Reduced sulfur compound oxidation by Thiobacillus caldus.

The oxidation of reduced inorganic sulfur compounds was studied by using resting cells of the moderate thermophile Thiobacillus caldus strain KU. The oxygen consumption rate and total oxygen consumed were determined for the reduced sulfur compounds thiosulfate, tetrathionate, sulfur, sulfide, and sulfite in the absence and in the presence of inhibitors and uncouplers. The uncouplers 2,4-dinitrophenol and carbonyl cyanide m-chlorophenyl-hydrazone had no affect on the oxidation of thiosulfate, suggesting that thiosulfate is metabolized periplasmically. In contrast, the uncouplers completely inhibited the oxidation of tetrathionate, sulfide, sulfur, and sulfite, indicating that these compounds are metabolized in the cytoplasm of T. caldus KU. N-Ethylmaleimide inhibited the oxidation of tetrathionate and thiosulfate at the stage of elemental sulfur, while 2-heptyl-4-hydroxyquinoline-N-oxide stopped the oxidation of thiosulfate, tetrathionate, and elemental sulfur at the stage of sulfite. The following intermediates in the oxidation of the sulfur compounds were found by using uncouplers and inhibitors: thiosulfate was oxidized to tetrathionate, elemental sulfur was formed during the oxidation of tetrathionate and sulfide, and sulfite was found as an intermediate of tetrathionate and sulfur metabolism. On the basis of these data we propose a model for the metabolism of the reduced inorganic sulfur compounds by T. caldus KU.     

Elemental sulfur as an intermediate of sulfide oxidation with oxygen byDesulfobulbus propionicus

The sulfate-reducing bacteriumDesulfobulbus propionicus oxidized sulfide, elemental sulfur, and sulfite to sulfate with oxygen as electron acceptor. Thiosulfate was reduced and disproportionated exclusively under anoxic conditions. When small pulses of oxygen were added to washed cells in sulfide-containing assays, up to 3 sulfide molecules per O₂ disappeared transiently. After complete oxygen consumption, part of the sulfide reappeared. The intermediate formed was identified as elemental sulfur by chemical analysis and turbidity measurements. When excess sulfide was present, sulfur dissolved as polysulfide. This process was faster in the presence of cells than in their absence. The formation of sulfide after complete oxygen consumption was due to a disproportionation of elemental sulfur (or polysulfide) to sulfide and sulfate. The uncoupler tetrachlorosalicylanilide (TCS) and the electron transport inhibitor myxothiazol inhibited sulfide oxidation to sulfate and caused accumulation of sulfur. In the presence of the electron transport inhibitor 2-n-heptyl-4-hydroxyquinoline-N-oxide (HQNO), sulfite and thiosulfate were formed. During sulfur oxidation at low oxygen concentrations, intermediary formation of sulfide was observed, indicating disproportionation of sulfur also under these conditions. It is concluded that sulfide oxidation inD. propionicus proceeds via oxidation to elemental sulfur, followed by sulfur disproportionation to sulfide and sulfate. Dedicated to Prof. Dr. Dr. h.c. Norbert Pfennig on the occasion of his 70th birthday     

Anaerobic oxidation of thiosulfate and elemental sulfur in Thiobacillus denitrificans

Thiobacillus denitrificans strain RT could be grown anaerobically in batch culture on thiosulfate but not on other reduced sulfur compounds like sulfide, elemental sulfur, thiocyanate, polythionates or sulfite. During growth on thiosulfate the assimilated cell sulfur was derived totally from the outer or sulfane sulfur. Thiosulfate oxidation started with a rhodanese type cleavage between sulfane and sulfone sulfur leading to elemental sulfur and sulfite. As long as thiosulfate was present elemental sulfur was transiently accumulated within the cells in a form that could be shown to be more reactive than elemental sulfur present in a hydrophilic sulfur sol, however, less reactive than sulfane sulfur of polythionates or organic and inorganic polysulfides. When thiosulfate had been completely consumed, intracellular elemental sulfur was rapidly oxidized to sulfate with a specific rate of 45 natom S°/min·mg protein. Extracellularly offered elemental sulfur was not oxidized under anaerobic conditions.     

