Carboxydotrophic anaerobic thermophiles have been isolated from various hydrothermal environments and are considered to be important carbon monoxide (CO) scavengers or primary producers. However, the ecological factors that influence the distribution, abundance and CO-oxidizing activities of these bacteria are poorly understood. A previous study detected the carboxydotrophic bacteria Carboxydothermus spp. in a hot spring sample and found that they constituted up to 10% of the total bacterial cells. In this study, we investigated environmental features, potential microbial CO-oxidation activities and the abundance of Carboxydothermus spp. in various hot springs to determine environmental factors that affect CO oxidizers and to see whether Carboxydothermus spp. are common in these environments. We detected potential microbial CO-oxidation activities in samples that showed relatively high values of total organic carbon, total nitrogen, oxidation-reduction potential and soil-water content. The abundance of Carboxydothermus spp. did not correlate with the presence of potential microbial CO-oxidation activities; however, Carboxydothermus spp. were detected in a wide range of environments, suggesting that these bacteria are widely distributed in spite of the relatively low population size. This study implies that thermophilic CO oxidizers occur in a wide range of environments and oxidize CO in somewhat oxidative environments rich in organic matter.

We developed methods and technology to identify oil and gas fields that are likely to support the restoration of methane deposits, and identified the main characteristics of microbes inhabiting depleted oil and gas fields. To evaluate the potential for microorganisms to inhabit oil and gas fields in Japan, we investigated the existence of methane-producing archaea (MPA) and hydrogen-producing bacteria (HPB) using PCR-DGGE (Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis ) analysis. Reservoir brine from Yabase oil field (Akita Pref., Japan), which was incubated under strictly anaerobic conditions at 50℃, actively produced methane, indicating that Yabase oil field is a suitable site for methane generation. Moreover, analysis of the enrichment culture revealed that it is possible for indigenous anaerobes inhabiting an oil field to generate methane from oil components. Additionally, findings established a methanogenic pathway composed of MPA such as Methanoculleus sp., Methanothermobacter thermoautotrophicus, and Methanosaeta sp. and hydrocarbon-degrading hydrogen-producing bacteria (HD-HPB) related to Thermotoga sp., Petrotoga sp. and Clostridiaceae str. These results strongly sug gest that Yabase oil field has the technological potential for the microbial restoration of methane.

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Research into the microbial restoration of methane deposits has been carried out since 2003. The objective of this research is to estimate the possibility of microbial restoration of methane deposits using subsurface sequestered CO2 and indigenous anaerobes in depleted oil and gas fields. The most important factors are the efficiency and velocity of methane conversion by indigenous anaerobes inhabiting a reservoir. Fluid samples (producing oil and water) from gas and oil fields in Japan were collected and analyzed in order to clarify the existence and survivability of indigenous hydrogen- and methane-producing anaerobes under severe reservoir conditions (high temperature and high pressure). PCR-DGGE analysis, a molecular biology method, was applied to reservoir samples such as reservoir brine, crude oil and producing water from the gas/oil fields. Some hydrogen-producing thermophilic bacteria (HPTB) and methane-producing thermophilic archaea (MPTA) which participate in the microbial restoration of natural gas were detected at the DNA level in some of the samples. Isolation of HPTB and MPTA was also attempted individually, and two strains of HPTB and one strain of methanogen were successfully separated. Subsequent to these findings, accelerated hydrogen- and methane-producing experiments, using glucose as a carbon source, have been conducted at the laboratory level to estimate the potential for microbial methane production under actual reservoir pressure and temperature (5MPa, 50°C) and the rock pore as micro culture space. Experiments, using the isolates described above and active anaerobes which were not isolated from the reservoir brine, indicated that microbial hydrogen- and methane-producing efficiency and velocity are relatively high even in various reservoir conditions. Furthermore, if a suitable and economical carbon source is available, depleted oil reservoirs are potentially good candidates to become subsurface microbial reactors, using hydrogen- and methane-producing indigenous anaerobes containing HPTB and MPTA to convert injected CO2 into methane. Introduction It is well known that CO2 is considered to be the major factor of global warming. Hence, the subsurface CO2 disposal and storage technology which has been studied around the world could become a "must" in the course of the next century in order to reduce the emission of greenhouse gases into the atmosphere1). On the other hand, SOx-free natural gas (methane) is an environmentally excellent form of energy and also one of the most desired energy sources in comparison with other fossil fuels2). Hence, natural gas consumption has risen significantly on a worldwide basis, and the development of perpetual sources of natural gas will become increasingly valued in the future3). Therefore, it is desirable to develop a technology which can resolve both issues, that is, a reduction of CO2 emissions and improved natural gas development.

Inpex and Chugai Technos have been working since 2006 to study methane-producing technology using