Sammendrag
A major part of anthropogenic N2O comes from denitrifying bacteria in agricultural systems and the emissions are predicted to continue to increase. Efficient mitigation options are largely lacking, and microbiologists, biogeochemists and biochemists need to join forces to develop novel ways to tackle this major greenhouse gas. Many denitrifying organisms can reduce nitrate to N2O but lack the last step of the denitrification pathway, reduction of N2O to N2. However, also organisms that perform complete denitrification differ remarkably in their emission of N2O and may act as sinks or sources of this gas, depending on their regulation. Research efforts during the past decades has revealed many details of the regulatory biology of denitrification, involving both transcriptional and post-transcriptional control mechanisms. Most of this knowledge comes from detailed studies of a limited number of model organism, but several more recent studies have included a wider range of environmental isolates. The emerging picture is a rather high phenotypic and genotypic richness among denitrifiers, leading to large variation in the accumulation of denitrification intermediate products, including N2O. The current knowledge is however biased because experiments were conducted under optimal conditions, while bacteria in most natural environments are exposed to fluctuating oxygen conditions and are starved for carbon substrates most of the time. There is urgent need for investigations of how such environmentally relevant stress factors affect the denitrification phenotype of different types of bacteria. Such knowledge is essential when selecting organisms suitable for developing methods to engineer soil microbial systems for N2O mitigation.
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