Pre-exposure to a variable temperature treatment improves the response of Acropora cervicornis to acute thermal stress

Abstract

Given that global warming is the greatest threat to coral reefs, coral restoration projects have expanded worldwide with the goal of replenishing habitats whose reef-building corals succumbed to various stressors. In many cases, however, these efforts will be futile if outplanted corals are unable to withstand warmer oceans and an increased frequency of extreme temperature events. Stress-hardening is one approach proposed to increase the thermal tolerance of coral genotypes currently grown for restoration. Previous studies have shown that corals from environments with natural temperature variability experience less bleaching when exposed to thermal stress, though it remains unclear if this localized acclimatization or adaptation to variable temperatures can be operationalized for enhancing restoration efforts. To evaluate this approach, fragments from six source colonies of nursery-raised Caribbean staghorn coral (Acropora cervicornis) were treated with a variable temperature regime (oscillating twice per day from 28 to 31ºC) or static temperatures (28ºC) in the laboratory for 89 days. Following this, fragments were subjected to a heat-stress assay (32ºC) for two weeks. Corals treated with variable temperatures manifested signs of severe thermal stress later than static temperature laboratory controls as well as untreated field controls collected from the nursery. Furthermore, there was a stark contrast in the physiological response to heat stress, whereby the laboratory and field control groups had a significantly higher incidence of rapid tissue sloughing and necrosis, while the variable temperature-treated corals succumbed to bleaching more gradually. Overall, our data show that pre-acclimation to a variable temperature regime improves acroporid thermotolerance. As corals continue to be outplanted back onto Florida's changing reef scape, understanding the molecular mechanisms underlying this enhanced thermal tolerance and its endurance in situ will be critical for future research and restoration applications.

Expression plasticity regulates intraspecific variation in the acclimatization potential of a reef-building coral

“…A significant positive change in thermal tolerance upon preexposure was found in two of our profiles (constant high and pulse) but was absent in the others (pulse high and pulse increase). Temperature variability can confer higher thermal tolerance on corals when compared to constant pre-exposures 42,43 , but this effect is not universal 32,33,44 and our results show substantial complexity even within variable temperature regimes, although hierarchical clustering suggests that constant exposures create a discrete expression response.…”

“…However, a few experiments have used moderate recovery periods on the order of 2 weeks and found that persistent gene expression changes explained improved thermal tolerance 29,34 , particularly in apoptosis and celldeath pathways. The relative importance of temperature variability is also disputed; regimes with more variation can confer higher thermal tolerance on corals than constant pre-exposures 42,43 , but this effect is not universal 32,33,44 .…”

Restoration and coral adaptation delay, but do not prevent, climate-driven reef framework erosion of an inshore site in the Florida Keys

“…Under SSP5-8.5 coupled with 2˚C adaptation, RAP max projections keep pace with SLR projections for an additional 17 years when restoration interventions are applied. Although global-scale action on climate change is critical for ensuring reef persistence, bold active restoration initiatives that incorporate stress hardening approaches to increase thermotolerance of corals 49 may create the necessary bridge to sustain the reef structure long enough for a low-carbon economy to become effective.…”

Rapid shifts in thermal reaction norms and tolerance of brooded coral larvae following parental heat acclimation

“…Despite the consensus that the genetic adaptation of reef corals might not keep pace with the current rate of ocean warming due to their long generation time, it has been increasingly recognized that acclimatization through phenotypic plasticity may to some extent buffer against the immediate impacts of ocean warming on reef corals (Hackerott et al, 2021; Torda et al, 2017; Voolstra et al, 2021). Several studies demonstrated that thermal priming and environmental hardening can enhance the acclimatization and resistance of reef corals to thermal stress (Brown et al, 2015; DeMerlis et al, 2022; Hackerott et al, 2021; Yu et al, 2020), either through symbiont shuffling or transcriptional reprogramming (Barshis et al, 2013; Kenkel & Matz, 2016; Silverstein et al, 2015). Furthermore, numerous recent studies showed that adult preconditioning can mitigate and even fully offset the negative effects of climate change stressors on their offspring in a wide range of marine taxa, suggesting that transgenerational and/or developmental plasticity may also serve as key acclimatory mechanisms by which marine organisms can persist under climate change (Donelson et al, 2018; Maboloc & Chan, 2021; Wong et al, 2018; Yin et al, 2019).…”

“…Several studies demonstrated that thermal priming and environmental hardening can enhance the acclimatization and resistance of reef corals to thermal stress (Brown et al, 2015;DeMerlis et al, 2022;Hackerott et al, 2021;Yu et al, 2020), either through symbiont shuffling or transcriptional reprogramming (Barshis et al, 2013;Kenkel & Matz, 2016;Silverstein et al, 2015). Furthermore, numerous recent studies showed that adult preconditioning can mitigate and even fully offset the negative effects of climate change stressors on their offspring in a wide range of marine taxa, suggesting that transgenerational and/or developmental plasticity may also serve as key acclimatory mechanisms by which marine organisms can persist under climate change (Donelson et al, 2018;Maboloc & Chan, 2021;Wong et al, 2018;Yin et al, 2019).…”