Thermal tolerance of prokaryote on surface of substrates

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One of the most crucial parameters that limits the habitable depth for extremophiles is temperature. The maximum temperature of growth for hyperthermophile is known to be 121 deg-C but some microbes live on a surface of substrates such as hydrothermal chimneys at temperatures more than 300 deg-C. Therefore, it is presumed that those attached to solid surface may be tolerable to extreme environment such as high temperature.We examined this hypothesis by in-situ sampling from geothermal reservoir using high-temperature, borehole fluid sampler which is tolerant to temperature as high as 300 deg-C. This borehole sampler enables us to get geothermal fluid in-situ to pressure-tight sample chamber (1250 ml) without drastic pressure change. Besides, all the parts can be sterilized either by autoclave or heating up to 200 deg-C.We applied the sampler to a geothermal well N18-OT-2 (1355m,>140 deg-C) that locates in Otari Village, Nagano Prefecture, central Japan. The fluid is rich in bicarbonate … More (2648 ppm), Na^+(1752 ppm) and Cl(1048 ppm). The oxygen isotopic content of the fluid (δ^<18>O<SMOW>=-8.3 permil) shows oxygen shift that is the sign of water rock interaction. The hydrogen isotopic content (^<18>D=-83 permil) is equivalent to local meteoric water. The dissolved organic carbon content (DOC) is as low as 4.55±0.53mg/L which is not high enough to assume intensive deep biosphere within the geothermal reservoir. Detailed geochemical and isotopic modeling indicate that the fluid was formed by 4:1 mixing between shallow, Ca-bearing hot spring water and CO_2 bearing deep geothermal fluid which may come form nearby Quaternary Kazafuki-dake volcano.It is worthy to note that a novel (phylum-level) thermophilic chemo-heterotrophic filamentous bacterium was obtained from the hot spring. Detailed information will be disclosed after publication. On the other hand, small number of cells that attached to the calcite scale of the well at the depth of 593 meters cannot be cultured or identified by CARD-FISH method due to their paucity. Less