Oxidation of H2, organic compounds and inorganic sulfur compounds coupled to reduction of O2 or nitrate by sulfate-reducing bacteria

All of fourteen sulfate-reducing bacteria tested were able to carry out aerobic respiration with at least one of the following electron donors: H₂, lactate, pyruvate, formate, acetate, butyrate, ethanol, sulfide, thiosulfate, sulfite. Generally, we did not obtain growth with O₂ as electron acceptor. The bacteria were microaerophilic, since the respiration rates increased with decreasing O₂ concentrations or ceased after repeated O₂ additions. The amounts of O₂ consumed indicated that the organic substrates were oxidized incompletely to acetate; only Desulfobacter postgatei oxidized acetate with O₂ completely to CO₂. Many of the strains oxidized sulfite (completely to sulfate) or sulfide (incompletely, except Desulfobulbus propionicus); thiosulfate was oxidized only by strains of Desulfovibrio desulfuricans; trithionate and tetrathionate were not oxidized by any of the strains. With Desulfovibrio desulfuricans CSN and Desulfobulbus propionicus the oxidation of inorganic sulfur compounds was characterized in detail. D. desulfuricans formed sulfate during oxidation of sulfite, thiosulfate or elemental sulfur prepared from polysulfide. D. propionicus oxidized sulfite and sulfide to sulfate, and elemental sulfur mainly to thiosulfate. A novel pathway that couples the sulfur and nitrogen cycles was detected: D. desulfuricans and (only with nitrite) D. propionicus were able to completely oxidize sulfide coupled to the reduction of nitrate or nitrite to ammonia. Cell-free extracts of both strains did not oxidize sulfide or thiosulfate, but formed ATP during oxidation of sulfite (37 nmol per 100 nmol sulfite). This, and the effects of AMP, pyrophosphate and molybdate on sulfite oxidation, suggested that sulfate is formed via the (reversed) sulfate activation pathway (involving APS reductase and ATP sulfurylase). Thiosulfate oxidation with O₂ probably required a reductive first step, since it was obtained only with energized intact cells.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - APS adenosine phosphosulfate or adenylyl sulfate     

Oxidation of inorganic sulfur compounds by obligatory organotrophic bacteria

New data obtained by the author and other researchers on two different groups of obligately heterotrophic bacteria capable of inorganic sulfur oxidation are reviewed. Among culturable marine and (halo)alkaliphilic heterotrophs oxidizing sulfur compounds (thiosulfate and, much less actively, elemental sulfur and sulfide) incompletely to tetrathionate, representatives of the gammaproteobacteria, especially from the Halomonas group, dominate. Some of denitrifying species from this group are able to carry out anaerobic oxidation of thiosulfate and sulfide using nitrogen oxides as electron acceptors. Despite the low energy output of the reaction of thiosulfate oxidation to tetrathionate, it can be utilized for ATP synthesis by some tetrathionate-producing heterotrophs; however, this potential is not always realized during their growth. Another group of marine and (halo)alkaliphilic heterotrophic bacteria capable of complete oxidation of sulfur compounds to sulfate mostly includes representatives of the alphaproteobacteria most closely related to nonsulfur purple bacteria. They can oxidize sulfide (polysulfide), thiosulfate, and elemental sulfur via sulfite to sulfate but neither produce nor oxidize tetrathionate. All of the investigated sulfate-forming heterotrophic bacteria belong to lithoheterotrophs, being able to gain additional energy from the oxidation of sulfur compounds during heterotrophic growth on organic substrates. Some doubtful cases of heterotrophic sulfur oxidation described in the literature are also discussed.     

