AbstractMe.tha'no.thrix. N.L. neut. n.methanummethane; Gr. fem. n.thrix, hair; N.L. fem. n.Methanothrix, methane (‐producing) hair.Halobacterota / Methanosarcinia / Methanotrichales / Methanotrichaceae / MethanothrixStraight, rod‐shaped cells with flat ends, usually 0.8–1.3 μm wide by 2.0–6.0 μm long enclosed in a tubular sheath. Forms short (∼5–25 μm) to long (>150 μm) flexible chains of cells within the sheath. Nonmotile. Gram‐stain‐negative. Lipids containmyo‐inositol, ethanolamine, and galactose as the polar head groups. Oxygen‐tolerant anaerobe. Organotrophic, splitting acetate into methane and CO2for energy generation. Some strains split formate into H2and CO2without producing methane. CO2can be reduced to methane in coculture withGeobacterspp. via direct interspecies electron transfer (DIET). Growth factors such as vitamins are stimulatory. Yeast extract is required, stimulatory or inhibitory, depending on the strain. NaCl is not required for growth. Optimal temperatures range from 34 to 37°C for mesophilic strains and 55 to 60°C for thermophilic strains; optimal pH range is 7.0–7.8. Gas vacuoles are generally found in thermophilic strains. Occur in both mesophilic and thermophilic anaerobic sludge digesters as well as anaerobic sediments. Synonymous with the genusMethanosaeta.DNA G + C content (mol%): 51–61 (genome).Type species:Methanothrix soehngeniiHuser et al. 1982, VL10.