Aerobic dissimilatory nitrate reduction to ammonium in Streptomyces hydrogenans EM-F5: metabolic insights, physiological activity, and driving factors

ABSTRACT Dissimilatory nitrate reduction to ammonium (DNRA) can retain nitrogen in the soil and reduce greenhouse gas emissions. However, bacterial anaerobic DNRA has certain drawbacks, such as the requirement for strictly anaerobic conditions, complex management, and high cost. A new actinomycete strain EM-F5 was isolated and identified as Streptomyces hydrogenans . Strain EM-F5 showed broad environmental adaptability (pH 6.20–9.20, 0.10–0.50 × 10 8 CFU/mL of inoculum size, 3.30–7.25 mg/L of dissolved oxygen, and 18℃–38℃). Using starch as the carbon source, the strain EM-F5 achieved 98.04% nitrate reduction and 54.89% ammonium production efficiency via aerobic DNRA, whereas only 37.35% of nitrate was converted under anaerobic conditions. Electron transport system activity (ETSA), adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide (NADH/NAD + ) reached up to 0.11 μgO 2 g −1 min −1 , 29.73 μmol/g, and 5.24, respectively, confirming the high efficiency of aerobic DNRA. The specific activity of nitrate reductase (NR) and nitrite reductase (NIR) was detected as 0.323 and 0.018 U/mg protein, respectively, confirming its key catalytic function in the DNRA pathway. Nitrogen balance (nitrogen loss of 25.46%) experiments further confirmed the aerobic DNRA pathway of strain EM-F5 was NO 3 − -N→NO 2 − -N→NH 4 + -N. These findings expand the current understanding of DNRA in actinomycetes and provide a promising microbial resource for nitrogen recycling in soil. IMPORTANCE Strain EM-F5 effectively performed dissimilatory nitrate reduction to ammonium (DNRA) with starch as the carbon source under aerobic conditions. Nitrate reduction was as high as 98.04%, and the ammonium production efficiency accounted for 54.89% of the total converted nitrate nitrogen. The aerobic DNRA pathway of strain EM-F5 was NO 3 − -N→NO 2 − -N→NH 4 + -N. Meanwhile, it converted 37.35% of nitrate in anaerobic environments. So far, there have been few studies that have demonstrated that actinomycetes can efficiently perform DNRA under aerobic conditions and also convert nitrate under anaerobic conditions. This provides a superior microbial source for treating highly nitrate-contaminated soil.