Utilization of sulfide and elemental sulfur by Ectothiorhodospira halochloris

Ectothiorhodospira halochloris grows photoheterotrophically with a variety of sulfur sources. During sulfide oxidation to elemental sulfur considerable amounts of polysulfides may be accumulated transiently. When grown on elemental sulfur no sulfate was produced by oxidation, but sulfide and polysulfide were formed by reduction. Only one soluble cytochrome c-551 was isolated and purified. It was a small acidic hemeprotein with a molecular weight of 6,300, an isoelectric point of 3.1 and a redox potential of-11 mV at pH 7.0. It showed three absorption maxima in the reduced state (=551 nm; =523 nm; =417 nm). The addition of various c-type cytochromes to a suspension of spheroplasts stimulated the velocity of sulfide oxidation. This stimulation was best with the small acidic cytochromes from E. halochloris or Ectothiorhodospira abdelmalekii. Sulfide oxidation was stopped by several uncoupling agents, ionophores and electron transport inhibitors. Antimycin A, rotenone and cyanide had no effect on sulfide oxidation.Dedicated to Prof. Dr. H. G. Schlegel on the occasion of his 60th birthday     

Oxidation of Inorganic Sulfur Compounds by Obligately Organotrophic Bacteria

New data obtained by the author and other researchers on two different groups of obligately heterotrophic bacteria capable of inorganic sulfur oxidation are reviewed. Among culturable marine and (halo)alkaliphilic heterotrophs oxidizing sulfur compounds (thiosulfate and, much less actively, elemental sulfur and sulfide) incompletely to tetrathionate, representatives of the gammaproteobacteria, especially from the Halomonas group, dominate. Some denitrifying species from this group are able to carry out anaerobic oxidation of thiosulfate and sulfide using nitrogen oxides as electron acceptors. Despite the low energy output of the reaction of thiosulfate oxidation to tetrathionate, it can be utilized for ATP synthesis by some tetrathionate-producing heterotrophs; however, this potential is not always realized during their growth. Another group of marine and (halo)alkaliphilic heterotrophic bacteria capable of complete oxidation of sulfur compounds to sulfate mostly includes representatives of the alphaproteobacteria which are most closely related to nonsulfur purple bacteria. They can oxidize sulfide (polysulfide), thiosulfate, and elemental sulfur via sulfite to sulfate but neither produce nor oxidize tetrathionate. All of the investigated sulfate-forming heterotrophic bacteria belong to lithoheterotrophs, being able to gain additional energy from the oxidation of sulfur compounds during heterotrophic growth on organic substrates. Some doubtful cases of heterotrophic sulfur oxidation described in the literature are also discussed.     

Respiratory oxidation of reduced sulfur compounds by intact cells of Thiobacillus tepidarius (type strain)

Thiobacillus tepidarius (type strain) was grown in microaerophilic conditions, on tetrathionate, thiosulfate or crystalline S^o. The rates of tetrathionate, thiosulfate, elemental sulfur (S^o) and sulfite oxidation of the different cultures were measured respirometrically, using exponentially growing cells, with an oxygen electrode. Cells growing on the three different sulfur compounds retain thiosulfate-, tetrathionate, and S^o-oxidizing activities (SOA), but lack respiratory sulfite-oxidizing activity. The SOA for all the cultures was almost totally inhibited by 50 M myxothiazol, an inhibitor of the quinone-cytochrome b region, and by 10 M of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP). Tetrathionate- and thiosulfate-oxidizing activities were moderately and weakly inhibited by 50 M totally inhibited (>95%) all respiratory activities. This study suggests that electrons released by S^o oxidation enter the respiratory chain in the quinone-cytochrome b region.Abbreviation SOA sulfur-oxidizing activity     

Characterization of a new platelet-forming purple sulfur bacterium,Amoebobacter pedioformis sp. nov.

The dominant purple sulfur bacterium of a reddish-colored waste water pond near Taichung, Taiwan, was isolated in pure culture, strain CML2. Individual cells were nearly spherical, nonmotile, and contained in their peripheral parts was vacuoles that appeared like elongated, curved tubes. Four to sixteen cells formed platelet-like aggregates reminiscent of Thiopedia rosea. The intracellular photosynthetic membrane system of the cells was of vesicular type; the photosynthetic pigments consisted of bacteriochlorophyll a and spirilloxanthin as the major carotenoid. The color of cell suspensions was pink to rosered. Under anaerobic conditions photolithoautotrophic growth occurred with sulfide, elemental sulfur or thiosulfate; sulfur globules were stored as an intermediary oxidation product. In the presence of sulfide, acetate, lactate and pyruvate were photoassimilated; strain CML2 lacked assimilatory sulfate reduction. Fastest photoautotrophic growth (11 h doubling time) was obtained at pH 7.5, 35°C and a light intensity of about 1000 lux (tungsten lamp). Chemolithoautotrophic growth in the dark was possible under reduced oxygen partial pressure with reduced sulfur compounds as respiratory substrates. The DNA base composition of strain CML2 was 65.5 mol% G+C. Strain CML2 is described as type strain of a new species, Amoebobacter pedioformis sp. nov., in the family Chromatiaceae.     

