The kingdom Archaea

Until presently the Archaea were regarded as a solely extremophilic species, growing in environmental niches unsuitable for other forms of life as extreme thermophiles psychrophiles, halophiles or natronophiles example for require salt to grow. With the advent of rRNA sequencing, we are now seeing species within the Euryarchaeota in arboreal forest soils in temperate lakes, indeed in most of the places we expect to see Bacteria. This should have been no great surprise since we have known about the methanogenic Archaea in our digestive systems for many decades. Despite having as great an ability to behave as chemolithotrophs or chemoorganotrophs as the Bacteria and possessing a greater diversity in nucleic acid replication, there are presently no known human Archaeal pathogens.

As has been emphasized in other sections the Archaea have various similarities physiologically to both the Bacteria and the Eukarya in Table 4. However, our understanding of the ecology, physiology and biochemistry of this kingdom does not start to approach the wealth of biological knowledge we have for such organisms as E. coli Saccharo¬ and E. coli myces cerevisiae.