Fire Phoenix plant mediated microbial degradation of pyrene: Increased expression of functional genes and diminishing of degraded products

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants, and bioremediation is an eco-friendly means of removing pollutants from contaminated soils. This study investigated mechanisms underlying plant mediated microbial degradation of pyrene. Fire Phoenix (Festuca spp.) plants were grown in soils contaminated by two levels of pyrene. Degraded products, expression of functional genes of microbial organisms, and pyrene removal rates were monitored before planting and after 60, 90, and 120 days of plant growth. Results showed that the removal rates of pyrene from soils contaminated with 22.4 mg/kg and 35.3 mg/kg of pyrene were 89.7% and 92.1%, respectively. Detected degraded products included 1-hydroxy pyrene, dihydroxylated pyrene, 1-hydroxy-2-naphthoic acid, and phthalic acid. As far as is known, this is the first report of 1-hydroxypyrene as an epoxidation product of pyrene. Pyrene degradation occurred through both double-oxygen and single-oxygen pathways in the rhizosphere of Fire Phoenix. The expression of microbial 16S rDNA, PAH-RHDα-GP, nidA, and nidB genes were analyzed. The copy numbers of 16S rDNA increased over time, and the highest occurred in soil with higher pyrene. Microbial nidA, nidB, and PAH-RHDα-GP genes were highly expressed in rhizosphere on day 60 of plant growth, and their copy numbers were 32, 49, and 320 times greater than those before plant cultivation. The higher levels of expression were significantly correlated with the residual amount of pyrene and the content of total PAH metabolites in the soil. Our research shows that that microbial RHDα-GP, nidA, and nidB genes are important factors influencing the degradation of pyrene and pyrene degradation products in rhizosphere. Based on the results, a two-pathway model for biodegradation of pyrene in Fire Phoenix rhizosphere is proposed.