Microbial Diversity and Activity in Waters of a Siberian Monitoring Well at the Deep-Bore-Hole Radioactive Waste Disposal Site TOMSK-7

Background
Microorganisms strongly influence the migration of radionuclides in the environment. For this reason analysis of natural bacterial and archaeal communities near radioactive wastes deposition sites is of great importance for risk assessments. In this study diversity was studied of the microorganisms indigenous for a monitoring well near the radioactive waste injection site Tomst-7 in Siberia. In addition, interactions of several bacterial isolates from this site with uranium and other metals was investigated.
Methods
Microbial diversity was studied applying the 16S rDNA7F-1513F for bacteria and 16S rDNA21f-958R for archaea. The diversity of the autotrophic bacteria was estimated by direct analysis of different RubisCO gene forms. Oligotrophic bacteria were cultured in low nutrient R2A medium. Interactions of the cultured bacterial isolates with U, Ni, Pb, As, and other metals were studied by using ICP-MS, flow cytometry and X-ray spectroscopic analyses.
Results
Our analyses demonstrated presence of a large number of diverse bacterial and archaeal groups at a depth of about 300m at the Siberian depository site Tomsk-7 where the radioactive wastes were injected. The most predominant bacterial populations were those of Betaproteobacteria mainly from the Rhodocyclus group, of Citophaga/Flavo-bacterium/Bacteroides, and of several novel “Cyanobacteria-like” groups. The RubisCO approach confirmed the Betaproteobacterial predominance in the samples studied.
Bacterial isolates were cultured from the samples belonging to Sphingomonas sp., Brevundimonas sp., Methylobacter sp., and a large microdiverse group of Actinobacteria closely related to Microbacterium oxydans. They tolerated U and other heavy metals in a species- and even strain-specific way. EXAFS analyses demonstrated that the isolates of Microbacterium sp. and of Sphingomonas sp. are complexing U(VI) at pH 4.5 via phosphorus (P) in a form of meta-autunite. The latter was connected to the liberation of inorganic phosphate by these strains due to their exo-phosphatase activity. At lower pH values the U is bound to organic phosphate residues.
Conclusions
The environment around the radioactive waste injection site Tomsk-7 possesses a large variety of microorganisms with a potential to bind and possibly transport radionuclides.