Response of benthic foraminifera to ocean acidification in their natural sediment environment: a long-term culturing experiment

Calcifying foraminifera are expected to be endangered by ocean acidification; however, the response of a complete community kept in natural sediment and over multiple generations under controlled laboratory conditions has not been constrained to date. During 6 months of incubation, foraminiferal assemblages were kept and treated in natural sediment with pCO2-enriched seawater of 430, 907, 1865 and 3247 µatm pCO2. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months of incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Omega calc was much higher in the sediment than in the water column in nearly all pCO2 treatments and remained close to saturation. As a result, the life cycle (population density, growth and reproduction) of living assemblages varied markedly during the experimental period, but was largely unaffected by the pCO2 treatments applied. According to the size-frequency distribution, we conclude that foraminifera start reproduction at a diameter of 250 µm. Mortality of living Ammonia aomoriensis was unaffected, whereas size of large and dead tests decreased with elevated pCO2 from 285 µm (pCO2 from 430 to 1865 µatm) to 258 µm (pCO2 3247 µatm). The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of CaCO3 weight. Living individuals had a calcium carbonate production rate of 0.47 g/m**2/a, whereas dead empty tests accumulated a rate of 0.27 g /m**2/a. Although Omega calc was close to 1, approximately 30% of the empty tests of Ammonia aomoriensis showed dissolution features at high pCO2 of 3247 µatm during the last 2 months of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. Our results emphasize that the sensitivity to ocean acidification of the endobenthic foraminifera Ammonia aomoriensis in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment.

钙化有孔虫（calcifying foraminifera）预计将受到海洋酸化的威胁，但截至目前，针对以天然沉积物为栖息基质、在受控实验室条件下历经多代的完整有孔虫群落的响应特征，尚未有明确定论。本实验在6个月的培育周期内，将有孔虫群落置于含天然沉积物的水体中，设置430、907、1865与3247 µatm的二氧化碳分压（pCO2）梯度，采用富CO₂海水进行处理。实验群落以阿穆尔卷虫（Ammonia aomoriensis）与希望虫属（Elphidium）物种为优势类群，胶结型有孔虫（agglutinated species）则较为稀有。培育6个月后，沉积物孔隙水（pore water）的碱度（alkalinity）远高于上覆海水。因此，几乎所有pCO2处理组中，沉积物内的文石饱和状态（Omega calc）均远高于水柱环境，且始终接近饱和水平。在此实验周期内，存活群落的生活史参数（种群密度、生长与繁殖）存在显著波动，但基本不受所施加的pCO2处理的影响。基于粒径频率分布（size-frequency distribution），我们推断有孔虫的繁殖起始粒径为250 µm。阿穆尔卷虫的存活个体死亡率未受显著影响，但大型死亡空壳的尺寸随pCO2升高而降低：从430至1865 µatm处理组的285 µm，降至3247 µatm组的258 µm。经测定，存活阿穆尔卷虫的总有机碳含量（total organic content）为碳酸钙（CaCO3）干重的4.3%。存活个体的碳酸钙产率为0.47 g/m²/a，而空壳体的堆积速率为0.27 g/m²/a。尽管文石饱和状态（Omega calc）接近1，但在培育的最后2个月中，3247 µatm高pCO2处理组中，约30%的阿穆尔卷虫空壳体出现了溶蚀特征（dissolution features）。与之相反，次优势种不整希望虫（Elphidium incertum）的壳体保持完整。本研究结果表明，栖息于天然沉积物中的底内底栖有孔虫（endobenthic foraminifera）阿穆尔卷虫，其对海洋酸化的敏感性远低于从沉积物中分离出的个体的实验响应结果。