The eubacteria are a diverse group that includes all the photosynthetic bacteria (except for the halobacteria) as well as many non-photosynthetic groups. Most eubacteria are mesophiles; however the eubacteria also include extreme thermophiles, such as Thermotoga maritima and Aquifex pyrophilus,which can grow up to 90°C and 95°C, respectively (Huber & Stetter, 1992). The lipids of eubacteria are primarily of the ester type although Thermotoga, Aquifex, and their relatives also contain branched ether lipids. E. coli is a well-known example of the eubacteria. Click on the picture at left for more information on E. coli.

The halobacteria are extreme salt-lovers. They are carbon heterotrophs that can use an unusual photosynthesis system, namely a light-driven proton pump based on bacteriorhodopsin. Like eubacteria, they contain the biochemical pathways for the synthesis of C₄₀ and C₅₀ carotenoids (Goodwin, 1980). Halobacterium salinarium is an example of a halophile. Click on the picture at left for more information on H. salinarium.

The methanogens are a phylogenetically diverse group, despite the fact that they share a common phenotype, namely they are strict anaerobes with the ability to chemically reduce carbon compounds to methane to provide energy. Associated with the methanogens is a phenotypically diverse group of organisms represented by such organism as Thermococcus celer, and Archaeoglobus fulgidus, and, possibly, Methanopyrus kandleri. This last organism grows at temperatures up to 112°C. Like methanogens, it reduces carbon compounds to methane, but it is not closely related to other methanogens but instead is intermediate between them and the next group of organisms (Burggraf, Stetter, Rouviere & Woese, 1991; Rivera & Lake, 1996). Methanococcus janaschii is one example of a methanogen. Click on the picture at left to find out more about M. janaschii.

The fourth prokaryotic gourp, the eocytes, consists of thermophilic, mostly sulfur-metabolizing organisms, many of which can grow at temperatures in excess of 100^oC. The eocytes include Sulfolobus, Desulfurococcus, Thermoproteus, Pyrodictium, Pyrobaculum, etc. Sulfolobus sulfataricus, oxidizes sulfur to H₂S. Others, such as Acidianus infernus, can oxidize or reduce sulfur to H₂SO₄ or to H₂S. The organisms with the highest maximum growth temperatures are Pyrodictium occultum (112°C), Pyrodictium abyssum (112°C), and Pyrolobus fumarii (113°C). The group is metabolically diverse, frequently hyperthermophilic, and phylogenetically monophyletic. Sulfolobus acidocaldarius is another interesting eocyte. Click on the picture at left for more information on S. acidocaldarius.