Phenanthrene and hexadecane biodegradation and degrader community dynamics in rhizosphere soil: Mechanisms responsible for enhancement

While many studies to date have demonstrated that plant-enhanced bioremediation in petroleum-contaminated soils is a promising and cost-effective soil remediation alternative, the mechanisms responsible for rhizoremediation have yet to be experimentally elucidated. The objectives of my dissertation work were [1] to determine if a predictable link between plant-enhanced phenanthrene biodegradation and degrader community dynamics in Avena barbata (slender oat) rhizosphere soil could be established, [2] to define the role(s) of individual slender oat rhizosphere components in accelerated phenanthrene biodegradation, and [3] to investigate contaminant specificity by comparing phenanthrene and hexadecane biodegradation and degrader community dynamics in slender oat rhizosphere soil.; Enhanced phenanthrene biodegradation rates were observed in soils planted with slender oat compared to unplanted bulk soil. Quantitative estimates of heterotrophic and phenanthrene degrader populations indicated that enrichment of degraders occurred during the mature stages of plant growth, primarily due to the maintenance of significantly larger degrader populations in rhizosphere soils. Data obtained from phenanthrene degrader isolates also indicated that the rhizosphere degraders were less diverse than bulk soil degraders. The rhizosphere was a more hospitable environment for phenanthrene degraders compared to bulk soil. Hydroponically collected slender oat root exudates had the largest individual impact on phenanthrene biodegradation enhancement and a combined root exudate + root debris-amended soil had the most rapid phenanthrene biodegradation rate. Root exudate-amended soils also supported the largest degrader populations compared to root amended and unamended soils. Hexadecane-contaminated rhizosphere soil did not support accelerated biodegradation rates compared to bulk soil indicating that slender oat-enhanced bioremediation is contaminant specific. Despite the absence of enhanced biodegradation, hexadecane degrader populations were selectively maintained in rhizosphere soils compared to bulk soils. No specific rhizosphere impact on microbial degraders in contaminated soils adequately explained the observed differences in biodegradation rate enhancement in phenanthrene versus hexadecane contaminated soils.; Each objective of my dissertation research was successfully accomplished. My results indicate that [1] plant-enhanced phenanthrene biodegradation enhancement in slender oat rhizosphere soil was carried out by a quantitatively larger and less diverse microbial community of degraders, [2] slender oat root exudates enhanced phenanthrene biodegradation rates by stimulating degrader populations, and [3] plant-enhanced petroleum biodegradation was contaminant-specific as hexadecane biodegradation was not accelerated in slender oat rhizosphere soil.