Do fluctuating temperature environments elevate coral thermal tolerance?

In reef corals, much research has focused on the capacity of corals to acclimatize and/or adapt to different thermal environments, but the majority of work has focused on distinctions in mean temperature. Across small spatial scales, distinctions in daily temperature variation are common, but the role of such environmental variation in setting coral thermal tolerances has received little attention. Here, we take advantage of back-reef pools in American Samoa that differ in thermal variation to investigate the effects of thermally fluctuating environments on coral thermal tolerance. We experimentally heat-stressed Acropora hyacinthus from a thermally moderate lagoon pool (temp range 26.5-33.3A degrees C) and from a more thermally variable pool that naturally experiences 2-3 h high temperature events during summer low tides (temp range 25.0-35A degrees C). We compared mortality and photosystem II photochemical efficiency of colony fragments exposed to ambient temperatures (median: 28.0A degrees C) or elevated temperatures (median: 31.5A degrees C). In the heated treatment, moderate pool corals showed nearly 50% mortality whether they hosted heat-sensitive (49.2 +/- A 6.5% SE; C2) or heat-resistant (47.0 +/- A 11.2% SE; D) symbionts. However, variable pool corals, all of which hosted heat-resistant symbionts, survived well, showing low mortalities (16.6 +/- A 8.8% SE) statistically indistinguishable from controls held at ambient temperatures (5.1-8.3 +/- A 3.3-8.3% SE). Similarly, moderate pool corals hosting heat-sensitive algae showed rapid rates of decline in algal photosystem II photochemical efficiency in the elevated temperature treatment (slope = -0.04 day(-1) +/- A 0.007 SE); moderate pool corals hosting heat-resistant algae showed intermediate levels of decline (slope = -0.039 day(-1) +/- A 0.007 SE); and variable pool corals hosting heat-resistant algae showed the least decline (slope = -0.028 day(-1) +/- A 0.004 SE). High gene flow among pools suggests that these differences probably reflect coral acclimatization not local genetic adaptation. Our results suggest that previous exposure to an environmentally variable microhabitat adds substantially to coral-algal thermal tolerance, beyond that provided by heat-resistant symbionts alone.