Detection of a variable intracellular acid-labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C(i) pool) is one strategy by which marine algae overcome the low abundance of dissolved CO2 (CO2 (aq) ) in modern seawater. To identify the environmental conditions under which algae accumulate an acid-labile C(i) pool, we applied a (14) C pulse-chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid-labile C(i) pools. C(i) pools are measureable in cells cultured in media with 2-30 µmol/l CO2 (aq), corresponding to a medium pH of 8.6-7.9. The absolute C(i) pool was greater for the larger celled diatoms. For both algal classes, the C(i) pool became a negligible contributor to photosynthesis once CO2 (aq) exceeded 30 µmol/l. Combining the (14) C pulse-chase method and (14) C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO2 (aq) . We showed that the C(i) pool decreases with higher CO2 :HCO3 (-) uptake rates.细胞内碳库（C(i) pool）的积累，是海洋藻类应对现代海水中溶解态二氧化碳（CO₂(aq)）丰度偏低的生存策略之一。为明确藻类积累酸不稳定碳库（acid-labile C(i) pool）的环境条件，我们将最初应用于甲藻的¹⁴C脉冲追踪法，应用于两类新的藻类：颗石藻与硅藻。该方法可在不改变培养基碳化学组成与培养细胞密度的前提下，定量测定细胞内的碳积累量。研究发现，硅藻威氏海链藻（Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle]）以及颗石藻赫氏颗石藻（Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler]）的钙化株系，均可形成显著的酸不稳定碳库。在CO₂(aq)浓度为2~30 μmol/L的培养基中培养的细胞，均可检测到C(i)库，对应的培养基pH范围为8.6~7.9。体积更大的硅藻，其绝对C(i)库含量更高。对于两类藻类而言，当CO₂(aq)浓度超过30 μmol/L时，C(i)库对光合作用的贡献可忽略不计。结合¹⁴C脉冲追踪法与¹⁴C不平衡法，我们得以评估赫氏颗石藻（E. huxleyi）与威氏海链藻（T. weissflogii）是否存在停止积累溶解无机碳（Dissolved Inorganic Carbon，DIC）的阈值，或是随着CO₂(aq)浓度升高而降低对碳酸氢盐摄取的依赖。我们证实，C(i)库会随着CO₂与HCO₃⁻摄取速率之比的升高而降低。