Surfactants effects on microbial activity during pentachlorophenol degradation by Sphingomonas chlorophenolicum strain RA2.

摘要

Biodegradation is an important approach for remediation of environmental contamination with organic compounds, but methods to monitor success still require further development. The dehydrogenase activity method (INT; p-iodo nitrophenyl phenyl tetrazolium chloride) was used as a measurement of microbial activity tested under various concentrations of a toxic substrate and surfactants. The INT method allowed as investigating the microbial activity changes during PCP (pentachlorophenol) degradation by Sphingomonas chlorophenolicum strain RA2 (RA2) in the presence of high surfactant concentration. As an uncoupler, PCP itself is toxic to bacteria in high concentrations. Biodegradation of glucose was also evaluated to compare results with a non-toxic substrate.;RA2 degraded up to 220 mg/L PCP within 150 hrs. INTF (p-iodo nitrophenyl phenyl tetrazolium chloride formazan) formation curve trends are somewhat correlated with the biomass growth curve measured by optical density. However, the INTF data shows an initial lag phase while the biomass growth and PCP degradation curves do not show this lag phase. Specific INTF data shows that per unit biomass RA2 has more respiration activity in the decay phase than in the exponential phase during biodegradation of high PCP concentrations. This may be due to the substrate toxicity of PCP. Yield coefficients for PCP are significantly different between the two methods, but similar for glucose. However, the endogenous decay coefficients iv derived from both methods are similar, with a trend to increasing values after degradation of higher initial PCP concentrations.;Surfactants may be injected into the subsurface to increase the desorption and dissolution of organic contaminants, where they can be extracted and/or are more bioavailable for biodegradation in situ or in ex situ treatment reactors. Non-ionic surfactants at concentrations above the critical micelle concentration (CMC) have been shown to inhibit bacterial degradation of PCP. A surfactant concentrations below the CMC, Tergitol NP-10 (TNP10, as a representative non-ionic surfactant) does not inhibit biodegradation of 100 mg/L of PCP by RA2. At the supra CMC levels of 3,000 mg/L of TNP10, RA2 can successfully degrade initial PCP concentrations up to 250 mg/L within 450 hrs. However, a significant lag time of more than 200 hours resulted from the surfactant addition, regardless of the initial PCP concentration. The surfactant inhibition mechanism at supra CMC levels is likely due to surfactant interactions with enzymes essential to PCP biodegradation. The surfactants may interfere with induction of the production of PCP degrading enzyme(s) within the RA2 bacteria.;A number of different surfactants were evaluated for their effects on substrate biodegradation by RA2. Glucose is a non-toxic substrate that is degraded by constitutive enzymes and will not partition into surfactant micelles. The surfactant inhibition effect on glucose degradation was, in order from most to least inhibition: the cationic surfactant CTAB; greater than the anionic surfactants SDS > SDBS; greater than the non-ionic surfactants Triton X 114 > Triton X 405 > Igepal CA 720 v and TNP 10 > Triton X 100. Thus, the non-ionic surfactants with mid-range hydrophobic:lipophilic balance (HLB) values were most biocompatible.;For tests with PCP as a representative hydrophobic toxic substrate, the surfactant inhibition effects on PCP biodegradation were: CTAB > SDS > Igepal CA 720 > Tergitol NP 10 > Triton X 114 > Triton X 100. In all cases, PCP biodegradation was inhibited at surfactant concentrations equal to or lower than the concentrations that inhibited glucose biodegradation.;There was some correlation between non-ionic surfactant HLB values and glucose degradation times. For the hydrophilic substrate, there was no change of degradation time with increasing surfactant concentration for both surfactants. However, the degradation of the hydrophobic substrate (PCP) was greatly affected by surfactant concentration. PCP degradation was slower due to the surfactants and different head group surfactant had very similar inhibition of PCP degradation (TNP 10 and Igepal CA 720). This could be due to surfactant sequestration of substrate or interference with membrane associated PCP-degrading enzymes.;This work contributes to the body of knowledge pertaining to biodegradation of PCP in both the presence and absence of surfactants. Information is provided on the possible efficacy of using INT measurements as a rapid surrogate method to acquire information on PCP biodegradation; however, this method has serious limitations for predicting RA2 growth and PCP degradation. More information is provided on how surfactants impact contaminant biodegradation, and possible mechanisms for these effects.