Competitive effects of the macroalgae Caulerpa taxifolia on key physiological processes in the scleratinian coral Turbinaria peltata under thermal stress

Abstract With the degradation of coral reefs induced by climate change and local environmental stressors, an increasing abundance of macroalgae is observed in many reefs. The sublethal damage caused by macroalgae on coral included inhibiting growth, development and reproduction. However, few studies have focused on the dynamics of the interspecific relationship between coral and macroalgae, which may change under climate change. Thus, the effects of thermal stress on the competition between the plate-like coral Turbinaria peltata and macroalgae Caulerpa taxifolia were explored in this study. The responses of corals exposed to the culture water of algae and those in direct contact with algae were also compared. The results showed that at ambient temperature (27 °C), coculture with C. taxifolia reduced the growth rate and protein content of T. peltata by 57.7~65.5% and 37.2~49.0%, respectively, and significantly increased the activities of antioxidant enzymes. Direct contact with C. taxifolia enhanced the activities of antioxidant enzymes further compared to coral exposed to algae. The density and chlorophyll a content of symbiotic zooxanthellae were not obviously influenced based on whether the coral T. peltata was cultured with C. taxifolia. When the temperature was increased by 3 °C, the growth rate of T. peltata decreased to the level of corals exposed to C. taxifolia at ambient temperature, whereas the same pattern was seen in the increase in antioxidant enzyme activity. Additionally, the density and chlorophyll a content of symbiotic zooxanthellae, as well as the feeding rate of coral, showed an evident decrease with increasing temperature. However, thermal stress was only shown in the feeding rate when T. peltata was in indirect contact with C. taxifolia. The interaction of macroalgae contact and thermal stress was evident in the growth rate, protein content, feeding rate and catalase activity (CAT) of coral compared to coral intact with algae under ambient temperature. These results indicated that the negative effects of the macroalgae C. taxifolia on the coral T. peltata are comparable to those of ocean warming, and the competition between corals and macroalgae may be more intense under thermal stress.