Formation and Conversion of Oxygen Metabolites by Lactococcus lactis subsp. lactis ATCC 19435 under Different Growth Conditions

A semidefined medium based on Casamino Acids allowed Lactococcus lactis ATCC 19435 to grow in the presence of oxygen at a slow rate (0.015 h⁻¹). Accumulation of H₂O₂ in the culture prevented a higher growth rate. Addition of asparagine to the medium increased the growth rate, whereby H₂O₂ accumulated only temporarily during the lag phase. H₂O₂ is an inhibitor for several glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase being the most sensitive. Strain ATCC 19435 contained NADH oxidase (maximum specific rate under aerobic conditions, 426 nmol of NADH min⁻¹ mg of protein⁻¹), which reduced oxygen to water, whereby superoxide was formed as a by-product. H₂O₂ originated from the dismutation of superoxide by superoxide dismutase. Although H₂O₂ was rapidly destroyed under high metabolic fluxes, neither NADH peroxidase nor any other enzymatic H₂O₂-reducing activity was detected. However, pyruvate, the end product of glycolysis, reacted nonenzymatically and rapidly with H₂O₂ and hence was a potential alternative for scavenging of this oxygen metabolite intracellularly. Indeed, intracellular concentrations of up to 93 mM pyruvate were detected in aerobic cultures growing at high rates. It is hypothesized that self-generated pyruvate may serve to protect L. lactis strain ATCC 19435 from H₂O₂.     

NADH Induces the Generation of Superoxide Radicals in Leaf Peroxisomes

In peroxisomes isolated from pea leaves (Pisum sativum L.) the production of superoxide free radicals (O₂⁻) by xanthine and NADH was investigated. In peroxisomal membranes, 100 micromolar NADH induced the production of O₂⁻ radicals. In the soluble fractions of peroxisomes, no generation of O₂⁻ radicals was observed by incubation with either NADH or xanthine, although xanthine oxidase was found located predominantly in the matrix of peroxisomes. The failure of xanthine to induce superoxide generation was probably due to the inability to fully suppress the endogenous Mn-superoxide dismutase activity by inhibitors which were inactive against xanthine oxidase. The generation of superoxide radicals in leaf peroxisomes together with the recently described production of these oxygen radicals in glyoxysomes (LM Sandalio, VM Fernández, FL Rupérez, LA del Río [1988] Plant Physiol 87: 1-4) suggests that O₂⁻ generation could be a common metabolic property of peroxisomes and further supports the existence of active oxygen-related rôles for peroxisomes in cellular metabolism.     

Phospholipid Composition of Desulfovibrio Species

The phospholipids of Desulfovibrio desulfuricans, Norway strain, D. vulgaris, and D. gigas were examined in relationship to their qualitative and quantitative composition. D. desulfuricans and D. vulgaris exhibited an essentially identical phospholipid composition consisting of phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, and lysophosphatidylserine. Phosphatidylserine (10.9%) was present in D. desulfuricans but was not detected in D. vulgaris. D. gigas was found to contain only two phospholipids, phosphatidylethanolamine (30%) and phosphatidylglycerol (70%). An ornithine-containing lipid was detected in D. gigas which was not present in the other two Desulfovibrio species.     

Whole-cell antigens of members of the sulfate-reducing genus Desulfovibrio

Antisera have been developed against the wholecell antigens of Desulfovibrio africanus Benghazi and Walvis Bay, D. vulgaris Hildenborough, D. salexigens British Guiana, D. gigas, and D. desulfuricans Essex 6. An enzymelinked immunoadsorption assay (ELISA) was developed to measure the reaction of these antisera with the homologous and heterologous antigens. The ELISA method demonstrated a reaction between pre-immune sera and cells of D. africanus, D. gigas and D. desulfuricans, suggesting the presence of a lectin-like substance on these cell surfaces. Extensive cross-reactions were seen between the antisera and heterologous cells, suggesting the sharing of a number of surface antigens amongst the Desulfovibrio. However, the pattern of these cross-reactions was different from that observed for an ELISA reaction developed for the cytochrome c₃ from various Desulfovibrio.Abbreviation ELISA enzyme-linked immunoadsorption assay     

Induction by Oxygen of Respiration and Phosphorylation of Anaerobically Grown Escherichia coli

