Metabolism of nitric oxide in soil and denitrifying bacteria

Abstract

Production and consumption of NO was measured under anaerobic conditions in a slightly alkaline and an acidic soil as well as in pure cultures of denitrifying Pseudomonas aeruginosa, P. stutzeri, P. fluorescens, Paracoccus denitrificans, Azospirillum brasilense, and A. lipoferum. Growing bacterial cultures reduced nitrate and intermediately accumulated nitrite, NO, N2O, but not NO2. Addition of formaldehyde inhibited NO production and NO consumption. In the presence of acetylene NO was reduced to N2O. Net NO release rates in denitrifying bacterial suspensions and in soil samples decreased hyperbolically with increasing NO up to mixing ratios of about 5 ppmv NO. This behaviour could be modelled by assuming a constant rate of NO production simultaneously with a NO consumption activity that increased with NO until Vmax was reached. The data allowed calculation of the gross rates (P) of NO production, of the rate constants (k), Vmax and Km of NO consumption, and of the NO compensation mixing ratio (mc). In soil, P was larger than Vmax resulting in net NO release even at high NO mixing ratios unless P was selectively inhibited by chlorate + chlorite or by aerobic incubation conditions. In bacteria, Vmax was somewhat larger than P resulting in net NO uptake at high NO mixing ratios. Both P and Vmax were dependent on the supply of electron donor (e.g. glucose). Both in soil (aerobic or anaerobic) and in pure culture, the Km values of NO consumption were in a similar low range of about 0.5–6.0 nM. Anaerobic soil and denitrifying bacteria exhibited mc values of 1.6–2.1 ppmv NO and 0.2–4.0 ppmv NO, respectively.