Seawater carbonate chemistry and calcification rates of Porites rus and Hydrolithon onkodes in experiments of Moorea

Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO2 in sea water, which causes declines in carbonate ion concentration [CO3]2- and increases in bicarbonate ion concentration [HCO3]-. To address this topic, we manipulated [CO3]2- and [HCO3]- to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. [CO3]2- played a significant role in light and dark calcification of P. rus, whereas [HCO3]- mainly affected calcification in the light. Both [CO3]2- and [HCO3]- had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with [CO3]2-. Our results show that the negative effect of declining [CO3]2- on the calcification of corals and algae can be partly mitigated by the use of [HCO3]- for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.评估海洋酸化（OA）对珊瑚礁影响的核心，在于厘清海水溶解二氧化碳如何调控钙化作用——该过程会导致海水碳酸根离子浓度[CO₃]²⁻下降，同时升高碳酸氢根离子浓度[HCO₃]⁻。为探究该议题，本研究通过调控[CO₃]²⁻与[HCO₃]⁻水平，测试其对多孔石珊瑚（Porites rus）和翁形石壳藻（Hydrolithon onkodes）钙化作用的影响；实验测量覆盖15天培养周期的全程，同时分别在日间与夜间开展检测。研究发现，[CO₃]²⁻对多孔石珊瑚的光照及黑暗钙化过程均具有显著影响，而[HCO₃]⁻主要作用于光照条件下的钙化过程。对于翁形石壳藻而言，[CO₃]²⁻与[HCO₃]⁻均对其钙化作用产生显著影响，但与[CO₃]²⁻的相关性最强。本研究结果表明，[CO₃]²⁻水平下降对珊瑚与藻类钙化作用的负面影响，可通过利用[HCO₃]⁻进行钙化或光合过程得到部分缓解。上述结果为两类概念模型提供了实证支撑，这些模型可作为后续研究的框架，用以阐释珊瑚与结壳珊瑚藻对海洋酸化的钙化响应机制。