Degradation of high molecular weight polycyclic aromatic hydrocarbons by phyllosphere bacteria and its enhancement by surfactant addition

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are dominant air pollutants in urban environment. Pyrene is a major high molecular weight (HMW) PAHs in roadside air and street dust, thus it was chosen as a representative of HMW-PAHs in this study. The HMW-PAHs can combine with airborne particulates and accumulate on plant leaves. The biodegradation of accumulated PAHs by phyllosphere bacteria would reduce their recirculation back into the atmosphere. The objectives of this research were to investigate the biodegradation potential of HMW-PAHs on ornamental plants and to use surfactant for increasing bioavailability of HMW-PAHs. This study monitored the abundance of airborne and phyllosphere bacteria as well as the concentrations of airborne PAHs around ornamental plant swaths on Prachachuen Road, Bangkok. The number of airborne pyrene-degrading bacteria ranged from 22-152 CFU m-3 air, and more bacteria were found in the proximity of the ornamental plant swath than along the roadside. On the other hand, the concentrations of particle-bound PAHs were much higher at roadside than those around plant swath. Pyrene-degrading bacteria averaged 5x104 CFU g-1 on the leaves of all tested plant species and accounted for approximately 7% of the total population. Four pyrene-degrading bacteria were isolated from I. coccinea to use as model phyllosphere bacteria. To increase the bioavailability of pyrene, a lipopeptide biosurfactant from Bacillus sp. GY19 was applied. Kocuria sp. IC3 showed the highest pyrene degradation in the medium containing biosurfactant. A small 2.5 L glass chamber containing I. coccinea twigs was then used for examining the efficiency of plant, phyllosphere bacteria and biosurfactant on removal of deposited pyrene at 30 µg g-1 leaf. After 14 days, leaves containing both Kocuria sp. IC3 and 0.1X CMC biosurfactant showed 100% pyrene removal with the most abundant bacteria. The system with biosurfactant alone also enhanced the activities of indigenous phyllosphere bacteria with 94% pyrene removal. Consequently, the bioaugmentation of leaves was not necessary. To investigate the activity of phyllosphere bacteria on reducing other PAHs found in the ambient air, the whole plant of I. coccinea was sprayed with biosurfactant and placed in a scale-up 96-L gas tight chamber containing vapor of mixed PAHs. The plant was able to reduce 64 – 86% of both airborne and deposited phenanthrene, pyrene and fluoranthene after 14 days, while only 20% of these PAHs was decreased in the control chamber. Consequently, the removal of pyrene and other PAHs could be achieved by spraying biosurfactant on ornamental shrubs. The acquired knowledge could be used to develop a bioremediation approach for improving air quality in urban areas.