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Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of

18Marine heat waves instigated by anthropogenic climate change are causing increasingly frequent 19 and severe coral bleaching events that often lead to widespread coral mortality. While 20 community-wide increases in coral mortality following bleaching events have been documented 21 on reefs around the world, the ecological consequences for conspecific individual colonies 22 exhibiting contrasting phenotypes during thermal stress (e.g. bleached vs. not bleached) are not 23 well understood. Here we describe the ecological outcomes of colonies of the two dominant reef-24 building coral species in Kāne ohe Bay, Hawai i, Montipora capitata and Porites compressa, 25 that exhibited either a bleaching susceptible phenotype (bleached) or resistant phenotype (non-26 bleached) following the second of two consecutive coral bleaching events in Hawai i in 2015. 27Conspecific pairs of adjacent bleaching susceptible vs. resistant corals were tagged on patch 28 reefs in two regions of Kāne ohe Bay with different seawater residence times and terrestrial 29 influence. The ecological consequences (symbiont recovery and mortality) were monitored for 30 two years following the peak of the bleaching event. Bleaching susceptible corals suffered higher 31 partial mortality than bleaching resistant corals of the same species in the first 6 months 32 following thermal stress. Surprisingly, P. compressa had greater resilience following bleaching 33 (faster pigment recovery and lower post-bleaching mortality) than M. capitata, despite having 34 less resistance to bleaching (higher bleaching prevalence and severity). These differences 35 indicate that bleaching susceptibility of a species is not always a good predictor of mortality 36 following a bleaching event. By tracking the fate of individual colonies of resistant and 37 susceptible phenotypes, contrasting ecological consequences of thermal stress were revealed that 38 were undetectable at the population level. Furthermore, this approach revealed individuals that 39 underwent particularly rapid recovery from mortality, including some colonies over a meter in 40 diameter that recovered all live tissue cover from >60% partial mortality within just one year. 41These coral pairs continue to be maintained and monitored in the field, serving as a "living 42 library" for future investigations on the ecology and physiology of coral bleaching. 43 44 45 46 102 2008; Bahr et al., 2017; Hughes et al., 2017). Kāne ohe Bay, Hawai i, located on the northeast 103 coast of O ahu, is an opportune system for investigating this question of how intraspecific 104 variability in coral responses to thermal bleaching events driven by climate change influence 105 coral survival. The two dominant reef-building coral species in the bay, Montipora capitata and 106Porites compressa, both exhibit differences in thermal performance within and between species 107 during bleaching (Grottoli et al., 2006; Cunning et al., 2016; Wall et al., 2019). Differences in 108 5 symbiont associations and nutritional plasticity a...

Urgent action is needed to prevent the demise of coral reefs as the climate crisis leads to an increasingly warmer and more acidic ocean. Propagating climate change–resistant corals to restore degraded reefs is one promising strategy; however, empirical evidence is needed to determine whether stress resistance is affected by transplantation beyond a coral’s native reef. Here, we assessed the performance of bleaching-resistant individuals of two coral species following reciprocal transplantation between reefs with distinct pH, salinity, dissolved oxygen, sedimentation, and flow dynamics to determine whether heat stress response is altered following coral exposure to novel physicochemical conditions in situ. Critically, transplantation had no influence on coral heat stress responses, indicating that this trait was relatively fixed. In contrast, growth was highly plastic, and native performance was not predictive of performance in the novel environment. Coral metabolic rates and overall fitness were higher at the reef with higher flow, salinity, sedimentation, and diel fluctuations of pH and dissolved oxygen, and did not differ between native and cross-transplanted corals, indicating acclimatization via plasticity within just 3 mo. Conversely, cross-transplants at the second reef had higher fitness than native corals, thus increasing the fitness potential of the recipient population. This experiment was conducted during a nonbleaching year, so the potential benefits to recipient population fitness are likely enhanced during bleaching years. In summary, this study demonstrates that outplanting bleaching-resistant corals is a promising tool for elevating the resistance of coral populations to ocean warming.

Urgent action is needed to prevent the demise of coral reefs as the climate crisis leads to an increasingly warmer and more acidic ocean. Propagating climate change resistant corals to restore degraded reefs is one promising strategy; however, empirical evidence is needed to determine if resistance is retained following transplantation within or beyond a coral’s natal reef. Here we assessed the performance of bleaching-resistant individuals of two coral species following reciprocal transplantation between environmentally distinct reefs (low vs high diel variability) to determine if stress resistance is retained following transplantation. Critically, transplantation to either environment had no influence on coral bleaching resistance, indicating that this trait was relatively fixed and is thus a useful metric for selecting corals for reef restoration within their native range. In contrast, growth was highly plastic, and native performance was not predictive of performance in the novel environment. Coral metabolism was also plastic, with cross transplants of both species matching the performance of native corals at both reefs within three months. Coral physiology (autotrophy, heterotrophy, and metabolism) and overall fitness (survival, growth, and reproduction) were higher at the reef with higher flow and fluctuations in diel pH and dissolved oxygen, and did not differ between native corals and cross-transplants. Conversely, cross-transplants at the low-variability reef had higher fitness than native corals, thus increasing overall fitness of the recipient population. This experiment was conducted during a non-bleaching year, which suggests that introduction of these bleaching-resistant individuals will provide even greater fitness benefits to recipient populations during bleaching years. In summary, this study demonstrates that propagating and transplanting bleaching-resistant corals can elevate the resistance of coral populations to ocean warming while simultaneously maintaining reef function as the climate crisis worsens.

Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of bleaching have been well documented, the consequences of heat stress for bleaching resistant individuals are not well understood. In addition, much remains to be learned about how heat stress affects cellular level processes that may be overlooked at the organismal level, yet are crucial for coral performance in the short term and ecological success over the long term. Here we compared the physiological and cellular responses of bleaching resistant and bleaching susceptible corals throughout the 2019 marine heatwave in Hawaii, a repeat bleaching event that occurred four years after the previous regional event. Relative bleaching susceptibility within species was consistent between the two bleaching events, yet corals of both resistant and susceptible phenotypes exhibited pronounced metabolic depression during the heatwave. At the cellular level, bleaching susceptible corals had lower intracellular pH than bleaching resistant corals at the peak of bleaching for both symbiont-hosting and symbiont-free cells, indicating greater disruption of acid-base homeostasis in bleaching susceptible individuals. Notably, cells from both phenotypes were unable to compensate for experimentally induced cellular acidosis, indicating that acid-base regulation was significantly impaired at the cellular level even in bleaching resistant corals and in cells containing symbionts. Thermal disturbances may thus have substantial ecological consequences, as even small reallocations in energy budgets to maintain homeostasis during stress can negatively affect fitness. These results suggest concern is warranted for corals coping with ocean acidification alongside ocean warming, as the feedback between temperature stress and acid-base regulation may further exacerbate the physiological effects of climate change.

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