Biogeochemical Processes In Ethanol Stimulated Uranium-contaminated Subsurface Sediments

Abstract

A laboratory incubation experiment was conducted with uranium-contaminated subsurface sediment to assess the geochemical and microbial community response to ethanol amendment. A classical sequence of terminal electron-accepting processes (TEAPs) was observed in ethanol-amended slurries, with NO3- reduction, Fe(III) reduction, SO4(2-) reduction, and CH4 production proceeding in sequence until all of the added 13C-ethanol (9 mM) was consumed. Approximately 60% of the U(VI) content of the sediment was reduced during the period of Fe(III) reduction. No additional U(VI) reduction took place during the sulfate-reducing and methanogenic phases of the experiment Only gradual reduction of NO3-, and no reduction of U(VI), took place in ethanol-free slurries. Stimulation of additional Fe(III) or SO4(2-) reduction in the ethanol-amended slurries failed to promote further U(VI) reduction. Reverse transcribed 16S rRNA clone libraries revealed major increases in the abundance of organisms related to Dechloromonas, Geobacter, and Herbaspirillum in the ethanol-amended slurries. Phospholipid fatty acids (PLFAs) indicative of Geobacter showed a distinct increase in the amended slurries, and analysis of PLFA 13C/12C ratios confirmed the incorporation of ethanol into these PLFAs. A increase in the abundance of 13C-labeled PLFAs indicative of Desulfobacter, Desulfotomaculum, and Desulfovibrio took place during the brief period of sulfate reduction that followed the Fe(III) reduction phase. Our results show that major redox processes in ethanol-amended sediments can be reliably interpreted in terms of standard conceptual models of TEAPs in sediments. However, the redox speciation of uranium is complex and cannot be explained based on simplified thermodynamic considerations.

16S rRNA Gene Microarray Analysis of Microbial Communities in Ethanol-Stimulated Subsurface Sediment

“…The 16S microarray data revealed qualitatively similar temporal patterns in microbial community structure compared to conventional 16S rRNA gene clone library data reported previously (29) for the slurry incubation experiment (Fig. 2).…”

“…Information on the status of microbial communities involved in U(VI) reduction and other electron-accepting pathways is important to understand the spatial/temporal dynamics and overall efficacy of in situ uranium bioremediation (1). Although 16S rRNA clone libraries and/or microarrays have been applied to various U(VI) reduction systems (2,4,7,17,28,29,40,41,44), knowledge remains scant of the quantitative coverage of these techniques, particularly in situations where multiple groups of organisms are active.…”

“…The samples analyzed came from an experiment designed to document patterns of microbial redox metabolism and community structure in response to the addition of an electron donor (ethanol) to stimulate U(VI) reduction activity (29). As part of that study, six conventional 16S rRNA gene clone libraries (90-134 clones each) were constructed from reversetranscribed 16S rRNA extracted from subsamples obtained at different time points during the experiment.…”

“…Although 16S microarrays have seen broad use in environmental microbiology, to date their ability to reveal changes in microbial community structure during redox transitions in space or time has not been established. A prior experiment (29) provided an ideal context for evaluating this question, as it was a controlled system for which both good supporting geochemical data on redox metabolism, and independent information on microbial community composition (i.e. clone libraries), were available.…”

Characterization of Microbial Communities in Subsurface Nuclear Blast Cavities of the Nevada Test Site

“…Aquatic chemistry and radiological analyses were performed by our LLNL collaborator. Briefly, activation products 14 C and 36 Cl were analyzed by accelerator mass spectrometry (AMS) and fission products Pumped or bailed water was obtained from subsurface radiological environments (Fig. 2).…”