Fate of elemental sulfur in an intertidal sediment

Abstract: Sediment from a tidal flat at Wedderwarden, near the mouth of the Weser estuary, northern Germany, was amended with elemental sulfur, and concentrations of metabolic end products were monitored. The production of both sulfate and sulfide was consistent with disproportionation as the most important fate of the added elemental sulfur. A population of bacteria conducting active elemental sulfur disproportionation was also enriched from the sediment. In the enrichments, containing both elemental sulfur and Fe oxides as a sulfide 'scrub', sulfide and sulfate were produced in a ratio of     , somewhat lower than the predicted ratio of     . The mismatch between predicted and observed production ratios is explained by the channelling of electrons into autotrophic or mixotrophic CO₂ fixation rather than sulfide formation. The production of organic carbon, in the correct amount to explain the observed sulfide to sulfate production ratio, was verified by organic carbon analysis. Finally, rates of sulfate reduction were identical in the elemental sulfur amended sediment, and in control sediment with no added sulfur. Hence, the heterotrophic bacterial community was completely unaffected by an active metabolism conducting elemental sulfur disproportionation.     

Stoichiometric modeling of oxidation of reduced inorganic sulfur compounds (Riscs) in Acidithiobacillus thiooxidans

The prokaryotic oxidation of reduced inorganic sulfur compounds (RISCs) is a topic of utmost importance from a biogeochemical and industrial perspective. Despite sulfur oxidizing bacterial activity is largely known, no quantitative approaches to biological RISCs oxidation have been made, gathering all the complex abiotic and enzymatic stoichiometry involved. Even though in the case of neutrophilic bacteria such as Paracoccus and Beggiatoa species the RISCs oxidation systems are well described, there is a lack of knowledge for acidophilic microorganisms. Here, we present the first experimentally validated stoichiometric model able to assess RISCs oxidation quantitatively in Acidithiobacillus thiooxidans (strain DSM 17318), the archetype of the sulfur oxidizing acidophilic chemolithoautotrophs. This model was built based on literature and genomic analysis, considering a widespread mix of formerly proposed RISCs oxidation models combined and evaluated experimentally. Thiosulfate partial oxidation by the Sox system (SoxABXYZ) was placed as central step of sulfur oxidation model, along with abiotic reactions. This model was coupled with a detailed stoichiometry of biomass production, providing accurate bacterial growth predictions. In silico deletion/inactivation highlights the role of sulfur dioxygenase as the main catalyzer and a moderate function of tetrathionate hydrolase in elemental sulfur catabolism, demonstrating that this model constitutes an advanced instrument for the optimization of At. thiooxidans biomass production with potential use in biohydrometallurgical and environmental applications. Biotechnol. Bioeng. 2013; 110: 2242–2251. © 2013 Wiley Periodicals, Inc.     

Sulfite-oxido-reductase is involved in the oxidation of sulfite in Desulfocapsa sulfoexigens during disproportionation of thiosulfate and elemental sulfur

The enzymatic pathways of elemental sulfur and thiosulfate disproportionation were investigated using cell-free extract of Desulfocapsa sulfoexigens. Sulfite was observed to be an intermediate in the metabolism of both compounds. Two distinct pathways for the oxidation of sulfite have been identified. One pathway involves APS reductase and ATP sulfurylase and can be described as the reversion of the initial steps of the dissimilatory sulfate reduction pathway. The second pathway is the direct oxidation of sulfite to sulfate by sulfite oxidoreductase. This enzyme has not been reported from sulfate reducers before. Thiosulfate reductase, which cleaves thiosulfate into sulfite and sulfide, was only present in cell-free extract from thiosulfate disproportionating cultures. We propose that this enzyme catalyzes the first step in thiosulfate disproportionation. The initial step in sulfur disproportionation was not identified. Dissimilatory sulfite reductase was present in sulfur and thiosulfate disproportionating cultures. The metabolic function of this enzyme in relation to elemental sulfur or thiosulfate disproportionation was not identified. The presence of the uncouplers HQNO and CCCP in growing cultures had negative effects on both thiosulfate and sulfur disproportionation. CCCP totally inhibited sulfur disproportionation and reduced thiosulfate disproportionation by 80% compared to an unamended control. HQNO reduced thiosulfate disproportionation by 80% and sulfur disproportionation by 90%.     

