Seawater carbonate chemistry during experiments with Stylophora pistillata, 2008@en

The decrease in the saturation state of seawater, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO32, HCO3, CO2), tanks were also maintained under similar pH, but with 2-mM HCO3 added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO2. Seawater acidification decreased coral calcification by ca. 0.1-mg CaCO3 g-1 d-1 for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO32 substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra- or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO32 from HCO3.海水酸化引发的海水饱和状态下降，被认为是导致海洋生物钙化作用降低的核心因素。为阐释该现象的生理学机制，本研究针对石珊瑚目热带珊瑚（scleractinian tropical coral）花柱珊瑚（Stylophora pistillata）的钙化与光合过程，探究了海水酸化的影响。将珊瑚断枝置于三种不同pH（7.6、8.0及8.2）的海水中恒温培养8天。为区分碳酸盐化学各组分（pH、碳酸根CO₃²⁻、碳酸氢根HCO₃⁻、二氧化碳CO₂）的独立效应，本研究还设置了pH一致但向海水中添加2mM碳酸氢盐（HCO₃⁻）的处理组。添加2mM碳酸氢盐可显著提升花柱珊瑚的光合速率，表明其生长环境原本处于碳限制状态。与之相反，光合过程对pH与二氧化碳分压（pCO₂）的变化并无响应。海水酸化每使pH降低0.1单位，珊瑚钙化量便减少约0.1mg CaCO₃·g⁻¹·d⁻¹。该相关性表明，海水酸化通过降低钙化过程所需的碳酸根底物可用性，对珊瑚钙化产生抑制作用。然而，珊瑚钙化量的下降也可能归因于胞外或胞内pH降低，或是培养基缓冲能力发生改变，进而阻碍了碳酸氢盐向碳酸根的转化供应。