Strong shift from HCO3- to CO2 uptake in Emiliania huxleyi with acidification: new approach unravels acclimation versus short-term pH effects

Effects of ocean acidification on Emiliania huxleyi strain RCC 1216 (calcifying, diploid life-cycle stage) and RCC 1217 (non-calcifying, haploid life-cycle stage) were investigated by measuring growth, elemental composition, and production rates under different pCO2 levels (380 and 950 µatm). In these differently acclimated cells, the photosynthetic carbon source was assessed by a (14)C disequilibrium assay, conducted over a range of ecologically relevant pH values (7.9-8.7). In agreement with previous studies, we observed decreased calcification and stimulated biomass production in diploid cells under high pCO2, but no CO2-dependent changes in biomass production for haploid cells. In both life-cycle stages, the relative contributions of CO2 and HCO3 (-) uptake depended strongly on the assay pH. At pH values =< 8.1, cells preferentially used CO2 (>= 90 % CO2), whereas at pH values >= 8.3, cells progressively increased the fraction of HCO3 (-) uptake (~45 % CO2 at pH 8.7 in diploid cells; ~55 % CO2 at pH 8.5 in haploid cells). In contrast to the short-term effect of the assay pH, the pCO2 acclimation history had no significant effect on the carbon uptake behavior. A numerical sensitivity study confirmed that the pH-modification in the (14)C disequilibrium method yields reliable results, provided that model parameters (e.g., pH, temperature) are kept within typical measurement uncertainties. Our results demonstrate a high plasticity of E. huxleyi to rapidly adjust carbon acquisition to the external carbon supply and/or pH, and provide an explanation for the paradoxical observation of high CO2 sensitivity despite the apparently high HCO3 (-) usage seen in previous studies.

本研究通过在不同二氧化碳分压（pCO₂）水平（380与950微大气压）下测定细胞生长、元素组成及生产速率，探究了海洋酸化对两株艾氏海球菌（Emiliania huxleyi）菌株RCC 1216（具钙化能力的二倍体生命周期阶段）与RCC 1217（非钙化的单倍体生命周期阶段）的影响。针对上述经不同驯化处理的细胞，本研究采用碳14（¹⁴C）不平衡检测法，在一系列生态相关的pH值范围（7.9~8.7）内评估了其光合碳源利用模式。与既往研究结果一致，我们观测到高二氧化碳分压环境下，二倍体细胞的钙化作用受到抑制、生物量生成得到促进，但单倍体细胞的生物量生成未出现二氧化碳依赖的显著变化。在两种生命周期阶段中，二氧化碳与碳酸氢根（HCO₃⁻）的摄取相对占比均随检测pH值发生显著改变：当pH值≤8.1时，细胞优先利用二氧化碳（二氧化碳摄取占比≥90%）；而当pH值≥8.3时，细胞对碳酸氢根的摄取比例逐步升高（二倍体细胞在pH 8.7时约45%的碳来自二氧化碳摄取，单倍体细胞在pH 8.5时约55%的碳来自二氧化碳摄取）。与检测pH值的短期调控效应不同，二氧化碳分压驯化历史对细胞的碳摄取行为无显著影响。数值敏感性分析证实，只要模型参数（如pH、温度）控制在典型测量不确定度范围内，碳14不平衡检测法的pH调整步骤可获得可靠实验结果。本研究结果表明，艾氏海球菌具有高度的表型可塑性，能够快速调整碳获取策略以适应外界碳供应与/或pH变化，并为解释既往研究中“尽管观测到较高的碳酸氢根利用比例，但艾氏海球菌仍对高二氧化碳环境具有显著敏感性”这一矛盾现象提供了理论依据。