Coral reef acclimatization to ocean acidification at CO2 seeps – CARiOCA

Climate change is threatening marine life worldwide. Coral reefs, which harbor 1/3 of world’s marine biodiversity and provide ecosystem goods and services to > 500 million people worldwide, are expected to experience severe negative impacts due to climate change. The increasing carbon dioxide (CO2) in the atmosphere is driving increased CO2 diffusion into the ocean. Current understanding of the effects of this ‘ocean acidification’ (OA) on marine biodiversity and ecosystem functioning is primarily based on short-term laboratory experiments on individual species. Although many of these experiments suggest dramatic impacts of OA, including the dissolution of carbonate structures and decreased coral calcification rates, short term experiments potentially overestimate such effects because they are too brief for organism acclimatization and/or adaptation to occur. Consequently, there is a pressing need for empirical data documenting the physiology, acclimatization, demographic rates and ecological functioning of coral communities that have been exposed to elevated pCO2 over multiple generations.

Natural CO2 seeps have been recently revealed as natural analogues to assess OA effects at the ecosystem level. Studies at CO2 seeps at d’Entrecasteaux Islands in Papua New Guinea have shown coral communities surviving at chronically high pCO2, although these communities are dominated by mound-shaped rather than branching coral species. The CARiOCA project proposes to use this unique site, which has seawater conditions similar to those expected at the end of this century, to identify the phenotypic traits that allow certain coral species to survive and reproduce in seawater naturally enriched in CO2. This will give new insights into the mechanisms that underlie acclimatization and adaptation to climate change. As physiological acclimation is critical in shaping species environmental tolerances this project will, for the first time, link long- and short-term changes in gene expression to changes in organismal physiology and demographic rates, and to shifts in the species composition of coral communities.

Coral responses will be investigated at different levels of organization: i) short- and long-term variation in differential gene expression; ii) variation in Symbiodinium communities; iii) variation in coral-associated Symbiodinium and host-associated bacterial communities; iv) colony-level changes in physiology of the coral holobiont; and v) scale-up from changes occurring at molecular and physiological levels to changes in population demographic rates. This approach allows us to establish the specific physiological traits that enable certain species to survive under high CO2 conditions.

Our objectives will be achieved via four integrated components: project management (WP #1), fieldwork (WP #2), molecular and microbial analyses (WP #3), and physiological modeling (WP #4). WP #1 will coordinate the project and build a common database to foster the production of joint papers. WP# 2 is dedicated to the acquisition of data during two field expeditions. WP #3 will implement molecular and microbial analyses. WP#4 will coordinate the analyses of the project data and develop and parameterize a ‘scope for growth’ model based on coral physiological energetics. This model will be validated using field observations of coral growth and reproduction from the study locations. CARiOCA will make a significant contribution to the preservation of coral reefs by providing innovative and mechanistic data that resolve, for the first time, whether and how corals acclimatize and/or adapt to OA. These results will allow robust projections of the future biodiversity and ecological functioning of coral reefs by identifying coral species that are likely to dominate coral reefs in a high CO2 world.