The effect of the presence of Syringodium filiforme on the physiology of Acropora cervicornis under ocean acidification conditions

Abstract

The increase of carbon emissions into the atmosphere is shifting the ocean chemistry, decreasing the pH and increasing the temperature, which can ultimately have detrimental effects on the physiology of calcifying organisms, such as corals. This study ran two short-term experiments, 5 and 2 weeks long, which evaluated the effects of Syringodium filiforme seagrass and of the projected oceanic pH in a century, pH 7.7, on the rates of respiration, photosynthesis, growth and calcification of Acropora cervicornis coral. Corals were placed in tanks with high pH seawater (pH 8.1) or low pH seawater (pH 7.6), where half of the tanks contained seagrass. In the five-week experiment, 13 corals under low pH condition suffered rapid tissue loss, some leading to mortality. The resulting corals showed no significant differences in coral physiology between treatment groups. The two-week experiment, however, showed increased growth rate in corals under low pH condition, and increased rate of photosynthesis in corals in tanks with seagrass present. Larger amount of CO2 available in low pH condition in short-term, may have increased the carbon fixation by A. cervicornis, leading to a greater growth rate. The presence of the seagrass, on other hand, may have increased the concentration of ions used by the zooxanthellae within A. cervicornis, increasing the rate of photosynthesis. The study supports previous suggestions that the response of coral physiology to both the presence of the seagrass and the ocean acidification conditions is species specific.