An Analogue study for heavy metal adsorption onto biofilm in the deep subsurface environment of the Horonobe area, Hokkaido, Japan

幌延の深部地下環境に形成されたバイオフィルムの重金属収着に関するアナログ研究

天野 由記   ; 伊勢 孝太郎; 伊藤 剛志; 根本 一昭; 舘 幸男  

Amano, Yuki; Ise, Kotaro; Ito, Tsuyoshi; Nemoto, Kazuaki; Tachi, Yukio

本研究では、幌延の地下施設内に形成されているバイオフィルムを対象として、その地球化学的、微生物学的特性研究を行った。バイオフィルム試料は、原位置の地球化学条件下において自然に形成されているものを用い、それらの重金属収着特性について評価を行った。その結果、バイオフィルム中には様々な重金属元素が高濃度で濃集しており、その元素濃度は地下水中のそれぞれの元素濃度と相関関係を示さなかったことから、バイオフィルムを構成している微生物の代謝活性の違いにより選択的に重金属元素を濃集していることが示唆された。バイオフィルム中の微生物群集組成について遺伝子解析を行った結果、Nitrospirae門、Candidate phylumとして知られているJS1やSM1、Proteobacteria門が優占することが示された。これらの微生物群集の代謝活性と重金属の収着特性に関するメカニズムについて今後評価を進めていく必要がある。

We characterized the geochemical and microbial structures of biofilm samples formed with groundwater from boreholes in the subsurface environment at Horonobe area, Japan. The biofilm samples were grown naturally into the sampling tubes made by nylon under in-situ geochemical conditions. The heavy metal sorption capacities of these biofilms were also determined to evaluate the effects of microorganims on radionuclides transport in HLW systems. From the results of heavy metal analyses, it is revealed that high content of elements was accumulated in the biofilms, and each elements was not accumulated in quantity in the same proportion and had different sorption properties. We could find no discernible correlation between those elements accumulated in the biofilms and the composition of groundwaters. It is assumed that these differences depend on the microbial community structure that composed of biofilms. That is to say, it is possible that microorganims that compose the biofilms selectively accumulate heavy metals according to their metabolic activity. Analyses of 16S rDNA utilizing next generation sequencing indicates that the biofilms were dominated by the phylum Nitrospirae, Proteobacteria, and JS1 and SM1 as Candidate phylum. Over 40% of the detected sequences belong to groups of uncultured microorganisms. These results possibly show the similar characteristics of natural biofilms formed on fracture-surfaces in the subsurface environments. Hence, it is necessary to evaluate mechanisms of interaction between heavy metals, microbial community structure and metabolic activities of biofilms.