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.

海洋酸化会对从生物个体到生态系统层面产生负面影响。八放珊瑚（Octocorals）在其骨骼中沉积高镁方解石；根据不同模型，相较于方解石或文石沉积生物，它们可能更容易受到碳酸根离子枯竭的影响。本研究在4周时间内，探究了柳珊瑚（gorgonian coral）Eunicea fusca对285至4568 ppm范围内CO₂浓度（pH值范围8.1-7.1）的响应。在各CO₂浓度水平下，柳珊瑚的生长和钙化分别通过线性延伸率、浮力重量变化百分比以及单个骨针中钙黄绿素（calcein）的掺入量来衡量。钙化与CO₂浓度之间存在显著负相关关系，线性模型回归分析可很好地解释浮力重量和钙黄绿素染色这两项指标的这一关系。总体而言，在所有pCO₂浓度下，生长和钙化均未停止；但在碳酸钙不饱和水平（>4500 ppm）下，部分八放珊瑚片段出现了负钙化现象，这表明可能存在溶解效应。这些结果凸显了柳珊瑚E. fusca对高浓度二氧化碳的敏感性，但也表明该物种在本世纪末预期的中期海洋酸化条件下仍能良好生存，这为海洋酸化对珊瑚礁群落动态的影响提供了重要信息。在“一切照旧”的CO₂排放情景下，随着石珊瑚（scleractinian corals）的衰退，大西洋-东太平洋区域的柳珊瑚有望多样化并繁盛，珊瑚礁群落可能会从石珊瑚主导转变为八放珊瑚/软珊瑚主导。