Remarkably high and consistent tolerance of a Red Sea coral to acute and chronic thermal stress exposures

Abstract

Global warming is resulting in unprecedented levels of coral mortality due to mass bleaching events and, more recently, marine heatwaves, where rapid increases in seawater temperature cause mortality within days. Here, we compare the response of a ubiquitous scleractinian coral, Stylophora pistillata, from the northern Red Sea to acute (7 h) and chronic (7-11 d) thermal stress events that include temperature treatments of 27 C (i.e., the local maximum monthly mean), 29.5 C, 32 C, and 34.5 C, and assess recovery of the corals following exposure. Overall, S. pistillata exhibited remarkably similar responses to acute and chronic thermal stress, responding primarily to the temperature treatment rather than duration or heating rate. Additionally, corals displayed an exceptionally high thermal tolerance, maintaining their physiological performance and suffering little to no loss of algal symbionts or chlorophyll a up to 32 C, before the host suffered from rapid tissue necrosis and mortality at 34.5 C. While there was some variability in physiological response metrics, photosynthetic efficiency measurements (i.e., maximum quantum yield Fv/Fm) accurately reflected the overall physiological response patterns, with these measurements used to produce the Fv/Fm effective dose (ED 50) metric as a proxy for the thermal tolerance of corals. This approach produced similar ED 50 values for the acute and chronic experiments (34.47 C vs. 33.81 C), highlighting the potential for acute thermal assays with measurements of Fv/Fm as a systematic and standardized approach to quantitively compare the upper thermal limits of reef-building corals using a portable experimental system.

Census of heat tolerance among Florida's threatened staghorn corals finds resilient individuals throughout existing nursery populations

“…Fragments of each coral colony were independently exposed to each of eight temperature stress profiles of increasing magnitude (with maximum temperatures between 30 and 38°C; figure 1b) for 7 h, after which maximum photochemical efficiency (F v /F m ) was measured as an indicator of each fragment's stress response. These data were used to construct a dose-response curve for each colony, from which the effective dose of heat stress required to reduce F v /F m by 50% (ED50 value; [41]) was calculated as a metric of each colony's thermal tolerance (figure 1c; details below).…”

“…Specifically, sets of portable experimental tanks termed coral bleaching automated stress systems (CBASS; [28]), can facilitate the application of standardized, rapid thermal tolerance assays on corals in remote field settings. Importantly, these rapid, acute assays can recapitulate outcomes of longer-term, ecologically relevant bleaching scenarios [24,41], indicating they can be used to generate meaningful, quantitative and comparable metrics of thermal tolerance. Such efforts are now beginning to be applied to broadly census thermal tolerance in wild coral populations [42], from which the identification of tolerant individuals could inform targeted nursery development [13], and other active conservation interventions.…”

Relative abundance of nitrogen cycling microbes in coral holobionts reflects environmental nitrate availability

“…The notion of seasonally changing N availability driving patterns in ratios of prokaryotic N cycling functional groups within coral holobionts is corroborated by the patterns of Symbiodiniaceae cell densities observed in both coral species. Similar to nirS to nifH gene abundance ratios, Symbiodiniaceae cell densities exhibited strong seasonal differences, comparable to Symbiodiniaceae cell densities of conspecifics originating from the Red Sea [ 21 , 40 , 42 , 43 ], that positively correlated with environmental nitrate and DIN concentrations ( figure 3 b , c ; electronic supplementary material, table S3). As the coral holobionts’ internal N concentrations were not measured in the present study, it is unknown whether environmental N was reflected by bioavailable N availability within the holobionts.…”

Extending the natural adaptive capacity of coral holobionts

“…Platforms for screening large numbers of individuals for increased thermotolerance have been developed in the form of standardized, mobile, and inexpensive acute heat stress assays, such as the Coral Bleaching Automated Stress System (CBASS) 41,44,49,92,93 . The underlying premise is that corals that exhibit increased thermal tolerance in acute assays are also more resistant and/or resilient during natural heat stress events 41,44 .…”