Spatial distribution of an uranium-respiring betaproteobacterium at the Rifle, CO field research site

Abstract

The Department of Energy's Integrated Field-Scale Subsurface Research Challenge Site (IFRC) at Rifle, Colorado was created to address the gaps in knowledge on the mechanisms and rates of U(VI) bioreduction in alluvial sediments. Previous studies at the Rifle IFRC have linked microbial processes to uranium immobilization during acetate amendment. Several key bacteria believed to be involved in radionuclide containment have been described; however, most of the evidence implicating uranium reduction with specific microbiota has been indirect. Here, we report on the cultivation of a microorganism from the Rifle IFRC that reduces uranium and appears to utilize it as a terminal electron acceptor for respiration with acetate as electron donor. Furthermore, this bacterium constitutes a significant proportion of the subsurface sediment community prior to biostimulation based on TRFLP profiling of 16S rRNA genes. 16S rRNA gene sequence analysis indicates that the microorganism is a betaproteobacterium with a high similarity to Burkholderia fungorum. This is, to our knowledge, the first report of a betaproteobacterium capable of uranium respiration. Our results indicate that this microorganism occurs commonly in alluvial sediments located between 3-6 m below ground surface at Rifle and may play a role in the initial reduction of uranium at the site.

Fig 1. Map of the study area indicating sampling wells and groundwater flow.

Fig 2. Changes in cell number in replicate microcosms with the various uranyl acetate additions (0–10 μM) after a 24-day incubation.

Error bars indicate the variability (SD) in the cell counts for each replicate.

Fig 3. Average proportion of soluble and insoluble uranium over time with uranyl acetate addition (0–3 μM) measured by ICP-mass spectrometry.

Fig 4. Maximum likelihood phylogenetic tree re-construction of Burkholderia type strains using 1366 bp of unambiguously aligned sequence of the 16S rRNA gene.

Burkholderia fungorum strain Rifle is indicated.

Fig 5. Spatial distribution of Burkholderia fungorum strain Rifle based on percent contribution to overall microbial community using the 166 bp MnlI peak in the bacterial TRFLP profile.