species [25, 32]. This process, also called codenitrification or BioNitrosation, is
facilitated by the presence of organics and metal ions. In addition, N 2 O is produced
by abiotically supported processes such as chemodenitrification [33–35].
The microbial trait of N 2 O reduction to N 2 was originally assigned to denitrifying species of the phylum Proteobacteria [36]. In denitrification, the sequential
reduction of nitrate via nitrite, NO and N 2 O to N 2 is stepwise catalysed by dedicated
enzymes. Typical denitrifiers are heterotrophs that use a variety of carbon compounds as electron and carbon source. Such electrons serve to reduce the
quinone/quinol pool and are eventually transferred to terminal reductases of
anaerobic respiration, namely nitrate reductase, NO-producing nitrite reductase
(either cytochrome cd 1 nitrite reductase NirS or Cu-containing nitrite reductase
NirK), N 2 O-producing NO reductase and N 2 O reductase (NosZ) [36]. NosZ is a
copper-containing enzyme that is described in Sect. 4 in more detail. Over the last
decade, it emerged that, based on phylogenetic analyses, each NosZ enzyme
belongs to one of the two distinct clades, which are nowadays called clade I and
clade II [28, 37, 38]. Furthermore, it turned out that this nomenclature can be
expanded to the corresponding NosZ-encoding nos gene clusters (NGCs) and
NosZ-harbouring microorganisms (see Table 1 for some properties of clade I and
clade II N 2 O reduction systems and Sects. 4 and 5 for more details).
Denitrification and DNRA (also known as respiratory nitrate/nitrite ammonification) are forms of microbial anaerobic respiration. The free energy of a redox
reaction catalysed in the course of anaerobic respiration is typically conserved in the
generation of a proton gradient over the cytoplasmic membrane (the proton motive
force, pmf), which is then used to drive ADP phosphorylation by ATP synthase.
According to this definition, the reduction of N 2 O to N 2 qualifies as anaerobic
respiration in its own right and is called here ‘N 2 O respiration’. N 2 O-respiring
microorganisms (NRBs) can be denitrifiers that catalyse N 2 O respiration as part of
their denitrification pathway or NosZ-containing nitrate/nitrite ammonifiers that use
N 2 O respiration to convert N 2 O that has been produced from endogenous NO
detoxification or by cells from the surrounding microbial community. Notably,
some denitrifiers and DNRA-performing microbes have even been shown to grow
by N 2 O respiration in the laboratory using N 2 O (supplied at various concentrations)
as the sole electron acceptor (see Sect. 2). In general, the reduction of nitrogen
oxyanions and nitrogen oxides such as nitrate, nitrite, NO and N 2 O is energetically
favourable, due to the rather positive midpoint redox potentials of the corresponding redox pairs (expressed in the E 0 ′ value, i.e. under standard conditions at
pH 7.0). For example, E 0 ′ of the N 2 O/N 2 couple amounts to + 1.36 V, which
makes N 2 O reduction by, for instance, hydrogen, formate or NAD(P)H highly
exergonic (see Sect. 2).
This chapter describes the physiology of NRBs and aspects of the molecular
basis of microbial N 2 O reduction/respiration at different levels (Sects. 2–5). Furthermore, it presents experimental approaches that aim to apply or engineer NRBs
in the context of humankind’s N 2 O emission problem (Sects. 6 and 7). The reader
might keep the following central questions in mind throughout the chapter.
188
J. Simon
facilitated by the presence of organics and metal ions. In addition, N 2 O is produced
by abiotically supported processes such as chemodenitrification [33–35].
The microbial trait of N 2 O reduction to N 2 was originally assigned to denitrifying species of the phylum Proteobacteria [36]. In denitrification, the sequential
reduction of nitrate via nitrite, NO and N 2 O to N 2 is stepwise catalysed by dedicated
enzymes. Typical denitrifiers are heterotrophs that use a variety of carbon compounds as electron and carbon source. Such electrons serve to reduce the
quinone/quinol pool and are eventually transferred to terminal reductases of
anaerobic respiration, namely nitrate reductase, NO-producing nitrite reductase
(either cytochrome cd 1 nitrite reductase NirS or Cu-containing nitrite reductase
NirK), N 2 O-producing NO reductase and N 2 O reductase (NosZ) [36]. NosZ is a
copper-containing enzyme that is described in Sect. 4 in more detail. Over the last
decade, it emerged that, based on phylogenetic analyses, each NosZ enzyme
belongs to one of the two distinct clades, which are nowadays called clade I and
clade II [28, 37, 38]. Furthermore, it turned out that this nomenclature can be
expanded to the corresponding NosZ-encoding nos gene clusters (NGCs) and
NosZ-harbouring microorganisms (see Table 1 for some properties of clade I and
clade II N 2 O reduction systems and Sects. 4 and 5 for more details).
Denitrification and DNRA (also known as respiratory nitrate/nitrite ammonification) are forms of microbial anaerobic respiration. The free energy of a redox
reaction catalysed in the course of anaerobic respiration is typically conserved in the
generation of a proton gradient over the cytoplasmic membrane (the proton motive
force, pmf), which is then used to drive ADP phosphorylation by ATP synthase.
According to this definition, the reduction of N 2 O to N 2 qualifies as anaerobic
respiration in its own right and is called here ‘N 2 O respiration’. N 2 O-respiring
microorganisms (NRBs) can be denitrifiers that catalyse N 2 O respiration as part of
their denitrification pathway or NosZ-containing nitrate/nitrite ammonifiers that use
N 2 O respiration to convert N 2 O that has been produced from endogenous NO
detoxification or by cells from the surrounding microbial community. Notably,
some denitrifiers and DNRA-performing microbes have even been shown to grow
by N 2 O respiration in the laboratory using N 2 O (supplied at various concentrations)
as the sole electron acceptor (see Sect. 2). In general, the reduction of nitrogen
oxyanions and nitrogen oxides such as nitrate, nitrite, NO and N 2 O is energetically
favourable, due to the rather positive midpoint redox potentials of the corresponding redox pairs (expressed in the E 0 ′ value, i.e. under standard conditions at
pH 7.0). For example, E 0 ′ of the N 2 O/N 2 couple amounts to + 1.36 V, which
makes N 2 O reduction by, for instance, hydrogen, formate or NAD(P)H highly
exergonic (see Sect. 2).
This chapter describes the physiology of NRBs and aspects of the molecular
basis of microbial N 2 O reduction/respiration at different levels (Sects. 2–5). Furthermore, it presents experimental approaches that aim to apply or engineer NRBs
in the context of humankind’s N 2 O emission problem (Sects. 6 and 7). The reader
might keep the following central questions in mind throughout the chapter.
188
J. Simon