Quantification of toxic and inhibitory impact of copper and zinc on mixed cultures of sulfate-reducing bacteria

The adverse effects of copper and zinc on an acetate-utilizing mixed cultures of sulfate-reducing bacteria (SRB) at concentrations below the toxic concentration (minimum metal concentration at which no sulfate reduction is observed) are reported in this paper. Mathematical models were developed to incorporate the toxic and inhibitory effects (defined as the reduction in bacterial population upon exposure to the metal and the decrease in the metabolic rate of sulfate reduction by the SRB, respectively) into the sulfate-reduction biokinetics. The characteristic toxicity and inhibition constants were obtained from the measurements of bacterial populations and dissolved metal concentrations in serum bottle studies conducted at 35 degrees C and pH 6.6. Both copper and zinc had toxic and inhibitory effects on SRB. The toxicity constants for copper and zinc were 10.6 and 2.9 mM(-1), respectively, indicating that exposure to copper resulted in a higher mortality of SRB than did exposure to zinc. The values of the inhibition constants were found to be 17.9 +/- 2.5 and 25.2 +/- 1.0 mM(-1) for copper and zinc, respectively. This implies that dissolved zinc was slightly more inhibitory to SRB than copper. The models presented in the paper can be used to predict the response of a sulfate-reduction bioreactor to heavy metals during acid mine drainage treatment.     

Culturable diversity of lithotrophic haloalkaliphilic sulfate-reducing bacteria in soda lakes and the description of Desulfonatronum thioautotrophicum sp. nov., Desulfonatronum thiosulfatophilum sp. nov., Desulfonatronovibrio thiodismutans sp. nov., and Desulfonatronovibrio magnus sp. nov

Soda lake sediments usually contain high concentrations of sulfide indicating active sulfate reduction. Monitoring of sulfate-reducing bacteria (SRB) in soda lakes demonstrated a dominance of two groups of culturable SRB belonging to the order Desulfovibrionales specialized in utilization of inorganic electron donors, such as formate, H₂ and thiosulfate. The most interesting physiological trait of the novel haloalkaliphilic SRB isolates was their ability to grow lithotrophically by dismutation of thiosulfate and sulfite. All isolates were obligately alkaliphilic with a pH optimum at 9.5–10 and moderately salt tolerant. Among the fifteen newly isolated strains, four belonged to the genus Desulfonatronum and the others to the genus Desulfonatronovibrio. None of the isolates were closely related to previously described species of these genera. On the basis of phylogenetic, genotypic and phenotypic characterization of the novel soda lake SRB isolates, two novel species each in the genera Desulfonatronum and Desulfonatronovibrio are proposed.     

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 degrees 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 micromol of sulfate/mg (dry weight) of SRB. h(-1)) at 0.25 mM sulfate to 5.0 +/- 1.1 micromol 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 micromol 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) (U(0) = 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.     

Competitive oxidation of volatile fatty acids by sulfate- and nitrate-reducing bacteria from an oil field in Argentina

Acetate, propionate, and butyrate, collectively referred to as volatile fatty acids (VFA), are considered among the most important electron donors for sulfate-reducing bacteria (SRB) and heterotrophic nitrate-reducing bacteria (hNRB) in oil fields. Samples obtained from a field in the Neuquén Basin, western Argentina, had significant activity of mesophilic SRB, hNRB, and nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB). In microcosms, containing VFA (3 mM each) and excess sulfate, SRB first used propionate and butyrate for the production of acetate, which reached concentrations of up to 12 mM prior to being used as an electron donor for sulfate reduction. In contrast, hNRB used all three organic acids with similar kinetics, while reducing nitrate to nitrite and nitrogen. Transient inhibition of VFA-utilizing SRB was observed with 0.5 mM nitrite and permanent inhibition with concentrations of 1 mM or more. The addition of nitrate to medium flowing into an upflow, packed-bed bioreactor with an established VFA-oxidizing SRB consortium led to a spike of nitrite up to 3 mM. The nitrite-mediated inhibition of SRB led, in turn, to the transient accumulation of up to 13 mM of acetate. The complete utilization of nitrate and the incomplete utilization of VFA, especially propionate, and sulfate indicated that SRB remained partially inhibited. Hence, in addition to lower sulfide concentrations, an increase in the concentration of acetate in the presence of sulfate in waters produced from an oil field subjected to nitrate injection may indicate whether the treatment is successful. The microbial community composition in the bioreactor, as determined by culturing and culture-independent techniques, indicated shifts with an increasing fraction of nitrate. With VFA and sulfate, the SRB genera Desulfobotulus, Desulfotignum, and Desulfobacter as well as the sulfur-reducing Desulfuromonas and the NR-SOB Arcobacter were detected. With VFA and nitrate, Pseudomonas spp. were present. hNRB/NR-SOB from the genus Sulfurospirillum were found under all conditions.     

