Change in nitrite conversion direction from oxidation to reduction in heterotrophic bacteria depending on the aeration conditions

For investigation of the effects of aeration on nitrite- and nitrate-transforming activities of various heterotrophic bacteria, a series of coefficients of the oxygen absorption rate (K_d, 8-99 × 10^-7 mol/ml·min·atm) in 500-ml shaking flasks were determined by varying plug types and culture volumes. Bacillus badius I-73, which neither shows denitrification activity nor utilize nitrate as a nitrogen source, consumed nitrite and accumulated nitrate at all K_d values at which experiments were conducted. In B. subtilis I-41, which does show denitrification activity, the manner of nitrite and nitrate conversion was influenced by the culture time and K_d, and the direction of conversion was changed from reduction to oxidation, as the K_d of the culture increased. Pseudomonas pavonaceae, another denitrification-positive strain, metabolized both nitrite and nitrate to more reduced compounds at low K_d, and the direction of conversion changed from reduction to oxidation at K_d = 20 × 10¹⁷ mol/ml·min·atm. Such switching behavior was also observed when P. pavonaceae was cultured continuously during variation of the aeration conditions with supply of pure oxygen. Many other denitrification-positive strains behaved similarly to P. pavonaceae, and showed their own critical K_d, the point at which the direction of nitrite metabolism changed. The results of intact-cell reaction experiments indicate that this switching might be caused by inhibition and repression of nitrite-reducing activity, and by stimulation of nitrite-oxidizing activity by oxygen.