Seawater carbonate chemistry and calcification during experiments with a coral Madracis auretenra, 2010@en

Physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO2 for calcification, not seawater carbonate. However, a number of investigators are attributing observed negative effects of experimental seawater acidification by CO2 or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state. Thus, there is a discrepancy between the physiological and geochemical views of coral biomineralization. Furthermore, not all calcifying organisms respond negatively to decreased pH or saturation state. Together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification. To distinguish the effects of pH, carbonate concentration and bicarbonate concentration on coral calcification, incubations were performed with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries. Carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries. Among treatment differences were highly significant. The corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH. Corals calcified at normal or elevated rates under low pH (7.6-7.8) when the seawater bicarbonate concentrations were above 1800 µm. Conversely, corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered. These results demonstrate that coral responses to ocean acidification are more diverse than currently thought, and question the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.珊瑚钙化（coral calcification）的生理数据与模型表明，珊瑚并非依赖海水碳酸盐（seawater carbonate），而是通过结合海水碳酸氢盐（seawater bicarbonate）与（主要为）呼吸源二氧化碳（respiratory CO₂）完成钙化过程。然而，诸多研究者将通过添加CO₂或盐酸实现的实验性海水酸化（experimental seawater acidification）所观测到的负面影响，归因于海水碳酸根离子浓度（seawater carbonate ion concentration）降低及随之而来的文石饱和度（aragonite saturation state）下降。这便在珊瑚生物矿化（coral biomineralization）的生理学与地球化学视角之间形成了认知分歧。 此外，并非所有钙化生物（calcifying organisms）都会随pH或饱和度降低产生负面响应。综合上述分歧来看，其他生理机制（physiological mechanisms）或许是不同酸化实验对钙化影响存在报道差异的原因——例如pH降低对钙或碳酸氢根离子转运（calcium or bicarbonate ion transport）的直接作用，以及生物调控内部pH（internal pH）的能力差异。 为厘清pH、碳酸根浓度与碳酸氢根浓度对珊瑚钙化的独立影响，本研究以金黄鹿角珊瑚（Madracis auretenra，即Wells 1973年定义的奇异鹿角珊瑚（Madracis mirabilis sensu Wells, 1973））为实验材料，在改良海水化学组成（modified seawater chemistries）体系中开展了培养实验（incubations）。相较于既往研究，本研究通过调控碳酸盐参数（carbonate parameters），更有效地分离了各参数的独立影响，共设置六种不同的海水化学体系。各处理组间差异（treatment differences）极显著：珊瑚对碳酸氢根浓度变化响应强烈，但对碳酸根浓度、文石饱和度或pH的响应并不稳定。当海水碳酸氢根浓度高于1800 µm时，即便处于低pH（7.6~7.8）环境，珊瑚仍能维持正常或升高的钙化速率（calcification rates）；反之，若碳酸氢根浓度降低，即便处于正常pH环境，珊瑚的钙化速率也会下降。 本研究结果表明，珊瑚对海洋酸化（ocean acidification）的响应比当前学界认知更为多样，同时也对“以碳酸根浓度或文石饱和度作为海洋酸化影响珊瑚钙化的唯一预测因子（sole predictor）”这一做法的可靠性提出了质疑。