PDF(Pubmed)
Abstract：
Hydrogen isotope ratios (δ2H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2H/1H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2(aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ2H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.
摘要：
氢同位素比率（δ2H）代表了代谢过程的重要天然示踪剂，但是缺乏控制水生光合生物中H-分馏过程的定量模型。这里,我们通过系统地控制温度来阐明藻类脂质中2H/1H分馏的潜在生理控制，光,和二氧化碳（aq）在连续培养的海藻植物中。我们分析了烯酮（α烯酮）中的氢同位素分馏，该物种和其他藻类特有的一类酰基脂质。我们发现α烯酮随着CO2（aq）的增加而强烈减少，并确认α烯酮与温度和光相关。基于已知的生物合成途径，我们开发了藻类酰基脂质的δ2H的细胞模型，以评估有助于这些控制分馏的过程。模拟表明，NADPH在叶绿体中的停留时间越长，NADPH与富含2H的细胞内水的交换越多，增加α烯酮。较高的叶绿体CO2（aq）和温度通过增强碳固定和脂质合成速率来缩短NADPH停留时间。在我们的培养物中，α烯酮与CO2（aq）的负相关表明，碳浓缩机制（CCM）在Rubisco位点并未实现CO2的恒定饱和，而是叶绿体CO2随外部CO2（aq）而变化。在现代和工业化前的海洋中，α烯酮与CO2（aq）的普遍负相关也表明，自然种群可能无法在CCM中达到Rubisco的恒定饱和度。与其重建生长水，α烯酮可能是阐明光合作用的碳限制的有力工具。
