Microbial Communities in Contaminated Sediments Associated with Bioremediation of Uranium to Submicromolar Levels

Center for Microbial Ecology, Michigan State University, East Lansing, Michigan 48824; Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305-4020; Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

	Abstract

Microbial enumeration, 16S rRNA gene clone libraries and chemical analysis were used to evaluate the in situ biological reduction and immobilization of uranium (VI) in a long term experiment (more than two years) conducted at a highly uranium contaminated site (up to 60 mg/l and 800 mg/kg solids) of the US Department of Energy in Oak Ridge, TN. Bioreduction was achieved by conditioning groundwater above ground and then stimulating growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria in situ through weekly injection of ethanol to the subsurface. After nearly two years of intermittent injection of ethanol, aqueous U levels fell below the US EPA maximum contaminant level (MCL) for drinking water and groundwater (<30 µg/l or 0.126 µM). Sediment microbial communities from the treatment zone were compared with those from a control well without biostimulation. Most Probable Number estimations indicated that microorganisms implicated in bioremediation accumulated in the sediments of the treatment zone but were either absent or in very low numbers in an untreated control area. Organisms belonging to genera known to include U(VI) reducers were detected, including Desulfovibrio, Geobacter, Anaeromyxobacter, Desulfosporosinus and Acidovorax spp. The predominant sulfate-reducing bacterial species were Desulfovibrio spp. while the iron reducers were represented by Ferribacterium spp and Geothrix spp. Diversity-based clustering revealed differences between treated and untreated zones and also within samples of the treated area. Spatial differences in community structure within the treatment zone were likely related to hydraulic pathway and electron donor metabolism during biostimulation.