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.

The effects of marine heatwaves on acute heat tolerance in corals

“…At the end of the temperature profiles, corals were maintained at ambient temperature for 11 h prior to physiological measurements. This 11‐h post‐heat stress sampling time point differs from other recent studies that sample immediately after ramping down from heat stress, though it has no effect on measures of photochemical yield (Nielsen et al, 2022 ).…”

Thermal tolerance traits of individual corals are widely distributed across the Great Barrier Reef

“…This may also be due to differences in the timing of measurement between traits as their decline occurs at different rates (Gardner et al, 2017). In A. tenuis, Fv/Fm values were found to be stable 0-24 hours after the end of a similar acute heat stress, while visual pigmentation and chlorophyll decreased up to 24 hours after the end of temperature ramp-down (Nielsen et al, 2022). Therefore delaying the timing of NDVI measurements in our study may have revealed higher divergence in chlorophyll retention between reefs.…”