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The inhibition of Bradyrhizobium sp. [Lupinus] denitrificatory growth by nitrite

The genetic diversity of bacteria from the genus Rhizobiace is the reason why particular strains of rhizobia have such large differences in their energy conservation pathways and nitrogen metabolism. As a consequence, rhizobia strains differ in their response to changing oxygen concentrations in the soil and in symbiotic habitats. Under microaerobic conditions, a number of symbiotic rhizobia strains activate a denitrification system which is able to produce ATP at 40-50% of the oxygen respiration level. Investigations performed in our laboratory showed that the Bradyrhizobium sp. (Lupinus) USDA3045 strain also possesses nitrate and nitrite reductases functioning in a dissimilatory manner. The aim of this study was to focus on the nutrition limits on nitrogen oxides related to dissimilatory growth. The nitrate respiration process was indicated in USDA3045 because an anaerobic culture of this strain was able to efficiently grow on a non-growth medium supplemented with nitrate as the sole nitrogen and energy source. The growth yield on a minimal medium with nitrite was similar. Besides having the ability to acquire energy from the dissimilation of nitrogen oxides, the strain appeared to be equally effective as a fermenter. The investigated strain rapidly consumed nitrates in the range of 1-7 mM, but at higher concentrations, NO 3-utilisation tended to stop shortly after 10 mM of nitrate had been taken up. Coincidental to this effect was the accumulation of extracellular nitrite (up to 7 mM), if the nitrate concentration in the medium was applied at levels higher than 10 mM. An exponential induction of nitrate reductase specific activity was observed in the range of 1-10mM of nitrate in the medium. It was followed by a 50% drop in activity at higher nitrate concentrations. Nitrite addition at 1mM was sufficient to cause a 2-fold stimulation of nitrite reductase specific activity. Nevertheless, the application of higher nitrite concentrations led to no additional increase in activity. It suggests the weak detoxification efficiency of the enzyme. It is consistent with the observation that concentrations of nitrite higher than 10mM could be partially taken up, but could not be utilised. In spite of subsequent significant excretion of nitrite, 70% growth inhibition of the investigated strain was observed. The presented data show that anaerobic nitrite transport is at a disproportionately high level relative to nitrite reduction in the USDA3045 strain.