Gastrointestinal and microbial responses to sulfate-supplemented drinking water in mice

There is increasing evidence that hydrogen sulfide (H2S), produced by intestinal sulfate-reducing bacteria (SRB), may be involved in the etiopathogenesis of chronic diseases such as ulcerative colitis and colorectal cancer. The activity of SRB, and thus H2S production, is likely determined by the availability of sulfur-containing compounds in the intestine. However, little is known about the impact of dietary or inorganic sulfate on intestinal sulfate and SRB-derived H2S concentrations. In this study, the effects of short-term (7 day) and long-term (1 year) inorganic sulfate supplementation of the drinking water on gastrointestinal (GI) sulfate and H2S concentrations (and thus activity of resident SRBs), and the density of large intestinal sulfomucin-containing goblet cells, were examined in C3H/HeJBir mice. Additionally, a PCR-denaturing gradient gel electrophoresis (DGGE)-based molecular ecology technique was used to examine the impact of sulfate-amended drinking water on microbial community structure throughout the GI tract. Average H2S concentrations ranged from 0.1 mM (stomach) to 1 mM (cecum). A sulfate reduction assay demonstrated in situ production of H2S throughout the GI tract, confirming the presence of SRB. However, H2S generation and concentrations were greatest in the cecum and colon. Sulfate supplementation of drinking water did not significantly increase intestinal sulfate or H2S concentrations, suggesting that inorganic sulfate is not an important modulator of intestinal H2S concentrations, although it altered the bacterial profiles of the stomach and distal colon of 1-year-old mice. This change in colonic bacterial profiles may reflect a corresponding increase in the density of sulfomucin-containing goblet cells in sulfate-supplemented compared with control mice.     

Comparison of microbial communities involved in souring and corrosion in offshore and onshore oil production facilities in Nigeria

Samples were obtained from the Obigbo field, located onshore in the Niger delta, Nigeria, from which oil is produced by injection of low-sulfate groundwater, as well as from the offshore Bonga field from which oil is produced by injection of high-sulfate (2,200 ppm) seawater, amended with 45 ppm of calcium nitrate to limit reservoir souring. Despite low concentrations of sulfate (0–7 ppm) and nitrate (0 ppm), sulfate-reducing bacteria (SRB) and heterotrophic nitrate-reducing bacteria (NRB) were present in samples from the Obigbo field. Biologically active deposits (BADs), scraped from corrosion-failed sections of a water- and of an oil-transporting pipeline (both Obigbo), had high counts of SRB and high sulfate and ferrous iron concentrations. Analysis of microbial community composition by pyrosequencing indicated anaerobic, methanogenic hydrocarbon degradation to be a dominant process in all samples from the Obigbo field, including the BADs. Samples from the Bonga field also had significant activity of SRB, as well as of heterotrophic and of sulfide-oxidizing NRB. Microbial community analysis indicated high proportions of potentially thermophilic NRB and near-absence of microbes active in methanogenic hydrocarbon degradation. Anaerobic incubation of Bonga samples with steel coupons gave moderate general corrosion rates of 0.045–0.049 mm/year, whereas near-zero general corrosion rates (0.001–0.002 mm/year) were observed with Obigbo water samples. Hence, methanogens may contribute to corrosion at Obigbo, but the low general corrosion rates cannot explain the reasons for pipeline failures in the Niger delta. A focus of future work should be on understanding the role of BADs in enhancing under-deposit pitting corrosion.     

