Award: OCE-1538495

Intellectual Merit The goal of this project was to study the cellular physiology of corals from reefs experiencing different environmental pH, carbon dioxide, and micronutrient conditions, and from different depths. Uniquely, this project combined (1) biochemical and microscopy experiments (to study coral cell physiology in unprecedented detail), (2) field and experimental aquarium experiments (to capture the physiology of wild corals and to test the effects of isolated environmental variables), and (3) cutting-edge biogeochemical technology (to characterize reef environmental conditions at high temporal and spatial resolution). (1) We focused on two novel cellular mechanisms that had been recently identified in corals through laboratory experiments. The first mechanism relies on "proton pump" enzymes in coral gastrodermal cells, and helps control photosynthesis by their symbiotic algae. The second mechanism uses "sodium/potassium" pump enzymes in coral calcifying cells, which provide the driving force for skeletal formation. We hypothesized that environmental conditions that are unfavorable for photosynthesis would induce upregulation of proton pump abundance, and that environmental conditions that are unfavorable for calcification would induce upregulation of sodium/potassium pumps. Conversely, favorable conditions would induce downregulation of those enzymes, with the associated benefit of energy savings. Since both enzymes are present in other cell types in addition to gastrodermal and calcifying cells, we used immunohistochemistry, a technique that allows visualizing specific enzymes in specific cells. In addition, we examined aerobic and anaerobic energy production capabilities. In an attempt to identify mechanistic reasons underlying species-specific vulnerability, we studied three coral species: Acropora cervicornis, Orbicella franksi, and Porites astroides. Our analyses have obtained several novel findings. One important finding is that, unlike corals grown under optimal aquaria conditions, wild corals display a previously unappreciated heterogeneity in protein expression patterns within a same cell type in different parts of the colony. For example, the VHA expression pattern in wild corals is highly patchy and can be vastly different in groups of cells only a hundred micrometers apart. Similarly, while all calcifying cells from laboratory corals express sodium/potassium pumps at high and similar abundance, their expression pattern in calcifying cells from wild corals is dependent on the position on the coral colony and their distance to seawater. These results indicate coral cell physiology is highly dependent on microenvironment conditions, in addition to "bulk seawater" conditions. And while these findings greatly complicate sample analysis and results interpretation, they unveiled an essential aspect of coral cell physiology that needs to be considered in future experiments and in assessments of coral responses to environmental stress. (2) Two field campaigns were conducted during October-November of 2015 and 2016 during which we collected coral samples from an open water reef and a lagoon reef, and from different depths (3, 5, and 8 meters). One important finding was a correlation between VHA abundance in gastrodermal cells and depth, indicating a role in promoting photosynthesis in light-saturating conditions. Another novel finding was an increased capacity for anaerobic metabolic energy production in P. asteroids from 8m depth, which suggests relatively higher resilience to environmental stress, and to hypoxia in particular. We also brought corals to an experimental aquarium and exposed them to different light, pH/CO2, and ammonia levels that were representative of the most extreme conditions found in the wild. The most significant finding was that elevated CO2 levels promoted phostosynthesis in both Porites and Orbicella corals, and that these conditions also resulted in VHA downregulation in gastrodermal cells and reduced oxidative damage, but only in Porites. These results again suggest that Porites will be relatively more resilient (and possibly benefited) by predicted future ocean acidification conditions. (3) While sampling corals from the field, we simultaneously characterized the environmental parameters in each sampling site using an array of pH, salinity, temperature, oxygen, and flow sensors together with discrete water sampling for carbonate chemistry and micro-nutrients such as nitrogen, phosphorus, and silica. This allowed us to directly correlate cellular physiology to the actual conditions corals were experiencing at the time of sampling and to better understand the prevalent environmental pressures corals experience at each sampling site. Broader impacts This NSF project trained two female PhD students, four Masters students (including two female, one of them Latina), and multiple undergraduate students who participated in field and laboratory activities either as their main thesis topic or as side projects. Results from this project were presented at undergraduate and graduate courses during 2016-2019, as well as at several international scientific venues. Uniquely, this project funded the 2018 public lecture "Coral doctors: the effects of global climate change on coral reefs?", which was attended by ~100 people and is available online and has ~25,000 views since June 2018. We also developed lesson plan for K-3, teacher training seminars, various outreach activities at the Scripps Birch Aquarium including a permanent museum exhibit, as well as activities with PBS TV show, Splash and Bubbles. Last Modified: 10/18/2019 Submitted by: Martin Tresguerres