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Denitrification as an N2O sink

Conthe, Monica, Lycus, Pawel, Arntzen, Magnus Ø., Ramos da Silva, Aline, Frostegård, Åsa, Bakken, Lars R., Kleerebezem, Robbert, van Loosdrecht, Mark C.M.
Water research 2019 v.151 pp. 381-387
activated sludge, ammonium, byproducts, carbon, denitrification, electrons, genes, greenhouse gas emissions, greenhouse gases, microbial communities, nitrates, nitrous oxide, nitrous-oxide reductase, oxidation, quantitative polymerase chain reaction, surveys, wastewater treatment
The strong greenhouse gas nitrous oxide (N2O) can be emitted from wastewater treatment systems as a byproduct of ammonium oxidation and as the last intermediate in the stepwise reduction of nitrate to N2 by denitrifying organisms. A potential strategy to reduce N2O emissions would be to enhance the activity of N2O reductase (NOS) in the denitrifying microbial community. A survey of existing literature on denitrification in wastewater treatment systems showed that the N2O reducing capacity (VmaxN2O→N2) exceeded the capacity to produce N2O (VmaxNO3→N2O) by a factor of 2–10. This suggests that denitrification can be an effective sink for N2O, potentially scavenging a fraction of the N2O produced by ammonium oxidation or abiotic reactions. We conducted a series of incubation experiments with freshly sampled activated sludge from a wastewater treatment system in Oslo and found that the ratio α = VmaxN2O→N2/VmaxNO3→N2O fluctuated between 2 and 5 in samples taken at intervals over a period of 5 weeks. Adding a cocktail of carbon substrates resulted in increasing rates, but had no significant effect on α. Based on these results – complemented with qPCR and metaproteomic data – we discuss whether the overcapacity to reduce N2O can be ascribed to gene/protein abundance ratios (nosZ/nir), or whether in-cell competition between the reductases for electrons could be of greater importance.