Responses of the tropical gorgonian coral Eunicea fusca to ocean acidification conditions

Ocean acidification can have negative repercussions from the organism to ecosystem levels. Octocorals deposit high-magnesium calcite in their skeletons, and according to different models, they could be more susceptible to the depletion of carbonate ions than either calcite or aragonite-depositing organisms. This study investigated the response of the gorgonian coral Eunicea fusca to a range of CO2 concentrations from 285 to 4,568 ppm (pH range 8.1-7.1) over a 4-week period. Gorgonian growth and calcification were measured at each level of CO2 as linear extension rate and percent change in buoyant weight and calcein incorporation in individual sclerites, respectively. There was a significant negative relationship for calcification and CO2 concentration that was well explained by a linear model regression analysis for both buoyant weight and calcein staining. In general, growth and calcification did not stop in any of the concentrations of pCO2; however, some of the octocoral fragments experienced negative calcification at undersaturated levels of calcium carbonate (>4,500 ppm) suggesting possible dissolution effects. These results highlight the susceptibility of the gorgonian coral E. fusca to elevated levels of carbon dioxide but suggest that E. fusca could still survive well in mid-term ocean acidification conditions expected by the end of this century, which provides important information on the effects of ocean acidification on the dynamics of coral reef communities. Gorgonian corals can be expected to diversify and thrive in the Atlantic-Eastern Pacific; as scleractinian corals decline, it is likely to expect a shift in these reef communities from scleractinian coral dominated to octocoral/soft coral dominated under a "business as usual" scenario of CO2 emissions.

海洋酸化可对从生物体到生态系统的各个层级带来负面影响。八放珊瑚（Octocorals）的骨骼会沉积高镁方解石（high-magnesium calcite），根据不同模型的预测，相较于方解石（calcite）沉积生物或文石沉积生物（aragonite-depositing organisms），八放珊瑚可能更易受到碳酸根离子耗竭的影响。本研究在为期4周的实验周期内，探究了棕黄柳珊瑚（Eunicea fusca）对285至4568 ppm CO₂浓度范围（对应pH区间8.1~7.1）的响应特征。本研究分别以线性延伸速率、个体骨针（sclerites）的浮力重量变化率以及钙黄绿素（calcein）掺入量，作为柳珊瑚（gorgonian coral）生长与钙化的衡量指标，在各CO₂浓度梯度下进行测定。钙化程度与CO₂浓度之间存在显著的负相关关系，通过对浮力重量与钙黄绿素染色结果开展线性模型回归分析，可很好地阐释这一相关性。总体而言，各分压CO₂（pCO₂）浓度下均未出现生长与钙化完全停止的情况；但在碳酸钙不饱和水平（>4500 ppm）下，部分八放珊瑚片段出现了钙化负值，提示可能存在溶解效应。本研究结果凸显了棕黄柳珊瑚（E. fusca）对高浓度CO₂的敏感性，但同时表明，在本世纪末预计的中期海洋酸化环境中，棕黄柳珊瑚仍可较好存活；该结果为探究海洋酸化对珊瑚礁群落动态的影响提供了重要参考依据。在大西洋-东太平洋海域，柳珊瑚有望实现物种分化并蓬勃生长；随着石珊瑚（scleractinian corals）种群衰退，在CO₂排放“一切照旧”的情景下，这类珊瑚礁群落很可能会从以石珊瑚为主导，转变为以八放珊瑚/软珊瑚（soft coral）为主导。