Effect of humic acid on aerobic denitrification by Achromobacter sp. strain GAD-3

Humic acid (HA), a common natural organic matter , could affect conventional anoxic denitrification. Aim of this study was to investigate effect of HA on the process of aerobic denitrification in Achromobacter sp. GAD-3, an aerobic denitrifying strain. The findings demonstrated that an increase in HA concentrations (≥5 mg L −1 ) promoted the aerobic denitrification process (excluding N 2 O reduction), manifesting as higher rates of nitrate removal (6.67–11.1 mg L −1  h −1 ) and lower levels of nitrite accumulation (30.2–20.7 mg L −1 ). This was attributed to the increased electron transfer activities and denitrifying reductase activities (including NAR, NIR and NOR) facilitated by HA. Accordingly, the expression of denitrification genes such as napA , cnorB , and nirS was enhanced by HA. Nonetheless, the nosZ gene and N 2 OR activity underwent suppression by HA, which was accountable for N 2 O emission. It is crucial to understand the HA mechanism towards aerobic denitrifiers for wastewater treatment plants to enhance nitrogen removal.