Microbial Diversity and Possible Activity in Nitrate- and Radionuclide-Contaminated Groundwater

This chapter illustrates the function of microbes in contaminated groundwater with radioactive waste in the region of Lake Karachai of East Ural, Russia. Since 1951, the lake has been used as a dumping site for low- and intermediate-level liquid radioactive waste of the Mayak Plant Association. Filtration of these solutions through the permeable lake floor resulted in a plume of contaminated groundwater with high concentrations of radionuclides, organic and mineral components such as acetate, oxalate, nitrate, and sulfate. The examined groundwater was sampled from depths of 20–100 m using bore holes, and Eh of the groundwater ranged from +50 to +310 mV; water was slightly alkaline with pH of 7.5–8.0. The water chemistry was characterized with very high concentration of nitrate amounting to 42700 mg/L. Among radionuclides predominated Strontium detected at high concentration in groundwater. The prokaryotic community of groundwater samples was analyzed by cultivation, and cloning-sequencing with 16S rRNA techniques. Due to mostly oxidative conditions in groundwater, high abundance of aerobic bacteria was detected, while the numbers and activity of sulfate-reducing and methanogenic prokaryotes were low. Analysis of the clone libraries of the 16S rRNA genes from weakly contaminated groundwater revealed a diverse microbial community represented mainly by the Patescibacteria (candidate phylum Parcubacteria), Proteobacteria, and by ammonium-oxidizing Thaumarchaeota. About 50 pure cultures isolated from groundwater samples belonged to 23 genera and 33 species of aerobic organotrophs and anaerobic denitrifying and iron-reducing bacteria. The presence of organic carbon sources (acetate), nitrates, and culturable denitrifying prokaryotes suggests a possibility of denitrification in contaminated groundwater. To elucidate microbial function in the absorption of radionuclides some microbial strains isolated from the groundwater were examined.