Impacts of ocean acidification on intertidal benthic foraminiferal growth and calcification

Foraminifera are expected to be particularly susceptible to future changes in ocean carbonate chemistry as a function of increased atmospheric CO2. Studies in an experimental recirculating seawater system were performed with a dominant benthic foraminiferal species collected from intertidal mudflats. We investigated the experimental impacts of ocean acidification on survival, growth/calcification, morphology and the biometric features of a calcareous species Elphidium williamsoni. Foraminifera were exposed for 6 weeks to four different pH treatments that replicated future scenarios of a high CO2 atmosphere resulting in lower seawater pH. Results revealed that declining seawater pH caused a decline in foraminiferal survival rate and growth/calcification (mainly through test weight reduction). Scanning electron microscopy image analysis of live specimens at the end of the experimental period show changes in foraminiferal morphology with clear signs of corrosion and cracking on the test surface, septal bridges, sutures and feeding structures of specimens exposed to the lowest pH conditions. These findings suggest that the morphological changes observed in shell feeding structures may serve to alter: (1) foraminiferal feeding efficiency and their long-term ecological competitiveness, (2) the energy transferred within the benthic food web with a subsequent shift in benthic community structures and (3) carbon cycling and total CaCO3 production, both highly significant processes in coastal waters. These experimental results open-up the possibility of modelling future impacts of ocean acidification on both calcification and dissolution in benthic foraminifera within mid-latitude intertidal environments, with potential implications for understanding the changing marine carbon cycle.有孔虫 (Foraminifera) 预计会对未来海洋碳酸盐化学的变化尤为敏感，而此类变化由大气CO₂浓度升高引发。本研究采用采集自潮间带泥滩的优势底栖有孔虫物种，在实验循环海水系统中开展相关实验，探究了海洋酸化对钙质物种威廉森希望虫 (Elphidium williamsoni) 的存活、生长/钙化、形态特征及生物计量学指标的实验性影响。研究将有孔虫置于四种模拟未来高CO₂大气情景下海水pH降低的不同pH梯度处理组中，暴露时长为6周。结果显示，海水pH下降会导致有孔虫的存活率与生长/钙化水平降低，主要表现为壳重减轻。实验结束后对活体标本开展的扫描电子显微镜 (Scanning Electron Microscopy) 图像分析表明，处于最低pH处理组的标本形态发生显著变化：其壳表、隔膜桥、缝合线及摄食结构均出现清晰的腐蚀与开裂迹象。本研究结果提示，壳类摄食结构的形态变化可能带来三方面改变：(1) 有孔虫的摄食效率及其长期生态竞争力；(2) 底栖食物网内的能量流动，进而引发底栖群落结构的改变；(3) 近岸海域的核心生物地球化学过程——碳循环与总碳酸钙 (CaCO₃) 生产。本实验结果为构建模型以探究海洋酸化对中纬度潮间带环境下底栖有孔虫钙化与溶解过程的未来影响提供了可能，同时也为理解不断变化的海洋碳循环提供了潜在的研究思路。