comparing such values since different growth media and conditions have been used
[19]. The parameters did not clearly correlate with the possession of either a clade I
or clade II NosZ or whether the organism is a nitrate/nitrite-reducing denitrifier or a
non-denitrifying DNRA bacterium. However, the ecological significance of these
findings remained unclear.
NRBs have been isolated from various, mostly anoxic or micro-oxic habitats
such as soil, activated sludge or animal gut or rumen, and many of these organisms
are facultatively anaerobic bacteria employing a respiratory chain involved in
(micro)aerobic respiration. This feature is thought to impart a certain degree of
aerotolerance on the cells and may shield N 2 O reductase from inactivation by
oxygen. In fact, several P. stutzeri strains have been described to perform N 2 O
reduction under microaerobic or even aerobic conditions [52–56]. This property is
exceptional since N 2 O reduction by NosZ usually requires anoxic conditions due to
the vulnerability of the NosZ copper centres to oxygen (see Sect. 4). Interestingly,
N 2 O reduction under (micro)oxic conditions has also been reported for clade II
organisms such as Azospira sp. and G. aurantiaca [42, 57].
3 The Genomic Level: Diversity of nos Gene Clusters
As stated in the Introduction, phylogenetic analyses indicated that NosZ enzymes
either belong to clade I or clade II, and it appeared that this classification is also
applicable to the corresponding nos gene clusters (NGCs). As a matter of fact, the
composition of clade I and clade II NGCs differs to a large extent, reflecting the
biochemical complexity of the corresponding N 2 O reduction systems (see Sects. 4
and 5).
Figure 2 shows three different NGCs that are representative for (i) a clade I NGC
from a denitrifier (Paracoccus denitrificans), (ii) a complex clade II NGC (W.
succinogenes) and (iii) a somewhat simpler clade II NGC from a species of the
phylum Firmicutes (Bacillus vireti) that lacks the nosG, -C1, -C2 and -H gene
assembly (Table 1). Note that DNRA bacteria such as W. succinogenes and B. vireti
typically contain a clade II rather than a clade I NGC. The clade I core NGC
comprises genes encoding NosR, -Z, -D, -F, -Y and -L, whereas the genes coding
for NosC and NosX are present only in some cases [see Table 2 for an overview of
the properties of different Nos proteins and their (proposed) function]. Of the core
Nos proteins, only NosR is found exclusively in clade I NGCs. In contrast, multiple
conserved genes that are specifically found in clade II NGCs have been described.
NosB is encoded in all known clade II NGCs, whereas the cluster of four genes
encoding NosG, -C1, -C2 and -H is found in many, but not all, clade II NGCs.
Mitigation of Laughing Gas Emissions …
191
[19]. The parameters did not clearly correlate with the possession of either a clade I
or clade II NosZ or whether the organism is a nitrate/nitrite-reducing denitrifier or a
non-denitrifying DNRA bacterium. However, the ecological significance of these
findings remained unclear.
NRBs have been isolated from various, mostly anoxic or micro-oxic habitats
such as soil, activated sludge or animal gut or rumen, and many of these organisms
are facultatively anaerobic bacteria employing a respiratory chain involved in
(micro)aerobic respiration. This feature is thought to impart a certain degree of
aerotolerance on the cells and may shield N 2 O reductase from inactivation by
oxygen. In fact, several P. stutzeri strains have been described to perform N 2 O
reduction under microaerobic or even aerobic conditions [52–56]. This property is
exceptional since N 2 O reduction by NosZ usually requires anoxic conditions due to
the vulnerability of the NosZ copper centres to oxygen (see Sect. 4). Interestingly,
N 2 O reduction under (micro)oxic conditions has also been reported for clade II
organisms such as Azospira sp. and G. aurantiaca [42, 57].
3 The Genomic Level: Diversity of nos Gene Clusters
As stated in the Introduction, phylogenetic analyses indicated that NosZ enzymes
either belong to clade I or clade II, and it appeared that this classification is also
applicable to the corresponding nos gene clusters (NGCs). As a matter of fact, the
composition of clade I and clade II NGCs differs to a large extent, reflecting the
biochemical complexity of the corresponding N 2 O reduction systems (see Sects. 4
and 5).
Figure 2 shows three different NGCs that are representative for (i) a clade I NGC
from a denitrifier (Paracoccus denitrificans), (ii) a complex clade II NGC (W.
succinogenes) and (iii) a somewhat simpler clade II NGC from a species of the
phylum Firmicutes (Bacillus vireti) that lacks the nosG, -C1, -C2 and -H gene
assembly (Table 1). Note that DNRA bacteria such as W. succinogenes and B. vireti
typically contain a clade II rather than a clade I NGC. The clade I core NGC
comprises genes encoding NosR, -Z, -D, -F, -Y and -L, whereas the genes coding
for NosC and NosX are present only in some cases [see Table 2 for an overview of
the properties of different Nos proteins and their (proposed) function]. Of the core
Nos proteins, only NosR is found exclusively in clade I NGCs. In contrast, multiple
conserved genes that are specifically found in clade II NGCs have been described.
NosB is encoded in all known clade II NGCs, whereas the cluster of four genes
encoding NosG, -C1, -C2 and -H is found in many, but not all, clade II NGCs.
Mitigation of Laughing Gas Emissions …
191