Genomic and Phenotypic Characterization of Chloracidobacterium Isolates Provides Evidence for Multiple Species

Chloracidobacteriumis the first and until now the sole genus in the phylumAcidobacteriota(formerlyAcidobacteria) whose members perform chlorophyll-dependent phototrophy (i.e., chlorophototrophy). An axenic isolate ofChloracidobacterium thermophilum(strain BT) was previously obtained by using the inferred genome sequence from an enrichment culture and diel metatranscriptomic profiling analysesin situto direct adjustments to the growth medium and incubation conditions, and thereby a defined growth medium forChloracidobacterium thermophilumwas developed. These advances allowed eight additional strains ofChloracidobacteriumspp. to be isolated from microbial mat samples collected from Mushroom Spring, Yellowstone National Park, United States, at temperatures of 41, 52, and 60°C; an axenic strain was also isolated from Rupite hot spring in Bulgaria. All isolates are obligately photoheterotrophic, microaerophilic, non-motile, thermophilic, rod-shaped bacteria.Chloracidobacteriumspp. synthesize multiple types of (bacterio-)chlorophylls and have type-1 reaction centers like those of green sulfur bacteria. Light harvesting is accomplished by the bacteriochlorophylla-binding, Fenna-Matthews-Olson protein and chlorosomes containing bacteriochlorophyllc. Their genomes are approximately 3.7 Mbp in size and comprise two circular chromosomes with sizes of approximately 2.7 Mbp and 1.0 Mbp. Comparative genomic studies and phenotypic properties indicate that the nine isolates represent three species within the genusChloracidobacterium. In addition toC. thermophilum, the microbial mats at Mushroom Spring contain a second species, tentatively namedChloracidobacterium aggregatum, which grows as aggregates in liquid cultures. The Bulgarian isolate, tentatively namedChloracidobacterium validum, will be proposed as the type species of the genus,Chloracidobacterium.Additionally,Chloracidobacteriumwill be proposed as the type genus of a new family,Chloracidobacteriaceae, within the orderBlastocatellales, the classBlastocatellia, and the phylumAcidobacteriota.