Isolation, Identification, Colonization, And Performance Of Phenanthrene-Degradation Endophytic Bacterium Pn2

Anthropogenic soil contamination has become a worldwide environmental problem in recent decades. Organic pollutants present in soil may be taken up by plants and translocated into shoots, which is the major pathway for toxic organic substances to reach the food chain. Because plants from the basis of human and animal food chains, potentially harmful organic contaminants could find their way into human and animal populations via this route. Clearly, increased understanding of how plants take up and metabolize organic contaminants from the soil could have considerable benefit for risk assessments.Endophytic bacteria usually live in plant tissues, protecting plants from external harsh environments or promoting plant growth. However, there is still limited information available on the endophytic bacteria-influenced metabolization of organic contaminants inner plants. Several species of endophytic PAH-degrading bacteria have been isolated from plants. Unfortunately, few literatures are available heretofore on the endophytic bacteria-influenced uptake and metabolization of PAHs by plants.In our research, an efficient phenanthrene-degrading endophytic bacterial strain Pn2was isolated from Alopecurus aequalis Sobol which grown in PAH-contaminated soils. And then the experiments on phenanthrene-degrading features and growth characteristics of the strain were investigated. After that, the influences of environmental conditions on phenanthrene degradation were evaluated and the conditions for phenanthrene degradation were optimized. Finally, Pn2was colonized into Cucumis sativus and Lolium multiflorum Lam through seed or root soaking. The effects of colonization on plant growth and phenanthrene uptake and metabolization were evaluated. Main novel findings are shown as follows:(1) Pn2was identified as Naxibacter sp. based on physiological characteristics and16S rDNA Sequence homology analysis. The experiments on phenanthrene-degrading features and growth characteristics of the strain showed that strain Pn2can utilize phenanthrene as the sole carbon source. At30℃and150rpm, the degrading ratio of phenanthrene (49.92 mg·L-1) in solution was98.78%after72hours rotary culture. Pn2grew well under the condition of25-37℃, pH6.0-8.0and1%-2%salinity. Strain Pn2grew aerobically, and the stronger aeration, the better Pn2growth. It was observed that Pn2can resist the low concentration of penicillin and chloramphenicol. Pn2had wide degradation spectrum, it can degrade naphthalene, acenaphthene, phenanthrene, pyrene, benzo[a]pyrene.(2) Under the conditions of the experiment, we optimize conditions for phenanthrene degradation by Pn2. The optimum conditions were30℃, pH7.0, phenanthrene concentration150mg·L-1, inoculation amounts15%,150rpm, external nitrogen and carbon source yeast powder and maltose. Strain Pn2can degrade96.86%of phenanthrene in24h and the period of adjustment was cut down to4h. The inoculation amount and pollution intensity positively influenced the phenanthrene degradation:the larger amount of Pn2inoculation, the higher phenanthrene degradation ratio was. With increasing of the pollution intensity, phenanthrene degradation ratio enlarged at first and reduced thereafter.150mg·L-1of phenanthrene is the optimum pollution intensity under the test condition.(3) Endophytic bacteria Pn2could colony well in Lolium multiflorum Lam and Cucumis sativus seedlings, and it could also be transferred into the stems and leaves from the roots. Strain Pn2can promote the absorption of phenanthrene and growth of Lolium multiflorum Lam, but it didn’t make effect on the plant growth of Cucumis sativus, and could even inhibit phenanthrene absorption by this plant. The phenanthrene concentrations of the roots of Lolium multiflorum Lam in control group were significantly higher than the colonization group. However, the phenanthrene concentration in the roots of Cucumis sativus seedlings in control group were lower than the colonization group. But the phenanthrene content of the stems and leaves in two groups of two plants had no obviously difference.