Seawater carbonate chemistry and calcification of coral Acropora hyacinthus@en

To investigate the effect of diel variation in pCO2 on coral calcification, branches of Acropora hyacinthus were collected in 2 habitats (upstream and downstream in a unidirectional flow) in a shallow back reef in Moorea, French Polynesia, where different diel amplitudes of pCO2 oscillation were expected. Corals were maintained for 6 wk under different pCO2 regimes (constant versus oscillatory), each delivered in 3 configurations: constant conditions of 400 µatm, 700 µatm, and 1000 µatm pCO2, or oscillatory conditions varying daily from 280 to 550 µatm, 550 to 1000 µatm, or 400 to 2000 µatm, with minima and maxima during the day and night, respectively. Calcification rates in all treatments tended to increase over time, and the interaction between Time and pCO2 regime (i.e. constant versus oscillating) was significant (or close to significant) for upstream corals due to higher calcification in oscillatory pCO2. A significant pCO2 regime effect was detected in the highest pCO2 for downstream corals, with higher calcification in the 400 to 2000 µatm oscillatory pCO2 treatment compared to the 1000 µatm constant pCO2 treatment. After 6 wk, calcification of A. hyacinthus was affected significantly by habitat, the pCO2 level, and the pCO2 regime. Calcification generally was reduced by high pCO2 and was ≥21% greater in 400 to 2000 µatm oscillatory pCO2 versus 1000 µatm constant pCO2 treatment. Increased calcification in the 400 to 2000 µatm oscillatory pCO2 treatment suggests that natural diel oscillations in pCO2 could play a role by reducing the locally negative effects of rising pCO2 associated with ocean acidification on coral calcification.为探究二氧化碳分压（pCO2）的昼夜变化对珊瑚钙化作用的影响，研究人员于法属波利尼西亚莫雷阿岛一处浅海礁后区域的两个生境（单向水流的上游与下游）采集了紫鹿角珊瑚（Acropora hyacinthus）的断枝，该区域预期存在不同幅度的pCO2昼夜振荡。研究人员将珊瑚置于不同pCO2处理模式（恒定式与振荡式）下培养6周，每种模式下设3种配置：恒定pCO2条件分别为400微大气压（µatm）、700 µatm与1000 µatm；振荡式条件则每日在两个数值间波动，分别为280~550 µatm、550~1000 µatm以及400~2000 µatm，且其分压最小值与最大值分别出现在日间与夜间。所有处理组的钙化速率均随培养时间呈上升趋势；对于上游生境的珊瑚而言，培养时间与pCO2处理模式（恒定式 vs 振荡式）的交互效应显著（或接近显著），这是因为振荡式pCO2条件下的珊瑚钙化速率更高。对于下游生境的珊瑚，在最高pCO2水平下检测到了显著的pCO2处理模式效应：相较于1000 µatm恒定pCO2处理组，400~2000 µatm振荡式pCO2处理组的珊瑚钙化速率更高。培养6周后，紫鹿角珊瑚的钙化作用显著受到生境、pCO2水平以及pCO2处理模式的影响。高pCO2通常会抑制珊瑚钙化，而400~2000 µatm振荡式pCO2处理组的钙化速率相较于1000 µatm恒定pCO2处理组高出21%及以上。400~2000 µatm振荡式pCO2处理组的钙化速率提升现象表明，自然状态下的pCO2昼夜振荡或可通过缓解海洋酸化引发的pCO2升高对珊瑚钙化的局部负面效应，发挥积极调控作用。