The changes occurring in the respiratory enzymes of anaerobically grown Escherichia coli strain B and E. coli 15 T⁻A⁻U⁻bar during exposure to oxygen were studied. Reduced nicotinamide adenine dinucleotide (NADH) oxidase activity reached its peak soon after O₂ exposure; cytochrome content and succinate oxidase activity increased more slowly, and these increases paralleled each other. The activities of isocitrate and malate dehydrogenases also increased, but the increase was less than that of the succinate and NADH oxidases; exposure to O₂ had no effect on the succinate and NADH dehydrogenase activities. On the other hand, the glycolytic activity decreased slowly after O₂ exposure. The incorporation of ³²P into acid-soluble organic phosphate esters paralleled the respiratory rate during the first 60 min after O₂ exposure, but continued to increase after the respiration reached a plateau. The sensitivity of ³²P incorporation to the uncoupler carbonyl cyanide m-chlorophenylhydrazone also increased with time. The observed relationship between the development of the respiratory chain and the energy-conserving mechanism during O₂ exposure is discussed. Synthesis of the respiratory enzymes upon exposure to oxygen was dependent on concomitant protein and ribonucleic acid synthesis but not on deoxyribonucleic acid synthesis.     

Biochemical and Immunological Study of Cell Envelope Proteins in Sulfate-Reducing Bacteria

The envelope proteins of 5 strains of the genus Desulfotomaculum and 12 strains of the genus Desulfovibrio were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. The Desulfovibrio strains exhibited a typical gram-negative cell envelope, whereas the cell envelope of Desulfotomaculum strains appeared to be gram-positive. A close relationship between strains of Desulfotomaculum nigrificans was observed. A comparison between different species of Desulfotomaculum revealed some degree of similarity between Desulfotomaculum nigrificans and Desulfotomaculum ruminis, whereas Desulfotomaculum orientis seemed unique. The strains of Desulfovibrio salexigens were quite different from the strains of the other species of Desulfovibrio. In two of the strains of Desulfovibrio desulfuricans, a species-specific antigen was observed. The strains of Desulfovibrio vulgaris, Desulfovibrio africanus, and Desulfovibrio gigas and one strain of Desulfovibrio desulfuricans exhibited a similar outer membrane protein profile and also showed very similar antigenic reactions.     

Occurrence of Nitrate Reductase and Molybdopterin in Xanthomonas maltophilia

Fifteen of 23 ATCC strains and 2 of 9 clinical isolates of Xanthomonas maltophilia, all of which grew aerobically on ammonia, but not nitrate, as a sole nitrogen source, reduced nitrate to nitrite. X. maltophilia failed to grow anaerobically on complex medium with or without nitrate, so it is considered an obligate aerobe. Nitrate-reducing strains contained reduced methyl viologen nitrate reductase (MVH-NR) with specific activities ranging from 49.2 to 192 U mg of protein⁻¹. Strain ATCC 17666 doubled its cell mass after 3 h of growth on nitrate broth under low aeration, possessed maximal MVH-NR activity, and converted the added nitrate to nitrite, which accumulated. Dissolved oxygen above 15% saturation greatly suppressed nitrite formation. All strains, except ATCC 14535, possessed between 0.25 and 5.05 pmol of molybdopterin mg of protein⁻¹ as measured by the Neurospora crassa nit-1 assay. The molybdopterin activity in the soluble fraction sedimented as a single symmetrical peak with an s_20,w of 5.1. Studies identified MVH-NR in selected strains as a membrane-bound protein. The deoxycholate-solubilized MVH-NR sedimented as a single peak in sucrose density gradients with an s_20,w of 8.8. The MVH-NR of X. maltophilia has the physical characteristics of a respiratory nitrate reductase and may enable cells to use nitrate as an electron sink under semiaerobic conditions.     

