The Phylogeny of Thermophiles and Hyperthermophiles and the Three Domains of Life

Abstract

The nature of the first cells and the environment in which they lived are two of the most interesting problems in evolutionary biology. All living things are descendents of these primordial cells and are divided into three fundamental lineages or domains, Archaea (formerly known as Archaebacteria), Bacteria (formerly known as Eubacteria), and the Eucarya (formerly known as Eukaryotes, Woese et al. 1990). The Archaea and Bacteria are prokaryotic domains whereas the Eucarya includes all other living things that have a nucleus (i.e., the genetic material is separated from the cytoplasm by a nuclear envelope). The observation of the three primary domains, first made on the basis of small subunit (i.e., 16S, 18S) ribosomal DNA (rDNA) sequence comparisons (Woese 1987), has created a framework with which the nature of the last common ancestor (LCA) can be addressed. In this review we present phylogenies of the prokaryotic domains to understand the origin and distribution of the thermophiles (organisms able to grow in temperatures > 45°C) and the hyperthermophiles (organisms able to grow in temperatures > 80°C). Hyperthermophiles are limited to the Archaea and Bacteria. In addition, we inspect the distribution of extremophiles within the cyanobacteria. The cyanobacteria are unique in being able to tolerate rapidly fluctuating environmental conditions. This capacity has presumably allowed some cyanobacteria to survive in niches unexplored by other prokaryotes and eukaryotes which have a more restricted environmental tolerance. We do not deal with eukaryotic extremophiles in this chapter because no multicellular plant or animal yet found can survive in temperatures of about 50°C and no protists can tolerate long-term exposure to temperatures in excess of 60°C (Madigan and Marrs 1997).