The genus Methylomonas accommodates aerobic, obligate methanotrophs, with their sole carbon and energy sources restricted to methane and methanol (Bowman, 2016). The first described methanotrophic bacterium, which was isolated and named as ‘Bacillus methanicus’ by Söhngen in 1906 (Söhngen, 1906), represents the type species of the genus Methylomonas, M. methanica (Whittenbury and Krieg, 1984). Members of this genus inhabit oxic-anoxic interfaces of lakes and rivers (Auman and Lidstrom, 2002, Bussmann et al., 2021), wetlands (Kip et al., 2011, Danilova and Dedysh, 2014), rice paddies (Dianou et al., 2012) and water-saturated soils. Methane (CH4) is oxidized by the particulate methane monooxygenase (pMMO) enzyme, which is localized in intracytoplasmic membranes (ICM) arranged as stacks of vesicular disks (type I ICM); the carbon is assimilated via the ribulose-monophosphate pathway (Whittenbury and Krieg, 1984, Bowman et al., 1993). In addition to pMMO, some Methylomonas species contain soluble methane monooxygenase (sMMO) (Auman and Lidstrom, 2002, Bussmann et al., 2021).

The genus Methylomonas belongs to the class Gammaproteobacteria, the family Methylococcaceae. At present, this genus includes 9 species with validly published names, including M. methanica (Whittenbury and Krieg, 1984); M. aurantiaca, M. fodinarum (Bowman et al., 1990); M. scandinavica (Kalyuzhnaya et al., 1999); M. koyamae (Ogiso et al., 2012); M. lenta (Hoefman et al., 2014); M. paludis (Danilova et al., 2013); M. albis and M. fluvii (Bussmann et al., 2021). Two additional species of this genus, ‘M. denitrificans’ (Kits et al., 2015) and ‘M. rhizoryzae’ (Zhu et al., 2020), have also been described but their names are not validly published yet. Members of these species are Gram-negative, rod-shaped or coccobacillary cells that produce pink or orange carotenoid pigments, are motile by means of a single flagellum, and grow between pH 5.0 and 9.0, with an optimum at pH 6.5-7.0. M. paludis is the only described acid-tolerant representative of this genus, which grows down to pH 3.8 (Danilova et al., 2013). All characterized Methylomonas species are mesophiles displaying growth optimum between 20 and 35 °C, although psychrotolerant members of this genus, such as M. scandinavica (Kalyuzhnaya et al., 1999) have also been described.

Due to their high growth rates on methane, Methylomonas species have attracted considerable attention as potential producers of a single-cell protein as well as various value-added products from C1 compounds (Koffas et al., 2003, Ye et al., 2007, Hur et al., 2016, Hur et al., 2017, Guo et al., 2017).

In this manuscript, we characterize two representatives of the genus Methylomonas, strains 12 and MP1T, which display extremely fast growth on methane and natural gas and, therefore, deserve attention as potential producers of a feed protein enriched with carotenoids. Since these bacteria are phylogenetically divergent from the currently described Methylomonas species, we propose a novel species to accommodate these methanotrophs.