These findings are in rough agreement with those of Narihiro et al. (2009) who reported a ratio range of 0.37-2.5 of 16S rRNA gene clones of bacteria to archaea at mesophilic temperatures, and close to 1 at thermophilic temperatures.The differences in these observations could be explained by different feed materials and conditions used in the abovementioned studies. In all, the bacteria to archaea ratio appears to be more stable in thermophilic processes, but still case specific and prone to large variations.