Mycelial morphology influenced aerobic DNRA in <i>Streptomyces mediolani</i> EM-B2: short rod-shaped mycelium showed markedly greater efficiency than long filamentous mycelium

ABSTRACT Microbial dissimilatory nitrate reduction to ammonium (DNRA) metabolism offers a novel eco-friendly ammonium production pathway. However, most reported anaerobic DNRA consortia showed low nitrite/oxygen tolerance, limited ammonium yield, and scarce reported pure-cultured DNRA functional actinomycete. This work revealed an aerobic morphology-dependent DNRA strain of Streptomyces mediolani EM-B2 with glucose as a carbon source. In terms of DNRA efficiency, the short rod-shaped mycelium was markedly superior, with 75.82% of the converted nitrate recovered as ammonium (63.61 mg/L) compared to only 46.61% (44.78 mg/L) for the long filamentous mycelium. To date, no comparable studies have reported such morphology-dependent metabolic differences in DNRA in actinomycetes. The strain EM-B2 could tolerate and utilize 300 mg/L of nitrite to produce 75.46 mg/L of ammonium nitrogen. Furthermore, nitrite reductase (NIR) played a key role in DNRA catalysis, with an enzyme specific activity of 0.024 U/mg protein. Combined analysis of metabolite and ¹⁵N-tracing experiments confirmed ammonium production primarily via the pathway of NO₃⁻ → NO₂⁻ → NH₄ + . This study provided a high-performance microbial agent and a technical strategy for mitigating nitrogen loss while enhancing soil nitrogen retention/recovery. IMPORTANCE In terms of DNRA efficiency, the short aerial hypha was markedly superior, with 75.82% of the converted nitrate recovered as ammonium (63.61 mg/L) compared to only 46.61% (44.78 mg/L) for the long filamentous mycelium. To date, no comparable studies have reported such morphology-dependent metabolic differences in DNRA in actinomycetes.