Microbial communities in a former pilot-scale uranium mine in Eastern Finland: Association with radium immobilization

The bacterial, fungal and archaeal communities were characterized in 17 top soil organic and mineral layer samples and in top sediment samples of the Paukkajanvaara area, a former pilot-scale uranium mine, located in Eno, Eastern Finland, using amplicon sequencing and qPCR. Soil and sediment samples were in addition analyzed for radium (²²⁶Ra), sulfate (SO₄ ²⁻), nitrate (NO₃ ⁻) and phosphate (PO₄ ³⁻) concentrations. New bacterial strains, representing Pseudomonas spp., were isolated from the mine and reference area and used in laboratory experiments on uptake and leaching of radium (Ra). The effect of these strains on the sulfate leaching from the soil samples was also tested in vitro. Between 6 × 10⁶ and 5 × 10⁸ copies g⁻¹ DW (dry weight) of bacterial 16S rRNA genes, 5 × 10⁵–1 × 10⁸ copies g⁻¹ DW archaeal 16S rRNA genes and 1 × 10⁵–1 × 10⁸ copies g⁻¹ DW fungal 5.8S rRNA genes were detected in the samples. A total of 814, 54 and 167 bacterial, archaeal and fungal genera, respectively, were identified. Proteobacteria, Euryarchaeota and Mortiriella were the dominant bacterial, archaeal and fungal phyla, respectively. All tested Pseudomonas spp. strains isolates from Paukkajanvaara removed Ra from the solution, but the amount of removed Ra depended on incubation conditions (temperature, time and nutrient broth). The highest removal of Ra (5320 L/kg DW) was observed by the Pseudomonas sp. strain T5-6-I at 37 °C. All Pseudomonas spp. strains decreased the release of Ra from soil with an average of 23% while simultaneously increasing the concentration of SO₄ ²⁻ in the solution by 11%. As Pseudomonas spp. were frequent in both the sequence data and the cultures, these bacteria may play an important role in the immobilization of Ra in the Paukkajanvaara mine area.