Clopyralid biodegradation and microbial inhibition kinetics by activated sludge mixed cultures and bacterial and fungal isolates

Clopyralid is among the most widely used herbicides worldwide. Discharging clopyralid-contaminated water into the environment can adversely affect human health and ecosystems. Research on the biological treatment of clopyralid-laden wastewater is crucial for enhancing process performance and preventing environmental contamination. This study investigates the biodegradation of clopyralid by an activated sludge (AS) culture to clarify its microbial degradation and inhibition kinetics. Furthermore, the inhibitory effect of clopyralid on isolated AS microorganisms (bacteria and fungi) was examined using paper disk and broth culture methods. The results demonstrate the potential of AS to biodegrade clopyralid. Clopyralid degradation rates increased with increasing herbicide concentration from 50 to 225 mg/L, then declined. At 300 mg/L, clopyralid biodegradation was completely inhibited. Luong's kinetics model for inhibitory substrates accurately described this biodegradation pattern. All cultured bacteria and fungi were inhibited at higher clopyralid doses. However, while most bacteria were inhibited at 1200 mg/L of clopyralid, fungi were inhibited at a 10-fold higher concentration. At this concentration range, clopyralid exhibited a bacteriostatic/fungistatic effect rather than a bactericidal/fungicidal one. That is, it did not cause lethal disruption of essential cellular functions. The findings of this study could inform strategies to enhance clopyralid biodegradation at high concentrations in AS reactors.