Nitrogen, Carbon, and Sulfur Metabolism in Natural Thioploca Samples

Filamentous sulfur bacteria of the genus Thioploca occur as dense mats on the continental shelf off the coast of Chile and Peru. Since little is known about their nitrogen, sulfur, and carbon metabolism, this study was undertaken to investigate their (eco)physiology. Thioploca is able to store internally high concentrations of sulfur globules and nitrate. It has been previously hypothesized that these large vacuolated bacteria can oxidize sulfide by reducing their internally stored nitrate. We examined this nitrate reduction by incubation experiments of washed Thioploca sheaths with trichomes in combination with ¹⁵N compounds and mass spectrometry and found that these Thioploca samples produce ammonium at a rate of 1 nmol min⁻¹ mg of protein⁻¹. Controls showed no significant activity. Sulfate was shown to be the end product of sulfide oxidation and was observed at a rate of 2 to 3 nmol min⁻¹ mg of protein⁻¹. The ammonium and sulfate production rates were not influenced by the addition of sulfide, suggesting that sulfide is first oxidized to elemental sulfur, and in a second independent step elemental sulfur is oxidized to sulfate. The average sulfide oxidation rate measured was 5 nmol min⁻¹ mg of protein⁻¹ and could be increased to 10.7 nmol min⁻¹ mg of protein⁻¹ after the trichomes were starved for 45 h. Incorporation of ¹⁴CO₂ was at a rate of 0.4 to 0.8 nmol min⁻¹ mg of protein⁻¹, which is half the rate calculated from sulfide oxidation. [2-¹⁴C]acetate incorporation was 0.4 nmol min⁻¹ mg of protein⁻¹, which is equal to the CO₂ fixation rate, and no ¹⁴CO₂ production was detected. These results suggest that Thioploca species are facultative chemolithoautotrophs capable of mixotrophic growth. Microautoradiography confirmed that Thioploca cells assimilated the majority of the radiocarbon from [2-¹⁴C]acetate, with only a minor contribution by epibiontic bacteria present in the samples.     

Stoichiometry of proton translocation coupled to substrate oxidation in plant mitochondria

The proton translocation coupled to the electron flux from succinate, exogenous NADH, and NAD⁺-linked substrates (malate and isocitrate) to cytochrome c and to oxygen was studied in purified potato (Solanum tuberosum) mitochondria using oxygen and ferricyanide pulse techniques. In the presence of valinomycin plus K⁺ (used as a charge compensating cation), optimum values of H⁺/2 e⁻ were obtained when low amounts of electron acceptors (oxygen or ferricyanide) were added to the mitochondria (1-2 nanogram [2 e⁻] equivalents per milligram protein). The stoichiometry of proton translocation to electron flux was unaffected in the presence of N-ethylmaleimide, an inhibitor of the Pi/H⁺ symport. With succinate as substrate, H⁺/2 e⁻ ratios were 4.0 ± 0.2 and 3.7 ± 0.3 with oxygen and ferricyanide as electron acceptors, respectively. With exogenous NADH, H⁺/2e⁻ ratios were 4.1 ± 0.9 and 3.4 ± 0.2, respectively. The proton translocation coupled to the oxidation of NAD⁺-linked substrates (malate, isocitrate) was dependent upon the presence of adenylates (ADP, AMP, or ATP). For malate (+ glutamate) oxidation the observed H⁺/2 e⁻ ratios were increased from 3.6 ± 2.2 to 6.5 ± 0.5 in the presence of 20 micromolar ADP.     

Isolation and characterization of sulfate-reducing bacteria from various marine environments

Summary Forty-one strains of non-sporulating sulfate-reducing bacteria were isolated from estuaries, deep sea and other saline environments. Their salinity requirements, utilization of significant carbon compounds, resistance against growth inhibition by Hibitane, optimal growth temperatures and growth temperature ranges were studied. The results include data on strains isolated from the Red Sea hot brine deep area. Basing on the determined characteristics the strains were identified as Desulfovibrio desulfuricans, D. vulgaris, D. salexigens, and D. desulfuricans var. aestuarii.     

Inactivation of Hydrogenase in Cell-free Extracts and Whole Cells of Chlamydomonas reinhardi by Oxygen

O₂ irreversibly inactivates hydrogenase from Chlamydomonas reinhardi. The mechanism for the inactivation involves the reaction of one molecule of hydrogenase with one molecule of O₂ (or two oxygen atoms) in the transition complex of the rate-limiting step. The second order rate constant for this reaction is 190 atmospheres⁻¹ minute⁻¹ (1.4 × 10⁵ molar⁻¹ minute⁻¹). At levels above 0.01 atmosphere O₂, the increased numbers of O₂ molecules may compete for the site of inactivation hindering the proper orientation for inactivation of any one O₂ molecule and resulting in lowered rates of inactivation.     

