# Acute Heat Priming Dampens Gene Expression Response to Thermal Stress in a Widespread Acropora Coral

## TL;DR

Coral corals exposed to short-term heat stress showed reduced gene expression responses to subsequent higher heat stress, suggesting a form of physiological resilience.

## Contribution

The study reveals that prior sub-bleaching heat exposure dampens the transcriptional response to subsequent thermal stress in corals.

## Key findings

- Primed corals showed a 28% decline in differentially expressed genes compared to naïve corals.
- Transcriptional responses in primed corals returned to baseline after recovery, with no visible physiological stress.
- Symbiotic partners also showed a dampened response in primed corals despite no decline in photosynthetic performance.

## Abstract

Physiological plasticity is fundamental for resisting environmental change. As climate change accelerates and environmental stressors become more frequent, understanding how habitat‐forming species shift their physiology to match their environment is essential for predicting broader ecosystem responses. In this study, we examined whether prior exposure to sub‐bleaching heat stress influenced the gene expression responses to a subsequent thermal challenge in a common reef‐building coral. We primed Acropora corals from the World Heritage‐listed Ningaloo Reef (WHNR) to acute (24 h) sub‐bleaching temperatures (+5°C from the mean monthly maximum MMM, 32°C) before subjecting them to a more intense thermal challenge (+6°C from MMM, 33°C), and assessed the physiological and transcriptional responses in both naïve (no prior preconditioning) and primed corals compared to controls. Both groups mounted large gene expression responses to heat stress (33°C), which returned to baseline after a recovery period (16 h) at control temperatures (27°C, MMM), with no visible signs of physiological stress. However, primed corals showed a dampened stress response relative to naïve corals, marked by a 28% decline in differentially expressed genes and an overall reduction in intensity of expression of those genes compared to controls. Similar patterns were observed in the symbiotic partners, which showed a dampened response within the primed corals compared to the controls, despite no detectable declines in photosynthetic performance within either treatment. Our results show that short‐term preconditioning of corals is associated with transcriptional dampening of key stress response genes, and that corals are capable of rapid transcriptional recovery and resilience to recurrent heat stress.

(a) Experimental design. Corals from 10 genotypes were distributed across two experimental blocks, each containing nine flow‐through tanks. Fragments from five genotypes were placed in each tank. (b) Temperature profiles and sampling time points in the heat stress assay, demonstrating ramp up from control conditions (27ºC, MMM) to the preconditioning treatment (33ºC) and the thermal challenge treatment (34ºC). Naïve treatment is shown in orange, the primed treatment in red and the control in blue. Sampling time points are indicated with dashed vertical lines: ‘T1’ represents the start of the thermal challenge, ‘T2’ represents the end of the heating hold and ‘T3’ represents the end of the recovery period. A single replicate tank of each temperature treatment (containing 5 colony fragments) from each experimental block (n = 2) was sampled at each time point.