Seawater carbonate chemistry and weight of two Atlantic corals Favia fragum and Porites astreoides during experiments, 2011

Rising concentrations of atmospheric CO2 are changing the carbonate chemistry of the oceans, a process known as ocean acidification (OA). Absorption of this CO2 by the surface oceans is increasing the amount of total dissolved inorganic carbon (DIC) and bicarbonate ion (HCO3) available for marine calcification yet is simultaneously lowering the seawater pH and carbonate ion concentration ([CO3]), and thus the saturation state of seawater with respect to aragonite. We investigated the relative importance of [HCO3] versus [CO3] for early calcification by new recruits (primary polyps settled from zooxanthellate larvae) of two tropical coral species, Favia fragum and Porites astreoides. The polyps were reared over a range of Oar values, which were manipulated by both acid-addition at constant pCO2 (decreased total [HCO3] and [CO3]) and by pCO2 elevation at constant alkalinity (increased [HCO3], decreased [CO3]). Calcification after 2 weeks was quantified by weighing the complete skeleton (corallite) accreted by each polyp over the course of the experiment. Both species exhibited the same negative response to decreasing [CO3] whether Oar was lowered by acid-addition or by pCO2 elevation--calcification did not follow total DIC or [HCO3]. Nevertheless, the calcification response to decreasing [CO3] was nonlinear. A statistically significant decrease in calcification was only detected between Omega aragonite = <2.5 and Omega aragonite = 1.1-1.5, where calcification of new recruits was reduced by 22-37% per 1.0 decrease in Omega aragonite. Our results differ from many previous studies that report a linear coral calcification response to OA, and from those showing that calcification increases with increasing [HCO3]. Clearly, the coral calcification response to OA is variable and complex. A deeper understanding of the biomineralization mechanisms and environmental conditions underlying these variable responses is needed to support informed predictions about future OA impacts on corals and coral reefs.

大气中二氧化碳浓度持续升高正在改变海洋的碳酸盐化学过程，这一过程被称为海洋酸化（Ocean Acidification, OA）。表层海水吸收此类二氧化碳后，可用于海洋钙化作用的总溶解无机碳（total dissolved inorganic carbon, DIC）与碳酸氢根离子（bicarbonate ion, HCO₃⁻）总量随之增加，但与此同时也会降低海水pH值与碳酸根离子浓度（[CO₃²⁻]），进而降低海水相对于文石的饱和状态。本研究针对两种热带珊瑚物种——蜂巢珊瑚（Favia fragum）与团块滨珊瑚（Porites astreoides）——由携带虫黄藻的幼虫附着形成的新生幼体（初级水螅体），探究了碳酸氢根离子浓度与碳酸根离子浓度对其早期钙化作用的相对重要性。研究人员通过两种方式调控文石饱和状态（Omega aragonite, Ωₐᵣ）：一是在恒定pCO₂条件下添加酸（此时总[HCO₃⁻]与[CO₃²⁻]均降低），二是在恒定碱度条件下提升pCO₂（此时总[HCO₃⁻]升高，[CO₃²⁻]降低），并将水螅体置于一系列Ωₐᵣ值环境中培育。实验持续2周后，通过称量每个水螅体所形成的完整骨骼（珊瑚萼部）的重量，量化其钙化程度。两种珊瑚均表现出对[CO₃²⁻]降低的一致负向响应，无论Ωₐᵣ是通过添加酸还是提升pCO₂实现降低——钙化程度并未随总DIC或[HCO₃⁻]的变化而改变。尽管如此，钙化程度随[CO₃²⁻]降低的响应呈非线性特征：仅当文石饱和状态从Ωₐᵣ<2.5降至Ωₐᵣ=1.1~1.5时，才检测到钙化程度的显著统计学下降，此时新生幼体的钙化程度每降低1.0个Ωₐᵣ单位，便会下降22%~37%。本研究结果与诸多此前报道的珊瑚钙化对OA呈线性响应的研究存在差异，同时也与那些认为钙化程度随[HCO₃⁻]升高而增强的研究结论相悖。显然，珊瑚钙化对海洋酸化的响应模式具有多样性与复杂性。为了能够对未来海洋酸化对珊瑚及珊瑚礁的影响做出合理预测，我们需要更深入地理解驱动这些可变响应的生物矿化机制与环境条件。