New findings on the high dissolved oxygen stimulation of dissimilatory nitrate/nitrite reduction to ammonium

Dissimilatory nitrate reduction to ammonium (DNRA) is a green technology for energy-efficient ammonium production. This reaction is performed by some microorganisms that contain functional nitrite reductase. In areas with nitrogen deficiencies, the unlimited DNRA potential of microorganisms has attracted much interest. However, the ability of actinomycetes to perform DNRA under aerobic conditions has been overlooked compared with that of bacteria. The metabolic process of microbial DNRA reportedly occurs mainly under anaerobic conditions. Here, we describe the ability of Streptomyces mediolani EM-B2 to produce high levels of ammonium under aerobic conditions. A higher ratio of nitrite to nitrate was more favorable for the DNRA process. The DNRA efficiency of strain EM-B2 reached 80.24 % under aerobic conditions, but this ability was lost under anaerobic conditions. These results show that efficient DNRA processes can occur under aerobic conditions. Strain EM-B2 can efficiently convert nitrate/nitrite into ammonium at a rate several times that of any previously reported wild-type aerobic DNRA microorganisms. This is the first report of actinomycetes production of ammonium in large quantities under aerobic conditions. This knowledge could be applied to manage nitrogen fertilizer use around the world.