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Halorhodospira (Hlr.) halochloris is a triply extremophilic phototrophic purple sulfur bacterium, as it is thermophilic, alkaliphilic, and extremely halophilic. The light‐harvesting‐reaction center (LH1–RC) core complex of this bacterium displays an LH1‐Qy transition at 1,016 nm, which is the lowest‐energy wavelength absorption among all known phototrophs. Here we report the cryo‐EM structure of the LH1–RC at 2.42 Å resolution. The LH1 complex forms a tricyclic ring structure composed of 16 αβγ‐polypeptides and one αβ‐heterodimer around the RC. From the cryo‐EM density map, two previously unrecognized integral membrane proteins, referred to as protein G and protein Q, were identified. Both of these proteins are single transmembrane‐spanning helices located between the LH1 ring and the RC L‐subunit and are absent from the LH1–RC complexes of all other purple bacteria of which the structures have been determined so far. Besides bacteriochlorophyll b molecules (B1020) located on the periplasmic side of the Hlr. halochloris membrane, there are also two arrays of bacteriochlorophyll b molecules (B800 and B820) located on the cytoplasmic side. Only a single copy of a carotenoid (lycopene) was resolved in the Hlr. halochloris LH1–α3β3 and this was positioned within the complex. The potential quinone channel should be the space between the LH1–α3β3 that accommodates the single lycopene but does not contain a γ‐polypeptide, B800 and B820. Our results provide a structural explanation for the unusual Qy red shift and carotenoid absorption in the Hlr. halochloris spectrum and reveal new insights into photosynthetic mechanisms employed by a species that thrives under the harshest conditions of any phototrophic microorganism known. [ABSTRACT FROM AUTHOR]

Roseiflexus sp. strains were cultivated from a microbial mat of an alkaline siliceous hot spring in Yellowstone National Park. These strains are closely related to predominant filamentous anoxygenic phototrophs found in the mat, as judged by the similarity of small-subunit rRNA, lipid distributions, and genomic and metagenomic sequences. Like a Japanese isolate, R. castenholzii, the Yellowstone isolates contain bacteriochlorophyll a, but not bacteriochlorophyll c or chlorosomes, and grow photoheterotrophically or chemoheterotrophically under dark aerobic conditions. The genome of one isolate, Roseiflexus sp. strain RS1, contains genes necessary to support these metabolisms. This genome also contains genes encoding the 3-hydroxypropionate pathway for C[O.sub.2] fixation and a hydrogenase, which might enable photoautotrophic metabolism, even though neither isolate could be grown photoautotrophically with [H.sub.2] or [H.sub.2]S as a possible electron donor. The isolates exhibit temperature, pH, and sulfide preferences typical of their habitat. Lipids produced by these isolates matched much better with mat lipids than do lipids produced by R. castenholzii or Chloroflexus isolates. doi: 10.1128/JB.01610-09

Despite the fact that heliobacteria are the only phototrophic representatives of the bacterial phylum Firmicutes, genomic analyses of these organisms have yet to be reported. Here we describe the complete sequence and analysis of the genome of Heliobacterium modesticaldum, a thermophilic species belonging to this unique group of phototrophs. The genome is a single 3.1-Mb circular chromosome containing 3,138 open reading frames. As suspected from physiological studies of heliobacteria that have failed to show photoautotrophic growth, genes encoding enzymes for known autotrophic pathways in other phototrophic organisms, including ribulose bisphosphate carboxylase (Calvin cycle), citrate lyase (reverse citric acid cycle), and malyl coenzyme A lyase (3-hydroxypropionate pathway), are not present in the H. modesticaldum genome. Thus, heliobacteria appear to be the only known anaerobic anoxygenic phototrophs that are not capable of autotrophy. Although for some cellular activities, such as nitrogen fixation, there is a full complement of genes in H. modesticaldum, other processes, including carbon metabolism and endosporulation, are more genetically streamlined than they are in most other low-G+C gram-positive bacteria. Moreover, several genes encoding photosynthetic functions in phototrophic purple bacteria are not present in the heliobacteria. In contrast to the nutritional flexibility of many anoxygenic phototrophs, the complete genome sequence of H. modesticaldum reveals an organism with a notable degree of metabolic specialization and genomic reduction.

