TAXA OF LIFE
DOMAIN ARCHAEA

INTRODUCTION

Barring viruses, living things seem to come in two different fundamental forms: prokaryotes and eukaryotes.  The separation is based on cellular structure or ultrastructure and some aspects of biochemistry.  However, the lineage of these two groups had been largely unexplored until Woese and Fox  (1977) chose the 16-S rRNA subunit to sequence in an attempt to determine the relationships of different prokaryotes and eukaryotes.  To the surprise of the scientific community, their results showed that life existed in three fundamental forms and two of them were prokaryotic.  Woese called the three groups Domains and named them Archaea, Bacteria, and Eukarya.  Within a common taxonomic hierarchy, then, kingdoms are clustered within domains.

The Archaea are prokaryotic organisms that have a number of unique cellular features which serve to distinguish them from the Bacteria, the other prokaryotic domain.  They differ in the form and structure of their ribosomes, the type and linkage of their lipids, structure of the cell covering, and a different type of RNA polymerase (Margulis and Schwartz 1998).  The neat package of three domains fails, however, because lateral gene transfer (a "mechanism" that is quite common in the prokaryotes) mixes the DNA of unrelated taxa.  It seems that small subunit rRNA nucleic acid is about the most conserved of the DNA sequences.  However, with enough time, most lineages likely have become somewhat scrambled.  With that warning I offer the following system of the Domain Archaea.  This system is based largely on Margulis and Schwartz (1998), with modifications from Garrity et al. (2001 and 2003) and Black (2002).  I raise the phylum designations of Margulis and Schwartz (1998) and Garrity et al. (2001) to kingdom-level.  So, the Archaea in this system have two kingdoms: the Euryarchaeota and the Crenarchaeota.  (Go here for a 16s rRNA molecular sequence cladogram of the Archaea based on data from Woese).

HIGHER LEVEL CLASSIFICATION

LITERATURE CITED

Black, J. G. 2002. Microbiology, Principles and Explorations. 5th ed. John Wiley and Sons, Inc. New York.

Garrity, G. M., M. Winters, and D. Searles. 2001. Bergey's manual of systematic bacteriology. 2nd ed. Springer-Verlag. New York.

Garrity, G. M., J. A. Bell, and T. G. Lilburn. 2003. Taxonomic Outline of the Prokaryotes. Bergey's Manual of Systematic Bacteriology. 2nd edition. Release 4.0. Springer-Verlag. New York.  pp. 1-397.

Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2^nd Edition. W. H. Freeman and Co.  New York.

Woese, C. R. and G. E. Fox. 1977. Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proceedings of the National Academy of Sciences USA . 74:5088-5090.