Thermal preconditioning modulates coral physiology and heat tolerance: A multi-species perspective
Posted on: 14 January 2025
Preprint posted on 22 July 2024
Beat the bleach: Thermal preconditioning modulates coral physiology and heat tolerance
Selected by Jasmine TaleviCategories: physiology
Background
Coral bleaching is a frequently used example to illustrate the severity of climate change. Coral bleaching can occur when the water becomes too warm for corals to tolerate, leading them to expel the microalgae living inside them. This results in corals losing their colour and turning white – termed “bleaching”. This also results in a loss of fuel for the corals as they rely on the microalgae’s photosynthetic abilities for food, eventually leading to death if they are unable to recover.
To continue thriving under changing environmental conditions, corals rely on adaptation whereby beneficial genes are continually passed down to the next generation, gradually increasing the frequency of this gene throughout the population. However, these shifts in genetic variation require a considerable period of time, which is currently being outpaced by the rapid environmental changes. Acclimatization, whereby an organism can change their physiology or behaviour in response to their environment, may instead provide a better mechanism to help corals cope with heat stress.
Acclimatization to heat stress can be initiated by pre-exposing corals to warm water, essentially “priming” their heat stress response for subsequent environmental challenges. This approached is termed “stress-hardening” and aims to accelerate the acclimatization of corals to future warming. The ideal pre-exposure conditions (ex. absolute temperature and duration) can vary between species and ultimately determine if this stress-hardening approach will help or hinder their thermal tolerance. To understand how preconditioning can influence the thermal tolerance of coral, Ferrara and collaborators selected six reef-building corals for this study. The corals were either preconditioned to “stable-high” (29 ºC), “variable-high” (27.5 – 30.5 ºC), or “stable-ambient” (26 ºC) conditions for >18 days, then exposed to an acute heat stress test and allowed to recover for 30 days. Several physiological characteristics were monitored throughout the experiment, including photosynthetic efficiency (ΔF/Fm’), tissue colour intensity, and survival rate (%) to assess how the different preconditioning regimes affected their baseline physiology, heat stress tolerance, long-term recovery, and to identify which regime was most effective
Key Findings
1) Stress hardening enhanced coral heat tolerance.
The authors found that thermal preconditioning can be applied to stress harden corals and enhance their ability to cope with an acute heat stress. It was observed that corals previously exposed to the “stable-high” and “variable-high” preconditioning regimes had consistently higher survival than the “stable-ambient” regime (Fig. 1). Between the two thermal stress conditioning regimes, the authors observed that the “variable-high” regime excelled over the “stable-high” treatment, which suggested that fluctuating conditions may promote better energy usage and recovery mechanisms.
2) The receptiveness to stress-hardening varied across coral species
The authors observed that thermal preconditioning was more effective for some species and less effective for others. For example, it was observed that the Octopus coral (Galaxea fascicularis) and staghorn coral (Acropora muricata) were most receptive to the stress-hardening effect of thermal precondition as the changes in photosynthetic efficiency and tissue colour were reduced in the “stable-high” and “variable- high” precondition treatments (Fig. 1). Alternatively, the hood coral (Stylophora pistillata) showed large declines in photosynthetic efficiency and tissue colour in all preconditioning treatments and were thus termed “non-receptive” to the stress-hardening approach (Fig. 1).
3) Thermal precondition regimes can shift coral baseline physiology
The authors observed that five of the six tested coral species preconditioned under “stable-high” temperature (ST, 29 °C) and “variable-high” temperature (VT, 29 ± 1.5 °C) had significantly lower photosynthetic efficiency and/or tissue color intensity compared to corals in the “stable-ambient” temperature regime (Fig. 1). The authors explain that this could signify that there has been a decrease in symbiotic microalgae quantity. While this acclimatization change may seem disadvantageous, having less microalgae could actually decrease the production of hazardous molecules during stress, such as reactive oxygen species, which may aid in resistance and resilience to heat stress.
Fig. 1. Coral survival rates and recovery following heat stress. Kaplan-Meier plots show changes in coral survival rates under ST, VT, and Ambient preconditioning regimes (A). The trajectories of effective quantum yield (ΔF/Fm’) (B) and tissue color intensity after the heat stress assays (C) for corals from the same preconditioning regimes reflect on the recovery of corals. Lines connect mean values (solid lines for heat treatment, dashed lines for control treatment), raw data points are included. These recovery lines are based on data from a constant sample size within each “population” of each preconditioning regime. The effective quantum yield of dead coral fragments was recorded as 0, while tissue color intensity was scored as 30 (the lowest score observed after the heat stress assay). Physiological parameters of alive P. verrucosa fragments were not scored due to uncertainties introduced by algal growth on necrotic tissue, which started dominating the corals between day 0 and 15.
Why I Highlighted This Preprint
I chose to highlight this preprint to share an encouraging finding amidst the despair of climate change. Being in the field of marine biology and physiology, I have repeatedly heard about the extensive coral bleaching events, which is often disheartening. This paper highlights new research to help improve the resilience of corals, with strategies that are very implementable. Additionally, I think this preprint is very well written and the results are presented in a logical order.
Future Directions
The continuation of this work could include replicating this study in the field, where these corals would be preconditioned in the laboratory, then planted in the field and monitored for survival, photosynthetic efficiency, and tissue colour over the duration of a year, or one summer season to capture the warmest period of the year.
Questions for Author
1) In the wild do these species of coral typically experience conditions similar to the stable ambient, stable high, or variable-high thermal regimes? Do you think the corals in the wild experience some form of pre-conditioning/stress-hardening?
2) For the species that did not respond to stress hardening, do you anticipate that another preconditioning regime could increase their thermal tolerance?
doi: https://doi.org/10.1242/prelights.39407
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