The temperature ranges of growth of archaea are strongly correlated with the guanine-plus-cytosine (G+C) contents of their 16S rRNA sequences. In order to estimate growth temperatures of uncultured archaea based on G+C content, the 16S rRNA sequences of archaeal species were collected from public databases. These values of growth temperature were plotted against G+C content of 16S rRNA, and then the regression line for estimating growth temperature was calculated. Growth temperatures of subsurface archaea are inferred from G+C content of the 16S rRNA gene sequences by the regression line. In future, growth temperatures of uncultured archaea and in situ subsurface temperatures will be roughly inferred from 16S rRNA sequences of archaea. This new method may be applicable to temperature estimation of subsurface environments for which it has been difficult to measure the actual temperature.