Oil field souring control by nitrate-reducing Sulfurospirillum spp. that outcompete sulfate-reducing bacteria for organic electron donors

Nitrate injection into oil reservoirs can prevent and remediate souring, the production of hydrogen sulfide by sulfate-reducing bacteria (SRB). Nitrate stimulates nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and heterotrophic nitrate-reducing bacteria (hNRB) that compete with SRB for degradable oil organics. Up-flow, packed-bed bioreactors inoculated with water produced from an oil field and injected with lactate, sulfate, and nitrate served as sources for isolating several NRB, including Sulfurospirillum and Thauera spp. The former coupled reduction of nitrate to nitrite and ammonia with oxidation of either lactate (hNRB activity) or sulfide (NR-SOB activity). Souring control in a bioreactor receiving 12.5 mM lactate and 6, 2, 0.75, or 0.013 mM sulfate always required injection of 10 mM nitrate, irrespective of the sulfate concentration. Community analysis revealed that at all but the lowest sulfate concentration (0.013 mM), significant SRB were present. At 0.013 mM sulfate, direct hNRB-mediated oxidation of lactate by nitrate appeared to be the dominant mechanism. The absence of significant SRB indicated that sulfur cycling does not occur at such low sulfate concentrations. The metabolically versatile Sulfurospirillum spp. were dominant when nitrate was present in the bioreactor. Analysis of cocultures of Desulfovibrio sp. strain Lac3, Lac6, or Lac15 and Sulfurospirillum sp. strain KW indicated its hNRB activity and ability to produce inhibitory concentrations of nitrite to be key factors for it to successfully outcompete oil field SRB.     

Advances In Biotreatment of Acid Mine Drainage and Biorecovery of Metals: 2. Membrane Bioreactor System for Sulfate Reduction

Several biotreatmemt techniques for sulfate conversion by the sulfate reducing bacteria (SRB) have been proposed in the past, however few of them have been practically applied to treat sulfate containing acid mine drainage (AMD). This research deals with development of an innovative polypropylene hollow fiber membrane bioreactor system for the treatment of acid mine water from the Berkeley Pit, Butte, MT, using hydrogen consuming SRB biofilms. The advantages of using the membrane bioreactor over the conventional tall liquid phase sparged gas bioreactor systems are: large microporous membrane surface to the liquid phase; formation of hydrogen sulfide outside the membrane, preventing the mixing with the pressurized hydrogen gas inside the membrane; no requirement of gas recycle compressor; membrane surface is suitable for immobilization of active SRB, resulting in the formation of biofilms, thus preventing washout problems associated with suspended culture reactors; and lower operating costs in membrane bioreactors, eliminating gas recompression and gas recycle costs. Information is provided on sulfate reduction rate studies and on biokinetic tests with suspended SRB in anaerobic digester sludge and sediment master culture reactors and with SRB biofilms in bench-scale SRB membrane bioreactors. Biokinetic parameters have been determined using biokinetic models for the master culture and membrane bioreactor systems. Data are presented on the effect of acid mine water sulfate loading at 25, 50, 75 and 100 ml/min in scale-up SRB membrane units, under varied temperatures (25, 35 and 40 °C) to determine and optimize sulfate conversions for an effective AMD biotreatment. Pilot-scale studies have generated data on the effect of flow rates of acid mine water (MGD) and varied inlet sulfate concentrations in the influents on the resultant outlet sulfate concentration in the effluents and on the number of SRB membrane modules needed for the desired sulfate conversion in those systems. The pilot-scale data indicate that the SRB membrane bioreactors systems can be applied toward field-scale biotreatment of AMD and for recovery of high purity metals and an agriculturally usable water.     

