Jeffrey L. Favinger

Rhodospirillum centenum, sp. nov., a thermotolerant cyst-forming anoxygenic photosynthetic bacterium

A novel non-sulfur purple photosynthetic bacterium, designated Rhodospirillum centenum, was isolated from an enrichment culture designed to favor growth of anoxygenic photosynthetic N2-fixing bacteria. R. centenum grows optimally at 40–42° C and has the capacity to produce cytoplasmic ‘R bodies’, refractile structures not observed hitherto in photosynthetic prokaryotes. The bacterium is also unusual among photosynthetic bacteria in that it forms desiccation-resistant cysts when grown aerobically in darkness with butyrate as the sole carbon source.

Redox mechanisms in “oxidant-dependent” hexose fermentation by Rhodopseudomonas capsulata

Abstract Trimethylamine N -oxide (TMAO) can function as an electron acceptor in the anaerobic metabolism of both Rhodopseudomonas capsulata and Escherichia coli . In both bacteria, anaerobic growth in the presence of TMAO induces a system that can reduce TMAO to trimethylamine (TMA). Comparative studies, however, show that TMAO reduction serves different purposes in the organisms noted. In E. coli , anaerobic growth on sugars does not require the presence of TMAO, but in cells induced for TMAO reductase, TMAO can act as the terminal electron acceptor for membrane-associated oxidative phosphorylation. Anaerobic dark growth of R. capsulata is dependent on the presence of TMAO (or an analog) and in this organism a soluble system catalyzes anaerobic oxidation of NADH with TMAO. The mechanism, in R. capsulata , appears to involve a flavoprotein of the flavodoxin type and presumably represents a system for maintenance of redox balance during anaerobic dark fermentation of hexoses and related compounds.

Increased activity of respiratory enzymes from photosynthetically grown Rhodopseudomonas capsulata in response to small amounts of oxygen

Assays were performed on the components of the respiratory chain enzyme complex and the citric acid cycle enzyme α-ketoglutarate dehydrogenase (EC 1.2.4.2) from membrane preparations of Rhodopseudomonas capsulata grown in continuous culture. Cells were grown photoheterotrophically then switched to a chemoheterotrophic growth mode. NADH-and succinate-dependent oxygen uptake activities and cytochrome c oxidase (EC 1.9.3.1) activity were 5–6-fold higher in chemotrophically grown cells. The activities of NADH dehydrogenase (EC 1.6.99.3) and succinate dehydrogenase (EC 1.3.99.1) did not differ greatly between the two growth modes, and it is suggested that in respiratory chain electron flow the cytochrome c oxidase segment catalyses the rate limiting step. In addition, a 5-fold increase in α-ketoglutarate dehydrogenase activity was observed after the transition from a phototrophic to chemotrophic growth mode. Oxidase and α-ketoglutarate dehydrogenase activities increased 3–4-fold in phototrophically growing cultures exposed to a low partial pressure of oxygen (50 Pa; 1 atmosphere = 1.013×105 Pa). The data suggest that this small amount of oxygen exerts a significant effect on aerobic bioenergetic enzymes, even during phototrophic growth of Rhodopseudomonas capsulata.

Taxonomic ambiguities: a case history.

Rhodospirillum centenum is a remarkable non-sulfur purple photosynthetic bacterium isolated in our laboratory in 1987. The first published description of the organism (Favinger et al., 1989) detailed an assortment of characterisics that placed it in the genus Rhodospirillum (morphology, in vivo absorbancy spectrum of photopigments, general physiology, nutritional requirements). We named the organism Rhodospirillum centenum in recognition of the fact that it was discovered on the 100th anniversary of the isolation of the first pure culture of an anoxyphototroph, Rhodospirillum rubrum. The latter became an important experimental organism whose study revealed many of the basic aspects of anoxygenic photosynthesis, including the discovery of photophosphorylation. Soon after isolation of the original type strain of Rhodospirillum centenum, it was deposited in the American Type Culture Collection as ATCC 43720 (non-phototactic variant) ; a phototactic variant was later deposited as ATCC 51521.

A novel directly coupled gradostat

The original bidirectional compound chemostat (gradostat) described by Lovitt and Wimpenny has been simplified by making a more compact apparatus in which chemical gradients are established by diffusion between adjacent culture chambers. The experimental model (diffusion coupled (DC) gradostat) consisted of five chambers whose contents could be agitated by turbines rotating in the horizontal plane on a common shaft. Two biological experiments were designed to reveal the value of the DC gradostat. A methylotroph (Methylophilus methylotrophus) grown in a methanol gradient showed expected changes in cell viability as a function of position in the five vessel array. Cells of two species of photosynthetic bacteria (Rhodobacter capsulata and Rhodopseudomonas marina/agilis) with different salt sensitivities could be mixed and subsequently separated by the DC gradostat operating with a NaCl gradient of 0-3% w/v.

Obligately halophilicChromatium vinosum from Hamelin Pool, Shark Bay, Australia

A representative of the purple sulfur bacteria was isolated from organic-rich intertidal sediments of Hamelin Pool, Shark Bay, Australia. The isolate, strain HPC, is nutritionally versatile, being capable of photoheterotrophic growth in the absence of reduced sulfur sources and of dark microaerophilic growth, either heterotrophically or lithotrophically. Vitamins are not required. Nine organic carbon substrates, including the C2−C5 fatty acids, support photoheterotrophic growth. The isolate is an obligate halophile capable of growth over a wide salinity range (0.5%–8.5% NaCl). On the basis of its morphology, physiology, pigmentation, and DNA base ratio, strain HPC is considered to be an obligately halophilic representative ofChromatium vinosum.