Abstract
Abstract. Nitrous oxide (N2O) is a potent greenhouse gas and produced in denitrification and nitrification by various microorganisms. Site preference (SP) of 15N in N2O, which is defined as the difference in the natural abundance of isotopomers 14N15NO and 15N14NO relative to 14N14NO, has been reported to be a useful tool to quantitatively distinguish N2O production pathways. To determine representative SP values for each microbial process, we firstly measured SP of N2O produced in the enzyme reaction of hydroxylamine oxidoreductase (HAO) purified from two species of ammonia oxidizing bacteria (AOB), Nitrosomonas europaea and Nitrosococcus oceani, and that of nitric oxide reductase (NOR) from Paracoccus denitrificans. The SP value for NOR reaction (−5.9 ± 2.1‰) showed nearly the same value as that reported for N2O produced by P. denitrificans in pure culture. In contrast, SP value for HAO reaction (36.3 ± 2.3‰) was a little higher than the values reported for N2O produced by AOB in aerobic pure culture. Using the SP values obtained by HAO and NOR reactions, we calculated relative contribution of the nitrite (NO2–) reduction (which is followed by NO reduction) to N2O production by N. oceani incubated under different O2 availability. Our calculations revealed that previous in vivo studies might have underestimated the SP value for the NH2OH oxidation pathway possibly due to a small contribution of NO2– reduction pathway. Further evaluation of isotopomer signatures of N2O using common enzymes of other processes related to N2O would improve the isotopomer analysis of N2O in various environments.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas and contributes indirectly to the destruction of the ozone layer in the stratosphere (Ravishankara et al, 2009)
The δ18O showed a variation among experiments A–D of N. oceani, it is not clear whether the δ18O of NH2OH was different between experiments A/B and C/D
Kept high enough to maintain the aerobic condition for the following reasons: (1) it is reported that the measured ratio of consumed O2 and produced NO−2 during aerobic incubation of N. oceani (1.35–1.43) is consistent with the ratio calculated from the stoichiometry of ammonia oxidation to nitrite (1.50) (Gundersen, 1966); (2) oxygen consumption by respiration using endogenous organics is low (Gundersen, 1966); and (3) a quantitative estimate based on the stoichiometry and the amount of the product (NO−2 and N2O) in this study indicates that the oxygen consumption was relatively small
Summary
Nitrous oxide (N2O) is a potent greenhouse gas and contributes indirectly to the destruction of the ozone layer in the stratosphere (Ravishankara et al, 2009). Production of N2O on earth is mainly controlled by microbial processes that include nitrification and denitrification (Stein and Yung, 2003). Autotrophic microorganisms like ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) oxidize ammonia to nitrite (NO−2 ) and produce N2O as a byproduct under mainly aerobic conditions (Casciotti et al, 2011). Various heterotrophic denitrifying microorganisms such as archaea, bacteria, and fungi reduce nitrate (NO−3 ) or NO−2 to dinitrogen (N2) as electron acceptors for anaerobic respiration and produce N2O as an intermediate under mainly anaerobic conditions (Hayastu et al, 2008). T. Yamazaki et al.: Isotopomeric characterization of nitrous oxide autotrophic/heterotrophic and oxic/anoxic processes. Investigation of N2O production pathways is useful to understand the nitrogen cycle and relevant microorganisms in various environments
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