Dissimilatory nitrate reductions in soil Neobacillus and Bacillus strains under aerobic condition.

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were thought to be carried-out by anaerobic bacteria constrained to anoxic conditions as they use nitrate (NO3-) as a terminal electron acceptor instead of molecular O2. Three soil bacilli, Neobacillus spp. strains PS2-9 and PS3-12 and Bacillus salipaludis PS3-36, were isolated from rice paddy field soil in Korea. The bacterial strains were selected as possible candidates performing aerobic denitrification and DNRA as they observed to reduce NO3- and produce extracellular NH4+ regardless of oxygen presence at the initial screening. Whole genome sequencing revealed that these strains possessed all the denitrification and DNRA functional genes in their genomes, including the nirK, nosZ, nirB, and nrfA genes, which were simultaneously cotranscribed under aerobic condition. The ratio between the assimilatory and dissimilatory NO3- reduction pathways depended on the availability of a nitrogen source for cell growth, other than NO3-. Based on the phenotypic and transcriptional analyses of the NO3- reductions, all three of the facultative anaerobic strains reduced NO3- likely in both assimilatory and dissimilatory pathways under both aerobic and anoxic conditions. To our knowledge, this is the first report that describes coexistence of NO3- assimilation, denitrification, and DNRA in a Bacillus or Neobacillus strain under aerobic condition. These strains may play a pivotal role in the soil nitrogen cycle.