Phenanthrene-degrader community dynamics in rhizosphere soil from a commonannual grass

Enhanced rates of phenanthrene biodegradation were observed in rhizosphere soils (17.2 and 15.5 mg/kg/d for initial and re-spiked additions, respectiv ely) planted with slender oat (A vena barbata Pott ex Link) compared with u nplanted bulk soil controls (12.4 and 10.7 mg/kg/d). Soil microbial populat ions were characterized using a modified most probable number (MPN) method to determine quantitative shifts in heterotrophic and phenanthrene degrader communities while principal component analysis (PCA) of fatty acid methyl ester (FAME) data from isolated phenanthrene degraders was used to identify qualitative differences and degrader community diversity. The average hete rotrophic bacterial population over time was about three times larger in rh izosphere soil than in bulk soil while phenanthrene degrading populations i ncreased by as much as an order of magnitude between 24 and 28 days after p lanting (DAP). Thus, phenanthrene degraders were selectively enriched in rh izosphere soil compared with bulk soil. The greatest selection for degrader s occurred during the later-stages of plant development from 24 to 32 DAP. A PCA plot of the FAME data from phenanthrene degrader isolates indicated t hat the rhizosphere degraders were less diverse than bulk soil degraders. T hese results give us some insight into the mechanisms responsible for enhan ced biodegradation and selective degrader enrichment in rhizosphere soils.