Coral reefs are essential habitats for marine life but are also the coastal ecosystem most impacted by climate change. Climate change is modifying the ocean’s environmental conditions through rising temperatures, acidity, and decreased dissolved oxygen, threatening the survival of coral reefs. Predicting how corals will react to upcoming climatic conditions is vital to forecast how these ecosystems might change in the future, but lab experiments are often limited by time, preventing long-term responses of corals to future conditions. This is why extreme reef environments, i.e. sites that already experience future-like conditions, are extremely valuable to investigate the response of corals to climate change. Such a site has been identified in 2017 in New Caledonia (lagoon of Bouraké, Southwest Pacific), and our current lab member Juliette had the opportunity to participate in a R/V field trip in 2020 with the goal of better understanding the metabolic specificities of corals thriving at this extreme site.
Coral Incubatioin set-up
The lagoon of Bouraké is of particular interest because it is one of the only reef environments identified where the three climate stressors (T, pH, DO) predicted to impact corals co-occur. However, despite these conditions, some coral species are healthy and give insight into how corals could persist in a changing ocean. To test whether the success of these “super corals” could be explained by changes in their metabolic rates, Juliette and Clément, supervised by Dr. Rodolfo-Metalpa (Institute of Research and Development, Nouméa) investigated the photosynthesis, respiration rates, and symbiotic characteristics of corals in the Bouraké lagoon and compared them with those of corals from a near-by control reef experiencing typical ocean conditions.
Juliette on board the R/V, analyzing seawater conditions.
Colonies from seven coral species were sampled at both the extreme and the control site and were incubated in conditions mimicking either present-day or future-like environmental conditions. For corals from both sites, and under both incubation conditions, photosynthesis, respiration and symbiont content were measured. This study allowed us to formulate several conclusions. First, symbiont density and chlorophyll content were as high or higher at the Bouraké site than at the control site. This is a demonstration of the good health of Bouraké corals, as corals tend to expel their symbionts when experiencing stress. Second, the incubations revealed that no changes in the metabolism of corals occurred when exposed to short-term changes of environmental conditions. Lastly, measurement of metabolic rates evidenced significant differences in the respiration and photosynthesis of corals from the extreme and the control site. However, these changes differed among species, indicating that mechanisms underlying these shifts were species-specific rather than ubiquitous. The specificities of the observed responses of corals developing in extreme environments suggests that no unique response will determine corals tolerance to future conditions.
Coral sampling by Clément at the extreme site of Bouraké.
Jacquemont, J., Houlbrèque, F., Tanvet, C. et al. Long-term exposure to an extreme environment induces species-specific responses in corals’ photosynthesis and respiration rates. Mar Biol 169, 82 (2022).
For access to the full version of the published article, follow this link: https://rdcu.be/cOt8M