Characteristics of Naturally Grown Biofilms in Deep Groundwaters and Their Heavy Metal Sorption Property in a Deep Subsurface Environment

Two biofilm samples were collected from anaerobic groundwater in the depth range of 158.8–199.4 m in a borehole drilled in the Tono area, Japan, to understand their effects on the migration behavior of heavy metals in subsurface environments. The depth range is featured geologically by the lignite formation of sedimentary rocks that bear a uranium ore and the underlying granitic formation. Microbiomes of the derived biofilms, as well as of the ambient bacterioplankton, were characterized based on 16S rRNA gene sequences (clones) or phylotypes, and their heavy metal sorption properties were examined with reference to geochemical features of groundwaters. Phylotypic compositions of the four microbiomes, i.e., of biofilm vs. planktonic bacteria as well as in granitic vs. sedimentary rock groundwaters showed significant differences. In addition, each microbiome was dominated by one or two distinctive phylotypes. In bacterioplankton, the phylotype related to a betaproteobacterial environmental clone dominated 54% of the sequenced clones derived from sedimentary rock groundwater, whereas those related to Denitratisoma oestradiolicum and Clostridium sp. dominated 45% and 37%, respectively, of the clones derived from granitic groundwater. In biofilms, the phylotypes related to Methylobacillus flagellatus and Ignavibacterium album accounted for 77% and 78% of the clones of the biofilms derived from the sedimentary rock and granitic groundwaters, respectively. Chemical and mineralogical analyses demonstrated that high amounts of heavy metals such as Fe, Ni, Cu, Zn, As, Cd, Pb, Th and U accumulated in the biofilms; and their sorption properties varied between biofilms presumably with influences of co-occurring Fe-hydroxides and sulfide minerals under the redox conditions of approximately ?360 to 0 mV in subsurface environments. The biofilm-mineral interaction provides an implication for possible retardation of radionuclide migration in subsurface hydrology, which is of practical interest in geological disposal systems for high-level radioactive waste.