There may be hope yet - Corals can adapt to climate change

Corals and their symbionts often live at the limit of their thermal tolerance, with small changes potentially resulting in mass bleaching. While many papers have shown that corals can bleach to adapt to change, it can take weeks/months for an effect to be seen. During this time, disease may kill the corals before they can recover.

However, a recent paper by Palumbi et al. (2014) showed that the transplantation of a fast growing, dominant coral species (Acropora hyacinthus) from cooler microclimes to areas experiencing highly variable (HV) temperatures (up to 35°C) adapted in less two years. Coral branches were transplanted from moderately variable (MV) sites, where the temperature rarely exceeds 32°C, to HV sites and grown in situ.

Coral branches from each site were bleached according to native conditions, with the retention of chlorophyll a in the symbionts used as a measure of resistance to thermal stress. The results showed that the MV corals had on average less heat resistance than the HV corals; the MV group retained only 45% of chlorophyll compared to 80% in HV.

To test for acclimatisation, the corals from MV were transported to HV conditions (vice versa) and then experimentally bleached at 12, 19 and 27 months. Of the 23 experiments, 22 showed that the corals acquired partial heat sensitivity of the pool they were transplanted to. This means that the corals from the MV group transported into HV conditions and bleached were able to retain more chlorophyll a in the new site than the native MV corals. What was interesting was that the HV corals that retain more chlorophyll a than the MV group when bleached actually dropped their levels to match the native MV group when transplanted.

An ANOVA using 16,728 genes revealed that 74 had changed significantly when comparing the genetically identical corals from each site. The gene expression had changed between sites, despite coming from genetic clones, purely based on environmental differences. Some genes were expressed purely based on the origin of the corals, regardless of where they were transplanted to. Based on these findings, there may be fixed constitutive expression levels for thermal resistance in both sites, and adapted gene expression acquired after transplantation.

Differences in the Symbiodinium clades present in each group after transplantation were measured by looking at transcriptome reads. Clades C and D are common in this species, with the HV group containing higher levels of D. Despite predictions that the clades may have changed, they determined that there was little shift in one clade over another even after transplantation.

Phenotypic change seen in the corals was also measured in order to put this acclimation effect in terms of evolution. This change is often studied by measuring the change in mean phenotype before versus after natural selection, divided by standard deviation. Estimates were made based on all data collected, which showed that the corals did in fact acclimate to higher temperatures.

Pooling all the data together, this study has shown “that acclimatisation can allow corals to acquire substantial high-temperature resistance more quickly than strong natural selection would produce”. Acclimation alone may not save corals world-wide, due to other increasing factors having an impact, however this paper could provide a method for preserving most corals. This is a good starting point for looking at how other species of coral found in shallow/deep waters may be affected by temperature change, and could show how they may adapt to changing conditions.

Palumbi, S. R., Barshis, D. J., Traylor-Knowles, N., & Bay, R. A. (2014). Mechanisms of reef coral resistance to future climate change. Science,344(6186), 895-898.

http://www.sciencemag.org/content/344/6186/895.full.pdf