Heat stress is an environmental factor that regularly challenges the well-being of living organisms. This study aims to examine the physiological changes happening in two reef-building coral species exposed to thermal stress under various light conditions. The two ecologically relevant heatwave scenarios were applied under ambient lights (high irradiance) and reduced light conditions (250 and < 10 μmol photons m⁻² s⁻¹). Corals were exposed to elevated temperatures of 32 °C (plus 6°) for a period of 1 up to 5 days corresponding to heatwaves reported on the Great Barrier Reef (GBR) that were associated with coral bleaching. We monitored changes in the physiological performance of these two coral species by measuring symbionts and corals’ physiological parameters including symbiont density, levels of multiple algal pigments (chlorophyll a and peridinin), as well as the changes in the host protein concentration. During the short-term heat stress, both species were with stable physiological performance with the only exception of Stylophora pistillata under ambient lights. These results show that S. pistillata was negatively influenced by a synergistic effect of temperature and high irradiance resulting in the first signs of bleaching after only 24h of thermal stress. Exposure to prolonged thermal stress, characterised with a slower rate of temperature increase, affected both coral species investigated and resulted in bleaching mainly by day 5. Interestingly, severe light reduction (<10 μmol photons m⁻² s⁻¹) made Acropora millepora corals more thermally sensitive and resulted in earlier signs of bleaching (on day 3). These findings indicate that there was a synergistic effect of very low lights and thermal stress that caused higher levels of bleaching in A. millepora. Our results revealed differential thermal sensitivity for two branching corals exposed to different thermal stress scenarios under various light irradiance conditions, naturally found in their existing habitats. Consequently, global warming may have a differential impact on coral reef biodiversity depending on light availability.

热应激是一种经常挑战活生物体健康的环境因素。这项研究旨在研究在不同光照条件下暴露于热应力下的两种造礁珊瑚物种发生的生理变化。在环境光（高辐照度）和减少的光照条件下（250和<10μmol光子m ^-2 s ^-1）应用了两种与生态相关的热波方案）。珊瑚暴露于32°C（+ 6°）的高温下长达1至5天，这与大堡礁（GBR）上报道的与珊瑚漂白相关的热浪相对应。我们通过测量共生体和珊瑚的生理参数（包括共生体密度，多种藻类色素（叶绿素a和peridinin）的水平以及宿主蛋白浓度的变化）来监测这两种珊瑚的生理性能变化。在短期热胁迫下，两个物种都具有稳定的生理性能，唯一的例外是Stylophora pistillata在环境光下。这些结果表明S. pistillata温度和高辐照度的协同作用会对温度产生负面影响，仅在热应力作用24小时后就会出现漂白的最初迹象。暴露于长时间的热应激中（以温度升高的速度较慢为特征），影响了所研究的两种珊瑚，主要在第5天就导致了漂白。有趣的是，严重的减光（<10μmol光子m ^-2 s ^-1）使Acropora millepora珊瑚对热更敏感，并导致较早的漂白迹象（第3天）。这些发现表明，非常低的光照和热应力具有协同作用，导致了A. millepora中较高水平的漂白。。我们的研究结果揭示了两种分支珊瑚在不同光照条件下暴露于不同热应力情况下的自然敏感性，它们自然存在于它们的现有栖息地中。因此，取决于光的可获得性，全球变暖可能对珊瑚礁生物多样性有不同的影响。