Hydrogen-Dependent Oxygen Reduction by Homoacetogenic Bacteria Isolated from Termite Guts

Although homoacetogenic bacteria are generally considered to be obligate anaerobes, they colonize the intestinal tracts of termites and other environments that are not entirely anoxic in space or time. In this study, we investigated how homoacetogenic bacteria isolated from the hindguts of various termites respond to the presence of molecular oxygen. All strains investigated formed growth bands in oxygen gradient agar tubes under a headspace of H₂-CO₂. The position of the bands coincided with the oxic-anoxic interface and depended on the O₂ partial pressure in the headspace; the position of the bands relative to the meniscus remained stable for more than 1 month. Experiments with dense cell suspensions, performed with Clark-type O₂ and H₂ electrodes, revealed a large capacity for H₂-dependent oxygen reduction in Sporomusa termitida and Sporomusa sp. strain TmAO3 (149 and 826 nmol min⁻¹ mg of protein⁻¹, respectively). Both strains also reduced O₂ with endogenous reductants, albeit at lower rates. Only in Acetonema longum did the basal rates exceed the H₂-dependent rates considerably (181 versus 28 nmol min⁻¹ mg of protein)⁻¹). Addition of organic substrates did not stimulate O₂ consumption in any of the strains. Nevertheless, reductive acetogenesis by cell suspensions of strain TmAO3 was inhibited even at the lowest O₂ fluxes, and growth in nonreduced medium occurred only after the bacteria had rendered the medium anoxic. Similar results were obtained with Acetobacterium woodii, suggesting that the results are not unique to the strains isolated from termites. We concluded that because of their tolerance to temporary exposure to O₂ at low partial pressures (up to 1.5 kPa in the case of strain TmAO3) and because of their large capacity for O₂ reduction, homoacetogens can reestablish conditions favorable for growth by actively removing oxygen from their environment.     

Methylmercury Resistance in Desulfovibrio desulfuricans Strains in Relation to Methylmercury Degradation

Two strains of Desulfovibrio desulfuricans, one known to synthesize monomethylmercury from ionic mercury, were grown to determine methylmercury toxicity and for comparison with an anaerobic strain of Clostridium pasteurianum, a H₂ producer, and with the broad-spectrum mercury-resistant Pseudomonas putida strain FB-1, capable of degrading 1 μg of methylmercury to methane and elemental mercury in 2 h. The CH₃HgCl resistance of D. desulfuricans strains was 10 times that of P. putida FB-1 and 100 times that of C. pasteurianum. The methylmercury resistance of D. desulfuricans was related to the disappearance of methylmercury from cultures by transformation to dimethylmercury, metacinnabar, methane, and traces of ionic mercury. During a 15-day experiment the kinetics of the two volatile compounds dimethylmercury [(CH₃)₂Hg] and methane were monitored in the liquid by a specific new technique with purge-and-trap gas chromatography in line with Fourier transform infrared spectroscopy and in the headspace by gas chromatography with flame ionization detection. Insoluble metacinnabar (cubic HgS) of biological origin was detected by X-ray diffractometry in the gray precipitate from the insoluble residue of the pellet of a 1-liter culture spiked with 100 mg of CH₃HgCl. This was compared with a 1-liter culture of D. desulfuricans LS spiked with 100 mg of HgCl₂. In a further experiment, it was demonstrated that insoluble, decomposable, white dimethylmercury sulfide [(CH₃Hg)₂S] formed instantly in the reaction of methylmercury with hydrogen sulfide. This organomercurial was extracted with chloroform and identified by gas chromatography in line with mass spectrometry. The D. desulfuricans strains were resistant to high concentrations of methylmercury because they produced insoluble dimethylmercury sulfide, which slowly decomposed under anaerobic conditions to metacinnabar and volatilized to dimethylmercury and methane between pHs 6.2 and 6.5 for high (4.5-g · liter^-1) or low (0.09-g · liter^-1) sulfate contents. Methane was produced from CH₃HgCl at a lower rate than by the broad-spectrum Hg-resistant P. putida strain FB-1.     

