Impact of ocean warming on coral thermal tolerance (ANCHOR)
Evolution, Anthropology
Microbial Ecology and Applied Microbiology
Final Report Abstract
The persistence of coral reef ecosystems is threatened by ongoing climate change. Even under more ‘optimistic’ scenarios of ~1.5°C warming (Representative Concentration Pathway RCP 2.6), the International Panel on Climate Change (IPCC) warns that 70-90% of reef ecosystems might be lost or severely degraded by the end of this century. Coral bleaching is the main driver of reef degradation globally. Bleaching is the process whereby stress (e.g. heat stress) causes coral to rapidly lose their microalgal endosymbionts via destabilization of the coral-algal symbiosis, leading to coral death if endosymbionts are not recovered rapidly. Loss of corals via bleaching causes ecological cascades that impact reef-associated fish, and ultimately the erosion of reef topography that together transform ecological and biogeochemical services. In recent years, researchers have identified an increasing number of coral populations, reef regions, and individual coral genotypes with enhanced bleaching resilience. Such differences in the susceptibility of corals to bleaching have been attributed to genomic specificities of the coral hosts and its microalgal symbionts, and the coral’s flexibility to form association with different microalgal symbiont species and bacteria. Thermally tolerant corals will most likely be the main reef builders and ecosystem sustainers of future reefs. Recent management efforts and investments have been prioritizing reefs with greater resilience, focusing on corals with inherently greater stress tolerance, recovery potential, or bleaching thresholds that are able to operate as climate change refugia. However, the viability and success of such reefs depends not only on their current resilience, but also on their ability to withstand bleaching during hot summer months on top of a general increase of sea surface temperatures (SSTs). In fact, it is unknown how corals that we consider less susceptible to bleaching under today’s conditions will cope with future increase in SSTs and how this will affect their ability to respond to severe episodes of heat stress (e.g., summer months). This project assessed how thermal tolerance (i.e., the ability to respond to short-term heat stress events) of coral species that are considered less susceptible to bleaching today, is affected by a long-term increase of baseline average seawater temperatures to +1.5°C and +3°C (in line with IPCC RCP 2.6 and 4.5 trajectories for reefs at the end of this century). To assess thermal tolerance, we have employed a newly developed portable experimental system termed CBASS (the Coral Bleaching Automated Stress System), which is used to conduct standardized short-term heat stress assays and concomitantly collect phenotypic diagnostics (photosynthetic efficiency, microalgal endosymbiont density) to reveal fine-scale differences in thermal tolerance. Results of the funded work clearly show that corals subjected to increased seawater temperatures gain thermal tolerance on the short run (2 months), but lose this enhanced thermal tolerance on the long run (6 months). By the end of the long-term experiment, following 6 months at elevated temperature and one month of recovery at control temperature, the tolerance of corals for acute heat stress diminished, suggesting that acclimation is not a universal avenue for corals to persist under climate change. Both medium and high temperature levels used in this study triggered a decrease in tolerance against further acute heat stress over time, suggesting that even the more optimistic IPCC trajectories will be too high for corals to acclimate on the long run; even species that are considered less susceptible to bleaching today. In addition to this phenotypic diagnosis, we explored the genetic and genomic underpinnings of the short-term gain and long-term loss of acute thermal tolerance by assessing changes in gene expression (host and microalgal symbionts) and in the composition of the microbial community (Symbiodiniaceae and bacteria). Analyses that will give insights into which molecular pathways are involved in the short term gain of thermal tolerance, and why corals are unable to maintain this acclimation, are still underway. Results of this study suggest that recent management efforts and investments that have been prioritizing the protection of reefs with greater inherent resilience will be in vain if temperatures continue to increase as climate change progresses.
