Fig. 3b, especially with respect to NosB acting as an interaction platform. Unfortunately, none of the mentioned fusion proteins has been characterized experimentally. Nonetheless, it seems attractive to postulate a complex made up from the
accessory Nos proteins that transiently interacts with NosZ and thereby facilitates
copper or electron transfer reactions necessary for maturation and/or activity
maintenance. The existence of a corresponding denitrification respirasome in
P. aeruginosa cells was indicated in a recent study that also reported interaction of
NosR and NosL with NosZ [81]. However, such a respirasome has not been isolated or structurally characterized and the stoichiometry of individual proteins or
protein complexes has not been elucidated.
6 Mitigation of N 2 O Emissions by Bioaugmentation
The use of NRBs to serve as an N 2 O sink in natural or man-made environments has
to be considered an attractive counteraction in addressing the problem of increasing
anthropogenic N 2 O emissions [19, 20, 28, 29]. In principle, many different
ecosystems such as activated sludge from wastewater treatment plants, agricultural
soil or digestates might be subject to microbial bioaugmentation approaches.
However, it has to be kept in mind that these habitats differ significantly not only in
their resident microbial communities but also in terms of environmental parameters
such as carbon and nitrogen availability, carbon-to-nitrogen ratio, oxygenation, pH
as well as copper content and copper complexation capability. In general, it seems
advisable to augment with habitat-adapted organisms that have been isolated from
the same or an ecologically similar environment to ensure cell survival and proliferation. Otherwise, the question has to be asked how long any unadapted bacteria
could possibly persist in a ‘foreign’ environment. Moreover, it appears reasonable
to use organisms for bioaugmentation that exhibit a net N 2 O consumption behaviour, which would favour DNRA organsism over many denitrifiers [29]. Cells
capable of N 2 O reduction in the presence of ambient oxygen are desirable or, as an
alternative, cells showing a fast reactivation of N 2 O reduction after oxygen inhibition appear suitable.
Table 3 summarizes some representative bioaugmentation studies that used
several different N 2 O-reducing bacteria, including clade I and clade II bacteria as
well as strains of Pseudomonas stutzeri capable of ‘aerobic denitrification’ (see
Sect. 2). The reported results allowed to conclude that N 2 O mitigation (i.e. reduced
N 2 O emissions after bioaugmentation) was indeed achievable in a laboratory scale
(microcosm experiments) as well as in mesocosm and field studies, at least in the
short run. At longer timescales, it remains to be seen whether any added cells will
be persistent enough to sustainably affect the fine balance between N 2 O production
and consumption in an established natural microbial community.
200
J. Simon
accessory Nos proteins that transiently interacts with NosZ and thereby facilitates
copper or electron transfer reactions necessary for maturation and/or activity
maintenance. The existence of a corresponding denitrification respirasome in
P. aeruginosa cells was indicated in a recent study that also reported interaction of
NosR and NosL with NosZ [81]. However, such a respirasome has not been isolated or structurally characterized and the stoichiometry of individual proteins or
protein complexes has not been elucidated.
6 Mitigation of N 2 O Emissions by Bioaugmentation
The use of NRBs to serve as an N 2 O sink in natural or man-made environments has
to be considered an attractive counteraction in addressing the problem of increasing
anthropogenic N 2 O emissions [19, 20, 28, 29]. In principle, many different
ecosystems such as activated sludge from wastewater treatment plants, agricultural
soil or digestates might be subject to microbial bioaugmentation approaches.
However, it has to be kept in mind that these habitats differ significantly not only in
their resident microbial communities but also in terms of environmental parameters
such as carbon and nitrogen availability, carbon-to-nitrogen ratio, oxygenation, pH
as well as copper content and copper complexation capability. In general, it seems
advisable to augment with habitat-adapted organisms that have been isolated from
the same or an ecologically similar environment to ensure cell survival and proliferation. Otherwise, the question has to be asked how long any unadapted bacteria
could possibly persist in a ‘foreign’ environment. Moreover, it appears reasonable
to use organisms for bioaugmentation that exhibit a net N 2 O consumption behaviour, which would favour DNRA organsism over many denitrifiers [29]. Cells
capable of N 2 O reduction in the presence of ambient oxygen are desirable or, as an
alternative, cells showing a fast reactivation of N 2 O reduction after oxygen inhibition appear suitable.
Table 3 summarizes some representative bioaugmentation studies that used
several different N 2 O-reducing bacteria, including clade I and clade II bacteria as
well as strains of Pseudomonas stutzeri capable of ‘aerobic denitrification’ (see
Sect. 2). The reported results allowed to conclude that N 2 O mitigation (i.e. reduced
N 2 O emissions after bioaugmentation) was indeed achievable in a laboratory scale
(microcosm experiments) as well as in mesocosm and field studies, at least in the
short run. At longer timescales, it remains to be seen whether any added cells will
be persistent enough to sustainably affect the fine balance between N 2 O production
and consumption in an established natural microbial community.
200
J. Simon