Microbial populations stimulated by lanthanoids in groundwater of Pliocene diatomaceous mudstone of northernmost Japan

Amano, Yuki   ; Endo, Takashi; Nemoto, Kazuaki; Tachi, Yukio  

High-level radioactive waste (HLW) from nuclear power generation must be disposed in a safe manner for at least 100,000 years until the radiation dose is reduced to the levels of natural radiation dose in subsurface environments. Interaction of long-lived radionuclides with subsurface microorganisms has been studied to evaluate the effect of radionuclides migration in subsurface environments. However, the microbial activities and their mechanisms of radionuclide transformation are poorly understood. In this study, we investigated microbial populations stimulated by lanthanoids as analogue elements for actinides in natural systems. Groundwater samples collected from boreholes in the subsurface environment at Horonobe Underground Research Laboratory, Japan, were incubated anaerobically with lanthanoids. During the incubation experiments, concentrations of lanthanoids, total Fe, Fe, S were measured by absorption spectrophotometry and ICP-MS. Microbial community compositions were analyzed by next generation sequencing the V4 region of the 16S rRNA gene on an Illumina Miseq. Phylogenetic analyses obtained from the original and incubated groundwaters revealed that microbial populations were changed from the original one. The original groundwater showed diverse microbial community structure with the dominant phylotypes clustered within the phylum Firmicutes, Proteobacteria, and Chloroflexi. In the lanthanoids added systems, microorganisms related to Chlostridium, Georgfuchsia or Rodobacter spp. increased by the incubation. These results indicate that interaction between microorganisms and actinides may effect on radionuclide migration in HLW systems.