Isolation And Characterization Of P-Nitrophenol Degrading Bacteria, Degradation Characteristics And Cell Surface Hydrophobicity

P-nitrophenol (PNP) is a widely distributed xenobiotic nitroaromatic compound, especially with the manufacture of dyes, drugs, explosives, industrial solvents and phosphororganic insecticides. P-nitrophenol may result in water pollution if entering water. Thus, it is considered as a priority pollutant by the Environmental Protection Agency (EPA) of USA. Its wide contamination to the environment has given rise to much public concern. Several studies have been conducted on the microbial degradation of p-nitrophenol; its major pathway of degradation has been prelimimarily demonstrated. However, few microorganisms that could completely mineralize p-nitrophenol efficiently and grow on p-nitrophenol as the sole carbon and nitrogen sources have been reported to date.In this study, two p-nitrophenol-degrading strains were isolated by enrichment culture and screening. They can grow on p-nitrophenol as the sole carbon and nitrogen sources. Their growth characteristics, degrading abilities, and mineralizing mechanisms were also investigated. Furthermore, their degrading enzymes were also studies. The main results are as follows:1. From Huayang pesticide industry's activated sludge, two bacterial strains CN2 and CN6 were isolated. Based on the results of phenotypic features, physiological-biochemical properties, and phylogenetic similarity of 16S rDNA gene sequences, the isolates were designated as Arthrobacter sp. CN2 and Rhodococcus sp. CN6. Their 16S rDNA partial sequences were deposited into the GenBank under accession No. EU266494 and EU266492, respectively.2. The growth and degrading abilities of each pure culture for the p-nitrophenol and 4-nitrocatecholdegradation was investigated by UV-Vis and HPLC. Under the condition of 30?C and pH 7, the strain CN6 could degrade 100mg/L p-nitrophenol or 4-nitrocatechol within 12h or 20h, respectively. Effects of different initial p-nitrophenol concentration, additional carbon source, nitrogen source, total inocula biomass amount, incubation temperatures, and initial pH were investigated to determine the optimal biodegradation conditions. Results showed that the isolate CN6 could effectively degrade p-nitrophenol when provided as the sole carbon source, with total inocula 0.15⁰.3g/L, at 15³5℃, pH 7⁹, and initial p-nitrophenol 25²00 mg/L; Addition of extra carbon source, such as glucose or sucrose, could greatly enhance the biomass, but hardly hasten the biodegradation of p-nitrophenol. Added nitrogen sources did not evidently impact the bacterial biomass formation and degradation ability.3. Degradation of different contaminants by Rhodococcus sp. CN6 was also investigated. It could utilize p-nitrophenol or 4-nitrocatechol as the sole carbon, nitrogen and energy sources, suggesting that the p-nitrophenol degradation involve the phenyl ring cleavage, and finally mineralized into CO₂ and H₂O. The strain could also degrade phenol, toluene and methyl parathion showing a board substrate range, which indicated its high potential use in bioremediation of contaminated environment.4. The cell surface hydrophobicity of the strain CN6 in different culture conditions were investigated. Results showed that its cell surface hydrophobicity increased with the rise of pH and the fall of temperature. When the concentration of p-nitrophenol was increased, the cell surface hydrophobicity of the strain CN6 accordingly increased. The high cell surface hydrophobicity could hasten the strain CN6 to adhere to organic contaminations, and hence accelerated their biodegradation.5. Enzymetic degradation assays indicated that crude enzyme extracts could rapidly degrade p-nitrophenol into nontoxic substances; they were effective at 20 40℃and pH 7 9, with the optimum pH 8 and temperature at 30℃for bacteria CN6. These enzyme extracts might be useful in rapid detoxification and cleanup of p-nitrophenol residues in the polluted sites and on the surface of fruits and vegetables.