Oxidation of dimethylsulfide to tetrathionate by Methylophaga thiooxidans sp. nov.: a new link in the sulfur cycle

A new pathway of dimethylsulfide (DMS) metabolism was identified in a novel species of Gammaproteobacteria, Methylophaga thiooxidans sp. nov., in which tetrathionate (S₄O₆^2?) was the end‐product of DMS oxidation. Inhibitor evidence indicated that DMS degradation was initiated by demethylation, catalysed by a corrinoid demethylase. Thiosulfate was an intermediate, which was oxidized to tetrathionate by a cytochrome‐linked thiosulfate dehydrogenase. Thiosulfate oxidation was coupled to ATP synthesis, and M. thiooxidans could also use exogenous thiosulfate as an energy source during chemolithoheterotrophic growth on DMS or methanol. Cultures grown on a variety of substrates oxidized thiosulfate, indicating that thiosulfate oxidation was constitutive. The observations have relevance to interactions among sulfur‐metabolizing bacteria in the marine environment. The production of tetrathionate from an organosulfur precursor is previously undocumented and represents a potential step in the biogeochemical sulfur cycle, providing a ‘shunt’ across the cycle.     

嗜酸硫氧化细菌消解元素硫的研究进展

浸矿酸性环境下,金属硫化矿在Fe3+作用下,经过硫代硫酸盐途径或多聚硫化氢途径而分解的过程中导致大量元素硫的累积,进而可能在金属硫化矿表面形成疏水元素硫层,阻碍金属离子的进一步浸出。酸性环境下,惰性元素硫的消解必须借助嗜酸硫氧化细菌来实现。该消解过程包括嗜酸硫氧化细菌对元素硫的吸附、转运以及氧化转化等过程。本文对近年来嗜酸硫氧化细菌消解元素硫过程的相关研究进行了全面评述,认为有关嗜酸硫氧化细菌消解元素硫的分子机制的清晰阐述还有待人们通过对消解过程的各个环节的分子机制进行大量研究来实现。     

Lithotrophic growth ofSulfurospirillum deleyianum with sulfide as electron donor coupled to respiratory reduction of nitrate to ammonia

Sulfurospirillum deleyianum grew in batch culture under anoxic conditions with sulfide (up to 5 mM) as electron donor, nitrate as electron acceptor, and acetate as carbon source. Nitrate was reduced to ammonia via nitrite, a quantitatively liberated intermediate. Four moles of sulfide were oxidized to elemental sulfur per mole nitrate converted to ammonia. The molar growth yield per mole sulfide consumed, Y_m, was 1.5 ± 0.2 g mol^–1 for the reduction of nitrate to ammonia. By this type of metabolism, S. deleyianum connected the biogeochemical cycles of sulfur and nitrogen. The sulfur reductase activity in S. deleyianum was inducible, as the activity depended on the presence of sulfide or elemental sulfur during cultivation with nitrate or fumarate as electron acceptor. Hydrogenase activity was always high, indicating that the enzyme is constitutively expressed. The ammonia-forming nitrite reductase was an inducible enzyme, expressed when cells were cultivated with nitrate, nitrite, or elemental sulfur, but repressed after cultivation with fumarate. Received: 13 March 1995 / Accepted: 29 May 1995     

Mechanism of oxidation of inorganic sulfur compounds by thiosulfate-grown Thiobacillus thiooxidans

