A rod-shaped extremely thermophilic methanogen is described, growing between 65 and 97 °C with an optimal temperature around 83 °C and a doubling time of 170 min. The GC-content of its DNA is 33 mol %. The isolated cell wall sacculus contains pseudomurein. The complex cell envelope exhibits two layers, each about 12 nm thick; the inner represents the pseudomurein sacculus and the outer a protein envelope. An enriched fraction of RNA polymerase does not react with antiserum against RNA polymerase from Methanobacterium thermoautotrophicum, indicating that the isolate belongs to a new family, the Methanothermaceae, within the order Methanobarteriales. The new organism is named Methanothermus fervidus.

Abstract. An angular plate-shaped weakly motile mesophilic methanogen was isolated from a swamp of drilling waste in Italy. Growth occurs on H 2 /C0 2 or on formate. Acetate is required in addition. The optimal doubling time is 7h at 40" C. The cell envelope is composed most likely of glycoprotein subunits in hexagonal arrangement. The GC-content of its DNA is 47.5 mol%. On the basis of DNA-RNA hybridization it was found to represent a new family, the Methanoplanaceae within the order Mcthanomicrobiales.Key words: Methanogens -Archaebacteria -Cell division -Glycoprotein -Acetate -Taxonomy Recently, a square-shaped flat bacterium was discovered in a saturated salt brine (Walsby 1980; Stoeckcnius 1981), which, however, cannot yet be cultivated in the laboratory. It was assumed (Walsby 1980) that the unusual shape may be explained by the absence of cell turgor in bacteria in a high ionic strength environment. From the composition of its envelope, Walsby (1980) speculates that this organism belongs to the archaebacteria.I lere, we report on the isolation and properties of another Hat archaebaclerium, which, however, grows at much lower ionic strength and which belongs to the methanogens. Materials and Methods StrainsMethanogenium marisnigri* DSM 1498, was obtained from the Deutsche Sammlung von Mikroorganismen, Göttingen. Cull arc ConditionsThe isolate M3 was cultivated by using the technique described by Halch and Wolfe (1976). If not mentioned otherwise, the isolate was grown in % *MCP medium, that is medium 3 of Halch el al. (1979), modified by the use of "Pepton aus Casein, tryptisch verdaut" (Merck) instead of Irypliease (HHl.) and by adjusting the pi I to (>.9 (11 2 S0 4 ).Twenty milliliter cultures were grown in stoppered pressurized UK) ml serum bottles (Hormioli. Italy) made of "type I IP-glass by incubation in water bath shakers (New Brunswick) at 140rpm and 37 "C..ihbrrvhitii'ns tilt*. I itiauiik* I losinc; SPS: Sodium dodccylsulfalc (Sodium lauryl sulfate) Methanogenium marisnigri, as a reference, was grown in the same medium. PlatingPolysilicate plates were prepared as described (Stettcr ct al. 1981) except that they were equilibrated with MG-medium containing penicillin, vancomycin, kanamycin (each 150 ug/ml) and tetracycline (100 ug/ml) to prevent eubacterial contaminations. Light MicroscopyThe cells were viewed and photographed with a Leitz Ortholuxll microscope, equipped with a vario-orlhomat camera system (Leitz). Fluorescence was observed in a Zeiss Standard fluorescence microscope with an excitation filler H436 and a selection filler LP 470. Electron MicroscopyFor thin sectioning, cell sediments were fixed in MG-medium not containing organic components with 20 g glutaraldehyde/l for 2h and post fixed with 10 g Os0 4 /l for 1 h. Durcupan (Fluka) epoxy resin was used for embedding and thin sections were contrasted with lead citrate (5 min), uranylacetate (5 min) and again with lead citrate (3 min). For shadowing, the cells were fixed on parlodion coated grids and shadowcasted (Edwards vacuum co...

Summary. DNAs from 16 species of archaebacteria including 6 novel isolates were hybridized with 16S rRNAs from 7 species representing different orders or groups of the urkingdom of archaebacteria.The yields, normalized for the number of genes per ¡ig of DNA, and the temperature stabilities of all hybrids were determined and related to each other.A taxonomic tree constructed from such fractional stability data reveals the same major divisions as that derived from comparative cataloging of 16S rRNA sequences. The extreme halophiles appear however as a distinct order besides the three known divisions of methanogens.The methanogens, the halophiles and Thermoplasma form one of two clearly recognizable branches of the archaebacterial urkingdom. The order represented by Sulfolobus and the related novel order Thermoproteales form the other branch.Three novel genera, Thermoproteus, Desulfurococcus and the "stiff filaments" represent three families of this order.The extremely thermophilic methanogen Methanothermusfervidus belongs to the Methanobacteriales. SN1, a methanogen from Italy, appears as another species of the genus Methanococcus. Another novel methanogen, M3, represents a genus or family of the order Methanomicrobidles.