Natural thermal stress-hardening of corals through cold temperature pulses in the Thai Andaman Sea

Abstract

Thermal variability can render corals stress resistant through a phenomenon coined as stress-hardening induced by environmental priming. Fluctuations that involve high temperature peaks have been commonly investigated, however, the effects of a stress-hardening stimulus generated by cold-water pulses has rarely been studied. Offshore island reefs in the Andaman Sea offer an ideal natural setting to study these effects, as cooling water of internal waves induce strong variability with peak intensity in January to June and absence in August to November. While western island shores are exposed to this stimulus, eastern shores remain sheltered. This study examined (1) whether corals from exposed reefs were more heat stress resistant compared to stimulus-sheltered conspecifics and (2) whether this trait can last in the absence of the stimulus. We quantified the thermal stress resistance in two ecologically important coral species, Pocillopora sp. and Porites sp., from the two island shores, during the two seasons. Coral bleaching intensity and photosynthetic efficiency of algal symbionts were measured as response variables after a short-term heat stress assay to assess thermal stress resistance. Stress responses of all stimulus-exposed corals were either undetectable (during the season of stimulus presence) or very weak (during stimulus absence), while corals from the stimulus-sheltered shore responded strongly to heat stress irrespective of the season. Hence, thermal resistance was overall greater in corals originating from the stimulus-exposed shore, but it was slightly diminished during the season of stimulus absence, emphasizing the relevance of stimulus recurrence in maintaining the resistance trait. We exemplify that the stimulus of fluctuating low temperature pulses successfully induced stress-hardening in corals. This suggests that priming stimuli do not necessarily need to transgress certain upper thermal thresholds, but can also touch on lower thresholds to be effective. Even more, we argue that cooling pulses might represent a safer stress-hardening regime, since warming-stress accumulation can be avoided. More research is required to obtain a better understanding of environmental priming, but current findings should encourage the development of artificial stress-hardening approaches to enhance coral resistance in reef restoration efforts.

Thermal preconditioning modulates coral physiology and heat tolerance: A multi-species perspective

“…Our aquarium experiments demonstrated that thermal preconditioning can be applied to stressharden corals and enhance their ability to cope with an acute heat stress pulse, contributing to the growing body of literature that has primarily reported this phenomenon from the field, including only few ex-situ studies (Ainsworth et al, 2016;Bay & Palumbi, 2015;Brown et al, 2023;Hackerott et al, 2021;Majerova et al, 2021;Martell, 2023;Wall et al, 2023). At the same time, our systematic and multi-species comparison has revealed that the receptiveness to the stress-hardening effects of thermal preconditioning strongly varied between the coral species.…”

“…Acclimatization relies on the adjustments of plastic phenotypic traits that can be induced by the changes in gene expression. These allow organisms to reduce stress responses by being better able to cope with extreme environmental conditions without permanent DNA modifications (DeMerlis et al, 2022;Majerova et al, 2021;Roik et al, 2023;Wall et al, 2023;Ziegler et al, 2017).…”

Trade-offs in a reef-building coral after six years of thermal acclimation

“…We suspect that the thermal history of the parental colonies in the field, as well as the early exposure to elevated temperatures of the offspring, have contributed to the acclimation success of corals in our experiment. Since exposure to thermal variability is a good predictor of high stress-resistance and large plasticity in corals (Hackerott et al, 2021;Rivest et al, 2017;M. Wall et al, 2023), the thermal history of the parents from the Luminao reef flat, which has a large thermal range, could be one explanation, why the offspring was able to acclimate to the new elevated temperature of 31 °C.…”

“…The increasing severity and frequency of deteriorating coral bleaching events (Donner et al, 2005;van Hooidonk et al, 2013) have been driving the development of proactive measures that aim to protect corals from thermal stress (van Oppen et al, 2015). Some of the anticipated approaches consider selection of thermally tolerant coral specimens for reef restoration (Humanes et al, 2021;Morikawa & Palumbi, 2019), while others intend to use thermal preconditioning treatments aiming to improve thermal tolerance of nursery corals (DeMerlis et al, 2022;Hancock et al, 2021;Henley et al, 2022;M. Wall et al, 2023).…”

“…While adaptation through trait selection is a lengthy process that requires generations of organisms to act on, some corals have indeed demonstrated a higher stress resistance compared to others, as well as the capacity to enhance this resistance within a lifetime. This phenomenon has been mostly observed in corals with a history of challenging thermal exposures or experience of highly variable environmental conditions in intertidal reefs, lagoonal reefs, or areas exposed to frequent upwelling (Brown et al, 2002;Buerger et al, 2015;Castillo et al, 2012;Oliver & Palumbi, 2011;M. Wall et al, 2023).…”