Fate of U(VI) added to a U mining waste pile sample and the resulting changes in the indigenous microbial community

Interactions of U(VI) with the indigenous bacterial and archael communities of the uranium mining waste pile Johanngeorgestadt in Germany were studied. For this several portions of one sample with a natural content of 40 mg U/kg were supplemented with 60 mg U/kg and 260 mg U/kg in form of uranyl nitrate. Part of the most contaminated portion was additionally incubated under anaerobic conditions. Bothe, selective sequential extraction (SSE) and X-ray absorption spectroscopic analyses, indicated that added U(VI) was bound as uranyl by inner-sphere sorption complexes. No reduction of U(VI) to U(IV) was observed even by the anaerobic treatment.
Analyses of the constructed 16S rRNA gene clone libraries of the untreated and of the U(VI)-supplemented samples revealed changes in the bacterial and in the archaeal parts of the natural microbial community. In the untreated sample the bacterial community was predominated by Alphaproteobacteria and Holophaga/Acidobacterium members. The number of the latter was strongly reduced by increasing the uranium content up to 100 mg U/kg. Instead, a large number of sequences were found representing Gamma-Pseudomonas spp., Arthrobacter ssp. (Actinobacteria) as well as Geobacter spp. (Deltaproteobacteria). The most contaminated portion containing approximately 300 mg U/kg was predominated by represenatives of Cytophaga/Flavobacterium/Bacteroides group. The anaerobic treatment of this sample induced also proliferation of Alpha- and Gammaproteobacteria in it.
The archaeal populations in the original waste sample were rather diverse, whereas the supplemented with U samples wee strongly predominated by a few particular Grenarchaeota species.
Our results indicate that the addition of U(VI) to a low contaminated uranium mining waste sample induces significant shifting in the indigenous microbial populations in dependence on the amount of U(VI) and the redox conditions.