Summary Coral thermal bleaching resilience can be improved by enhancing photosymbiont thermal tolerance via experimental evolution. While successful for some strains, selection under stable temperatures was ineffective at increasing the thermal threshold of an already thermo‐tolerant photosymbiont (Durusdinium trenchii). Corals from environments with fluctuating temperatures tend to have comparatively high heat tolerance. Therefore, we investigated whether exposure to temperature oscillations can raise the upper thermal limit of D. trenchii. We exposed a D. trenchii strain to stable and fluctuating temperature profiles, which varied in oscillation frequency. After 2.1 yr (54–73 generations), we characterised the adaptive responses under the various experimental evolution treatments by constructing thermal performance curves of growth from 21 to 31°C for the heat‐evolved and wild‐type lineages. Additionally, the accumulation of extracellular reactive oxygen species, photophysiology, photosynthesis and respiration rates were assessed under increasing temperatures. Of the fluctuating temperature profiles investigated, selection under the most frequent oscillations (diurnal) induced the greatest widening of D. trenchii's thermal niche. Continuous selection under elevated temperatures induced the only increase in thermal optimum and a degree of generalism. Our findings demonstrate how differing levels of thermal homogeneity during selection drive unique adaptive responses to heat in a coral photosymbiont.

中文翻译：

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突破极限：通过不同的选择机制扩大珊瑚光共生体的温度耐受性


摘要 通过实验进化增强光共生体的热耐受性，可以提高珊瑚的热漂白恢复能力。虽然对某些菌株来说是成​​功的，但稳定温度下的选择对于提高已经耐热的光共生体（Durusdinium Trenchii）的热阈值是无效的。来自温度波动环境的珊瑚往往具有相对较高的耐热性。因此，我们研究了暴露于温度振荡是否可以提高 D. Trenchii 的热上限。我们将 D. Trenchii 菌株暴露于稳定和波动的温度曲线，其振荡频率不同。 2.1 年（54-73 代）后，我们通过构建热进化和野生型谱系从 21 到 31°C 的生长热性能曲线，描述了在各种实验进化处理下的适应性反应。此外，在温度升高的情况下评估了细胞外活性氧的积累、光生理学、光合作用和呼吸速率。在研究的波动温度曲线中，最频繁振荡（昼夜）下的选择导致了 D. Trenchii 热生态位的最大扩展。高温下的连续选择导致了热最适值和一定程度的普遍性的唯一增加。我们的研究结果表明，选择过程中不同水平的热均匀性如何驱动珊瑚光共生体对热的独特适应性反应。