Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (PML B92/11), 2000

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO2 concentrations (1.1-53.5 micromol L-1). The carbon isotope fractionation of POC (POC) varied by ca. 7 per mil and was positively correlated with aqueous CO2 concentration [CO2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of POC and in the slope of the relationship of POC with growth rate and [CO2aq]. Also, a significantly greater offset was obtained between the delta13C of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8 per mil). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in 13C by 2.3 per mil to 4.1 per mil, phytol was depleted in 13C by 1.9 per mil, and the major sterol 24-methylcholesta-5,22E-dien-3beta-ol was depleted in 13C by 8.5 per mil. This large spread of delta13C values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have delta13C values differing by just a few per mil. Increases in [CO2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U37k- used for reconstructions of paleo sea surface temperature.

本研究测定了在宽范围CO₂浓度（1.1~53.5 μmol·L⁻¹）下的稀释分批培养体系中生长的颗石藻（coccolithophorid）赫氏颗石藻（Emiliania huxleyi）的体相有机质（bulk organic matter, POC）、长链烯酮（alkenones）、甾醇（sterols）、脂肪酸（fatty acids）及植醇（phytol）的稳定碳同位素组成。体相有机质的碳同位素分馏（carbon isotope fractionation）波动幅度约为7‰，且与水溶液CO₂浓度[CO₂aq]呈正相关。该结果验证了氮限制恒化器培养条件下同种藻类观测到的总体趋势，但体相有机质的绝对数值及其与生长速率、[CO₂aq]的关系斜率均存在显著差异。此外，本研究中长链烯酮与体相有机质的δ¹³C（delta13C）差值较既往研究更大（本研究为5.4‰，既往研究为3.8‰），表明同位素偏移幅度可能依赖于培养条件。相较于体相有机质，各单体脂肪酸的δ¹³C值偏轻2.3‰~4.1‰，植醇偏轻1.9‰，主要甾醇24-甲基胆甾-5,22E-二烯-3β-醇（24-methylcholesta-5,22E-dien-3beta-ol）偏轻8.5‰。同一藻类不同脂类组分（lipid classes）的δ¹³C值存在如此大的差异，提示在有机地球化学研究中，若仅依据δ¹³C值相差数‰就将不同脂类归为不同来源，需格外谨慎。水溶液CO₂浓度升高会导致单细胞内长链烯酮含量及其占总有机碳的比例显著上升，但对用于古海表温度（paleo sea surface temperature）重建的U₃₇ₖ⁻值无系统性影响。