Phyllosphere-mediated bacterial degradation of airborne phenol

Abstract

Bacterial degradation of organic pollutants in soil has been studied extensively but little is known about the availability and degradation of pollutants by bacterial resident on leaf surfaces. Here, we examined availability and degradation of phenol on leaves by introduced cells of a known phenol degrader and by natural microbial communities. Phenol-responsive gfp-expressing Pseudomonas fluorescens A506 bioreporter cells detected airborne phenol as well as phenol that had been adsorbed by leaf surfaces. The degradation of airborne 14 Cphenol by leaf-associated bacteria showed that Pseudomonas sp. strain CF600 released eight times more 14 CO2 than a non-degrading mutant after introduction onto leaves. This study provided the first direct evidence of degradation of an organic air pollutant by leaf-associated bacteria. We evaluated phenol degradation by natural microbial communities on green ash leaves that were collected from a field site rich in airborne organic pollutants. We found that significantly more phenol was mineralized by these leaves when the communities were present than by these leaves following surface sterilization. Thus, phenol-degrading organisms were present in these natural communities and were metabolically capable of phenol degradation. We isolated and identified phenol-degrading bacterial isolates within these natural microbial communities. Ten phenol-degrading bacterial isolates were obtained that were most closely related to the genera Alcaligenes, Acinetobacter and Rhodococcus. The genetic characterization of these isolates revealed little diversity as phenol hydroxylase (PH) enzymes of all of the Gram-negative isolates were type II PHs, which have a moderate affinity for phenol, and these isolates harbored the ortho pathway of phenol degradation. Lastly, we attempted to understand the mechanism of phenol toxicity and tolerance in bacteria by comparing the transcriptional profiles of cells exposed to phenol and cells exposed to agents causing membrane and oxidative stress using the leaf-associated bacterial species, Pseudomonas syringae pv. tomato. Phenol resulted in the induction of heat shock genes that were not induced by the other stresses tested. The role of these genes in cellular and membrane protein damage in response to phenol is discussed.