A Novel A3 Group Aconitase Tolerates Oxidation and Nitric Oxide

Achromobacter denitrificans YD35 is an NO₂⁻-tolerant bacterium that expresses the aconitase genes acnA3, acnA4, and acnB, of which acnA3 is essential for growth tolerance against 100 mm NO₂⁻. Atmospheric oxygen inactivated AcnA3 at a rate of 1.6 × 10⁻³ min⁻¹, which was 2.7- and 37-fold lower compared with AcnA4 and AcnB, respectively. Stoichiometric titration showed that the [4Fe-4S]²⁺ cluster of AcnA3 was more stable against oxidative inactivation by ferricyanide than that of AcnA4. Aconitase activity of AcnA3 persisted against high NO₂⁻ levels that generate reactive nitrogen species with an inactivation rate constant of k = 7.8 × 10⁻³ min⁻¹, which was 1.6- and 7.8-fold lower than those for AcnA4 and AcnB, respectively. When exposed to NO₂⁻, the acnA3 mutant (AcnA3Tn) accumulated higher levels of cellular citrate compared with the other aconitase mutants, indicating that AcnA3 is a major producer of cellular aconitase activity. The extreme resistance of AcnA3 against oxidation and reactive nitrogen species apparently contributes to bacterial NO₂⁻ tolerance. AcnA3Tn accumulated less cellular NADH and ATP compared with YD35 under our culture conditions. The accumulation of more NO by AcnA3Tn suggested that NADH-dependent enzymes detoxify NO for survival in a high NO₂⁻ milieu. This novel aconitase is distributed in Alcaligenaceae bacteria, including pathogens and denitrifiers, and it appears to contribute to a novel NO₂⁻ tolerance mechanism in this strain.     

Transition from Anaerobic to Aerobic Growth Conditions for the Sulfate-Reducing Bacterium Desulfovibrio oxyclinae Results in Flocculation

A chemostat culture of the sulfate-reducing bacterium Desulfovibrio oxyclinae isolated from the oxic layer of a hypersaline cyanobacterial mat was grown anaerobically and then subjected to gassing with 1% oxygen, both at a dilution rate of 0.05 h⁻¹. The sulfate reduction rate under anaerobic conditions was 370 nmol of SO₄²⁻ mg of protein⁻¹ min⁻¹. At the onset of aerobic gassing, sulfate reduction decreased by 40%, although viable cell numbers did not decrease. After 42 h, the sulfate reduction rate returned to the level observed in the anaerobic culture. At this stage the growth yield increased by 180% compared to the anaerobic culture to 4.4 g of protein per mol of sulfate reduced. Protein content per cell increased at the same time by 40%. The oxygen consumption rate per milligram of protein measured in washed cell suspensions increased by 80%, and the thiosulfate reduction rate of the same samples increased by 29% with lactate as the electron donor. These findings indicated possible oxygen-dependent enhancement of growth. After 140 h of growth under oxygen flux, formation of cell aggregates 0.1 to 3 mm in diameter was observed. Micrometer-sized aggregates were found to form earlier, during the first hours of exposure to oxygen. The respiration rate of D. oxyclinae was sufficient to create anoxia inside clumps larger than 3 μm, while the levels of dissolved oxygen in the growth vessel were 0.7 ± 0.5 μM. Aggregation of sulfate-reducing bacteria was observed within a Microcoleus chthonoplastes-dominated layer of a cyanobacterial mat under daily exposure to oxygen concentrations of up to 900 μM. Desulfonema-like sulfate-reducing bacteria were also common in this environment along with other nonaggregated sulfate-reducing bacteria. Two-dimensional mapping of sulfate reduction showed heterogeneity of sulfate reduction activity in this oxic zone.     

Oxygen radical generation by isolated microsomes from soybean seedlings

The generation of active oxygen species by microsomes isolated from soybean seedlings was studied. NADPH-dependent superoxide anion production was 5.0 ± 0.4 nmol · min⁻¹ mg⁻¹ of microsomal protein. Hydrogen peroxide generation by microsomes was 1.40 ± 0.05 nmol · min⁻¹ mg⁻¹ of protein. Hydroxyl radical production, in the presence of ferric EDTA, evaluated through the generation of formaldehyde from dimethyl sulfoxide or tert-butyl alcohol was 0.50 ± 0.04 and 0.44 ± 0.03 nmol · min⁻¹ mg⁻¹, respectively. NADH proved to be suitable as cofactor for oxygen radical generation by microsomes from soybean seedlings. Because transition metals are implicated in radical generation by biological systems, the ability of microsomal membranes to reduce iron complexes was studied. Ferric ATP, ferric citrate, ferric ADP, ferric diethylenetriamine pentaacetic acid, and ferric EDTA were efficiently reduced in the presence of either NADPH or NADH as cofactor. The pattern of effectiveness of the different ferric complexes, on superoxide anion, hydrogen peroxide, and hydroxyl radical production, was similar to that found with animal microsomes. The data presented here indicate that microsomal ability to catalyze oxygen radical generation must be considered as an important contribution to cellular radical steady-state concentrations in cells from soybean seedlings.     

