Soda lakes is a unique type of inland highly alkaline and saline habitat dominated by a specialized group of haloalkaliphilic bacteria and archaea (Grant and Jones, 2016). Intensive microbiology and molecular ecology studies of the soda lakes in the past three decades uncovered functionally rich and taxonomically diverse prokaryotic communities (Sorokin et al., 2015, Sorokin, 2017, Haines et al., 2023, Sorokin et al., 2014 Vavourakis et al., 2016; Vavourakis et al., 2019; Zhao et al., 2020, Zorz et al., 2019). One of the primary functional blocks of the carbon-cycling microbial communities is polymer degradation, and cellulose is a major carbohydrate polymer produced by plants and algae. So far, little was known about the identity of haloalkaliphilic prokaryotes involved in mineralization of cellulose in soda lakes. Only recently it was discovered that in hypersaline soda lakes aerobic natronoarchaeal genera Natronobiforma and Natronolimnobius can use cellulose as growth substrate (Sorokin et al., 2015, Sorokin et al., 2018) whose genomes encode multiple copies of endo-cellulases from the GH5 and GH9 families (Elcheninov et al., 2023). However, the identity of cellulose-mineralizing bacteria in moderately saline soda lakes (whereby natronoarchaea are scarcely present) remained unclear. The only example of such bacteria, known so far, was a moderately salt-tolerant, anaerobic, cellulosome-forming Clostridium alkalicellulosi (lately reclassified as Herbivorax alkalicellulosi) from a soda lake Hadyn in Tyva Republic (Russia) (Zhilina et al., 2005; Zvereva et al., 2006).

Our recent efforts in obtaining a better understanding of the bacterial cellulose mineralization in moderately saline soda lakes resulted in enrichment of three haloalkaliphilic anaerobic bacterial cultures capable of utilization of various forms of insoluble cellulose as growth substrate. The primary cellulotrophic bacteria in these cultures were represented by new members of the Clostridiales, Halanaerobiales and Fibrobacterota (Sorokin et al., 2024). The latter has just been described as a new genus and species Cellulosispirillum alkaliphilum (Sorokin et al., 2025). In this work we present results of phenotypic, phylogenetic and functional genomic analyses of the endospore-forming cellulotrophic Bacillota strains ANBcel31T and ANBcel28Ts, and propose to classify them as a new species in the genus Herbivorax and a new genus and species in the order Halanaerobiales, respectively. The ANBcel31T is represented by a pure culture, while the ANBcel28Ts originated from a stable binary consortium with a cellobiose-utilizing satellite, and was unable to grow alone in liquid cultures with cellulose or cellobiose. Thus, it is classified under the SeqCode rules (Rosselló-Móra et al., 2023).