OCE-PRF: Impacts of endolithic microbial sulfur cycling on coral holobiont ecophysiology, biomineralization, and geochemistry

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Reef-building corals construct skeletons and build complex structures upon which entire reef ecosystems depend. A diverse group of microbes, known as endoliths, live in coral skeletons and are in close proximity to the coral host. Some of these endolithic microbes could influence coral calcification, by changing pH and alkalinity, and may also provide nutrients to the coral host. However, little is known about the microbes inhabiting coral skeletons. Our limited knowledge about coral endoliths leaves a large gap in our understanding of coral physiology. Without a full understanding of coral physiology, it is difficult to make predictions of how corals and the structure they create will respond to climate change. This work will investigate diversity and activity of microbes in coral skeletons, with a focus on microbes that could affect coral calcification. In addition, as coral skeletons are often used to reconstruct past climate, this work will determine how these microbes impact the chemical signatures of coral skeletons and climate proxy interpretation. Through research programs at the Marine Biological Laboratory, high school and undergraduate level students will be involved in laboratory analysis and aquarium-based experiments. Findings and products will be shared in open-access publications, open-access data and code repositories, and through local and national (virtual) public outreach events. By improving our understanding of microbial diversity and activity in coral skeletons, this research will improve our understanding how corals will respond to environmental change, allowing us to better preserve coral reef ecosystems and the important economic services that they provide.Despite decades of research on coral biology and biomineralization, many basic mechanisms of coral growth and physiology remain debated in the literature, including the role of biology in coral calcification. Coral biology is known to affect calcification, and the skeletal microbial community likely plays a role in calcification, bioerosion, and nutrient cycling within the coral. By combining microbial physiology, biogeochemistry, and geochemistry techniques, this research will take an integrative and interdisciplinary approach towards understanding the coral skeletal microbial community and its influence on coral physiology and calcification. Microbial sulfur cycling rates in coral skeletons will be measured and paired with taxonomic identification through a combination of amplicon and metagenomic sequencing approaches. Key microbial taxa will be localized using fluorescence in situ hybridization and confocal microscopy. Particular focus will be given to the spatial distribution of specific functional groups within the skeleton, including taxa performing anoxygenic photosynthesis and sulfate reduction, two pathways known to affect pH and alkalinity. Microsensor and isotope tracer experiments will be used to quantify microbial activity, as well as any potential transfer of nutrients between endoliths or from endoliths to the coral tissue. After constraining the diversity, distribution, and activity of these microbes, geochemical signatures of coral skeletons containing known endolithic communities will be used to study the effect of endoliths on coral paleoclimate proxies. This work could lead to the development of new environmental proxies or correction factors for existing proxies, by constraining how endolith-driven changes in pH, alkalinity, and carbon cycling alter calcification and skeletal geochemistry. By providing a deeper and more holistic understanding of coral endolithic microbes, findings from this work will improve our understanding of corals and their ability to create structure, recycle nutrients, and support ecologically and economically critical ecosystems, as well as our understanding of microbes adapted to extreme habitats.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.