Award: OCE-1558868
Award Title: Investigating Structural Changes In Reef-Associated Biodiversity Along A Natural Gradient In Ocean Acidification
Outcomes Report
Carbon dioxide released into the atmosphere through burning of fossil fuels has widely recognized impacts on climate. The oceans absorb roughly ¼ of all carbon dioxide emissions and, when dissolved into seawater the gas lowers the ocean pH, causing a chemical change toward more acidity. Ocean acidification has a broad range of mainly negative consequences for marine life. For example, it can directly impact the physiology of organisms by slowing growth or reproduction or making it more difficult to build a shell or skeleton. Even organisms not particularly sensitive to pH may still be indirectly affected by impacts on the habitats or the food chains they depend on. On coral reefs, indirect impacts can be particularly dire for the numerous species that depend on the limestone structure provided by corals for habitat, as both corals and their limestone skeletons are sensitive to ocean acidification. This study investigated changes in community structure of coral reef-dwelling organisms caused by low pH, focusing in particular on the many small species that hide in the cracks and crevices of reefs. These very small and diverse organisms provide critical food resources for larger organisms and are thus a crucial component of reef ecosystems. To study the impacts of ocean acidification on these organisms, we used a natural gradient of carbon dioxide concentration created by volcanic CO2 seeps that acidify the surrounding waters on reefs in Papua New Guinea. We put out standardized sampling tools called Autonomous Reef Monitoring Structures (ARMS) to collect organisms without damaging the reefs. These consist of stacks of PVC plates separated by spacers, that can be thought of as small apartment houses on reefs. Many different kinds of organisms settle into and grow on ARMS, both animals and algae as well bacteria. We placed the ARMS at two localities, and at each locality we chose three different pH levels (low pH, medium pH, and normal pH). We left a total of 30 ARMS on the reefs for two years and then collected them to study the communities that had settled and grown on them under the different pH conditions. Small cryptic organisms can be very hard to identify. To solve this challenge, we used DNA sequencing to obtain a molecular signature and to tell which organisms were present. For larger invertebrates (>2mm in size), we used an approach called DNA Barcoding to obtain a "molecular barcode" for every individual separately. For the many tiny organisms, given the sheer diversity and abundance of life forms we used a method called metabarcoding that is able to sequence all the organisms mixed together. In both cases the sequences can be matched against databases to determine what kind of organisms each DNA sequence came from. We were able to detect a large number of different kinds of organisms. For the larger invertebrates, we detected over 300 species, and with metabarcoding we detected the presence of 47 major animal and algal groups (phyla), 29 bacterial and 4 archaea phyla. Such a comprehensive survey of the effects of ocean acidification has not previously been attempted. Overall, both abundance and diversity were strongly reduced in higher carbon dioxide conditions and associated higher acidity, with declines as high as 34-56% and 42-45% respectively for bigger invertebrate animals. Other groups displayed profound shifts, with crustose algae giving way to fleshier red and brown algae as well as diatom mats. Bacteria also displayed notable declines in diversity and shifts in community composition. Overall, community compositions were very different between control and high CO2 conditions, suggesting that ongoing ocean acidification might have deep consequences for many marine groups as well as ecosystem functioning. This work is particularly important because it integrates the effects of acidification on the complex interactions in real-world communities, something that cannot be accomplished in the laboratory. This project also demonstrated the ability of standardized and non-destructive ARMS sampling tools, coupled with molecular approaches, to capture fine-scale differences in biodiversity patterns associated with environmental changes. Through participation with the Smithsonian YES! (Youth Engagement through Science) program, this project helped expose students traditionally underrepresented in STEM careers to research, by providing immersion science internships. A total of eight YES! students were involved in fundamental scientific research (marine ecology and environmental change) and were taught cutting-edge laboratory techniques (e.g., DNA sequencing, metabarcoding). ARMS also formed part of a hands-on learning experience at the Smithsonian Natural History Museum. Materials and results from this research were used to produce the Reefs Unleashed National Museum of Natural History school program, aimed at grades 6-12. During the lifetime of this project, the school program ran 72 times for 1,617 students. Seventeen collaborators were engaged in the production of important research as well as education and outreach experiences during this project. Last Modified: 10/28/2021 Submitted by: Laetitia M Plaisance