Chloracidobacterium validum sp. nov., a thermophilic chlorophotoheterotrophic bacterium of the phylum Acidobacteriota from an alkaline hot spring microbial mat, represents Chloracidobacterium gen. nov., Chloracidobacteriaceae fam. nov. and Chloracidobacteriales ord. nov.

A novel chlorophototrophic bacterium, strain BV2-C T , was isolated from a microbial mat (~40 °C) at Rupite hot springs, Bulgaria. Cells were rod-shaped, Gram-negative, with numerous fimbriae/pili, but non-motile. Optimal growth occurred under micro-oxic conditions in the light (20–50 µmol photons m −2 s −1 ) at 45 °C (range 35–50 °C) and pH 7.2 (range pH 5.5–9.0). Bacteriochlorophyll c , bacteriochlorophyll a P , echinenone and γ -carotene were the major pigments in cells capable of producing chlorosomes and type-1 homodimeric reaction centres. Dense liquid culture appeared greenish-brown, with cells forming small clumps and aggregates. Strain BV2-C T grows photoheterotrophically using organic compounds as carbon source (i.e. a mixture of 20 proteinogenic amino acids or peptone) and thioglycolate as reduced sulphur source. Strain BV2-C T obligately required branched-chain amino acids ( l -isoleucine, l -leucine and l -valine) and l -lysine for growth. It is closely related to Chloracidobacterium thermophilum and ‘ Candidatus Chloracidobacterium aggregatum ’ with 16S rRNA gene sequence identities of 96.97 and 97.78%, respectively. The genome of strain BV2-C T comprises two circular chromosomes of 2,659,040 and 1,000,103 bp (total genome, 3.65 Mb) and G+C content of 59.9 mol%. Phenotypic, phylogenetic and genomic analyses demonstrated that Chloracidobacterium validum strain BV2-C T (=DSM 113832 T =JCM 39534 T ) represents a novel species of the genus ‘ Candidatus Chloracidobacterium’, which we hereby propose as Chloracidobacterium gen. nov. with Chloracidobacterium validum sp. nov. as the type species. Based on phylogenomic analyses, we also propose a new family, Chloroacidobacteriaceae fam. nov., and a new order, Chloracidobacteriales ord. nov., within the class Blastocatellia . We further propose the name ‘ Ca. C. aggregatum ’ for pro-valid publication with the genome sequence of strain E as nomenclatural type.