assembly [73]. However, the enzyme was not connected to an ETC, probably
hindered by the fact that E. coli neither forms a Rieske/cytochrome bc complex nor
a small monohaem cytochrome c.
Although a lot has been learnt recently about the physiology and applicability of
NRBs and their N 2 O reduction systems, there are still many unknowns, especially
in the context of the ecological role of clade I and clade II NRBs [106]. In natural
habitats, many clade II nos gene clusters were found in the uncultured majority of
microorganisms, based on metagenomics and metatranscriptomics data [28]. Furthermore, distinct primer sets can be applied to amplify (partial) nosZ genes of
either clade I or clade II [106–108]. Therefore, it seems reasonable to isolate or
enrich novel NRBs along the strategies mentioned above. The use of N 2 O as sole
electron acceptor in chemostat enrichment cultures using activated sludge samples
with acetate as carbon and energy source resulted in an N 2 O-reducing community,
in which clade I NRBs dominated over clade II NRBs [109]. Interestingly, many
enriched clade II organisms from various samples were classified as Betaproteobacteria (order Rhodocyclales) closely related to bacteria of the genera
Dechloromonas, Azospira, Thauera or Comamonas ([42, 43, 109–111]; Sascha
Hein and Jörg Simon, unpublished results). Therefore, another strategy to obtain
efficient and resilient N 2 O-respiring organisms could be the isolation or
chemostat-based enrichment of new N 2 O-reducing microbes under selective conditions such as permanent microaerobiosis, a growth-limiting N 2 O concentration
and low copper content. Sampling sites might include soil, activated sludge,
digestates, the rumen and other parts of digestive systems or future hotspots of
greenhouse gas emission such as thawing permafrost or marine oxygen minimum
zones. Growth media should favour respiratory metabolism, for example, by
offering non-fermentable carbon and electron sources (formate or acetate, for
instance). Otherwise, N 2 O-tolerant fermentative bacteria might outcompete NRBs.
8 Concluding Remarks and Perspectives
In the twenty-first century, humanity faces severe global challenges. Two major
problems are climate change (and the need to reduce greenhouse gas emissions) and
the anthropogenic disturbance of biogeochemical nutritent cycles, and it becomes
increasingly clear that we cannot continue to adopt a ‘business as usual’ approach
[3, 10, 112]. Practical solutions are urgently required to restore a balanced and
sustainable nitrogen cycle on Earth, and climate- and nitrogen-smart food systems
are needed to feed the 9–10 billion people predicted for 2050. In this context, the
application potential of greenhouse gas-converting microorganisms such as
methane-oxidizing or N 2 O-reducing bacteria and archaea is clearly underexplored
[20, 113, 114].
204
J. Simon
hindered by the fact that E. coli neither forms a Rieske/cytochrome bc complex nor
a small monohaem cytochrome c.
Although a lot has been learnt recently about the physiology and applicability of
NRBs and their N 2 O reduction systems, there are still many unknowns, especially
in the context of the ecological role of clade I and clade II NRBs [106]. In natural
habitats, many clade II nos gene clusters were found in the uncultured majority of
microorganisms, based on metagenomics and metatranscriptomics data [28]. Furthermore, distinct primer sets can be applied to amplify (partial) nosZ genes of
either clade I or clade II [106–108]. Therefore, it seems reasonable to isolate or
enrich novel NRBs along the strategies mentioned above. The use of N 2 O as sole
electron acceptor in chemostat enrichment cultures using activated sludge samples
with acetate as carbon and energy source resulted in an N 2 O-reducing community,
in which clade I NRBs dominated over clade II NRBs [109]. Interestingly, many
enriched clade II organisms from various samples were classified as Betaproteobacteria (order Rhodocyclales) closely related to bacteria of the genera
Dechloromonas, Azospira, Thauera or Comamonas ([42, 43, 109–111]; Sascha
Hein and Jörg Simon, unpublished results). Therefore, another strategy to obtain
efficient and resilient N 2 O-respiring organisms could be the isolation or
chemostat-based enrichment of new N 2 O-reducing microbes under selective conditions such as permanent microaerobiosis, a growth-limiting N 2 O concentration
and low copper content. Sampling sites might include soil, activated sludge,
digestates, the rumen and other parts of digestive systems or future hotspots of
greenhouse gas emission such as thawing permafrost or marine oxygen minimum
zones. Growth media should favour respiratory metabolism, for example, by
offering non-fermentable carbon and electron sources (formate or acetate, for
instance). Otherwise, N 2 O-tolerant fermentative bacteria might outcompete NRBs.
8 Concluding Remarks and Perspectives
In the twenty-first century, humanity faces severe global challenges. Two major
problems are climate change (and the need to reduce greenhouse gas emissions) and
the anthropogenic disturbance of biogeochemical nutritent cycles, and it becomes
increasingly clear that we cannot continue to adopt a ‘business as usual’ approach
[3, 10, 112]. Practical solutions are urgently required to restore a balanced and
sustainable nitrogen cycle on Earth, and climate- and nitrogen-smart food systems
are needed to feed the 9–10 billion people predicted for 2050. In this context, the
application potential of greenhouse gas-converting microorganisms such as
methane-oxidizing or N 2 O-reducing bacteria and archaea is clearly underexplored
[20, 113, 114].
204
J. Simon