Identifying the optimum temperature for the most efficient photosynthetic rate in the scleractinian coral Stylophora pistillata in the Gulf of Eilat, Red Sea

Abstract

The Gulf of Eilat (GoE), Red Sea, is a unique region that harbours one of the northernmost coral reef ecosystems in the world, and possesses high biodiversity and endemism. Global climate change is associated with an increase in sea surface temperatures that can lead to bleaching and mortality in scleractinian corals. However the corals in the GoE differ to other reefs at the same high latitude because of their unique resilience to thermal stress, which suggests that this region may be a refuge for reefs under a period of environmental change. This study aimed to determine the temperature at which the photosynthetic rate of the branching coral Stylophora pistillata is at its optimum capacity within this refuge. The effects of gradually increasing temperatures on the photochemical capability and oxygen evolution in the endosymbiotic zooxanthellae were measured, as well as the effect of the current ambient (26°C ) and the known bleaching threshold (32°C) temperatures on the zooxanthellae density and chlorophyll content. As photosynthetic rate is widely considered to be a good indicator of coral health, these results suggest that photosynthesis in S. pistillata functions most efficiently between 27-29°C, and that there is a significant effect on this rate at the known bleaching threshold at 32°C. There were also higher values of both zooxanthellae density and chlorophyll content at 26°C compared to 32°C. Comparing these results to predictions of future SSTs in the GoE suggests that the local average summer maxima will reach ~28°C by 2030, and ~29°C by 2050. This provides guidelines for scientists and conservation managers as to when corals will reach their optimum temperature and enabling improved ability to predict and mitigate the effects of other anthropogenic stressors, so that coral reefs can thrive in their optimal environment.Keywords: climate change, coral reefs, coral health, photosynthesis, optimum temperature, Gulf of Eilat, Red Sea