Does heat tolerance vary with rates of oxygen production in photosymbiotic cnidarians?

Oxygen acquisition and delivery to tissues is believed to be a key factor in heat tolerance, but testing this link has been challenging owing to methodological limitations to separate processes related to oxygen acquisition and oxygen delivery. In this study, we altered tissue oxygenation by manipulating light intensity using cnidarians that host endosymbiotic algae as model species. We first verified that light intensity determines net photosynthetic rates, showing that all species produced oxygen at the highest light intensity, and that chemically inhibiting photosynthesis successfully reduced oxygen production. We then tested the prediction that heat tolerance would be higher at higher light intensities and lower in specimens that no longer have internal oxygen production due to photosynthesis (chemical inhibition). Overall, photosynthetic specimens had a higher heat tolerance than inhibited specimens and increased light intensity improved heat tolerance for two of the three species we examined. Because inhibited specimens had lower heat tolerances, we conclude that oxygen dynamics are involved in shaping heat tolerance. Interestingly, light intensity also affected oxygen uptake and heat tolerance in some of the chemically inhibited specimens, indicating that either we did not achieve complete inhibition of photosynthesis or that light modulates aspects of cnidarian metabolism that are related to thermal tolerance, but which may extend beyond oxygen dynamics and the photosynthesis occurring in their algae.