The genus Undibacterium belongs to the family Oxalobacteraceae has been reported in diverse environments, including gastrointestinal tracts (Sun et al., 2019; Wu et al., 2021), rainfalls sprinkle cloud (Péguilhan et al., 2021), rapid sand filters of drinking water treatment plants (Vandermaesen et al., 2017), aquifer systems (Abiriga et al., 2021; Lehosmaa et al., 2021; Patel et al., 2024) and soils (Wolf et al., 2015). The Undibacterium strains could be as endosymbionts in freshwater crustaceans (Mioduchowska et al., 2018), seeds of maize (Liu et al., 2013b) and pest species (Liu et al., 2024b). The Undibacterium strains also play an important role during the ferrous iron oxidation (Gülay et al., 2018), the degradation of commercially available biodegradable plastic films (Morohoshi et al., 2018) and the activation and transfer of cadmium in soils (Huang et al., 2023). The Undibacterium strains even could tolerate the drinking water chlorination (Jia et al., 2015) and monochloramine in membrane-based water treatment systems (Yang et al., 2017). The Undibacterium strains also could degrade cellulase (Nahm et al., 2017) and produce a novel extracellular polyhydroxyalkanoate depolymerase (Morohoshi et al., 2020).

At present, the genus Undibacterium contains 26 validly published species (https://lpsn.dsmz.de/genus/undibacterium). Most species were isolated from freshwater environments (Chen et al., 2017; Du et al., 2015; Eder et al., 2011; Lu et al., 2021; Kämpfer et al., 2007; Kim et al., 2014a; Phurbu et al., 2021; Sheu et al., 2014a; Sheu et al., 2014b; Van Le et al., 2024). Undibacterium jejuense (Kim et al., 2014a), Undibacterium arcticum (Li et al., 2016) and Undibacterium terreum (Liu et al., 2013a) were isolated from soils. Only Undibacterium piscinae was isolated from Korean shiner intestine (Lee et al., 2019) and Undibacterium danionis was isolated from a zebrafish (Kämpfer et al., 2016). Since we reported 11 novel species of genus Undibacterium isolated from streams in 2021 (Lu et al., 2021), only Undibacterium cyanobacteriorum was reported in 2024 (Van Le et al., 2024). The previous Undibacterium species were determined mainly based on the 16S rRNA gene sequence similarity and DNA-DNA hybridization values among closely related strains, so the taxonomic status of some Undibacterium species were questionable when the microbial taxonomy entered the genomic era and used the whole genome sequence data for re-examining these species (Chun et al., 2018; Riesco and Trujillo, 2024). In 2023, Ma et al. noticed the taxonomic problems of the Oxalobacteraceae strains and did the detailed genome-based analyses of family Oxalobacteraceae (Ma et al., 2023). The genus Undibacterium was divided into four clades in their phylogenomic tree and each of the four clades was proposed to represent an independent genus. Three new genera names were proposed in their study, including Paraundibacterium, Neoundibacterium and Affinundibacterium. It was a little pity that these novel genera names were still not valid and Ma et al. only used 23 Undibacterium genomes in their study. During the investigation of microbial resources from the southwestern China in 2023, 25 Undibacterium-related strains were isolated from alpine lakes. The genomes of these 25 strains are determined in this study and combined with 26 genomes of valid Undibacterium species and 20 genomes of GTDB proposed Undibacterium species to re-examine the taxonomic status of this genus. The genomic characteristics of these 25 Undibacterium-related strains adapted to alpine lakes are also identified in this study.