Dimethyl sulfoxide as an electron acceptor for anaerobic growth

Dimethylsulphoxide and trimethylamine oxide respiration of Proteus vulgaris. Evidence for a common terminal reductase system

Dimethylsulphoxide (DMSO) and trimethylamine oxide (TMAO) sustained anaerobic growth of Proteus vulgaris with the non-fermentable substrate lactate. Cytoplasmic membrane vesicles energized by electron transfer from formate to DMSO displayed anaerobic uptake of serine, which was hindered by metabolic inhibitors known to destroy the proton motive force. This showed that DMSO reduction was coupled with a chemiosmotic mechanism of energy conversion; similar data for TMAO respiration have been presented previously. All biochemical tests applied indicated that the oxides were reduced by the same reductase system. The DMSO and TMAO reductase activities showed the same mobility on ion-exchange chromatography, and polyacrylamide disc gel electrophoresis (pH 8.9), gradient gel electrophoresis, and gel isoelectric focusing; mol. wt. and pI determined were 95,000 and 4.6, respectively. DMSO inhibited reduction of [14C]TMAO in vesicles. The reductase was inducible to a certain extent; both oxides being equally efficient as inducers. TMAO was reduced at a higher rate than DMSO, explaining faster growth of cells and increased uptake of serine in vesicles with TMAO as electron acceptor. Comparative studies with Escherichia coli also gave evidence for common TMAO and DMSO reductase systems.

Fermentation and anaerobic respiration by Rhodospirillum rubrum and Rhodopseudomonas capsulata

Rhodospirillum rubrum and Rhodopseudomonas capsulata were able to grow anaerobically in the dark either by a strict mixed-acid fermentation of sugars or, in the presence of an appropriate electron acceptor, by an energy-linked anaerobic respiration. Both species fermented fructose without the addition of accessory oxidants, but required the initial presence of bicarbonate before fermentative growth could begin. Major products of R. rubrum fermentation were succinate, acetate, propionate, formate, hydrogen, and carbon dioxide; R. capsulata produced major amounts of lactate, acetate, succinate, hydrogen, and carbon dioxide. R. rubrum and R. capsulata were also capable of growing strictly through anaerobic, respiratory mechanisms. Nonfermentable substrates, such as succinate, malate, or acetate, supported growth only in the presence of an electron acceptor such as dimethyl sulfoxide or trimethylamine oxide. Carbon dioxide and dimethyl sulfide were produced during growth of R. rubrum and R. capsulata on succinate plus dimethyl sulfoxide. Molar growth yields from cultures grown anaerobically in the dark on fructose plus dimethyl sulfoxide were 3.8 to 4.6 times higher than values obtained from growth on fructose alone and were 56 to 60% of the values obtained from aerobic, respiratory growth with fructose. Likewise, molar growth yields from anaerobic, respiratory growth conditions with succinate plus dimethyl sulfoxide were 51 to 54% of the values obtained from aerobic, respiratory growth with succinate. The data indicate that dimethyl sulfoxide or trimethylamine oxide as a terminal oxidant is approximately 33 to 41% as efficient as O(2) in conserving energy through electron transport-linked respiration.

Are sulfur isotope ratios sufficient to determine the antiquity of sulfate reduction?

Sulfur isotope fractionation values have been measured in sedimentary sulfides of varying ages, The 'Antiquity and evolutionary status of bacterial sulfate reduction...' has been inferred from these measurements by Schidlowski (1979). However, under experimental conditions, the isotope values vary widely due to inadequately controlled variables. Thus the direct extrapolation of sulfur isotope fractionation values measured in the laboratory to those measured in sedimentary rocks is unwarranted. New sulfur transforming microbes have been described and recent measurements indicate that inorganic processes affect sulfur isotope fractionation values. This information is summarized here; at present sulfur isotope fractionation values are insufficient to determine the antiquity of sulfate reduction.

Physiology of dark fermentative growth of Rhodopseudomonas capsulata

The photosynthetic bacterium Rhodopseudomonas capsulata can grow under anaerobic conditions with light as the energy source or, alternatively, in darkness with D-fructose or certain other sugars as the sole source of carbon and energy. Growth in the latter mode requires an "accessory oxidant" such as trimethylamine-N-oxide, and the resulting cells contain the photosynthetic pigments characteristic of R. capsulata (associated with intracytoplasmic membranes) and substantial deposits of poly-beta-hydroxybutyrate. In dark anaerobic batch cultures in fructose plus trimethylamine-N-oxide medium, trimethylamine formation parallels growth, and typical fermentation products accumulate, namely, CO2 and formic, acetic, and lactic acids. These products are also found in dark anaerobic continuous cultures of R. capsulata; acetic acid and CO2 predominate when fructose is limiting, whereas formic and lactic acids are observed at elevated concentrations when trimethylamine-N-oxide is the limiting nutrient. Evidence is presented to support the conclusions that ATP generation during anaerobic dark growth of R. capsulata on fructose plus trimethylamine-N-oxide occurs by substrate level phosphorylations associated with classical glycolysis and pyruvate dissimilation, and that the required accessory oxidant functions as an electron sink to permit the management of fermentative redox balance, rather than as a terminal electron acceptor necessary for electron transport-driven phosphorylation.