Oil Field Souring Control by Nitrate-Reducing Sulfurospirillum spp. That Outcompete Sulfate-Reducing Bacteria for Organic Electron Donors

Nitrate injection into oil reservoirs can prevent and remediate souring, the production of hydrogen sulfide by sulfate-reducing bacteria (SRB). Nitrate stimulates nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and heterotrophic nitrate-reducing bacteria (hNRB) that compete with SRB for degradable oil organics. Up-flow, packed-bed bioreactors inoculated with water produced from an oil field and injected with lactate, sulfate, and nitrate served as sources for isolating several NRB, including Sulfurospirillum and Thauera spp. The former coupled reduction of nitrate to nitrite and ammonia with oxidation of either lactate (hNRB activity) or sulfide (NR-SOB activity). Souring control in a bioreactor receiving 12.5 mM lactate and 6, 2, 0.75, or 0.013 mM sulfate always required injection of 10 mM nitrate, irrespective of the sulfate concentration. Community analysis revealed that at all but the lowest sulfate concentration (0.013 mM), significant SRB were present. At 0.013 mM sulfate, direct hNRB-mediated oxidation of lactate by nitrate appeared to be the dominant mechanism. The absence of significant SRB indicated that sulfur cycling does not occur at such low sulfate concentrations. The metabolically versatile Sulfurospirillum spp. were dominant when nitrate was present in the bioreactor. Analysis of cocultures of Desulfovibrio sp. strain Lac3, Lac6, or Lac15 and Sulfurospirillum sp. strain KW indicated its hNRB activity and ability to produce inhibitory concentrations of nitrite to be key factors for it to successfully outcompete oil field SRB.     

Purification and properties of 16 alpha-hydroxyprogesterone dehydroxylase from Eubacterium sp. strain 144

Eubacterium sp. strain 144 converts 16 alpha-hydroxyprogesterone to 17-isoprogesterone. The first step of this reaction is catalyzed by 16 alpha-hydroxyprogesterone dehydroxylase (16 alpha-dehydroxylase). This enzyme was purified 40-70-fold and characterized. 16 alpha-Dehydroxylase was found to be active in two molecular weight forms of Mr 181 000 and 326 000. A subunit relative molecular weight of 42 400 was determined by sodium dodecyl sulfate gel electrophoresis of the purified enzyme. Although active with both 16 alpha-hydroxyprogesterone and 16 alpha-hydroxypregnenolone, the affinity of 16 alpha-dehydroxylase for the latter steroid was twice that of the former based on the apparent Km values. Evidence of possible substrate inhibition at high concentrations was seen with 16 alpha-hydroxypregnenolone. 16-Ketoprogesterone was found to be a competitive inhibitor of 16 alpha-dehydroxylase with respect to both steroid substrates. Although generally unaffected by low concentrations of non-ionic detergents, 16 alpha-dehydroxylase activity was stimulated 3-7-fold by sodium dodecyl sulfate and inhibited strongly by cetyltrimethylammonium bromide.     

Kinetics of Acetate Utilization by Two Thermophilic Acetotrophic Methanogens: Methanosarcina sp. Strain CALS-1 and Methanothrix sp. Strain CALS-1

The kinetics of acetate utilization were examined for washed concentrated cell suspensions of two thermophilic acetotrophic methanogens isolated from a 58°C anaerobic digestor. Progress curves for acetate utilization by cells of Methanosarcina sp. strain CALS-1 showed that the utilization rate was concentration independent (zero order) above concentrations near 3 mM and that acetate utilization ceased when a threshold concentration near 1 mM was reached. Acetate utilization by cells of Methanothrix sp. strain CALS-1 was concentration independent down to 0.1 to 0.2 mM, and threshold values of 12 to 21 μM were observed. Typical utilization rates in the concentration-independent stage were 210 and 130 nmol min⁻¹ mg of protein⁻¹ for the methanosarcina and the methanothrix, respectively. These results are in agreement with a general model in which high acetate concentrations favor Methanosarcina spp., while low concentrations favor Methanothrix spp. However, acetate utilization by these two strains did not follow simple Michaelis-Menton kinetics.     

