Perspective research in the field of NRBs might be guided based on the following knowledge gaps and thoughts.
• The detailed function of each Nos protein except NosZ needs to be elucidated.
• With the exception of NosZ and apo-NosL, structural information on Nos proteins is generally lacking. Data on Nos proteomics/complexomics and the cellular organization of the Nos system is scarce.
• Only few organisms have been described for which nos gene cluster mutants are
available, for example, W. succinogenes, P. stutzeri and P. denitrificans.
• The mechanism of NosZ reactivation following oxygen inhibition is unclear.
• The ecological significance of the abundance and function of clade I and clade II
systems in natural habitats is unresolved.
• Despite some indications, it is unclear whether N 2 O could be sensed by clade II
NRBs in order to up-regulate the Nos system [42, 51]. In denitrifying clade I
organisms, nitrate and NO (but not N 2 O) are known to induce the corresponding
respiratory system [18].
• In soil, the combination between efficient N 2 O reduction and dinitrogen fixation
appears to be especially desirable and, in this respect, Bradyrhizobium species
appear be especially useful [115, 116].
• Organisms that couple methane oxidation to N 2 O reduction are yet to be
described. Nonetheless, these organisms might exist as the corresponding
reaction is exergonic and methane oxidation in anoxic environments has been
reported [20, 117, 118].
Hopefully, future research along these points will help to find a sustainable and
environmentally friendly solution to mitigate N 2 O emissions from different habitats.
Very likely, microbiology holds the key.
Acknowledgements The author thanks the co-workers of his cited publications for their contributions. Laboratory work is funded by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation; projects 324263958 and 418104137) and the Forum for Interdisciplinary
Research (FiF) at TU Darmstadt.
References
1. IPCC (2014) Climate change 2014: synthesis report. Contribution of working groups I, II
and III to the fifth assessment report of the intergovernmental panel on climate change.
IPCC, Geneva, Switzerland
2. Ravishankara AR, Daniel JS, Portmann RW (2009) Nitrous oxide (N 2 O): the dominant
ozone-depleting substance emitted in the 21st century. Science 326:123–125
3. Rockström J, Steffen W, Noone K et al (2009) A safe operating space for humanity. Nature
461:472–475
4. Schilt A, Baumgartner M, Blunier T (2010) Glacial-interglacial and millennial scale
variations in the atmospheric nitrous oxide concentration during the last 800,000 years.
Quaternary Sc Rev 29:182–192
Mitigation of Laughing Gas Emissions …
205
• The detailed function of each Nos protein except NosZ needs to be elucidated.
• With the exception of NosZ and apo-NosL, structural information on Nos proteins is generally lacking. Data on Nos proteomics/complexomics and the cellular organization of the Nos system is scarce.
• Only few organisms have been described for which nos gene cluster mutants are
available, for example, W. succinogenes, P. stutzeri and P. denitrificans.
• The mechanism of NosZ reactivation following oxygen inhibition is unclear.
• The ecological significance of the abundance and function of clade I and clade II
systems in natural habitats is unresolved.
• Despite some indications, it is unclear whether N 2 O could be sensed by clade II
NRBs in order to up-regulate the Nos system [42, 51]. In denitrifying clade I
organisms, nitrate and NO (but not N 2 O) are known to induce the corresponding
respiratory system [18].
• In soil, the combination between efficient N 2 O reduction and dinitrogen fixation
appears to be especially desirable and, in this respect, Bradyrhizobium species
appear be especially useful [115, 116].
• Organisms that couple methane oxidation to N 2 O reduction are yet to be
described. Nonetheless, these organisms might exist as the corresponding
reaction is exergonic and methane oxidation in anoxic environments has been
reported [20, 117, 118].
Hopefully, future research along these points will help to find a sustainable and
environmentally friendly solution to mitigate N 2 O emissions from different habitats.
Very likely, microbiology holds the key.
Acknowledgements The author thanks the co-workers of his cited publications for their contributions. Laboratory work is funded by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation; projects 324263958 and 418104137) and the Forum for Interdisciplinary
Research (FiF) at TU Darmstadt.
References
1. IPCC (2014) Climate change 2014: synthesis report. Contribution of working groups I, II
and III to the fifth assessment report of the intergovernmental panel on climate change.
IPCC, Geneva, Switzerland
2. Ravishankara AR, Daniel JS, Portmann RW (2009) Nitrous oxide (N 2 O): the dominant
ozone-depleting substance emitted in the 21st century. Science 326:123–125
3. Rockström J, Steffen W, Noone K et al (2009) A safe operating space for humanity. Nature
461:472–475
4. Schilt A, Baumgartner M, Blunier T (2010) Glacial-interglacial and millennial scale
variations in the atmospheric nitrous oxide concentration during the last 800,000 years.
Quaternary Sc Rev 29:182–192
Mitigation of Laughing Gas Emissions …
205