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.大气二氧化碳（CO₂）浓度持续升高正改变海洋碳酸盐化学体系，该过程被定义为海洋酸化（OA）。表层海水吸收此类CO₂后，可供海洋钙化作用利用的总溶解无机碳（DIC）与碳酸氢根离子（HCO₃⁻）总量持续增加，但与此同时却会降低海水pH值与碳酸根离子浓度（[CO₃²⁻]），进而削弱海水文石饱和状态。本研究以两种热带珊瑚物种——蜂巢珊瑚（Favia fragum）与团块滨珊瑚（Porites astreoides）——的新招募个体（即由虫黄藻共生幼虫沉降形成的原初水螅体）为研究对象，探究了碳酸氢根离子（HCO₃⁻）与碳酸根离子（[CO₃²⁻]）对其早期钙化过程的相对重要性。实验通过两种手段调控文石饱和状态（Ω_ar）：其一为在恒定二氧化碳分压（pCO₂）条件下添加酸液，该操作会同时降低总溶解无机碳（DIC）与碳酸根离子（[CO₃²⁻]）浓度；其二为在恒定总碱度条件下升高pCO₂，该操作会提升碳酸氢根离子（HCO₃⁻）浓度并降低碳酸根离子（[CO₃²⁻]）浓度。实验周期为2周，通过称量每个原初水螅体所形成的完整骨骼（珊瑚杯）质量，量化其钙化程度。结果显示，无论通过添加酸液还是升高pCO₂来降低文石饱和状态，两种珊瑚均表现出一致的负向响应——钙化速率并未随总溶解无机碳（DIC）或碳酸氢根离子（HCO₃⁻）浓度变化产生对应改变。尽管如此，钙化速率对碳酸根离子（[CO₃²⁻]）降低的响应并非线性。仅当文石饱和状态（Ω_ar）从≤2.5降至1.1~1.5区间时，才检测到钙化速率存在统计学意义上的显著下降，此时新招募水螅体的钙化速率随Ω_ar每降低1.0，便会下降22%~37%。本研究结果与诸多既往研究存在显著差异：部分既往研究报道了珊瑚钙化对海洋酸化（OA）的线性响应，另有研究则发现钙化速率随碳酸氢根离子（HCO₃⁻）浓度升高而提升。显而易见，珊瑚钙化对海洋酸化（OA）的响应具有多变性与复杂性。为能够基于科学依据预测未来海洋酸化（OA）对珊瑚及珊瑚礁生态系统的影响，我们亟需更深入地阐明驱动这些可变响应的生物矿化机制与环境调控条件。