Thermodesulfobacterium hveragerdense sp. nov., and Thermodesulfovibrio islandicus sp. nov., two thermophilic sulfate reducing bacteria isolated from a Icelandic hot spring

Two thermophilic non-sporeforming sulfate-reducing bacteria (SRB) were isolated from microbial mats collected from an Icelandic hot spring. Strain JSP was a gram negative rod, with an average cell size of 2.8 x 0.5 microm. No flagella were found. Growth occurred between 55 and 74 degrees C with an optimum between 70 and 74 degrees C at pH 7.0. The G+C content was 40 mol%. Strain R1Ha3 was a gram negative vibrio-shaped rod with an average cell size of 1.7 x 0.4 microm. Motility was observed mediated by one polar flagellum. The growth optimum at pH 7.0 was 65 degrees C, and growth occurred between 45 and 70 degrees C. The G+C content was 38 mol%. In the presence of sulfate, both strains used lactate, pyruvate and H2 as electron donors. In addition, strain R1Ha3 used formate. Pyruvate was the only substrate supporting fermentative growth of both strains. Growth occurred with sulfate as well as thiosulfate as electron acceptors. Furthermore, strain R1Ha3 reduced nitrate and strain JSP reduced sulfite. Neither of the strains were able to oxidize lactate completely to CO2 and neither of the strains contained desulfoviridin. 16S rDNA sequencing placed strain JSP in the genus Thermodesulfobacterium and strain R1Ha3 in the genus Thermodesulfovibrio. Based on the DNA-DNA hybridization studies and differences in morphology and physiology to their closest relatives the two new isolates were considered as new species. Strain JSP is named Thermodesulfobacterium hveragerdense and strain R1Ha3 Thermodesulfovibrio islandicus.     

Molecular characterization of sulfate-reducing bacteria in a New England salt marsh

Sulfate reduction, mediated by sulfate-reducing bacteria (SRB), is the dominant remineralization pathway in sediments of New England salt marshes. High sulfate reduction rates are associated with the rhizosphere of Spartina alterniflora when plants elongate aboveground. The growth process concurrently produces significant amounts of new rhizome material belowground and the plants leak dissolved organic compounds. This study investigated the diversity of SRB in a salt marsh over an annual growth cycle of S. alterniflora by exploring the diversity of a functional gene, dissimilatory sulfite reductase (dsrAB). Because the dsrAB gene is a key gene in the anaerobic sulfate-respiration pathway, it allows the identification of microorganisms responsible for sulfate reduction. Conserved dsrAB primers in polymerase chain reaction (PCR) generated full-length dsrAB amplicons for cloning and DNA sequence analysis. Nearly 80% of 380 clone sequences were similar to genes from Desulfosarcina and Desulfobacterium species within Desulfobacteraceae. This reinforces the hypothesis that complete oxidizers with high substrate versatility dominate the marsh. However, the phylotypes formed several clades that were distinct from cultured representatives, indicating a greater diversity of SRB than previously appreciated. Several dsrAB sequences were related to homologues from gram-positive, thermophilic and non-thermophilic Desulfotomaculum species. One dsrAB lineage formed a sister group to cultured members of the delta-proteobacterial group Syntrophobacteraceae. A deeply branching dsrAB lineage was not affiliated with genes from any cultured SRB. The sequence data from this study will allow for the design of probes or primers that can quantitatively assess the diverse range of sulfate reducers present in the environment.     

Catalytic properties and mode of action of three endo-beta-glucanases from Talaromyces emersonii on soluble beta-1,4- and beta-1,3;1,4-linked glucans

In this paper, we present the first detailed analysis of the modes of action of three purified, thermostable endo-beta-D-glucanases (EG V-VII) against a range of soluble beta-linked glucans. Studies indicated that EG V-VII, purified to homogeneity from a new source, the thermophilic fungus Talaromyces emersonii, are strict beta-glucanases that exhibit maximum activity against mixed-link 1,3;1,4-beta-D-glucans. Time-course hydrolysis studies of 1,4-beta-D-glucan (carboxymethylcellulose; CMC), 1,3;1,4-beta-D-glucan from barley (BBG) and lichenan confirmed the endo-acting nature of EG V-VII and verified preference for 1,3;1,4-beta-D-glucan substrates. The results suggest that EG VI and EG VII belong to EC 3.2.1.6, as both enzymes also exhibit activity against 1,3-beta-glucan (laminaran), in contrast to EG V. Although cellobiose, cellotriose and glucose were the main glucooligosaccharide products released, the range and relative amount of each product was dependent on the particular enzyme, substrate and reaction time. Kinetic constants (Km, Vmax, kcat and kcat/Km) determined for EG V-VII with BBG as substrate yielded similar Km and Vmax values for EG V and EG VI. EG VII exhibited highest affinity for BBG (Km value of 9.1 mg ml(-1)) and the highest catalytic efficiency (kcat/Km of 12.63 s(-1) mg(-1) ml).     

