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 Chloracido

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 <i>c</i>, bacteriochlorophyll <i>a</i> _P, echinenone and <i>γ</i>-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 <i>Chloracidobacterium thermophilum</i> and '<i>Candidatus Chloracidobacterium aggregatum</i>' 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 <i>Chloracidobacterium validum</i> strain BV2-C^T (=DSM 113832^T=JCM 39534^T) represents a novel species of the genus '<i>Candidatus</i> Chloracidobacterium', which we hereby propose as <i>Chloracidobacterium</i> gen. nov. with <i>Chloracidobacterium validum</i> sp. nov. as the type species. Based on phylogenomic analyses, we also propose a new family, <i>Chloroacidobacteriaceae</i> fam. nov., and a new order, <i>Chloracidobacteriales</i> ord. nov., within the class <i>Blastocatellia</i>. We further propose the name '<i>Ca. C. aggregatum</i>' for pro-valid publication with the genome sequence of strain E as nomenclatural type.