Symbiotic dinoflagellates associated with corals in Qatar: characterization and physiological response to environmental change

Abstract

Studying coral reefs’ responses to environmental and anthropogenic stressors is crucial to ensure the development of appropriate conservation and restoration programs, especially in areas such as the Arabian Gulf where they are subjected to extreme seawater temperature stress and experiencing consistent losses in coverage and distribution. Coral bleaching events witnessed during the last decades in the Arabian Gulf are believed to be the result of the effect of heat events occurring more frequently in the region as a result of the global ocean warming. When corals are under thermal stress, their symbiotic zooxanthellae are affected, and their photosynthetic activity decreases until the stress disappears or until the zooxanthellae are expelled from the coral. To study the response of thermal stress on the zooxanthellae associated with the coral host, I exposed three ecologically important coral species from Qatar, Porites lutea, Dipsastraea pallida, and Acropora downingi, to three levels of temperature in laboratory aquaria. The three chose species are known to have differential responses to heat events. Response parameters were assessed during and after the experiment using fluorescence technique (Pulse Amplitude Modulation – PAM fluorometry and flow cytometry) to evaluate the coral health by measuring the photosynthetic activity of Symbiodinium zooxanthellae. Samples of the three coral species were collected from an offshore reef of Qatar (Um Al-Arshan) iv and used to test the effects of different levels of temperature under controlled artificial conditions. Our results suggested that high levels of temperature 35°C affect consistently the photosynthetic activity of zooxanthellae within coral tissue of the three species. Acropora downingi was the most sensitive species to the thermal stress. Indeed the measured photosynthetic activity in the 35°C treatment was reduced after 17 days, the coral host bleached after 21 days, and all the colonies died after 24 days. The photosynthetic activity of Dipsastraea pallida was overall lower than the other studied species. At 35°C, it started to decrease after 17 days, but D. pallida showed the less decrease in photosynthetic activity throughout the experiment. Finally, Porites lutea showed the highest decrease in photosynthetic activity at 35°C. No decrease in photosynthetic activity was detected at 30°C for the three species. The enumeration and physiological characterization of zooxanthellae extracted from the corals subjected to three different temperature treatment (25ºC, 30ºC and 35ºC) were conducted using both flow cytometry and investigation under microscope. Results showed a consistent reduction of the ratio healthy/unhealthy cells among all three considered species when temperature was increased.