Efficient aerobic nitrate removal and nitrogen metabolism of the salt-tolerant Halomonas alkaliphila HYJ1

The rapid expansion of the mariculture industry has significantly increased the production of nitrogen-rich wastewater, highlighting an urgent need for more efficient treatment technologies. However, the high salinity in mariculture wastewater severely inhibits microbial metabolism, underscoring the critical necessity for introducing high-efficient salt-tolerant strains to enhance the nitrogen removal process. To address this challenge, a salt-tolerant bacterial strain, Halomonas alkaliphila HYJ1, with remarkable aerobic nitrate removal capability was successfully isolated and identified in this study. Under salinity concentration ranging from 2 % to 5 %, strain HYJ1 achieved 100 % nitrate removal efficiency within 20 h. The optimal conditions included sodium acetate as carbon source, C/N ratio of 4-8, pH 9.0, and dissolved oxygen levels of 10-30 %. The whole-genome analysis revealed that strain HYJ1 harbored key genes including narHIG, napAB, nasA, nirD, glnA, gltB, gdhA and amt, but lacked the conventional denitrifying genes responsible for reducing nitrite to N₂, suggesting the strain employs a dissimilatory/assimilatory nitrate reduction pathway: NO-3-N → NO-2-N→ NH+ 4-N → glutamate. Transcriptional analysis further indicated that the strain accumulates abundant compatible solutes and regulates Na⁺/H⁺ antiporters to resist salinity stress. Bioaugmentation tests demonstrated that strain HYJ1 significantly enhanced nitrate removal efficiency by 92.9 % within 1 h in simulated mariculture wastewater treatment systems by attaching to and being retained on hydrophilic polyurethane carriers, highlighting its potential for sustainable nitrogen management in high-salinity aquaculture systems.