Seawater carbonate chemistry and Emiliania huxleyi mass and size, 2011

About one-third of the carbon dioxide (CO2) released into the atmosphere as a result of human activity has been absorbed by the oceans, where it partitions into the constituent ions of carbonic acid. This leads to ocean acidification, one of the major threats to marine ecosystems and particularly to calcifying organisms such as corals, foraminifera and coccolithophores. Coccolithophores are abundant phytoplankton that are responsible for a large part of modern oceanic carbonate production. Culture experiments investigating the physiological response of coccolithophore calcification to increased CO2 have yielded contradictory results between and even within species. Here we quantified the calcite mass of dominant coccolithophores in the present ocean and over the past forty thousand years, and found a marked pattern of decreasing calcification with increasing partial pressure of CO2 and concomitant decreasing concentrations of CO3. Our analyses revealed that differentially calcified species and morphotypes are distributed in the ocean according to carbonate chemistry. A substantial impact on the marine carbon cycle might be expected upon extrapolation of this correlation to predicted ocean acidification in the future. However, our discovery of a heavily calcified Emiliania huxleyi morphotype in modern waters with low pH highlights the complexity of assemblage-level responses to environmental forcing factors.人类活动排放至大气中的二氧化碳（CO₂），约有三分之一被海洋吸收，随后在海水中分解为碳酸的组成离子。这一过程引发了海洋酸化，其已成为威胁海洋生态系统的主要威胁之一，尤其对钙化生物（如珊瑚、有孔虫以及颗石藻（coccolithophores））造成严重影响。颗石藻是一类分布广泛的浮游植物，贡献了现代海洋中相当一部分的碳酸盐生成量。针对二氧化碳浓度升高对颗石藻钙化生理响应的培养实验，在不同物种间甚至同一物种内部均得到了相互矛盾的研究结果。本研究对现代海洋以及过去四万年间优势颗石藻的方解石质量进行了定量测定，发现钙化程度随二氧化碳分压升高、碳酸根（CO₃²⁻）浓度降低呈现出显著下降的规律。分析结果显示，不同钙化程度的物种及形态型，其海洋分布格局与碳酸盐化学环境密切相关。若将这一相关性外推至未来预估的海洋酸化场景，预计将对海洋碳循环产生显著影响。然而，本研究在低pH值现代海域中发现了一种钙化程度极高的赫氏颗石藻（Emiliania huxleyi）形态型，这一发现凸显了群落级生物对环境驱动因子响应的复杂性。