Cellulosispirillum alkaliphilum, gen. nov., sp. nov., an obligately anaerobic cellulotrophic member of the phylum Fibrobacterota from soda lakes

Intensive microbiology studies of the past several decades of soda lakes, uncovered a rich functional diversity of haloalkaliphilic microbes driving carbon, nitrogen and sulfur cycles in these unique double-extreme habitats. One of the unexpected finding was a discovery there of aerobic extremely halophilic cellulotrophic natronoarchaea. Yet, little is still known about the identity of the soda lake bacteria able to use native cellulose as growth substrate, except for a single case of an anaerobic clostridium. In this work we present results of phenotypic and functional genomic analysis of an anaerobic bacterium, strain ANBcel5^T, enriched from hypersaline Siberian soda lakes with amorphous cellulose as growth substrate. Phylogenetic analysis placed the isolate into the family Chitinispirillaceae in the phylum Fibrobacterota as a new genus and species lineage with the 16S rRNA gene identity and Relative Evolutionary Divergence (RED) to its only known species Chitinispirillum alkaliphilum ACht6–1^T of 95.2 % and 0.847, respectively. In contrast, despite obvious morphological resemblance to ACht6–1^T, strain ANBcel5^T is a narrow cellulose-utilizing fermentative anaerobe fermenting cellulose and cellobiose to acetate, H₂ and succinate. It is a moderately salt-tolerant obligate alkaliphile growing optimally at 0.6 M total Na⁺ as carbonates and pH 9.5. Functional genome analysis of the isolate revealed the presence of multiple genes encoding extracellular endocellulases from the GH families 5 and 9, three sodium-translocating membrane complexes and osmolytes Nε-acetyl-β-lysine and glycine betaine biosynthesis. The bacterium is proposed to be classified as Cellulosispirillum alkaliphilum gen. nov., sp. nov. (DSM 113825 = UQM 41584).