Standardized short-term acute heat stress assays resolve historical differences in coral thermotolerance across microhabitat reef sites.

Christian R Voolstra,Anny Cárdenas,Gabriela Perna,Nils Rädecker,Benjamin C C Hume,Daniel J Barshis,Carol Buitrago‐López

doi:10.1111/gcb.15148

Abstract

Coral bleaching is one of the main drivers of reef degradation. Most corals bleach and suffer mortality at just 1-2°C above their maximum monthly mean temperatures, but some species and genotypes resist or recover better than others. Here, we conducted a series of 18-hr short-term acute heat stress assays side-by-side with a 21-day long-term heat stress experiment to assess the ability of both approaches to resolve coral thermotolerance differences reflective of in situ reef temperature thresholds. Using a suite of physiological parameters (photosynthetic efficiency, coral whitening, chlorophyll a, host protein, algal symbiont counts, and algal type association), we assessed bleaching susceptibility of Stylophora pistillata colonies from the windward/exposed and leeward/protected sites of a nearshore coral reef in the central Red Sea, which had previously shown differential mortality during a natural bleaching event. Photosynthetic efficiency was most indicative of the expected higher thermal tolerance in corals from the protected reef site, denoted by an increased retention of dark-adapted maximum quantum yields at higher temperatures. These differences were resolved using both experimental setups, as corroborated by a positive linear relationship, not observed for the other parameters. Notably, short-term acute heat stress assays resolved per-colony (genotype) differences that may have been masked by acclimation effects in the long-term experiment. Using our newly developed portable experimental system termed the Coral Bleaching Automated Stress System (CBASS), we thus highlight the potential of mobile, standardized short-term acute heat stress assays to resolve fine-scale differences in coral thermotolerance. Accordingly, such a system may be suitable for large-scale determination and complement existing approaches to identify resilient genotypes/reefs for downstream experimental examination and prioritization of reef sites for conservation/restoration. Development of such a framework is consistent with the recommendations of the National Academy of Sciences and the Reef Restoration and Adaptation Program committees for new intervention and restoration strategies.

Highlights

Coral bleaching caused by ocean warming is among the major drivers of global coral reef decline (De’ath, 2012; Hughes et al, 2017, 2018), and recent years have witnessed the worst global coral bleaching events on record (Hughes et al, 2017)
We focused on thermal tolerance as it is straightforward to measure in portable short-term heat stress experimental setups that can be deployed across large spatial scales
4.2 | Performance of different physiological measures of experimental heat stress as a proxy for in situ thermal tolerance. Results from both the Coral Bleaching Automated Stress System (CBASS) and CLASSIC experimental setups using dark-adapted maximum quantum yield (Fv/Fm) as a proxy for thermotolerance suggest that corals from the protected site are less photosynthetically stressed at higher temperatures than corals from the exposed site (Figure 3). Both experimental setups showed higher retention of Fv/Fm in corals from the protected versus exposed site at increasing temperatures (Figure 3), suggesting that corals that perform better in short-term acute heat stress assays fare better in long-term heat stress experiments

Summary

Introduction

Coral bleaching caused by ocean warming is among the major drivers of global coral reef decline (De’ath, 2012; Hughes et al, 2017, 2018), and recent years have witnessed the worst global coral bleaching events on record (Hughes et al, 2017). We measured dark-adapted photosynthetic efficiency of coral fragments across different temperatures from the exposed and protected sites and inferred that better retention of Fv/Fm at higher temperatures is representative of higher thermal tolerance (Figure 3).

Results

Conclusion