Thiobacillus thiooxidans was grown at pH 5 on thiosulfate as an energy source, and the mechanism of oxidation of inorganic sulfur compounds was studied by the effect of inhibitors, stoichiometries of oxygen consumption and sulfur, sulfite, or tetrathionate accumulation, and cytochrome reduction by substrates. Both intact cells and cell-free extracts were used in the study. The results are consistent with the pathway with sulfur and sulfite as the key intermediates. Thiosulfate was oxidized after cleavage to sulfur and sulfite as intermediates at pH 5, the optimal growth pH on thiosulfate, but after initial condensation to tetrathionate at pH 2.3 where the organism failed to grow. N-Ethylmaleimide (NEM) inhibited sulfur oxidation directly and the oxidation of thiosulfate or tetrathionate indirectly. It did not inhibit the sulfite oxidation by cells, but inhibited any reduction of cell cytochromes by sulfur, thiosulfate, tetrathionate, and sulfite. NEM probably binds sulfhydryl groups, which are possibly essential in supplying electrons to initiate sulfur oxidation. 2-Heptyl-4-hydroxy-quinoline N-oxide (HQNO) inhibited the oxidation of sulfite directly and that of sulfur, thiosulfate, and tetrathionate indirectly. Uncouplers, carbonyl cyanide-m-chlorophenylhydrazone (CCCP) and 2,4-dinitrophenol (DNP), inhibited sulfite oxidation by cells, but not the oxidation by extracts, while HQNO inhibited both. It is proposed that HQNO inhibits the oxidation of sulfite at the cytochrome b site both in cells and extracts, but uncouplers inhibit the oxidation in cells only by collapsing the energized state of cells, delta muH+, required either for electron transfer from cytochrome c to b or for sulfite binding.     

Oxidation of reduced sulphur compounds by intact cells of Thiobacillus acidophilus

Oxidation of reduced sulphur compounds by Thiobacillus acidophilus was studied with cell suspensions from heterotrophic and mixotrophic chemostat cultures. Maximum substrate-dependent oxygen uptake rates and affinities observed with cell suspensions from mixotrophic cultures were higher than with heterotrophically grown cells. ph Optima for oxidation of sulphur compounds fell within the pH range for growth (pH 2–5), except for sulphite oxidation (optimum at pH 5.5). During oxidation of sulphide by cell suspensions, intermediary sulphur was formed. Tetrathionate was formed as an intermediate during aerobic incubation with thiosulphate and trithionate. Whether or not sulphite is an inter-mediate during sulphur compound oxidation by T. acidophilus remains unclear. Experiments with anaerobic cell suspensions of T. acidophilus revealed that trithionate metabolism was initiated by a hydrolytic cleavage yielding thiosulphate and sulphate. A hydrolytic cleavage was also implicated in the metabolism of tetrathionate. After anaerobic incubation of T. acidophilus with tetrathionate, the substrate was completely converted to equimolar amounts of thiosulphate, sulphur and sulphate. Sulphide- and sulphite oxidation were partly inhibited by the protonophore uncouplers 2,4-dinitrophenol (DNP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) and by the sulfhydryl-binding agent N-ethylmaleimide (NEM). Oxidation of elemental sulphur was completely inhibited by these compounds. Oxidation of thiosulphate, tetrathionate and trithionate was only slightly affected. The possible localization of the different enzyme systems involved in sulphur compound oxidation by T. acidophilus is discussed.     

Glutathione reductase/glutathione is responsible for cytotoxic elemental sulfur tolerance via polysulfide shuttle in fungi

Fungi that can reduce elemental sulfur to sulfide are widely distributed, but the mechanism and physiological significance of the reaction have been poorly characterized. Here, we purified elemental sulfur-reductase (SR) and cloned its gene from the elemental sulfur-reducing fungus Fusarium oxysporum. We found that NADPH-glutathione reductase (GR) reduces elemental sulfur via glutathione as an intermediate. A loss-of-function mutant of the SR/GR gene generated less sulfide from elemental sulfur than the wild-type strain. Its growth was hypersensitive to elemental sulfur, and it accumulated higher levels of oxidized glutathione, indicating that the GR/glutathione system confers tolerance to cytotoxic elemental sulfur by reducing it to less harmful sulfide. The SR/GR reduced polysulfide as efficiently as elemental sulfur, which implies that soluble polysulfide shuttles reducing equivalents to exocellular insoluble elemental sulfur and generates sulfide. The ubiquitous distribution of the GR/glutathione system together with our findings that GR-deficient mutants derived from Saccharomyces cerevisiae and Aspergillus nidulans reduced less sulfur and that their growth was hypersensitive to elemental sulfur indicated a wide distribution of the system among fungi. These results indicate a novel biological function of the GR/glutathione system in elemental sulfur reduction, which is distinguishable from bacterial and archaeal mechanisms of glutathione- independent sulfur reduction.     

