Allochromatium tepidum, sp. nov., a hot spring species of purple sulfur bacteria.

We describe a new species of purple sulfur bacteria (Chromatiaceae, anoxygenic phototrophic bacteria) isolated from a microbial mat in the sulfidic geothermal outflow of a hot spring in Rotorua, New Zealand. This phototroph, designated as strain NZ, grew optimally near 45 °C but did not show an absorption maximum at 915 nm for the light-harvesting-reaction center core complex (LH1-RC) characteristic of other thermophilic purple sulfur bacteria. Strain NZ had a similar carotenoid composition as Thermochromatium tepidum, but unlike Tch. tepidum, grew photoheterotrophically on acetate in the absence of sulfide and metabolized thiosulfate. The genome of strain NZ was significantly larger than that of Tch. tepidum but slightly smaller than that of Allochromatium vinosum. Strain NZ was phylogenetically more closely related to mesophilic purple sulfur bacteria of the genus Allochromatium than to Tch. tepidum. This conclusion was reached from phylogenetic analyses of strain NZ genes encoding 16S rRNA and the photosynthetic functional gene pufM, from phylogenetic analyses of entire genomes, and from a phylogenetic tree constructed from the concatenated sequence of 1090 orthologous proteins. Moreover, average nucleotide identities and digital DNA:DNA hybridizations of the strain NZ genome against those of related species of Chromatiaceae supported the phylogenetic analyses. From this collection of properties, we describe strain NZ here as the first thermophilic species of the genus Allochromatium, Allochromatium tepidum NZT, sp. nov.

Complete genome of the thermophilic purple sulfur Bacterium Thermochromatium tepidum compared to Allochromatium vinosum and other Chromatiaceae.

The complete genome sequence of the thermophilic purple sulfur bacterium Thermochromatium tepidum strain MCT (DSM 3771T) is described and contrasted with that of its mesophilic relative Allochromatium vinosum strain D (DSM 180T) and other Chromatiaceae. The Tch. tepidum genome is a single circular chromosome of 2,958,290 base pairs with no plasmids and is substantially smaller than the genome of Alc. vinosum. The Tch. tepidum genome encodes two forms of RuBisCO and contains nifHDK and several other genes encoding a molybdenum nitrogenase but lacks a gene encoding a protein that assembles the Fe-S cluster required to form a functional nitrogenase molybdenum-iron cofactor, leaving the phototroph phenotypically Nif-. Tch. tepidum contains genes necessary for oxidizing sulfide to sulfate as photosynthetic electron donor but is genetically unequipped to either oxidize thiosulfate as an electron donor or carry out assimilative sulfate reduction, both of which are physiological hallmarks of Alc. vinosum. Also unlike Alc. vinosum, Tch. tepidum is obligately phototrophic and unable to grow chemotrophically in darkness by respiration. Several genes present in the Alc. vinosum genome that are absent from the genome of Tch. tepidum likely contribute to the major physiological differences observed between these related purple sulfur bacteria that inhabit distinct ecological niches.

Cultivation and characterization of snowbound microorganisms from the South Pole.

Little is known about microbial ecosystems of interior Antarctica, if indeed such ecosystems exist. Although considerable research has assessed microorganisms indigenous to coastal regions of Antarctica, particularly their lakes, ponds, and soils, to our knowledge only one characterized bacterium, a strain of Pseudomonas, has been isolated from South Pole ice or snow. Metagenomic community analyses described in this work and elsewhere reveal that a diversity of bacteria exists in inland polar snows, yet attempts to culture and characterize these microbes from this extreme environment have been few to date. In this molecular and culture-dependent investigation of the microbiology of inland Antarctica, we enriched and isolated two new strains of bacteria and one strain of yeast (Fungi) from South Pole snow samples. The bacteria were of the genera Methylobacterium and Sphingomonas, and the yeast grouped with species of Naganishia (class Tremellocytes). In addition to phylogenetic analyses, characterization of these isolates included determinations of cell morphology, growth as a function of temperature, salinity tolerance, and carbon and energy source versatility. All organisms were found to be cold-adapted, and the yeast strain additionally showed considerable halotolerance. These descriptions expand our understanding of the diversity and metabolic activities of snowbound microorganisms of interior Antarctica.