The temperature ranges of growth of archaea are strongly correlated with the guanine-plus-cytosine (G+C) contents of their 16S rRNA sequences (PGC). In order to estimate minimum (Tmin), optimal (Topt), and maximum (Tmax) growth temperatures of uncultured archaea based on PGC, the 16S rRNA gene sequences of 207 archaeal species were collected from public databases, and their Tmin, Topt and Tmax were extracted from description papers and reviews. These values of growth temperatures were plotted against PGC, and then the regression lines for estimating Tmin, Topt and Tmax were calculated. We PCR-amplified the archaeal 16S rRNA gene fragments from the hot water samples, cloned the fragments, and determined the sequences. Growth temperatures of environmental archaea were inferred from G+C content of the 16S rRNA gene sequences by the regression lines. In the terrestrial hot springs (74°C and 85°C), both estimated growth temperatures of archaea were higher than in situ temperatures of hot spring waters. Even from tepid hydrothermal fluid (40°C) we obtained a significant number of archaeal genes indicating high growth temperatures. These results suggested that hot subsurface environments exist under those hydrothermal and geothermal regions. In this study, growth temperatures of uncultured archaea and in situ subsurface temperatures were roughly inferred from 16S rRNA gene sequences of archaea that were transported from the subsurface biosphere. This new method based on microbial molecular information may be applicable to temperature estimation of subsurface environments for which it has been difficult to measure the actual temperature with appropriate instrumentation.
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