Mechanism of Nonionic Surfac- Tant Inhibition of Pentachlo- Rophenol Biodegradation

Several potential mechanisms of surfactant-induced inhibition of pentachlorophenol (PCP) biodegra-dation were tested using a pure bacterial culture of Sphingomonas chlorophenolicum sp. Strain RA2. Concentrations of the surfactant Tergitol NP-10 (TNP10) over 200 mg/L inhibit biodegradation of PCP at concentrations below 100 mg/L. At PCP concentrations above 200 mg/L, TNP10 reduced the substrate inhibition effect of PCP, resulting in faster PCP degradation rates at higher concentrations of TNP10. Measurements of surfactant confirm that TNP10 is not degraded by RA2. Calculations of a micelle partition coefficient (K m) show that the differences in initial PCP degradation rates can be explained by accounting for the amount of PCP available to the cell in the aqueous solution. Also, TNP10 between 500 and 5000 mg/L did not slow glucose degradation or endogenous oxygen uptake rates, indicating a lack of general cellular toxicity from the TNP10. A model is discussed based on these results where PCP is sequestered into micelles at high TNP10 concentrations to become less available to the bacterial cell and resulting in observed inhibition as opposed to a mechanism of general cellular toxicity. Under substrate toxicity conditions, the same mechanism serves to increase the rate of PCP biodegradation by reducing aqueous PCP concentrations to less toxic levels.