Perez-Garcia Octavio, Chandran Kartik, Villas-Boas Silas G, Singhal Naresh
Department of Civil and Environmental Engineering, University of Auckland, 20 Symonds Street, Auckland, New Zealand.
Department of Earth and Environmental Engineering, Columbia University, New York, New York.
Biotechnol Bioeng. 2016 May;113(5):1124-36. doi: 10.1002/bit.25880. Epub 2015 Nov 26.
Over the coming decades nitrous oxide (N2O) is expected to become a dominant greenhouse gas and atmospheric ozone depleting substance. In wastewater treatment systems, N2O is majorly produced by nitrifying microbes through biochemical reduction of nitrite (NO2(-)) and nitric oxide (NO). However it is unknown if the amount of N2O formed is affected by alternative NO redox reactions catalyzed by oxidative nitrite oxidoreductase (NirK), cytochromes (i.e., P460 [CytP460] and 554 [Cyt554 ]) and flavohemoglobins (Hmp) in ammonia- and nitrite-oxidizing bacteria (AOB and NOB, respectively). In this study, a mathematical model is developed to assess how N2O formation is affected by such alternative nitrogen redox transformations. The developed multispecies metabolic network model captures the nitrogen respiratory pathways inferred from genomes of eight AOB and NOB species. The performance of model variants, obtained as different combinations of active NO redox reactions, was assessed against nine experimental datasets for nitrifying cultures producing N2O at different concentration of electron donor and acceptor. Model predicted metabolic fluxes show that only variants that included NO oxidation to NO2(-) by CytP460 and Hmp in AOB gave statistically similar estimates to observed production rates of N2O, NO, NO2(-) and nitrate (NO3(-)), together with fractions of AOB and NOB species in biomass. Simulations showed that NO oxidation to NO2(-) decreased N2O formation by 60% without changing culture's NO2(-) production rate. Model variants including NO reduction to N2O by Cyt554 and cNor in NOB did not improve the accuracy of experimental datasets estimates, suggesting null N2O production by NOB during nitrification. Finally, the analysis shows that in nitrifying cultures transitioning from dissolved oxygen levels above 3.8 ± 0.38 to <1.5 ± 0.8 mg/L, NOB cells can oxidize the NO produced by AOB through reactions catalyzed by oxidative NirK.
在未来几十年里，一氧化二氮（N₂O）预计将成为主要的温室气体和大气臭氧层消耗物质。在废水处理系统中，N₂O主要由硝化微生物通过亚硝酸盐（NO₂⁻）和一氧化氮（NO）的生化还原作用产生。然而，尚不清楚所形成的N₂O量是否受到氨氧化细菌（AOB）和亚硝酸盐氧化细菌（NOB）中氧化型亚硝酸盐氧化还原酶（NirK）、细胞色素（即P460 [CytP460] 和554 [Cyt554]）以及黄素血红蛋白（Hmp）催化的替代性NO氧化还原反应的影响。在本研究中，开发了一个数学模型来评估此类替代性氮氧化还原转化如何影响N₂O的形成。所开发的多物种代谢网络模型捕捉了从8种AOB和NOB物种的基因组推断出的氮呼吸途径。针对产生不同浓度电子供体和受体的N₂O的硝化培养物的9个实验数据集，评估了作为活性NO氧化还原反应的不同组合而获得的模型变体的性能。模型预测的代谢通量表明，只有在AOB中包含CytP460和Hmp将NO氧化为NO₂⁻的变体，对N₂O、NO、NO₂⁻和硝酸盐（NO₃⁻）的观测产生速率以及生物量中AOB和NOB物种的比例给出了统计学上相似的估计。模拟表明，将NO氧化为NO₂⁻可使N₂O的形成减少60%，而不改变培养物的NO₂⁻产生速率。包括NOB中Cyt554和cNor将NO还原为N₂O的模型变体并未提高实验数据集估计的准确性，这表明硝化过程中NOB产生的N₂O为零。最后，分析表明，在硝化培养物从溶解氧水平高于3.8±0.38 mg/L转变为<1.5±0.8 mg/L的过程中，NOB细胞可以通过氧化型NirK催化的反应氧化AOB产生的NO。
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