Abstract
A novel thermophilic, microaerophilic, anoxygenic, and chlorophototrophic member of the phylum Acidobacteria, Chloracidobacterium thermophilum strain BT, was isolated from a cyanobacterial enrichment culture derived from microbial mats associated with Octopus Spring, Yellowstone National Park, Wyoming. C. thermophilum is strictly dependent on light and oxygen and grows optimally as a photoheterotroph at irradiance values between 20 and 50 μmol photons m-2 s-1. C. thermophilum is unable to synthesize branched-chain amino acids (AAs), l-lysine, and vitamin B12, which are required for growth. Although the organism lacks genes for autotrophic carbon fixation, bicarbonate is also required. Mixtures of other AAs and 2-oxoglutarate stimulate growth. As suggested from genomic sequence data, C. thermophilum requires a reduced sulfur source such as thioglycolate, cysteine, methionine, or thiosulfate. The organism can be grown in a defined medium at 51∘C (Topt; range 44–58∘C) in the pH range 5.5–9.5 (pHopt = ∼7.0). Using the defined growth medium and optimal conditions, it was possible to isolate new C. thermophilum strains directly from samples of hot spring mats in Yellowstone National Park, Wyoming. The new isolates differ from the type strain with respect to pigment composition, morphology in liquid culture, and temperature adaptation.
Highlights
Growth physiology of Chloracidobacterium thermophilum and homodimeric type-1 photochemical reaction centers
Historical Context for Growth and Isolation of C. thermophilum Prior to the cultivation studies reported here that led to an axenic culture, C. thermophilum had been studied in several ways to gain information about this unusual bacterium
These studies classified C. thermophilum as an anoxygenic, chlorophototrophic Acidobacterium, and the data strongly indicated that this organism relies on organic carbon source(s), reduced sulfur source(s) and oxygen for BChl, carotenoid, and tyrosine biosynthesis
Summary
Growth physiology of Chloracidobacterium thermophilum and homodimeric type-1 photochemical reaction centers. At the time of its initial discovery, “Ca. C. thermophilum” extended the number of bacterial phyla known to have members capable of Chl -dependent phototrophic growth (i.e., chlorophototrophy) from five to six These include the phyla Cyanobacteria, Chloroflexi, Chlorobi, Proteobacteria, Firmicutes, and Acidobacteria (Bryant et al, 2007). Using physical methods (e.g., low-speed centrifugation) to obtain enriched populations of cells, it was possible to obtain highly enriched DNA preparations for C. thermophilum This allowed the complete genome of the organism to be determined (Garcia Costas et al, 2012a). The genome lacked genes for nitrate reductase, nitrite reductase, nitrogenase, and assimilatory sulfate reduction Based upon these findings, Garcia Costas et al (2012a) concluded that C. thermophilum was an aerobic anoxygenic photoheterotroph. A metatranscriptomic study of the microbial mats from which the organism was enriched over a complete diel cycle suggested that C. thermophilum might require alternating oxic and anoxic conditions for optimal growth or might prefer constantly microoxic conditions (Liu et al, 2011, 2012)
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