Me.thy.lo.coc.ca.ce'ae. N.L. masc. n. Methylococcus type genus of the family; ‐aceae ending to denote a family; N.L. gem. pl. n. Methylococcaceae the Methylococcus family.

Proteobacteria / Gammaproteobacteria / Methylococcales / Methylococcaceae

Cells are Gram‐negative, cocci to rods, 0.5–4.0 × 0.5–2.0 μm. Some species appear as sheathed filaments up to 2 cm long and 1–6 µm wide. Species may be motile via a single, unsheathed polar flagellum or nonmotile. Form type I intracytoplasmic membranes appearing as stacks of vesicular discs. Cells usually contain polyhydroxyalkanoate and polyphosphate granules. Some species form cyst‐like resting bodies that provide different degrees of desiccation resistance but not heat resistance. Do not form spores or exospores. Some species are mildly thermophilic, while some others are psychrophilic. Most described species are mesophiles. Most species grow best around neutral pH. Occur widely in terrestrial and marine environments that receive methane flux. Marine species require Na⁺ ions for growth. Do not occur in hypersaline environments. Strictly aerobic. Some species grow best at low O₂ tensions. Most species are catalase and oxidase positive. Type I (type Ia and Ib) methanotrophs. Usually only utilize methane and methanol as sole sources of carbon and energy. Methane is always oxidized by particulate methane monooxygenase. Soluble methane monooxygenase activity is rare. Carbon is mainly assimilated via the ribulose monophosphate pathway. Some strains can couple methanotrophy to denitrification (forming nitrous oxide) when at low O₂ tensions. Some strains can fix CO₂ via ribulose 1,5‐diphosphate carboxylase. Several species can fix atmospheric nitrogen. Most species can use both inorganic and organic forms of nitrogen. Main fatty acids include saturated and monounsaturated fatty acids of C₁₄ or C₁₆ chain lengths with type depending on the genus and species. Main lipoquinones are either ubiquinone‐8 or 18‐methylene‐ubiquinone‐8. Main polar lipids include phosphatidylglycerol and/or phosphatidylethanolamine.

DNA G + C content (mol%): 40–69 (T_m, HPLC, genome analysis).

Type genus: Methylococcus Foster and Davis 1966, 1929^AL emend. Bowman, Sly, Nichols and Hayward 1993, 748^VP.