Micro-CT analysis of the Caribbean octocoral Eunicea flexuosa subjected to elevated pCO2

Rising anthropogenic carbon dioxide has resulted in a drop in ocean pH, a phenomenon known as ocean acidification (OA). These acidified waters have many ramifications for diverse marine biota, especially those species which precipitate calcium carbonate skeletons. The permanence of coral reef ecosystems is therefore closely related to OA stress as habitat-forming corals will exhibit reduced calcification and growth. Relatively little is known concerning the fate of other constituent taxa which may either suffer concomitant declines or be competitively favoured in acidified waters. Here, we experimentally (49 d) test the effects of next century predictions for OA (pH = 7.75, pCO2 = 1081 µatm) vs. near-present-day conditions (pH = 8.01, pCO2 = 498 µatm) on the common Caribbean octocoral Eunicea flexuosa. We measure linear extension of this octocoral and use a novel technique, high-resolution micro-computed tomography, to measure potential differences in the morphology of calcified internal skeletal structures (sclerites) in a 2 mm apical section of each branch. Despite the use of highly accurate procedures, we found no significant differences between treatments in either the growth of E. flexuosa branches or the structure of their sclerites. Our results suggest a degree of resilience to OA stress and provide evidence that this octocoral species may persist on Caribbean coral reefs, despite global change.

日益加剧的人为二氧化碳排放已引发海洋pH值降低，该现象被称为海洋酸化（OA）。酸化海水对多样的海洋生物类群存在诸多影响，尤其对那些能够沉积碳酸钙骨骼的物种而言。因此，珊瑚礁生态系统的存续与海洋酸化胁迫紧密相关——作为生境构建者的造礁珊瑚，其钙化速率与生长速度均会出现下降。而对于其余类群成员在酸化水体中的存续命运——它们或随种群衰退，或获得竞争优势——目前人们的认知仍较为匮乏。本研究开展了为期49天的受控实验，对比未来百年预测的海洋酸化环境（pH=7.75，pCO₂=1081 µatm）与近当代环境条件（pH=8.01，pCO₂=498 µatm）对常见加勒比海八放珊瑚Eunicea flexuosa的影响。我们测定了该八放珊瑚的线性延伸量，并采用一项创新技术——高分辨率微型计算机断层扫描（high-resolution micro-computed tomography）——对每一分枝2毫米顶端区段内的钙化内部骨骼结构（骨针，sclerites）的形态差异进行检测。尽管采用了高精度的实验流程，我们并未发现两组处理在E. flexuosa分枝生长或其骨针结构上存在显著差异。本研究结果表明该八放珊瑚对海洋酸化胁迫具备一定抗逆性，并证实即便面临全球变化，该物种仍有可能在加勒比海珊瑚礁生态系统中存续。