Nitrogen and oxygen isotopic fractionation during microbial nitrite reduction

Nitrogen and oxygen isotopic fractionation during microbial nitrite reduction
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微生物亚硝酸盐还原过程中的氮和氧同位素分馏

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发表时间：
2016
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通讯作者：
K. Casciotti
K. Casciotti
中科院分区：
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文献类型：
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作者：
T. S. Martin;K. Casciotti

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微生物还原亚硝酸盐在氮循环中起着重要作用，在反硝化过程中产生第一个气态产物。亚硝酸盐还原在环境中的作用可以通过亚硝酸盐的稳定同位素测量来评估。在这里，我们给出了在含铜亚硝酸盐还原酶(Cu-NIR)和含细胞色素CD1亚硝酸盐还原酶(Fe-NIR)催化亚硝酸盐还原过程中氮(N)和氧(O)同位素分馏的估计。用瑞利分馏模型计算了分批培养实验中亚硝酸盐浓度和同位素组成随时间变化的N和O同位素效应，分别为15ε和18ε。3株携带铜-近红外的反硝化菌，15ε = 22 ± 2‰和18ε = 2 ± 2‰(95%可信区间)。对于携带铁-近红外的三株反硝化菌，15ε = 8 ± 2和18ε = 6 ± 2‰(95%可信区间)。这些对亚硝酸盐还原的同位素效应是显著不同的。此外，15ε和18ε并不像假设的那样表现出1：1的关系。这两种酶的同位素效应之间的差异可能是由于酶与亚硝酸盐结合的机制不同。铜-近红外光谱与O原子结合，而铁-近红外光谱只与N原子结合，使N--O键断裂，对O产生比铜-近红外光谱更大的同位素效应。利用这些新的N同位素效应在氧最小区域内还原亚硝酸盐，N循环模型比以前的模型产生更高的亚硝酸盐氧化速率。
Microbial nitrite reduction plays an important role in the nitrogen cycle, producing the first gaseous product in the denitrification pathway. The role of nitrite reduction in the environment can be assessed using stable isotope measurements of nitrite. Here, we present estimates for nitrogen (N) and oxygen (O) isotope fractionation during nitrite reduction catalyzed by copper‐containing nitrite reductase (Cu‐NIR) and cytochrome cd1‐containing nitrite reductase (Fe‐NIR). A Rayleigh fractionation model was used to calculate the N and O isotope effects, 15ε and 18ε respectively, from time‐course measurements of nitrite concentration and isotopic composition in batch culture experiments. For three strains of denitrifier carrying the Cu‐NIR, 15ε = 22 ± 2‰ and 18ε = 2 ± 2‰ (95% confidence interval). For three strains of denitrifier carrying the Fe‐NIR, 15ε = 8 ± 2 and 18ε = 6 ± 2‰ (95% confidence interval). These isotope effects for nitrite reduction are significantly different from each other. Furthermore, 15ε and 18ε do not show a 1 : 1 relationship, as has been assumed. The difference between the isotope effects for these two families of enzymes is likely due to a mechanical difference in how the enzymes bind nitrite. The Cu‐NIR binds to both O atoms and the Fe‐NIR only binds to the N, allowing either NO bond to be cleaved and imparting a larger isotope effect for O than for the Cu‐NIR. Utilizing these new N isotope effects for nitrite reduction in oxygen minimum zone N cycle models results in higher rates of nitrite oxidation than previously modeled.