Investigation of microbial population biology during the degradation of organic contaminant mixtures

Spills of contaminants such as oil, diesel, gasoline, and chlorinated solvents result in the introduction complex mixtures of organic contaminants to the environment. I sought to discover relationships between complex mixtures of organic contaminants and niche diversity of contaminant-degrading microorganisms and to determine how the interactions of nonpolar organic contaminants within mixtures influence microbial community structure and function. Denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified 16S rRNA gene fragments was combined with traditional cultivation to investigate whether the changing composition of hydrocarbon mixtures could be related to changes in microbial populations that might represent distinct functional groups. Microbial communities associated with the biodegradation of crude oil and a 4-compound hydrocarbon mixture underwent similar patterns of succession based on DGGE analysis, suggesting that different bacterial populations appeared to be associated with the degradation of n- and isoprenoidalkanes. Since both n- and isoprenoid-alkanes are important nonaqueous phase liquids (NAPLs) that govern the partitioning of other contaminants (e.g. aromatics) in crude oil, enrichments were conducted with phenanthrene partitioned into heptadecane, pristane and a 1:1 mixture (v:v) of heptadecane and pristane. Analysis of these enrichments indicated that the presence of heptadecane as a NAPL results in different kinetics of phenanthrene degradation and community structure in comparison to enrichments with phenanthrene partitioned into pristane. DGGE analyses were also applied to the study of methyl tert-butyl ether (MTBE)-degrading bacteria from a gasoline-contaminated aquifer (Ronan, MT). A consortium was cultivated that could utilize MTBE as a sole carbon and energy source and two MTBE-degrading bacteria were isolated from this consortium. Molecular analyses of the consortium combined with physiological characterization of the isolates suggested that microorganisms in the consortium may be more fit for MTBE degradation than the cultivated isolates. These findings indicate that contaminant mixture complexity may be important in determining microbial niche diversity, which in turn drives selection of contaminant-degrading microbial communities. In addition, describing and explaining the microbial population biology of contaminant degradation is critical to achieving a predictive knowledge of contaminant fate and transport in situ.