Site-directed mutations of arginine 65 at the periphery of the active site cleft of yeast 3-phosphoglycerate kinase enhance the catalytic activity and eliminate anion-dependent activation.

The function of arginine 65, a conserved residue located at the periphery of the active site cleft in yeast 3-phosphoglycerate kinase (PGK), has been investigated by site-directed mutagenesis. Mutant enzymes with glutamine, serine and alanine at position 65 all have very similar kinetic properties. The maximum velocities, determined in the absence of sulfate anion, are approximately 100% higher than the Vmax of wild-type PGK. The Km values are increased 2- to 3-fold for ATP and 5- to 6-fold for 3-phosphoglycerate (3PG). These results demonstrate that arginine 65 is not essential for catalysis. In contrast to wild-type enzyme, the mutants are not activated by sulfate ions. In addition, steady-state kinetic experiments indicate that the mutants are no longer activated by high concentrations of either 3PG or ATP. The dissociation constants for anions were determined by spectral titrations of the R65Q mutant labeled with a chromophoric probe. The Kd for 3PG is increased 6-fold, as compared to wild-type PGK, whereas the Kd for ATP is essentially unchanged. The Kd for sulfate is decreased less than 2-fold. The suppression of substrate- and sulfate-dependent activation suggests that arginine 65 participates in the regulatory mechanism responsible for activation of the enzyme.     

Neural network prediction of thermophilic (65 degrees C) sulfidogenic fluidized-bed reactor performance for the treatment of metal-containing wastewater

The performance of a fluidized-bed reactor (FBR) based sulfate reducing bioprocess was predicted using artificial neural network (ANN). The FBR was operated at high (65 degrees C) temperature and it was fed with iron (40-90 mg/L) and sulfate (1,000-1,500 mg/L) containing acidic (pH = 3.5-6) synthetic wastewater. Ethanol was supplemented as carbon and electron source for sulfate reducing bacteria (SRB). The wastewater pH of 4.3-4.4 was neutralized by the alkalinity produced in acetate oxidation and the average effluent pH was 7.8 +/- 0.8. The oxidation of acetate is the rate-limiting step in the sulfidogenic ethanol oxidation by thermophilic SRB, which resulted in acetate accumulation. Sulfate reduction and acetate oxidation rates showed variation depending on the operational conditions with the maximum rates of 1 g/L/d (0.2 g/g volatile solids (VS)/d) and 0.3 g/L/d (0.06 g/g VS/d), respectively. This study presents an ANN model predicting the performance of the reactor and determining the optimal architecture of this model; such as best back-propagation (BP) algorithm and neuron numbers. The Levenberg-Marquardt algorithm was selected as the best of 12 BP algorithms and optimal neuron number was determined as 20. The developed ANN model predicted acetate (R=0.91), sulfate (R=0.95), sulfide (R=0.97), and alkalinity (R=0.94) in the FBR effluent. Hence, the ANN based model can be used to predict the FBR performance, to control the operational conditions for improved process performance.     

Methylmercury and methane production potentials in North Carolina Piedmont stream sediments

