures (ITS: A1, B1) at ambient and elevated temperature. At 32°C, there was a significant decrease in both the maximum efficiency of PSII measured by chlorophyll fluorescence (Fv/Fm) and the efficiency of oxygen evolution (α) in both phylotypes: however, there was a two-fold greater decrease in α versus Fv/Fm. The relationship between oxygen evolution and electron transport was linear at low to moderate light levels, above which electron transport rates overestimates oxygen production. This study suggests that fluorescence often overestimates true values of photosynthetic activity and that care must be taken in interpreting field fluorescence data.
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