Written by Tim Bateman

Extreme environments are often subject to habitat conditions that can provide a window into the future of climate change on reefs. Reefs that have consistently elevated temperature, decreased pH, deoxygenation, or some combination of these environmental parameters can be used to study how corals will adapt as climate change progresses. Recently, three different reefs have been the subject of studies that have investigated aspects of coral physiology likely to be impacted by climate change, outlining how researchers can take advantage of these extreme environments to better understand potential future environments. Corals on these reefs are consistently experiencing temperatures, pH, and oxygen levels that are expected to be common on the majority of the world’s tropical reefs before the end of the century according to the most recent IPCC report. This makes them an incredible place to conduct research, allowing scientists insight into how coral physiology may change in the future

The Rock Islands of Palau, Micronesia contain a series of inland waterways which include the extreme environment of Nikko Bay. Nikko Bay is consistently ~2°C warmer and 0.15 pH units lower than offshore reefs surrounding Palau (Fig. 1). Despite these conditions, which typically cause bleaching and reduce calcification on other reefs, a broad diversity of corals thrives in this extreme environment. One study found that a critical aspect allowing these corals to survive is the identity of their algal symbiont, the thermally tolerant species Durusdinium trenchii. D. trenchii is often considered less beneficial as a symbiont because, under certain conditions, it provides less energy to its host than other species. However, this is not the case for the corals in Nikko Bay. Hoadley et al. 2019 delved further into the potential trade-offs of associating with this thermally tolerant species by comparing the physiology and photobiology of corals from Nikko Bay to the same species from nearby offshore reefs. They consistently found that Nikko Bay corals displayed a muted stress response when heated while offshore corals suffered significant bleaching in the same conditions. Additionally, the response of the photobiology of Nikko Bay corals differed by species when heated, while offshore species all suffered similar patterns of photoinactivation and symbiont loss. Nikko Bay corals had species-specific ways of regulating photosynthetic processes allowing them to continue fixing carbon and providing energy to the coral host despite the increased temperature. These patterns of symbiont association and thermal tolerance inform us that the symbiont plays an essential role in coral survival in extreme environments and trade-offs associated with thermal tolerance do not necessarily persist across habitats or species.

Fig. 1 Physiology and photobiology experiments being carried out in Nikko Bay.

Climate change will also cause seawater acidification by increasing dissolved carbon dioxide (CO 2 ) concentrations on reefs. Elevated CO 2 reduces pH, making the water more acidic, but also provides more dissolved inorganic carbon, which algal symbionts can use for photosynthesis, potentially providing more food to the coral host. In Papua New Guinea, CO 2 bubbles from the ocean floor at a series of seeps at reef sites reducing pH to levels that commonly reduce coral calcification and increase skeletal dissolution (Fig. 2). Experiments aimed at investigating ocean acidification often struggle to capture the effects on ecologically relevant timescales making natural study sites like this invaluable. Dr. Kenkel and a team of researchers sought to elucidate the molecular mechanisms underpinning acclimation and adaptation to ocean acidification and took advantage of this window into climate conditions. It was found that the algal symbionts displayed a greater and more varied gene expression response compared to the coral hosts. This emphasizes the importance of CO 2 to photosynthetic symbionts as well as the possibility that hosts will have to adjust to changes in their environment as well as changes in the physiology of their symbionts.

Our last set of ‘future reefs’ is found on a remote island in the Coral Sea. New Caledonia is home to a recently described extreme shallow lagoon reef with conditions that combine extremely variable temperatures with pH and oxygen levels that many reefs will be experiencing by the end of the century; a lagoon with a potential view into the future. Patterns of photosynthetic strategy and dominant symbiont type outlined the strategies necessary for maintaining high coral cover and diversity inside the lagoon. Further investigation by Camp et al. revealed that differences in the microbiome are also necessary to support corals in this extreme environment. Corals from the lagoon were host to a wider diversity of microbes than their offshore counterparts, which likely facilitate the physiological differences found in the original study. The corals from this extreme environment tell us that microbial associations required for survival in future tropical reef conditions will be different and more varied than those seen on most healthy reefs today.

Extreme environments have proved invaluable to researchers thus far, and their continued exploitation as natural experiments could inform us of how reefs will look in the future. To date, they have helped elucidate how specific aspects of coral physiology could change to adapt corals to life under these conditions. The significance of different photobiological strategies, gene expression, and microbial associations are just three examples of what has been gleaned thus far but there is still much work undergoing and to be done! With a view through these windows into climate change on reefs, scientists will continue to contribute to the understanding reefs in the future.

Glossary

Photobiology: the study of the effects of light and photosynthesis on living organisms

Photoinactivation: the retardation or prevention of photosynthetic processes by excess light energy

Gene Expression: the process by which the information in a gene, the sequence of DNA base pairs, is made into a functional gene product, such as protein or RNA

References:

Nikko Bay

Hoadley, K. D., Lewis, A. M., Wham, D. C., Pettay, D. T., Grasso, C., Smith, R., Kemp, D. W., LaJeunesse, T. C. and Warner, M. E. (2019). Host-symbiont combinations dictate the photo-physiological response of reef-building corals to thermal stress. Scientific Reports 9, 15.

Papua New Guinea

Kenkel, C. D., Moya, A., Strahl, J., Humphrey, C. and Bay, L. K. (2018). Functional genomic analysis of corals from natural CO 2 -seeps reveals core molecular responses involved in acclimatization to ocean acidification. Global Change Biology 24, 158-171.

New Caledonia

Camp, E. F., Edmondson, J., Doheny, A., Rumney, J., Grima, A. J., Huete, A. and Suggett, D. J. (2019). Mangrove lagoons of the Great Barrier Reef support coral populations persisting under extreme environmental conditions. Marine Ecology Progress Series 625, 1-14.

Camp, E. F., Nitschke, M. R., Rodolfo-Metalpa, R., Houlbreque, F., Gardner, S. G., Smith, D. J., Zampighi, M. and Suggett, D. J. (2017). Reef-building corals thrive within hot-acidified and deoxygenated waters. Scientific Reports 7, 9.Camp, E. F., Suggett, D. J., Pogoreutz, C., Nitschke, M. R., Houlbreque, F., Hume, B. C. C., Gardner, S. G., Zampighi, M., Rodolfo-Metalpa, R. and Voolstra, C. R. (2020). Corals exhibit distinct patterns of microbial reorganization to thrive in an extreme inshore environment. Coral Reefs.