Seawater carbonate chemistry and biological processes during experiments with coral Oculina arbuscula, 2010

Anthropogenic elevation of atmospheric pCO2 is predicted to cause the pH of surface seawater to decline by 0.3-0.4 units by 2100 AD, causing a 50% reduction in seawater [CO3] and undersaturation with respect to aragonite in high-latitude surface waters. We investigated the impact of CO2-induced ocean acidification on the temperate scleractinian coral Oculina arbuscula by rearing colonies for 60 days in experimental seawaters bubbled with air-CO2 gas mixtures of 409, 606, 903, and 2,856 ppm pCO2, yielding average aragonite saturation states (Omega aragonite) of 2.6, 2.3, 1.6, and 0.8. Measurement of calcification (via buoyant weighing) and linear extension (relative to a 137Ba/138Ba spike) revealed that skeletal accretion was only minimally impaired by reductions in Omega aragonite from 2.6 to 1.6, although major reductions were observed at 0.8 (undersaturation). Notably, the corals continued accreting new skeletal material even in undersaturated conditions, although at reduced rates. Correlation between rates of linear extension and calcification suggests that reduced calcification under Omega aragonite = 0.8 resulted from reduced aragonite accretion, rather than from localized dissolution. Accretion of pure aragonite under each Omega aragonite discounts the possibility that these corals will begin producing calcite, a less soluble form of CaCO3, as the oceans acidify. The corals' nonlinear response to reduced Omega aragonite and their ability to accrete new skeletal material in undersaturated conditions suggest that they strongly control the biomineralization process. However, our data suggest that a threshold seawater [CO3] exists, below which calcification within this species (and possibly others) becomes impaired. Indeed, the strong negative response of O. arbuscula to Omega aragonite= 0.8 indicates that their response to future pCO2-induced ocean acidification could be both abrupt and severe once the critical Omega aragoniteis reached.

人为活动导致的大气二氧化碳分压（pCO2）升高预计将使2100年的表层海水pH下降0.3至0.4个单位，造成海水中碳酸根离子([CO₃²⁻])浓度降低50%，并使高纬度表层海水出现文石（aragonite）饱和状态不足的现象。本研究将温带石珊瑚杯形珊瑚（Oculina arbuscula）的群落置于充有409、606、903及2856 ppm pCO2的空气-二氧化碳混合气的实验海水中培育60天，以此探究二氧化碳诱导的海洋酸化对该物种的影响，对应实验海水的平均文石饱和状态（Omega aragonite）分别为2.6、2.3、1.6及0.8。通过浮力称重法（buoyant weighing）测定钙化速率，并以¹³⁷Ba/¹³⁸Ba同位素尖峰（¹³⁷Ba/¹³⁸Ba spike）作为参照测定线性延伸速率后，研究人员发现：当文石饱和状态从2.6降至1.6时，骨骼沉积仅受到轻微抑制；而当饱和状态降至0.8时，则出现显著的钙化速率下降。值得注意的是，即使在文石饱和不足的条件下，该珊瑚仍能持续沉积新的骨骼物质，只是速率有所降低。线性延伸速率与钙化速率的相关性表明，当文石饱和状态为0.8时，钙化速率降低是由文石沉积减少所致，而非局部骨骼溶解。在各文石饱和状态下均仅沉积纯文石这一结果，排除了海洋酸化时该珊瑚会转而沉积溶解度更低的碳酸钙（CaCO3）同质异形体方解石（calcite）的可能性。该珊瑚对文石饱和状态降低的非线性响应，以及在饱和不足条件下仍能沉积骨骼的能力，表明其对生物矿化（biomineralization）过程具有极强的调控作用。然而，本研究数据表明存在一个临界海水碳酸根离子浓度阈值，低于该阈值时，该物种（可能还包括其他物种）的钙化过程会受到抑制。事实上，杯形珊瑚在文石饱和状态为0.8时出现的强烈负响应表明，一旦达到临界文石饱和状态，该物种对未来二氧化碳诱导的海洋酸化的响应可能会既突然又剧烈。