Nitrous oxide (N2O) production by Alcaligenes faecalis during feast and famine regimes

The environmentally harmful compound, nitrous oxide (N2O), can accumulate as an intermediate in the process of denitrification. One important parameter, which can influence this accumulation, is the feeding regime sensed by the bacteria involved, in which an unbalanced supply of electron donor and acceptor may occur. When pulse additions of C-compounds (acetate, butyrate and malate) were given to denitrifying cultures of Alcaligenes faecalis strain TUD, the production rate of N2O was reduced from 9.9–18.5% to 1.8–10.4% of the total nitrite converted, as long as the C-substrate was in excess. However, as soon as the availability of carbon compounds became exhausted and the culture entered starvation, N2O was one of the main products of denitrification and production increased to 32–64% of the total N-feed. Under dynamic feeding conditions, the culture was able to adapt to the fluctuating conditions and the ratio of N2O to nitrite decreased. However, during starvation the ratio of N2O to nitrite was still high (± 27%), indicating that with prolonged starvation, the overall N2O emission will increase. Competition between the enzymes of denitrification for electrons from the cytochrome c pool could explain the emission of N2O, if the enzyme N2O-reductase has a lower affinity for the electron-donor than the other reductases.