Thermostabilization of Proteins by Diglycerol Phosphate, a New Compatible Solute from the Hyperthermophile Archaeoglobus fulgidus

Diglycerol phosphate accumulates under salt stress in the archaeon Archaeoglobus fulgidus (L. O. Martins, R. Huber, H. Huber, K. O. Stetter, M. S. da Costa, and H. Santos, Appl. Environ. Microbiol. 63:896–902, 1997). This solute was purified after extraction from the cell biomass. In addition, the optically active and the optically inactive (racemic) forms of the compound were synthesized, and the ability of the solute to act as a protecting agent against heating was tested on several proteins derived from mesophilic or hyperthermophilic sources. Diglycerol phosphate exerted a considerable stabilizing effect against heat inactivation of rabbit muscle lactate dehydrogenase, baker's yeast alcohol dehydrogenase, and Thermococcus litoralis glutamate dehydrogenase. Highly homologous and structurally well-characterized rubredoxins from Desulfovibrio gigas, Desulfovibrio desulfuricans (ATCC 27774), and Clostridium pasteurianum were also examined for their thermal stabilities in the presence or absence of diglycerol phosphate, glycerol, and inorganic phosphate. These proteins showed different intrinsic thermostabilities, with half-lives in the range of 30 to 100 min. Diglycerol phosphate exerted a strong protecting effect, with approximately a fourfold increase in the half-lives for the loss of the visible spectra of D. gigas and C. pasteurianum rubredoxins. In contrast, the stability of D. desulfuricans rubredoxin was not affected. These different behaviors are discussed in the light of the known structural features of rubredoxins. The data show that diglycerol phosphate is a potentially useful protein stabilizer in biotechnological applications.     

Continuous Synthesis and Excretion of the Compatible Solute Ectoine by a Transgenic, Nonhalophilic Bacterium

The compatible solute 1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid (ectoine) acts in microorganisms as an osmotic counterweight against halostress and has attracted commercial attention as a protecting agent. Its production and application are restricted by the drawbacks of the discontinuous harvesting procedure involving salt shocks, which reduces volumetric yield, increases reactor corrosion, and complicates downstream processing. In order to synthesize ectoine continuously in less-aggressive media, we introduced the ectoine genes ectABC of the halophilic bacterium Chromohalobacter salexigens into an Escherichia coli strain using the expression vector pASK-IBA7. Under the control of a tet promoter, the transgenic E. coli synthesized 6 g liter⁻¹ ectoine with a space-time yield of 40 mg liter⁻¹ h⁻¹, with the vast majority of the ectoine being excreted.     

Modeling Reduction of Uranium U(VI) under Variable Sulfate Concentrations by Sulfate-Reducing Bacteria

The kinetics for the reduction of sulfate alone and for concurrent uranium [U(VI)] and sulfate reduction, by mixed and pure cultures of sulfate-reducing bacteria (SRB) at 21 ± 3°C were studied. The mixed culture contained the SRB Desulfovibrio vulgaris along with a Clostridium sp. determined via 16S ribosomal DNA analysis. The pure culture was Desulfovibrio desulfuricans (ATCC 7757). A zero-order model best fit the data for the reduction of sulfate from 0.1 to 10 mM. A lag time occurred below cell concentrations of 0.1 mg (dry weight) of cells/ml. For the mixed culture, average values for the maximum specific reaction rate, V_max, ranged from 2.4 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h⁻¹) at 0.25 mM sulfate to 5.0 ± 1.1 μmol of sulfate/mg (dry weight) of SRB · h⁻¹ at 10 mM sulfate (average cell concentration, 0.52 mg [dry weight]/ml). For the pure culture, V_max was 1.6 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h⁻¹ at 1 mM sulfate (0.29 mg [dry weight] of cells/ml). When both electron acceptors were present, sulfate reduction remained zero order for both cultures, while uranium reduction was first order, with rate constants of 0.071 ± 0.003 mg (dry weight) of cells/ml · min⁻¹ for the mixed culture and 0.137 ± 0.016 mg (dry weight) of cells/ml · min⁻¹ (U₀ = 1 mM) for the D. desulfuricans culture. Both cultures exhibited a faster rate of uranium reduction in the presence of sulfate and no lag time until the onset of U reduction in contrast to U alone. This kinetics information can be used to design an SRB-dominated biotreatment scheme for the removal of U(VI) from an aqueous source.