Verrucomicrobiota is ubiquitous and ecologically important bacterial phylum. Initially observed in freshwater aquatic habitats in 1935, these bacteria were not isolated in pure culture until 1970, when the first strain was isolated from a lake in Michigan (Henrici and Johnson, 1935, DeBont et al., 1970). In further studies the representatives of the phylum were found in many various environments, where they are playing different roles – from autotrophs such as methanotrophic members of Methylacidiphilaceae (Khadem et al., 2011) to heterotrophs with aerobic or anaerobic mode of life which is a characteristic of an absolute majority of the phylum. Although Verrucomicrobiota members most commonly are detected in mesophilic marine or terrestrial habitats (Hedlund, 2010), they also could thrive in extreme environments: cold waters of Antarctica (Schlesner et al., 2006), soda lakes (Humayoun et al., 2003), submarine hydrothermal systems (Hirayama et al., 2007), mud volcanoes (Alain et al., 2006) and hot springs of Russia, New Zealand, Italy, and USA (Op den Camp et al., 2009, Kochetkova et al., 2022). Finally, the phylum representatives have also been found in various anthropogenic ecosystems (rice paddies, leachate from a municipal solid-waste landfill, acid rock drainage etc.) as well as digestive tract of various animals (termites, sea cucumbers, mice) and human (Wang et al., 2005).

The taxonomy of the group was established in 1995 when the verrucomicrobial representatives were formally assigned to a separate order Verrucomicrobiales (Ward-Rainey et al., 1995), which was soon reclassified to a new class Verrucomicrobiae (Hedlund et al., 1997), and then ranked as a phylum (Garrity and Holt, 2001). According to the current GTDB classification (Parks et al., 2022) phylum Verrucomicrobiota consists of three classes: Kiritimatiellae, Lentisphaeria and Verrucomicrobiae. The latter consists of seven orders, including “Pedosphaerales”, for which formerly used order name has not been validly published.

To date, the order ”Pedosphaerales” is presented by two cultivated members – “Pedosphaera parvula” strain Ellin514 (Sangwan et al., 2005, Kant et al., 2011) and Limisphaera ngatamarikiensis strain NGM72.4T (Anders et al., 2015). There is little information about phenotypic properties of “P. parvula”: it is only known that it is represented by mesophilic aerobic chemoorganotrophic saccharolytic Gram-negative cocci, isolated from pasture soils (Hedlund, 2010, Kant et al., 2011). L. ngatamarikiensis is a moderately thermophilic aerobic chemoorganotroph, isolated from geothermally heated subaqueous clay sediments, also represented by Gram-negative cocci (Anders et al., 2015). Little is known about the ”Pedosphaerales” distribution. Besides “P. parvula” and L. ngatamarikiensis, representatives of the “Pedosphaerales” were also detected in microbial consortia of rice paddy soil in China (Wang et al., 2019), fens and bogs in Russia (Ivanova et al., 2020; Dedysh et al., 2021) and marine environments (Tully et al., 2018, Howe et al., 2023).

In this work, a novel thermophilic representative of the phylum Verrucomicrobiota, Fontisphaera persica strain B-154T, isolated from a hot spring of Baikal region is described. Based on phylogenomic analysis of the F. persica and its relatives a novel order Limisphaerales is proposed to replace the “Pedosphaerales” order. The novel order includes two novel families Limisphaeraceae and Fontisphaeraceae for which Limisphaera and Fontisphaera are the type genera, respectively.