Coral calcification under daily oxygen saturation and pH dynamics reveals the important role of oxygen@en

Coral reefs are essential to many nations, and are currently in global decline. Although climate models predict decreases in seawater pH (0.3 units) and oxygen saturation (5 percentage points), these are exceeded by the current daily pH and oxygen fluctuations on many reefs (pH 7.8-8.7 and 27-241% O2 saturation). We investigated the effect of oxygen and pH fluctuations on coral calcification in the laboratory using the model species Acropora millepora. Light calcification rates were greatly enhanced (+178%) by increased seawater pH, but only at normoxia; hyperoxia completely negated this positive effect. Dark calcification rates were significantly inhibited (51-75%) at hypoxia, whereas pH had no effect. Our preliminary results suggest that within the current oxygen and pH range, oxygen has substantial control over coral growth, whereas the role of pH is limited. This has implications for reef formation in this era of rapid climate change, which is accompanied by a decrease in seawater oxygen saturation owing to higher water temperatures and coastal eutrophication.珊瑚礁对诸多国家至关重要，目前却正经历全球性衰退。尽管气候模型预测海水pH值将下降0.3个单位、氧饱和度将下降5个百分点，但许多珊瑚礁当前的每日pH与氧波动幅度已远超这一预测：波动范围分别为pH 7.8~8.7以及27%~241%的氧饱和度。本研究以模式物种微孔轴孔珊瑚（Acropora millepora）为实验材料，在实验室环境下探究了氧与pH波动对珊瑚钙化作用的影响。实验结果显示，在常氧条件下，海水pH升高可使光照环境下的钙化速率大幅提升（增幅达178%），但高氧环境会完全抵消这一正向效应；而在低氧条件下，黑暗环境下的钙化速率会受到显著抑制（抑制幅度为51%~75%），此时pH则无明显影响。本研究的初步结果表明，在当前的氧与pH波动范围内，氧浓度对珊瑚生长具有显著的调控作用，而pH的调控作用相对有限。这一发现对于快速气候变化时代的珊瑚礁形成研究具有重要启示意义——当前气候变化伴随海水温度升高与沿岸富营养化，进而导致海水氧饱和度下降。