Novel Thermophilic Species of Limisphaera Demonstrate Facultatively Anaerobic Mode of Life and Utilize a Wide Range of Carbohydrates

Two novel facultatively anaerobic thermophilic members of the genus Limisphaera (Verrucomicrobiota), strains 4302-coT and VF2T, were isolated from terrestrial hot springs of Geyser Valley (Kamchatka) and the Lake Baikal region, Russia. Cells were motile cocci, forming aggregates; strain 4302-coT grew optimally at a range of 60–65 °C and pH 7.4 and strain VF2T at 55 °C and pH 8.5. Both were chemoorganoheterotrophs utilizing mono-, di-, and polysaccharides, including cellulose, xylan, xanthan gum, arabinan, gluco-, and galactomannans. Both grew anaerobically by fermentation or by respiration with nitrite, nitrate, ferric citrate, or ferrihydrite; dissimilatory iron reduction is reported here for the first time within the phylum Verrucomicrobiota. Their genomes encoded more than 100 glycoside hydrolases and more than 15 polysaccharide lyases, indicating high hydrolytic potential. Additionally, the metabolic pathways of sugar catabolism, fermentation, and both aerobic and anaerobic respiration were reconstructed. A GH51 glycoside hydrolase of strain 4302-coT phylogenetically related to endoglucanases was heterologously expressed and shown to be active at 70 °C with unusually broad specificity, hydrolyzing both β-1,4 and β-1,3 linkages. Based on phylogenomic analysis and phenotypic features, the strains are assigned to two novel species, for which the names Limisphaera vallis sp. nov. and Limisphaera sibirica sp. nov. are proposed.