Grouping of archaea according to their standardised amino acid usage. The left panel is a pictorial representation of relative amino acid usage in the respective archaea. The over- representation and underrepresentation of amino acid residues in the organisms are shown in green and red colored blocks, respectively. The right panel depicts the complete linkage clustering on the Euclidean distances between the relative abundances of different amino acid residues in the encoded proteins of organisms. Organism abbreviations are listed in Additional file 1. The panel in between shows the short names of the archaea, their genomic GC content, temperature adaptation and oxygen requirements respectively.