UC San Diego

Currently, the only way to identify the extent of coral stress is through visually observable decreases in coral coloration as a product of coral bleaching. However, by the time bleaching has been initiated, the coral may already be in such a deleterious state that it may be difficult for it to recover if or when the stress factors are removed. Having the ability to identify when the coral is in the early stages of stress would be beneficial in managing and, eventually, restoring coral reefs. Fluorescent proteins make up roughly 20% of the total soluble proteins in corals but, despite the abundance of fluorescent proteins in corals, their physiological roles are still not well understood. However, they have previously been hypothesized to function as photo- protective ROS scavengers, photo-enhancers, and light- driven proton pumps. This study aims to quantify changes in fluorescence in corals exposed to CO₂-induced ocean acidification (OA) conditions over the course of 31 days with three realistic pH conditions under controlled laboratory settings. This study shows that the intensity of green fluorescence declined in the coral's chronic response to decreased pH (increased CO₂), while the color under bright field did not pale. Towards the end of the experiment, fluorescence in the corals under OA treatment showed no increase in fluorescence and did not reach the level of the control corals. With regards to their calcification, it was found that corals exposed to OA were able to grow just as well as the corals kept in control conditions for most of the experiment, suggesting that corals have mechanisms to cope with short-term OA. However, growth did decline after a one-month of exposure to OA. This study also found that SOD activity increased rapidly and consistently maintained a high level of scavenging in response to OA. On the other hand, exposure to OA was not correlated to the overall abundance of VHA, an enzyme proposed to be involved in carbon-concentrating for zooxanthellae photosynthesis. This may imply that corals are able to buffer the effects of OA for their zooxanthellae in order to maintain their symbiotic relationship. Overall, this study highlights the effectiveness of green fluorescence in corals as an early proxy to stress levels and an indicator to potential long- term physiological effects