Paper Reviewed
Silverstein, R.N., Cunning, R. and Baker, A.C. 2015. Change in algal symbiont communities after bleaching, not prior heat exposure, increases heat tolerance of reef corals. Global Change Biology 21: 236-249.

Writing as background for their work, Silverstein et al. (2015) state that "mutualistic organisms can be particularly susceptible to climate change stress, as their survivorship is often limited by the most vulnerable partner." But on the other hand, they note that "stress-tolerant symbionts can also sometimes mitigate bleaching." And in studying this phenomenon, they conducted "repeat bleaching and recovery experiments on the coral Montastraea cavernosa," using quantitative PCR and chlorophyll fluorometry to assess the structure and function of the Symbiodinium communities within the coral hosts.

This work revealed, as they describe it, that "during an initial heat exposure (32°C for 10 days), corals hosting only stress-sensitive symbionts (Symbiodinium C3) bleached, but recovered (at either 24°C or 29°C) with predominantly (>90%) stress-tolerant symbionts (Symbiodinium D1a), which were not detected before bleaching (either due to absence or extreme low abundance)." In addition, the three U.S. researchers report that "corals that were initially bleached without heat by a herbicide (DCMU, at 24°C) also recovered predominantly with D1a symbionts," noting that they too "similarly lost fewer symbionts during subsequent thermal stress." And one more thing of note: they report that "increased thermo-tolerance was also not observed in C3-dominated corals that were acclimated for 3 months to warmer temperatures (29°C) before heat stress."

In considering these several observations, Silverstein et al. logically concluded that the observed "increased thermo-tolerance post-bleaching resulted from symbiont community composition changes, not prior heat exposure," which conclusion was additionally supported by the fact that initially undetectable D1a symbionts only became dominant after bleaching. And they thereby clearly demonstrated that even "symbionts that are extremely rare, or absent, in corals prior to bleaching can become dominant after disturbance and influence overall host physiology," thus providing ever more evidence of the great tenacity of corals in their ongoing fight to survive heat-induced bleaching.