Coral reefs are increasingly threatened by climate change, mass bleaching events and ocean acidification (OA). Coral calcification, a process that is critical to build and maintain the structure of tropical coral reefs, is highly sensitive to both warming and acidifying oceans. However, in contrast to the impacts of OA on coral calcification, significant knowledge gaps remain regarding how coral biomineralization mechanisms are impacted by heat stress and bleaching. Using a combined physiological and geochemical approach, we investigated how a marine heatwave impacted coral symbiotic status (chlorophyll a , algal symbiont density), the carbonate chemistry of the coral calcifying fluid (via δ 11 B and B/Ca) and skeletal trace element composition in the branching coral Acropora aspera . Importantly, we recorded in situ temperature throughout the bleaching event and recovery as well as coral symbiotic status during peak bleaching and after 7 months of recovery. We show that heat-stressed Acropora corals continued to upregulate the pH of their calcifying fluid (cf); however, dissolved inorganic carbon upregulation inside the cf was significantly disrupted by heat stress. Similarly, we observed suppression of the typical seasonality in the trace element (TE) temperature proxies Sr/Ca, Mg/Ca, Li/Ca and Li/Mg, indicating disruption of important calcification mechanisms, Rayleigh fractionation and reduced growth rates. Anomalies in TE/Ca ratios were still observed 7 months after peak bleaching, even though algal symbiont densities and chlorophyll a concentrations were fully restored at this point. Interestingly, the biomineralization response to heat stress did not differ between thermally distinct reef habitats harbouring coral populations with different heat tolerance, nor between heat-stressed colonies with different severity of bleaching. Our findings suggest that coral biomineralization mechanisms in Acropora are highly sensitive to heat stress, showing similar patterns of biogeochemical stress response as other coral taxa.

中文翻译：

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热应激对造礁珊瑚的关键钙化机制的影响不同

珊瑚礁正日益受到气候变化、大规模白化事件和海洋酸化 (OA) 的威胁。珊瑚钙化是建立和维持热带珊瑚礁结构的关键过程，它对海洋变暖和酸化高度敏感。然而，与 OA 对珊瑚钙化的影响相反，关于珊瑚生物矿化机制如何受到热应激和漂白的影响，仍然存在重大的知识差距。我们使用生理学和地球化学相结合的方法，研究了海洋热浪如何影响珊瑚共生状态（叶绿素 a 、藻类共生体密度）、珊瑚钙化液的碳酸盐化学（通过 δ 11 B 和 B/Ca）和骨骼微量元素组成在分枝珊瑚Acropora aspera中。重要的，我们记录了整个漂白事件和恢复过程中的原位温度，以及漂白高峰期和恢复 7 个月后的珊瑚共生状态。我们表明，热应激的鹿角珊瑚继续上调其钙化液的 pH 值（cf）；然而，热应激显着破坏了 cf 内溶解的无机碳上调。同样，我们观察到微量元素 (TE) 温度代理 Sr/Ca、Mg/Ca、Li/Ca 和 Li/Mg 的典型季节性受到抑制，表明重要的钙化机制、瑞利分馏和生长速率降低。尽管此时藻类共生体密度和叶绿素 a 浓度已完全恢复，但在漂白峰值后 7 个月仍观察到 TE/Ca 比率异常。有趣的是，对热应激的生物矿化反应在具有不同耐热性的珊瑚种群的热不同珊瑚礁栖息地之间没有差异，在具有不同漂白严重程度的热应激群落之间也没有差异。我们的研究结果表明，鹿角珊瑚的珊瑚生物矿化机制对热应激高度敏感，显示出与其他珊瑚类群相似的生物地球化学应激反应模式。