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 N₂O 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 N₂OR activity underwent suppression by HA, which was accountable for N₂O emission. It is crucial to understand the HA mechanism towards aerobic denitrifiers for wastewater treatment plants to enhance nitrogen removal.