The impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2 (pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcus with low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans.

中文翻译：

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不同pH和CO2条件下海洋真核和原核藻类硝酸盐同化过程中同位素分馏的变化

藻类硝酸盐同化过程中环境因素对氮（N）和氧（O）同位素效应的影响导致沉积氮同位素记录和硝酸盐同位素在了解海洋氮循环的现场应用中存在不确定性。预计海洋酸化会改变氮循环，包括硝酸盐同化，但藻类硝酸盐同化过程中 N 和 O 同位素的影响如何随海水 pH 值和 CO 分压的变化而变化2（p一氧化碳2）仍然未知。我们测量了硝酸盐同化过程中的 N 和 O 同位素效应以及海洋硅藻的生理状态魏氏海链藻和聚球藻属在不同pH值（8.1或7.8）下p一氧化碳2（400 或 800μ大气压）条件。pH值低和/或高p一氧化碳2在硅藻同化硝酸盐过程中，N 和 O 同位素效应同样降低，可能是由于细胞硝酸盐流出/吸收比率的降低以及硝酸盐吸收的同位素效应的降低，而它们并没有影响那些聚球藻属细胞内硝酸盐浓度低且硝酸盐流出有限。我们的结果提供了令人信服的实验证据，表明海洋真核和原核浮游植物在低 pH 和/或高 pH 下硝酸盐同化过程中 N 和 O 同位素效应的不同变化p一氧化碳2。这些发现为藻类硝酸盐同化过程中同位素效应变异性的环境控制提供了新的见解，并且对于提高对酸化海洋中氮和氧同位素工具的预测性理解具有重要意义。