Elucidating the mechanisms underlying heterotrophic nitrification-aerobic denitrification and cold tolerance in Pseudomonas fragi EH-H1 under weakly acidic conditions

Low temperatures and weakly acidic environments seriously inhibit heterotrophic nitrification and aerobic denitrification (HN-AD) functional bacteria. In this study, Pseudomonas fragi EH-H1 efficiently performed HN-AD at 15 °C under pH 6.2. Ammonium and nitrate removal efficiencies of 100% (20 h) and 99.02% (30 h) were achieved, with corresponding total nitrogen removal rates of 97.74% and 97.14%, respectively. Nitrogen balance analysis and N₂O measurements indicated that strain EH-H1 may eliminate nitrogen through the pathways: NH₄⁺→gaseous nitrogen and NO₃⁻→NO₂⁻→gaseous nitrogen. Genome-wide annotation of genes associated with nitrogen metabolism (glnA, gltB, gdhA, nasA, nirBD, narK and norR) demonstrated that strain EH-H1 contained a fully functional assimilatory nitrate reduction system. Concurrently, the genes of cspA, caps, ibpA, gadB and gadC that were related to cold and acid tolerance were detected. These results suggested that the genes for HN-AD in strain EH-H1 under low-temperature and weakly acidic conditions may differ from those of conventional HN-AD bacteria.