Written by Melissa Naugle
When we hear the term coral bleaching, most of the time we associate it with the massive die off of corals. When corals bleach, they lose their symbiotic partner, microscopic algae called zooxanthellae (Fig. 1). As a response to stressful changes in their surroundings, zooxanthellae will abandon their coral host, leaving behind a pale and hungry coral skeleton. Often times, the corals never recover their symbiotic algae and die. As headlines of coral bleaching inundate our news channels and social media, it is hard not to feel overwhelmed and depressed. While climate change-induced mass bleaching is certainly not positive news, there remains a glimmer of hope: perhaps some bleaching could be a good thing?
Before explaining how bleaching can actually benefit the coral, lets review some background information (Fig. 2).
In order to live and grow, corals depend on their photosynthetic symbiotic algae (symbionts) to provide energy for them. These algae are of the genus Symbiodinium but can be broken down into different clades (or lineages). Coral hosts typically prefer one clade and carry it at higher densities but will also carry other clades at backgrounds levels. The proportions of different clades that live inside a coral make up its symbiont community. Symbiodinium clades differ from each other in their geographic ranges, host preference, and most importantly, their thermotolerance.
When corals endure high temperatures, they have the option to respond by adjusting their symbiont communities. For example, they can ‘shuffle’ their existing symbiont community (increasing the proportion of one clade, decreasing the proportion of another) or they can take up new symbionts from surrounding seawater. In the latter, corals will often experience some bleaching, which kicks out existing symbionts and makes room for new ones to come in. This process is explained by the adaptive bleaching hypothesis which views bleaching as a strategy used by corals to adjust their symbiont community in favor of a more thermally tolerant combination, termed symbiont shuffling. Hence, bleaching that does not kill the coral host could be an opportunity to find some better suited symbionts. For example, researchers have found that switching symbiont communities can increase a coral’s thermotolerance by 1-1.5° C (Fig. 3).
Since climate change appears to be outpacing coral evolution and bleaching is occurring on a massive scale, corals have to depend on other ways to withstand hotter temperatures. One way they may do so is through adjusting their symbiont communities to better suit their changing environment. In the midst of the bleak news surrounding corals, this process may provide some hope. In a recent projection of future coral bleaching, researchers found a 14% decrease in bleaching when they accounted for strategies such as symbiont shuffling. Future, a recent study found that corals that adjusted their symbiont communities could pass these changed communities onto their offspring. However, there are limits to how far this process will take corals battling climate change. If ocean temperatures continue to rapidly rise, corals will no longer be able to find Symbiodinium that are able to tolerate such high temperatures.
Further research into the adaptive bleaching hypothesis will provide more information about how symbiont shuffling can buffer the effects of climate change. In all the negative press about coral bleaching, symbiont shuffling may be able to raise our spirits and extend coral resilience.
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