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The goal of the present work was to investigate the physicochemical and radiochemical conditions and the composition of the microbial community in the groundwater of a suspended surface repository for radioactive waste (Russia) and to determine the possibility of in situ groundwater bioremediation by removal of nitrate ions. Groundwater in the repository area (10-m depth) had elevated concentrations of strontium, tritium, nitrate, sulfate, and bicarbonate ions. High-throughput sequencing of the V3–V4/V4 region of the 16S rRNA gene revealed the presence of members of the phyla Proteobacteria (genera Acidovorax, Simplicispira, Thermomonas, Thiobacillus, Pseudomonas, Brevundimonas, and uncultured Oxalobacteraceae), Firmicutes (genera Bacillus and Paenibacillus), and Actinobacteria (Candidatus Planktophila, Gaiella). Canonical correspondence analysis suggested that major contaminant – nitrate, uranium, and sulfate shaped the composition of groundwater microbial community. Groundwater samples contained culturable aerobic organotrophic, as well as anaerobic fermenting, iron-reducing, and denitrifying bacteria. Pure cultures of 33 bacterial strains belonging to 15 genera were isolated. Members of the genera Pseudomonas, Rhizobium, Cupriavidus, Shewanella, Ensifer, and Thermomonas reduced nitrate to nitrite and/or dinitrogen. Application of specific primers revealed the nirS and nirK genes encoding nitrite reductases in bacteria of the genera Pseudomonas, Rhizobium, and Ensifer. Nitrate reduction by pure bacterial cultures resulted in decreased ambient Eh. Among the organic substrates tested, sodium acetate and milk whey were the best for stimulation of denitrification by the microcosms with groundwater microorganisms. Injection of these substrates into the subterranean horizon (single-well push-pull test) resulted in temporary removal of nitrate ions in the area of the suspended radioactive waste repository and confirmed the possibility for in situ application of this method for bioremediation.

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A microbial enhanced oil recovery (MEOR) technique was tested at low-temperature heavy oil reservoirs (Russia). The bioaugmentation approach used is based on the introduction of hydrocarbon-oxidizing bacteria into the oilfield in combination with an injection of oxygen as a H 2 O 2 solution in order to initiate the first stage of hydrocarbon oxidation and of (NH 4 ) 2 HPO 4 as a source of biogenic elements. Before the pilot trials, the microorganisms of petroleum reservoirs were investigated by high-throughput sequencing, as well as by culture-base and radioisotope techniques. Molecular studies revealed the differences in microbial composition of the carbonate and terrigenous oil reservoirs and the communities of injection and formation water. Aerobic bacteria Rhodococcus erythropolis HO-KS22 and Gordonia amicalis 6-1 isolated from oilfields oxidized oil and produced biosurfactants. Fermentative enrichment and pure cultures produced considerable amounts of low fatty acids and alcohols from sacchariferous substrates. In core-flooding tests, 43.0-53.5% of additional heavy oil was displaced by aerobic bacteria, producing biosurfactants, and 13.4-45.5% of oil was displaced by fermentative bacteria, producing low fatty acids, alcohols, and gas. A total of 1250 t additional oil was recovered as a result of the application of an MEOR technique at the Cheremukhovskoe heavy oil reservoir and Vostochno-Anzirskoe reservoir with light conventional oil.