545

We have already noted that Archaea and Bacteria are prokaryotic, but differ in their membrane and wall composition, molecular mechanisms for transcription and translation, and many other biological features (see Table 26.1). As a result, the two groups tend to thrive in very different environments. Many archaeons tolerate environmental extremes such as heat and acidity. In fact, for many environmental conditions, archaeons define the known limits of life. What do these disparate environments have in common? One proposal is that archaeons thrive in environments where the energy available to fuel cell activities is limited—where there is no light, or a limited abundance of fuels and oxidants for chemoautotrophy or respiration, or both. That is, archaeons commonly live where energy resources are too low to support bacteria and eukaryotes.

Bacteria and Archaea diverged from their last common ancestor more than 3 billion years ago. Just as Bacteria have evolved a phylogenetic diversity that defies easy categorization, Archaea also include many distinct types of organism. Many microbiologists recognize three major divisions of Archaea, called the Crenarchaeota, Euryarchaeota, and Thaumarchaeota, but a few poorly known groups may indicate further major branches within the domain (for example, the Korarchaeota and Nanoarchaeota noted in Fig. 26.17). Indeed, some microbiologists now propose that several of these groups together form a distinct clade nicknamed TACK (Thaumarchaeota + Aigarchaeota + Crenarchaeota + Korarchaeota; the upper main branch in Fig. 26.17). Like the Bacteria, Archaea include forms able to respire aerobically or anaerobically, as well as chemoautotrophs and fermenters. There are, however, metabolic differences between the two domains. No Archaea carry out photosynthesis using chlorophyll and related pigments, but some Archaea exhibit forms of energy metabolism unknown in bacteria or eukaryotes.