Experimental considerations of acute heat stress assays to quantify coral thermal tolerance

Abstract Thermal tolerance is variable in corals, yet intrinsic and extrinsic drivers of tolerance are not well understood. Understanding the distribution and abundance of heat tolerant corals across seascapes is imperative for predicting responses to climate change and to support novel management actions. Rapid and high-throughput methods to measure heat-induced coral bleaching sensitivity are increasingly required to understand current and predict future responses to climate change. Experimental evaluations of coral heat and bleaching tolerance typically involve ramp-and-hold experiments run across days to weeks within aquarium facilities with limits to colony replication. Field-based acute heat stress assays have emerged as an alternative experimental approach to rapidly quantify heat tolerance in a large number of samples yet the role of key methodological considerations on the stress response measured remains unresolved. Here, we quantify the effects of coral fragment size, sampling time point, and physiological measures on the acute heat stress response in adult corals. The effect of fragment size differed between species (Acropora tenuis and Pocillopora damicornis). Most physiological parameters measured here declined over time (tissue colour, chlorophyll-a and protein content) from the onset of heating, with the exception of maximum photosynthetic efficiency (Fv/Fm), which was stable up to 24h post heating. Based on our experiments, we identified photosynthetic efficiency, tissue colour change, and host-specific assays such as catalase activity as key physiological measures for rapid quantification of thermal tolerance. We recommend that future applications of acute heat stress assays include larger fragments (>9cm2) where possible and sample between 10 - 14h after the end of heat stress. A validated high-throughput experimental approach combined with cost-effective genomic and physiological measurements underpins the development of markers and maps of heat tolerance across seascapes and ocean warming scenarios.