Clarifying Microbial Nitrous Oxide Reduction under Aerobic Conditions: Tolerant, Intolerant, and Sensitive

Abstract

Some bacteria can reduce N 2 O in the presence of O 2 , whereas others cannot. It is unclear whether this trait of aerobic N 2 O reduction is related to the phylogeny and structure of N 2 O reductase.

Bacterial denitrification drives elevated N 2 O emissions in arid southern California drylands

“…N 2 O reduction to N 2 is an anaerobic process not often measured in dryland ecosystems (30,47), but many denitrifiers have both NO 3 − -and N 2 O-reducing genes (4,48), such that the same organisms that reduce NO 3 − may also reduce N 2 O when wetting establishes anoxic conditions. Even if soils do not maintain anoxic microsites, a growing number of nondenitrifying organisms have been shown to reduce N 2 O under aerobic conditions, allowing for N 2 O reduction in aerated soils (48,49). Thus, while chemodenitrification may have occurred, bacterial denitrification and N 2 O reduction to N 2 best explain the N 2 O isotope values we observed, indicating that anaerobic microbial processes play an important role in regulating N 2 O emissions after wetting dry soils.…”

Contribution of Microorganisms with the Clade II Nitrous Oxide Reductase to Suppression of Surface Emissions of Nitrous Oxide

“…Some available data point to a higher affinity for nitrous oxide and less inhibition by oxygen. 4,19,50 However, our data point to much more complex environmental controls of the distribution and activity. Also, since most of the Clade II containing organisms identified in our metagenomic survey are not represented in any of the major culture collections, a future emphasis on cultivation and isolation of environmentally relevant representatives will be key to constraining models to accurately predict net emission of nitrous oxide from the soil to the atmosphere.…”

“…Produced by both biotic and abiotic processes, the only known sink for nitrous oxide below the stratosphere is the microbial reduction to N 2 by the nitrous oxide reductase (NosZ) enzyme. Although nitrous oxide is a thermodynamically more favorable electron acceptor ( E 0 = 1.77 V) than oxygen ( E 0 = 0.815 V), competition experiments with characterized facultative anaerobes have shown that nitrous oxide reduction is not always the preferred electron acceptor over a wide range of oxygen concentrations. − This could reflect the stoichiometric differences in energy yield for the alternative substrates since oxygen has a higher energy yield than nitrous oxide on a mole of oxidant basis and may be the more relevant limiting substrate in many environments. Regardless of mechanism, what would appear to be a highly favorable electron acceptor even in the presence of oxygen is lost to the atmosphere from many environments, including soils (0.0006 ± 0.0023 μmol m –2 s –1 [mean ± standard deviation] − ), marine systems (0.0019 ± 0.0035 μmol m –2 s –1 − ), and freshwater systems (0.0029 ± 0.0068 μmol m –2 s –1 ).…”

Nitrous oxide respiration in acidophilic methanotrophs

“…This means that even when exposed to high levels of O 2 , the N 2 OR in the suboxic CH 4 + O 2 + N 2 O-adapted cells remained functional and could reduce N 2 O at high rates. Other bacterial strains’ N 2 OR activities have been reported at DO concentrations between 100 and 260 µM 93, 94 , indicating that their N 2 OR activity is similarly O 2 -tolerant 93 as that of strain T4. According to the findings of Wang and colleagues 93 , N 2 O reducers with an O 2 tolerant N 2 OR maintain low internal O 2 concentrations in their cells by rapidly consuming O 2 , allowing the N 2 OR to remain active.…”

“…While O 2 is well known to impair N 2 OR activity 34, 92 , some bacterial strains have been reported to reduce N 2 O in the presence of O 2 93, 94 . We therefore tested the capacity of strains IT6 and T4 to reduce N 2 O in the presence of O 2 by using resting cells of anoxic CH 3 OH + N 2 O-cultures.…”