Nitriliruptoria is one class in the phylum Actinomycetota with a few species were described. It has a distinct phyletic lineage and signature 16S rRNA gene sequence nucleotides and other characteristics such as showing Gram-stain-positive rods, containing MK-9(H4) as the major quinone and having A1γ type of the cell-wall peptidoglycan (Kurahashi et al., 2010, Sorokin et al., 2009). The class was originally described as the subclass Nitriliruptoridae (Kurahashi et al., 2010) and then emended to the rank of the class (Ludwig et al., 2012). Until now, only eight species belong to five orders are validly published (https://lpsn.dsmz.de/genus/thermus). Almost all species of Nitriliruptoria were isolated from saline environment which made them were all halotolerant or halophilic strains. Among them, Nitriliruptor alkaliphilus and Profundirhabdus halotolerans of Nitriliruptorales were isolated from soda lakes (Sorokin et al., 2009) or sea water (Liu et al., 2023). Egibacter rhizosphaerae of Egibacterales was isolated from rhizosphere (Zhang et al., 2016). Egicoccus halophilus of Egicoccales was isolated from soil (Zhang et al., 2016). Euzebya tangerine; Euzebya rosea and Euzebya pacifica of Euzebyales were isolated from epidermis of trepang (Kurahashi et al., 2010) or seawater (Yin et al., 2018, Jian et al., 2021). And Salsipaludibacter albus of Salsipaludibacterales was isolated from solar saltern (Almeida et al., 2022). The genome sizes of these species range from 3.98 to 5.56 Mbp, with higher G + C contents (68.9–72.6 mol%).

In this study, two facultative anaerobic strains (SYSU M7M538T and SYSU M7M531) which were isolated from the oral of Steller sea lion showed different characteristics with other Nitriliruptoria species. For example, they can only tolerant very few NaCl concentration (2 %) and have reduced genome (2.1 and 2.3 Mbp) with lower G + C content (52.5 and 52.7 %). Analyses revealed they have 16S rRNA gene sequences similarity less than 89 % and ANI value less than 65 % compared with other species of Nitriliruptoria. Here, the phylogenetic, phenotypic, and genome feature evidences revealed these two strains represent a novel species of a novel genus, a novel family and a novel order of the class Nitriliruptoria, for which the names Stomatohabitans albus gen. nov., sp. nov., Stomatohabitantaceae fam. nov. and Stomatohabitantales ord. nov. are proposed.