Eight strains of chemoorganotrophic bacteria which were isolated from the water column of Lake Hoare, McMurdo Dry Valleys, Antarctica by using cold enrichment temperatures revealed that at least two major phyla of bacteria inhabit Lake Hoare.

A report on the vertical distribution of cultivable strains of planktonic sulfur-oxidizing bacteria from the water column of the permanently ice-covered Lake Fryxell, Antarctica is described. The results support the hypothesis that biological sulfur cycling is occurring in this polar lake.

Research has been conducted on the filamentous bacteria containing bacteriochlorophylls c and a. The enrichment of these bacteria from the hypersaline microbial mats has been carried out and the results are described.

A PCR primer that targets the pufM gene in purple phototrophic bacteria is capable of amplifying DNA from purple sulfur phototrophs, purple nonsulfur phototrophs, and Chloroflexus species. The pufM gene codes for the M subunit of the photosynthetic reaction center, which is universal among purple phototrophic bacteria. The primer did not amplify cyanobacterial DNA despite the similarity between the purple bacterial reaction center and photosytem II in green plants.

Rhodobacter capsulatus can thrive on short- to long-chain fatty acids as its carbon source. However, negative PhaC strains of Rhodobacter capsulatus failed to thrive on solid media with high levels of certain fatty acids but negative wild-type PhaA and PhaB strains thrived on the same media. This finding indicates that polyhydroxyalkanoates (PHA) detoxify fatty acids on such media and could pave the way for the cloning of PHA biosynthetic enzymes.

The phaA, phaB and phaC genes of Rhodobacter capsulatus that are responsible for polyhydroxyalkanoate (PHA) biosynthesis have been isolated and inactivated. The phaC-lacZ and phaAB-lacZ reporter genes were found to be expressed even in the absence of the nitrogen or carbon source. Mutant genes of R. capsulatus could still synthesize the PHAs even if these genes have been eliminated from the transcriptional nitrogen regulatory pathway.

Escherichia coli plasmids pDSK519 and pGSS33 were transferred by conjugation into the thermophilic green sulfur bacterium Chlorobium tepidum. C. tepidum offspring expressed kanamycin and ampicillin-chrophenicol resistances encoded by the plasmids. Methods for growth of C. tepidium on agar, isolation of plating strains, and antibiotic-resistant mutants of wild-type C.tepidum cells were also investigated.

Chlorobium tepidum is a recently described thermophilic bacterium which grows faster than any known green sulfur bacterium. Growth and acetylene reduction experiments confirmed that this bacterium is nitrogen fixing. Moreover, nitrogen fixation occured up to temperatures of 60 degrees, celsius, the highest recorded for anoxygenic phototrophic bacteria. Enzyme assays also showed the ATP-dependent GS-GOGAT system as the major route for ammonia and molecular nitrogen assimilation.

A degenerate primer set based on the bchY gene is designed to detect anoxygenic bacteria containing either type 1 or type 2 photosynthetic reaction centers in a single PCR. The new primers are validated in silico by using the GenBank nucleotide database as well as by PCR on pure strains and environmental DNA.

The biodiversity of Archaea in the permanently frozen Lake Fryxell in the Antarctica is explored. Two clusters of methanogens were detected in the sediments, and another cluster of possibly methanotrophic Euryarchaeota was detected in the anoxic water column just above the sediments.

The populations of sulfate-reducing bacteria in Lake Fryxell, both 16 rRNA gene and metabolic primer sets targeting the dsrA gene for the dissimilatory sulfite reductase alpha subunit were employed to analyze environmental DNA obtained from the water column and sediments of Lake Fryxell were investigated. Several clone groups were highly localized with respect to lake depth and therefore experienced specific physiochemical conditions.

Research has been conducted on phototrophic purple bacteria from the permanently frozen Antarctic Lake Fryxell. The authors have investigated the biodiversity, ecology and metabolic activity of these phototrophs, and report that this lake is inhibited by a broad diversity of purple nonsulfur bacteria.