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

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.人为二氧化碳侵入表层海洋，正在改变海水碳酸盐组分，这一过程通常被称为海洋酸化（Ocean Acidification）。南大洋高纬度海域是受影响最严重的主要区域之一。颗石藻（Coccolithophores）是一类重要的浮游植物类群，负责全球海洋中绝大多数远洋碳酸钙生产，其分布范围从热带海域延伸至极地海域。赫氏颗石藻（Emiliania huxleyi）是数量上最为丰富的颗石藻物种，存在多种不同的生态型。本研究测试了海洋酸化对3株从南大洋分离得到的赫氏颗石藻生态型的影响，通过生长、光合作用、钙化作用、细胞几何形态以及化学计量比等指标衡量其响应。在相同温度（14℃）与持续光照（110 µmol photons/m²/s）条件下培养时，这3株生态型对海水碳酸盐化学表现出不同的敏感性。在未来海洋酸化情景下，颗粒无机碳与有机碳比值（PIC:POC）在‘过度钙化’形态型A（A o/c）、形态型A以及形态型B/C中分别下降38%~44%、47%~51%与71%~98%。所有生态型的钙化速率均出现下降，其中冷水适应型（形态型B/C）的敏感性最为显著，在二氧化碳分压（pCO2）高于1000 µatm时几乎完全停止钙化。本研究建议，未来针对南大洋赫氏颗石藻细胞的调查应具备通过分子与显微技术识别‘裸细胞’的能力。这3株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进而南大洋碳循环产生影响。