biochemical reaction mechanisms in sulfur oxidation by chemosynthetic bacteria

Summary Aspects of the biochemistry of the oxidation of inorganic sulfur compounds are discussed in thiobacilli but chiefly inThiobacillus denitrificans. Almost all of the thiobacilli (e.g. T. denitrificans, T. neapolitanus, T. novellus, andThiobacillus A ₂) were capable of producing approximately 7.5 moles of sulfuric acid aerobically from 3.75 moles of thiosulfate per gram of cellular protein per hr. By far the most prolific producer of sulfuric acid (or sulfates) from the anaerobic thiosulfate oxidation with nitrates wasT. denitrificans which was capable of producing 15 moles of sulfates from 7.5 moles of thiosulfate with concomitant reduction of 12 moles of nitrate resulting in the evolution of 6 moles of nitrogen gas/g protein/hr. The oxidation of sulfide was mediated by the flavo-protein system and cytochromes ofb, c, o, anda-type. This process was sensitive to flavoprotein inhibitors, antimycin A, and cyanide. The aerobic thiosulfate oxidation on the other hand involved cytochromec : O₂ oxidoreductase region of the electron transport chain and was sensitive to cyanide only. The anaerobic oxidation of thiosulfate byT. denitrificans, however, was severely inhibited by the flavoprotein inhibitors because of the splitting of the thiosulfate molecule into the sulfide and sulfite moieties produced by the thiosulfate-reductase. Accumulation of tetrathionate and to a small extent trithionate and pentathionate occurred during anaerobic growth ofT. denitrificans. These polythionates were subsequently oxidized to sulfate with the concomitant reduction of nitrate to N₂. Intact cell suspensions catalyzed the complete oxidation of sulfide, thiosulfate, tetrathionate, and sulfite to sulfate with the stoichiometric reduction of nitrate, nitrite, nitric oxide, and nitrous oxide to nitrogen gas thus indicating that NO₂ ⁻, NO, and N₂O are the possible intermediates in the denitrification of nitrate. This process was mediated by the cytochrome electron transport chain and was sensitive to the electron transfer inhibitors. The oxidation of sulfite involved cytochrome-linked sulfite oxidase as well as the APS-reductase pathways. The latter was absent inT. novellus andThiobacillus A ₂. In all of the thiobacilli the inner as well as the outer sulfur atoms of thiosulfate were oxidized at approximately the same rate by intact cells. The sulfide oxidation occurred in two stages: (a) a cellular-membrane-associated initial and rapid oxidation reaction which was dependent upon sulfide concentration, and (b) a slower oxidation reaction stage catalyzed by the cellfree extracts, probably involving polysulfides. InT. novellus andT. neapolitanus the oxidation of inorganic sulfur compounds is coupled to energy generation through oxidative phosphorylation, however, the reduction of pyridine nucleotides by sulfur compounds involved an energy-linked reversal of electron transfer. Paper read at the Symposium on the Sulphur Cycle, Wageningen, May 1974. Summary already inserted on p. 189 of the present volume.     

Assimilatory sulfur metabolism in Rhodopseudomonas sulfoviridis

Rhodopseudomonas sulfoviridis is unable to grow with sulfate as sole sulfur source. Radioactively labelled sulfate is not incorporated into the cells. Growth only occurs in the presence of reduced sulfur compounds, such as sulfide, thiosulfate, elemental sulfur and cysteine. ATP sulfurylase, adenylylsulfate kinase, O-acetylserine sulfhydrylase and cysteine desulfhydrase are present. Adenylylsulfate sulfotransferase and thiosulfonate reductase are lacking. The enzymes of the sulfate-activating system are not derepressed by O-acetylserine.Non common Abbreviations APS Adenosine 5-phosphosulfate - PAPS 3-phosphoadenosine 5-phosphosulfate