A novel chlorophototrophic bacterium, strain BV2-CT, 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-CT 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-CT 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-CT 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-CT (=DSM 113832T=JCM 39534T) 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.

Department of Biochemistry and Molecular Biology The Pennsylvania State University University Park PA 16802 USA

Department of Biological Sciences Tokyo Metropolitan University 1 1 Minami Osawa Hachioji Tokyo 192 0397 Japan

Department of Chemistry and Biochemistry Montana State University Bozeman MT 59717 USA

Laboratory of Anoxygenic Phototrophs Institute of Microbiology CAS 37901 Třeboň Czechia

Leibniz Institute DSMZ German Collection of Microorganisms and Cell Cultures GmbH Inhoffenstraße 7B 38124 Braunschweig Germany

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