Environmental controls on the Emiliania huxleyi calcite mass

Although ocean acidification is expected to impact (bio)calcification by decreasing the seawater carbonate ion concentration, [CO3]2-, there exists evidence of non-uniform response of marine calcifying plankton to low seawater [CO3]2-. This raises questions on the role of environmental factors other than acidification and on the complex physiological responses behind calcification. Here we investigate the synergistic effect of multiple environmental parameters, including temperature, nutrient (nitrate and phosphate) availability, and seawater carbonate chemistry on the coccolith calcite mass of the cosmopolitan coccolithophore Emiliania huxleyi, the most abundant species in the world ocean. We use a suite of surface (late Holocene) sediment samples from the South Atlantic and southwestern Indian Ocean taken from depths lying well above the modern lysocline. The coccolith calcite mass in our results presents a latitudinal distribution pattern that mimics the main oceanographic features, thereby pointing to the potential importance of phosphorus and temperature in determining coccolith mass by affecting primary calcification and possibly driving the E. huxleyi morphotype distribution. This evidence does not necessarily argue against the potentially important role of the rapidly changing seawater carbonate chemistry in the future, when unabated fossil fuel burning will likely perturb ocean chemistry beyond a critical point. Rather our study highlights the importance of evaluating the combined effect of several environmental stressors on calcifying organisms to project their physiological response(s) in a high CO2 world and improve interpretation of paleorecords.

尽管海洋酸化被认为会通过降低海水碳酸根离子浓度（[CO3]²⁻）影响（生物）钙化（(bio)calcification），但已有证据表明海洋钙化浮游生物对低海水[CO3]²⁻的响应并不均一。这引发了关于酸化以外环境因子的作用，以及钙化背后复杂生理响应的疑问。本研究探讨了温度、营养盐（硝酸盐与磷酸盐）可利用性及海水碳酸盐化学等多种环境参数对广布性颗石藻（coccolithophore）——全球海洋中丰度最高的赫胥黎艾密里藻（Emiliania huxleyi）——颗石方解石质量的协同效应。我们使用了一套取自南大西洋及西南印度洋的表层（晚全新世）沉积物样品，其采样深度远高于现代溶跃面（lysocline）。结果显示，颗石方解石质量呈现出与主要海洋学特征相似的纬度分布模式，这表明磷与温度可能通过影响初级钙化（primary calcification）并潜在驱动赫胥黎艾密里藻形态型（morphotype）分布，在决定颗石质量方面具有重要作用。这些证据并非否定未来快速变化的海水碳酸盐化学的潜在重要作用——若化石燃料燃烧持续无节制，海洋化学状态可能会被扰动至临界点以上。相反，本研究强调了评估多种环境胁迫因子（environmental stressors）对钙化生物的联合效应的重要性，这对于预测高CO₂世界中它们的生理响应及改进古记录（paleorecords）的解读至关重要。