Evaluation and mutagenesis of an assimilatory nitrate reduction bacterium Priestia megaterium BZ-95

Excessive NO3--N and NO2--N accumulation threatens aquaculture water quality, highlighting the need for aerobic microorganisms capable of converting dissolved inorganic nitrogen into biomass-associated organic nitrogen. This study aimed to characterize the assimilatory NO3-/NO2- reduction mechanism of Priestia megaterium BZ-95 and to evaluate its potential for aquaculture wastewater treatment.

A NO3--reducing bacterium, P. megaterium BZ-95, was investigated using whole-genome analysis, gene expression profiling, and nitrogen-removal assays. Genome analysis revealed two nitrogen-assimilation modules, namely a NO3- assimilation cluster (nasC-nirB_2-narT) and a nitrite assimilation operon (nirB_1-nirD-nirC-cysG), where as no denitrification or DNRA genes were detected. In NO3--N medium, NO3--N decreased from 72.59 to 8.07 mg/l within 24 h, accompanied by transient NH4+ accumulation. In NO2--N medium, NO2--N decreased from 70.00 to 0.009 mg/l within 21 h, with no detectable NH4+ accumulation. In mixed-N medium, NO3--N and NO2--N decreased from 30.00 to 3.33 mg/l and from 30.00 to 0.99 mg/l, respectively, within 24 h. Gene expression analysis showed that NO3- mainly induced the nasC-nirB_2-narT module, whereas NO2- strongly induced the nirB_1-nirD-nirC-cysG module. In addition, the mutant BZ-95D4 showed a 61.08% higher NO3--N removal capacity than the parental strain BZ-95.

Priestia megaterium BZ-95 possesses a dual-module assimilatory nitrogen-reduction system with distinct responses to NO3- and NO2-. These findings improve understanding of microbial nitrate assimilation and indicate that BZ-95, particularly the improved strain BZ-95D4, is a promising candidate for aquaculture wastewater treatment.