Responses of the Emiliania huxleyi Proteome to Ocean Acidification@en

Ocean acidification due to rising atmospheric CO2 is expected to affect the physiology of important calcifying marine organisms, but the nature and magnitude of change is yet to be established. In coccolithophores, different species and strains display varying calcification responses to ocean acidification, but the underlying biochemical properties remain unknown. We employed an approach combining tandem mass-spectrometry with isobaric tagging (iTRAQ) and multiple database searching to identify proteins that were differentially expressed in cells of the marine coccolithophore species Emiliania huxleyi (strain NZEH) between two CO2 conditions: 395 (~current day) and ~1340 p.p.m.v. CO2. Cells exposed to the higher CO2 condition contained more cellular particulate inorganic carbon (CaCO3) and particulate organic nitrogen and carbon than those maintained in present-day conditions. These results are linked with the observation that cells grew slower under elevated CO2, indicating cell cycle disruption. Under high CO2 conditions, coccospheres were larger and cells possessed bigger coccoliths that did not show any signs of malformation compared to those from cells grown under present-day CO2 levels. No differences in calcification rate, particulate organic carbon production or cellular organic carbon: nitrogen ratios were observed. Results were not related to nutrient limitation or acclimation status of cells. At least 46 homologous protein groups from a variety of functional processes were quantified in these experiments, of which four (histones H2A, H3, H4 and a chloroplastic 30S ribosomal protein S7) showed down-regulation in all replicates exposed to high CO2, perhaps reflecting the decrease in growth rate. We present evidence of cellular stress responses but proteins associated with many key metabolic processes remained unaltered. Our results therefore suggest that this E. huxleyi strain possesses some acclimation mechanisms to tolerate future CO2 scenarios, although the observed decline in growth rate may be an overriding factor affecting the success of this ecotype in future oceans.大气二氧化碳浓度升高引发的海洋酸化，预计会对重要的钙化海洋生物的生理机能产生影响，但其变化的本质与幅度尚未明确。在颗石藻（coccolithophores）中，不同物种与品系对海洋酸化的钙化响应存在差异，但其背后的生化机制仍未明晰。本研究采用串联质谱（tandem mass-spectrometry）结合同量异位素标记（iTRAQ）与多数据库检索的方法，对两种二氧化碳浓度条件下海洋颗石藻物种艾氏颗石藻（Emiliania huxleyi，品系NZEH）细胞内的差异表达蛋白进行鉴定：两种条件分别为395×10⁻⁶（约当前大气水平）与约1340×10⁻⁶体积分数的二氧化碳。与维持在当前二氧化碳浓度条件下的细胞相比，暴露于高二氧化碳浓度条件下的细胞，其细胞内颗粒无机碳（particulate inorganic carbon，CaCO3）、颗粒有机氮与颗粒有机碳含量更高。该结果与高二氧化碳浓度下细胞生长速率减慢的观测结果一致，提示细胞周期受到干扰。在高二氧化碳浓度条件下，球石囊体积更大，细胞所拥有的颗石尺寸也更大，且与当前二氧化碳浓度下培养的细胞相比，未出现任何畸形迹象。研究未观测到钙化速率、颗粒有机碳生成量或细胞有机碳氮比存在差异。上述结果与细胞的营养限制或驯化状态无关。本实验共定量到至少46个来自不同功能通路的同源蛋白组，其中组蛋白H2A、H3、H4以及叶绿体30S核糖体蛋白S7这4种蛋白在所有高二氧化碳浓度暴露的生物学重复中均呈现下调表达，这或许反映了生长速率的下降。本研究提供了细胞应激响应的相关证据，但许多关键代谢过程相关的蛋白表达未发生改变。因此本研究结果表明，该艾氏颗石藻品系具备一定的驯化机制以应对未来的二氧化碳浓度情景，尽管观测到的生长速率下降，可能成为影响该生态型在未来海洋中生存竞争力的主导因素。