Methanogen Diversity in Deep Subsurface Gas-Associated Water at the Minami-Kanto Gas Field in Japan

Methanogen diversity and methanogenic potential in formation water obtained from the Minami-kanto gas field in Japan were investigated by using 16S rRNA gene libraries and culture-based enrichment methods, respectively. This region is the largest gas field that produces natural gases of dissolved-in-water type in Japan. Although the microbial population density was below statistical quantification limits (1 × 10⁴ cells ml⁻¹), autofluorescent coccoid and rod-shaped cells indicative of methanogens were observed. The represented genera in the archaeal 16S rRNA genes libraries were comprised of Methanobacterium, Methanospirillum, Methanocalculus, Methanococcus, Methanolobus and Methanosaeta. The dominant archaeal sequences were related to the hydrogenotrophic methanogens in the genus Methanobacterium. Of the methanogenic substrates tested using the formation water-based medium,H2-CO2 yielded the highest methane production. These results strongly suggest that the formation water of the Pleistocene strata in the gas fields harbor viable hydrogenotrophic methanogens and have possibly been making a contribution to ongoing methanogenesis.

Keywords:

• gas field
• methanogen
• hydrogenotrophic
• acetotrophic
• formation water
• archaeal methane

Hanako Mochimaru is also affiliated with the Institute for Biological Resources and Functions, National Institute of Advanced Industrial Science and Technology (AIST).

We gratefully acknowledge Hiroshi Iwamoto of Kanto Natural Gas Development Co., Ltd. for contributing the samples and for information about gas wells. We are also grateful to Nobuyuki Kaneko, Susumu Sakata and Shuichi Tokuhashi of Advanced Industrial Science and Technology for their helpful comments about the stable carbon isotopic composition of the CH₄ and the geological structure of the natural gas fields in Japan.