Paper Reviewed

Thomas, L., Rose, N.H., Bay, R.A., Lopez, E.H., Morikawa, M.K., Ruiz-Jones, L. and Palumbi, S.R. 2018. Mechanisms of thermal tolerance in reef-building corals across a fine-grained environmental mosaic: Lessons from Ofu, American Samoa. Frontiers in Marine Science 4: Article 434, doi: 10.3389/fmars.2017.00434.

In discussing the rationale for their review paper, Thomas et al. (2018) write that "for reef-building corals, understanding the relative roles of acclimatization and adaptation in generating thermal tolerance is fundamental to predicting the response of coral populations to future climate change." Indeed it is. And thus they proceed to summarize findings in this regard based on research conducted for well over a decade in the back-reef pools on Ofu, Manu'a Islands Group, American Samoa.

The back-reef pools on Ofu are ideal for studying the thermal tolerance of corals in a natural setting. Corals inhabiting such pools experience a wide range of temperature and irradiance values across a tidal cycle, the most variable of which reach temperatures of 34°C or higher and daily thermal fluctuations of up to 6°C. Two pools in particular, a highly variable pool experiencing temperatures that range from 24.5 to 35°C and a moderately variable pool with temperature variations of 25-32°C, have served as ideal settings for researchers to investigate the subject of thermal tolerance; they are adjacent to one another (~500 m apart) and both sustain a diverse assemblage of corals that are "nearly identical in species diversity and percent live coral cover."

So what did the authors' review of the subject reveal?

Focusing on corals of the Acropora genus, Thomas et al. report that "both acclimatization and adaptation occur strongly and define thermal tolerance differences between pools." Transplant experiments, for example, showed that individual corals were able to shift their physiology to "become more heat resistant when moved [from the cooler pool] into the warmer pool." Such physiological shifts often occurred quickly, within as little as a week. Transcriptome-wide data on gene expression provided insight on how such shifts occur, revealing that "a wide variety of genes are co-regulated in expression modules that change expression after experimental heat stress, after acclimatization, and even after short term environmental fluctuations." Furthermore, Thomas et al. note that coral symbionts varied between pools and species, adding that "the thermal tolerance of a coral is a reflection of individual host genotype and specific symbiont types."

The seven researchers conclude their review by "placing this work in the context of parallel research going on in other species, reefs, and ecosystems around the world and into the broader framework of reef coral resilience in the face of climate change." In doing so, they conclude that although some reef-building coral populations have experienced wide-spread thermal bleaching and die-off, "in many ways corals have the necessary tool-kit to cope with near-future climate change." And they present the following four mechanisms in support of their resilience claim:

1. Corals generally have large effective population sizes with high levels of genetic diversity. Such genetic variation, they say, "is a key component of the adaptive capacity to environmental change as higher levels of genetic diversity provide a greater probability of achieving allelic combinations that confer beneficial phenotypes in the new environment."

2. Coral species span strong temperature gradients, which likely has conferred an abundance of genetic variation in traits associated with thermal tolerance.

3. The primary reproductive mode utilized by corals is broadcast spawning, with larvae capable of dispersing large distances. Consequently, coral populations generally have high levels of gene flow, so "the exchange of beneficial genetic variants among populations spread across large areas is high." Furthermore, a high dispersal capacity "means that they have a high capacity for colonizing novel habitats that become suitable as isotherms shift poleward.

4. Coral populations show remarkable capacity for phenotypic plasticity and can rapidly shift their physiology to cope with repeated stress events. Phenotypic plasticity "can also be adaptive, and recent studies show that this trait provides resilience to frequently encountered environmental variation."

Commenting on the above tools of resilience, Thomas et al. say they "set the stage for short-scale local adaptation over space but can also allow rapid evolution over time." And thus this layered capacity of coral adaptability should help to ensure their population persistence well into the future, as it has for hundreds of millions of years in the past.