Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado 80309, United States.
Department of Geosciences, Princeton University, Princeton, New Jersey 08544, United States.
Environ Sci Technol. 2021 Apr 20;55(8):5537-5546. doi: 10.1021/acs.est.0c07816. Epub 2021 Mar 9.
Dissimilatory nitrate reduction (DNR) to nitrite is the first step in denitrification, the main process through which bioavailable nitrogen is removed from ecosystems. DNR is catalyzed by both cytosolic (Nar) and periplasmic (Nap) nitrate reductases and fractionates the stable isotopes of nitrogen (N, N) and oxygen (O, O), which is reflected in residual environmental nitrate pools. Data on the relationship between the pattern in oxygen vs nitrogen isotope fractionation (ε/ε) suggests that systematic differences exist between marine and terrestrial ecosystems that are not fully understood. We examined the ε/ε of nitrate-reducing microorganisms that encode Nar, Nap, or both enzymes, as well as gene deletion mutants of Nar and Nap to test the hypothesis that enzymatic differences alone could explain the environmental observations. We find that the distribution of ε/ε fractionation ratios of all examined nitrate reductases forms two distinct peaks centered around an ε/ε proportionality of 0.55 (Nap) and 0.91 (Nar), with the notable exception of the Bacillus Nar reductases, which cluster isotopically with the Nap reductases. Our findings may explain differences in ε/ε fractionation between marine and terrestrial systems and challenge current knowledge about Nar ε/ε signatures.
异化硝酸盐还原（DNR）为亚硝酸盐是反硝化的第一步，反硝化是生物可利用氮从生态系统中去除的主要过程。DNR 由细胞质（Nar）和周质（Nap）硝酸盐还原酶催化，并使氮（N，N）和氧（O，O）的稳定同位素分馏，这反映在残留的环境硝酸盐库中。关于氧与氮同位素分馏（ε/ε）模式之间关系的数据表明，海洋和陆地生态系统之间存在系统差异，但尚未完全理解。我们研究了编码 Nar、Nap 或两种酶的硝酸盐还原微生物的 ε/ε，以及 Nar 和 Nap 的基因缺失突变体，以检验酶差异单独是否可以解释环境观察的假设。我们发现，所有被检测的硝酸盐还原酶的 ε/ε 分馏比的分布形成了两个明显的峰值，中心位于 ε/ε 比例为 0.55（Nap）和 0.91（Nar），芽孢杆菌 Nar 还原酶是一个显著的例外，它与 Nap 还原酶在同位素上聚类。我们的发现可以解释海洋和陆地系统之间 ε/ε 分馏的差异，并挑战当前关于 Nar ε/ε 特征的知识。
