Differing responses of three Southern Ocean Emiliania huxleyi ecotypes to changing seawater carbonate chemistry@en

The invasion of anthropogenic carbon dioxide into the surface ocean is altering seawater carbonate speciation, a process commonly called ocean acidification. The high latitude waters of the Southern Ocean are one of the primary and most severely affected regions. Coccolithophores are an important phytoplankton group, responsible for the majority of pelagic calcium carbonate production in the world's oceans, with a distribution that ranges from tropical to polar waters. Emiliania huxleyi is numerically the most abundant coccolithophore species and appears in several different ecotypes. We tested the effects of ocean acidification on 3 carefully selected E. huxleyi ecotypes isolated from the Southern Ocean. Their responses were measured in terms of growth, photosynthesis, calcification, cellular geometry, and stoichiometry. The 3 ecotypes exhibited differing sensitivities in regards to seawater carbonate chemistry when cultured at the same temperature (14°C) and continuous light (110 µmol photons/m2/s). Under future ocean acidification scenarios, particulate inorganic to organic carbon ratios (PIC:POC) decreased by 38-44, 47-51 and 71-98% in morphotype A 'over-calcified' (A o/c), A and B/C, respectively. All ecotypes reduced their rate of calcification, but the cold-water adapted ecotype (morphotype B/C) was by far the most sensitive, and almost ceased calcification at partial pressure of carbon dioxide ( pCO2) levels above 1000 µatm. We recommend that future surveys for E. huxleyi cells in the Southern Ocean should include the capability of recognising 'naked cells' by molecular and microscopic tools. The distinct differences in the physiological responses of these 3 dominant Southern Ocean coccolithophore ecotypes are likely to have consequences for future coccolithophore community structures and thereby the Southern Ocean carbon cycle.人为二氧化碳（anthropogenic carbon dioxide）侵入表层海洋，正改变海水碳酸盐化学形态（seawater carbonate speciation），该过程通常被称为海洋酸化（ocean acidification）。南大洋的高纬度海域是全球海洋酸化影响最主要、最严重的区域之一。颗石藻（Coccolithophores）是一类重要的浮游植物类群，主导全球海洋中绝大多数的浮游碳酸钙生产，其分布范围覆盖热带至极地海域。赫氏颗石藻（Emiliania huxleyi）是数量占绝对优势的颗石藻物种，存在多种不同的生态型。我们针对3株从南大洋分离、经严格筛选的赫氏颗石藻生态型，开展了海洋酸化影响的受控实验。实验从生长速率、光合作用能力、钙化作用、细胞几何形态以及化学计量比多个维度，测定了各生态型的响应特征。在相同温度（14℃）与持续光照（110 μmol 光子·米⁻²·秒⁻¹）条件下培养时，这3种生态型对海水碳酸盐化学环境表现出显著不同的敏感性。在未来海洋酸化情景下，钙化过度型A ('A o/c')、A型以及B/C型的颗粒无机碳与有机碳比值（PIC:POC）分别下降38%~44%、47%~51%与71%~98%。所有生态型的钙化速率均出现下降，但适应冷水环境的B/C形态型生态型敏感性最强，当二氧化碳分压（pCO2）超过1000 μatm时，其钙化作用几乎完全停止。我们建议，未来针对南大洋赫氏颗石藻细胞的调查研究，应具备通过分子与显微技术识别“裸细胞（naked cells）”的能力。这3种占优势的南大洋颗石藻生态型在生理响应上的显著差异，可能会对未来颗石藻群落结构产生影响，进而改变南大洋碳循环过程。