Methylated mercury (MeHg) can be produced by all microbes possessing the genes hgcA and hgcB, which can include sulfate-reducing bacteria (SRB), iron-reducing bacteria (FeRB), methane-producing archaea (MPA), and other anaerobic microbes. These microbial groups compete for substrates, including hydrogen and acetate. When sulfate is in excess, SRB can outcompete other anaerobic microbes. However, low concentrations of sulfate, which often occur in stream sediments, are thought to reduce the relative importance of SRB. Although SRB are regarded as the primary contributors of MeHg in many aquatic environments, their significance may not be universal, and stream sediments are poorly studied with respect to microbial Hg methylation. We evaluated suppression of methanogenesis by SRB and the potential contributions from SRB, MPA and other MeHg producing microbes (including FeRB) to the production of MeHg in stream sediments from the North Carolina Piedmont region. Lower methanogenesis rates were observed when SRB were not inhibited, however, application of a sulfate-reduction inhibitor stimulated methanogenesis. Greater MeHg production occurred when SRB were active. Other MeHg producing microbes (i.e., FeRB) contributed significantly less MeHg production than SRB. MPA produced MeHg in negligible amounts. Our results suggest that SRB are responsible for the majority of MeHg production and suppress methanogenesis in mid-order stream sediments, similar to other freshwater sediments. Further investigation is needed to evaluate the generality of these findings to streams in other regions, and to determine the mechanisms regulating sulfate and electron acceptor availability and other potential factors governing Hg methylation and methane production in stream sediments.     

On true and apparent Michaelis constants in enzymology. I. Differences

Differences between both true and apparent rate constants and Michaelis constants have been examined. Rate constants of elementary stages of real mechanisms are true ones. True Michaelis constant Km is expressed by equation Km = (k(-1) + k2)/k. True constants may be determined for reliable mechanism only for which the equation of initial rate was obtained which displays physical sense of these constants and permits to find the method of their calculation. The true constant values are independent of concentration of reactants, activators, inhibitors, extraneous agents and pH. The apparent rate constants are such constants of the composite reaction which are observed when this reaction is described by the equation of simple reaction. Michaelis constant calculated by a half of the ultimate constant is an apparent constant. The apparent constants may be functions of several true rate constants and/or concentrations of reacting substances. The evident physical sense of apparent constants being absent, only formal relation between the reaction rate and reactant concentration independent of the investigated mechanism is provided.     

Toxicity of nitrite toward mesophilic and thermophilic sulphate-reducing, methanogenic and syntrophic populations in anaerobic sludge

The various problems associated with treating sulphate-containing wastewaters stem inherently from successful competitive interactions between sulphate reducing bacteria (SRB) and other bacteria involved in the process, resulting in the formation of H₂S. Prevention of in-reactor sulphide generation by use of specific SRB inhibitors presents a potential solution. Nitrite has been reported to be a specific inhibitor of SRB but its possible toxicity to syntrophic and methanogenic members of the anaerobic consortium has not been investigated. In batch activity and toxicity tests, under both mesophilic and thermophilic conditions, nitrite, at concentrations of up to 150 mg L^–1, was found to be ineffective as a specific inhibitor of SRB, and was also shown to have an inhibitory effect on the activity of syntrophic and methane-producing bacteria in mesophilic and thermophilic digester sludge samples.     

The use of magnesium peroxide for the inhibition of sulfate-reducing bacteria under anoxic conditions

Sulfate-reducing bacteria (SRB), which cause microbiologically influenced material corrosion under anoxic conditions, form one of the major groups of microorganisms responsible for the generation of hydrogen sulfide. In this study, which is aimed at reducing the presence of SRB, a novel alternative approach involving the addition of magnesium peroxide (MgO₂) compounds involving the use of reagent-grade MgO₂ and a commercial product (ORC™) was evaluated as a means of inhibiting SRB in laboratory batch columns. Different concentrations of MgO₂ were added in the columns when black sulfide sediment had appeared in the columns. The experimental results showed that MgO₂ is able to inhibit biogenic sulfide. The number of SRB, the sulfide concentration and the sulfate reducing rate (SRR) were decreased. ORC™ as an additive was able to decrease more effectively the concentration of sulfide in water and the SRB-control effect was maintained over a longer time period when ORC™ was used. The level of oxidation–reduction potential (ORP), which has a linear relationship to the sulfide/sulfate ratio, is a good indicator of SRB activity. As determined by fluorescence in-situ hybridization (FISH), most SRB growth was inhibited under increasing amounts of added MgO₂. The concentration of sulfide reflected the abundance of the SRB. Utilization of organic matter greater than the theoretical SRB utilization rate indicated that facultative heterotrophs became dominant after MgO₂ was added. The results of this study could supply the useful information for further study on evaluating the solution to biocorrosion problems in practical situations.