Hydrocarbon Bioremediative Potential of (Per)Chlorate-Reducing Bacteria

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Abstract
Petroleum contamination of soils and sediments is a national concern due to the toxicity and recalcitrance of the aromatic components in the absence of oxygen. Oxygen can be introduced into the anaerobic zone of a contaminated environment by injection of gaseous O2 to stimulate biodegradation, but this process is costly and inefficient. Other more soluble electron acceptors, such as nitrate or sulfate, may alternatively be used, but the rates of oxidation are slow and not all hydrocarbons are degraded. Here we report that chlorite dismutation by (per)chlorate-reducing bacteria may offer an alternative source of oxygen for contaminant degradation. The dismutation of chlorite is an intermediate step in the microbial reduction of chlorate. Chlorite dismutation can stimulate the rapid oxidation of aromatic hydrocarbons such as benzene or naphthalene in anoxic environments by supplying oxygen to the aerobic hydrocarbon-oxidizing population. Benzene, which is extremely recalcitrant under anaerobic conditions, is rapidly degraded to CO2 even in pristine soils with no prior exposure to hydrocarbons. The (per)chlorate-reducing bacteria grew rapidly in a broad diversity of environmental conditions and survived in significant numbers over the long term in inoculated sediments. In addition, the (per)chlorate-reducer Dechlorimonas agitatus strain CKB could survive starvation, forming a stable ultramicrobacterium with a cell size less than one-tenth that of the vegetative cells. Such ultramicrobacterial cells can readily pass through small pore sizes of subsurface environments preventing near-well plugging in bioaugmentation strategies. The ultramicrobacterial cells formed could readily be recovered as vegetative cells and could be used to stimulate hydrocarbon oxidation after only 68 hours recovery. Our results suggest that chlorite in the presence of (per)chlorate-reducing bacteria may be used as an effective in situ remediation strategy for petroleum contamination.

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