Energy reserves and symbiont dynamics of Acropora aspera during and after a natural bleaching event in northwest Australia

In 2015-16, a strong El Niño event triggered the most devastating global coral mass bleaching event to date. It also impacted the pristine inshore Kimberley region in northwest Australia which had escaped major bleaching in the past. In this extreme macrotidal region, intertidal (IT) corals are regularly exposed to air and experience strong daily temperature fluctuations (up to 7°C) during spring low tides whereas corals in the subtidal (ST) are typically submerged and experience only moderate temperature changes. Previous work has shown that IT corals have both a higher heat tolerance and recovery capacity than ST corals; however, the physiological mechanisms underlying both heat tolerance and recovery capacity of these corals remain largely unknown. For this study, samples of the common reef-building coral species Acropora aspera were collected from both IT and ST environments during and seven months after the peak bleaching in 2016 and assessed for symbiont density, chlorophyll a (chl a), tissue biomass and energy reserves (lipid, protein and carbohydrate). Despite being exposed to similar heat stress during peak bleaching (~4.6 degree heating weeks), analyses of symbiont density and chl a found that bleaching susceptibility and severity was higher in the subtidal than intertidal. This confirmed findings from published coral health surveys during peak bleaching, suggesting that the coral holobiont was more heat-sensitive in the subtidal than intertidal. Unexpectedly, energy reserves were only catabolized in bleached corals in the intertidal (-41%) but not subtidal corals that suffered extensive mortality. This demonstrates that maintaining energy reserves during bleaching cannot guarantee survival. The preservation of energy reserves in bleached heat-sensitive corals is highly unusual, with one potential explanation being linked to the reproductive cycle of Kimberley corals. The more heatresistant IT corals may have spawned just before the first sampling time point, resulting in depleted energy reserves that were potentially unrelated to heat stress. In contrast, energy reserves in heat-sensitive ST corals may have already been too low for reproduction, thus failure to spawn may have enabled them to maintain moderate levels of energy reserves during bleaching. In addition, other factors such as the (in-) ability to access existing lipid reserves in IT vs. ST corals, habitat-specific fitness trade-offs and differential heterotrophic capacity may also have influenced the different recovery trajectories of Kimberley corals. However, the fast recovery of IT corals gives hope for reef habitats that suffered from extensive mortality during the 2016 bleaching event in the Kimberley and IT corals may thus be considered for proactive reef management such as assisted evolution or translocation. Overall, the findings of this study demonstrate that corals from extreme environments can provide important insights into the mechanisms underlying coral heat tolerance