Four strains of aerobic, mesophilic, acidophilic bacteria that produced bacteriochlorophyll (BChl) a were isolated from acidic hot springs and mine drainage. The characteristics of the four isolates were almost identical. The isolates were strictly aerobic and chemo-organotrophic. They were gram-negative, non-motile cocci and coccobacilli, formed salmon-pink colonies on solidified media and produced BChl a and carotenoids only under aerobic growth conditions. The cells also produced small amounts of zinc-substituted BChl a when grown in the presence of 1 mM zinc sulfate. Anaerobic growth in the light was not found, but aerobic growth was stimulated by continuous incandescent illumination. The isolates grew in a pH range of 3.5-6.0, with pH optima of 4.5-5.0. A phylogenetic analysis based on 16S rDNA sequences showed that the isolates clustered in the major acidophilic group of the class Proteobacteria, which includes species of the genera Acidiphilium and Rhodopila. The anaerobic phototrophic bacterium Rhodopila globiformis was the closest relative to the new isolates (95% level of sequence similarity). The G+C content of the genomic DNA of the isolates was 69.1-69.8 mol%. On the basis of these results, it was concluded that the four isolates should be classified into a new genus and a new species, for which the name Acidisphaera rubrifaciens is proposed. The type strain is strain HS-AP3T (= JCM 10600T).


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