Plant leaves and their associated bacteria for the degradation of PAH deposited on leaf surface

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are priority pollutants that continuously generated by the incomplete combustion of organic fuels from traffic and industrial processes. Interest in using plant leaves as a mechanism for biodegradation of atmospheric PAHs is motivated by the fact that leaves are exposed to the atmosphere and microbes are the most abundant inhabitants of the phyllosphere. In leaves, PAHs become potentially available to microbial degradation as it accumulates on plants surfaces by gaseous and particle-bound deposition. The objectives of this study were to identify an ornamental plant that contained high number of PAH-degrading bacteria and to apply this plant and its associated bacteria to reduce the deposited PAHs. The study used phenanthrene as a modeled PAS. Six plant species were analyzed; namely Wrightia religiosa Benth. ex Kurz, Pereskia grandiflora Haw., Hibiscus rosa-sinensis L., Excoecaria cochinchinensis Lour. Var. cochinchinensis, Ixora sp., and Hymenocallis littoralis Salisb. Their leaf samples were collected from two locations in Bangkok that had high and low amounts of air pollutants. The concentrations of phenanthrene-degrading bacteria on leaves were ranged from 3.5x10[superscript 4] to 1.95x10[superscript 7] CFU/g. There was no significant relationship between the number of bacteria and location, moisture content, or wax content of the leaves and thus colonization of phenanthrene-degrading bacteria probable depended on other plant characteristics. From the six species of plants screened, the leaves of the Ixora sp. Showed the highest concentration of phenanthrene-degrading bacteria. Further laboratory studies were carried out using Ixora sp. to determine the rate of phenanthrene degradation by indigenous bacteria and added Sphingomonas sp. P2, a known PAH-degrader. The results indicated that the leaf associated bacteria effectively reduced the amount of phenanthrene that accumulated on the leaves. In addition, the augmentation with Sphingomonas sp. P2 increased the rate of phenanthrene degradation. This was detected along with a rapid increase in number of phenanthrene-degrading bacteria on the augmented leaves. The survival of the augmented bacteria on leaves, however, requires further investigation. The study also isolated four leaf indigenous bacteria and found that they could completely degrade 100 ppm of phenanthrene in liquid medium within 9 days. Finally, using 16S rDNA gene sequencing, one of the indigenous bacteria was identified as Deinococcus sp. The application of Ixora sp. and their associated bacteria would be a natural and environmentally friendly approach to reduce the amount of atmospheric PAHs.