Inhibition Of Sulfate Reduction Research Articles

Erratum for Barnes et al., "Inhibition of Sulfate Reduction and Cell Division by Desulfovibrio desulfuricans Coated in Palladium Metal".

Erratum for Barnes et al., "Inhibition of Sulfate Reduction and Cell Division by Desulfovibrio desulfuricans Coated in Palladium Metal".

Sulfate‐ and iron‐dependent anaerobic methane oxidation occurring side‐by‐side in freshwater lake sediment

AbstractAnaerobic oxidation of methane (AOM) is a potentially important methane sink in lake sediments, but the biogeochemistry and microbial ecology of this process are understudied. Potential electron acceptors for AOM include Fe(III) and sulfate; however, it is not clear to which extent low sulfate concentrations constrain the coupling of AOM to sulfate reduction, nor if Fe(III) reduction drives AOM directly or via a cryptic sulfur cycle. We investigated AOM pathways in the sediment of iron‐rich Danish Lake Ørn through anoxic sediment slurry incubations with additions of 13C‐labeled methane as a substrate, sulfate and Fe(III) as potential electron acceptors, and molybdate as an inhibitor of sulfate reduction. The experiments demonstrated the co‐occurrence of sulfate‐ and iron‐dependent modes of AOM, with the former supported by recycling of sulfate coupled to iron reduction. Quantitative PCR analysis demonstrated the abundance of archaea of the ANME‐2d clade (Ca. Methanoperedenaceae) as likely drivers of AOM. Our study demonstrates that sulfate‐dependent AOM can consume methane at sulfate concentrations typical for freshwater systems and emphasizes the importance of sulfur and iron cycling in the regulation of methane emission from freshwater sediments.

Assessing the Effect of Humic Substances and Fe(III) as Potential Electron Acceptors for Anaerobic Methane Oxidation in a Marine Anoxic System.

Marine anaerobic methane oxidation (AOM) is generally assumed to be coupled to sulfate reduction, via a consortium of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). ANME-1 are, however, often found as single cells, or only loosely aggregated with SRB, suggesting they perform a form of AOM independent of sulfate reduction. Oxidized metals and humic substances have been suggested as potential electron acceptors for ANME, but up to now, AOM linked to reduction of these compounds has only been shown for the ANME-2 and ANME-3 clades. Here, the effect of the electron acceptors anthraquinone-disulfonate (AQDS), a humic acids analog, and Fe3+ on anaerobic methane oxidation were assessed by incubation experiments with anoxic Black Sea water containing ANME-1b. Incubation experiments with 13C-methane and AQDS showed a stimulating effect of AQDS on methane oxidation. Fe3+ enhanced the ANME-1b abundance but did not substantially increase methane oxidation. Sodium molybdate, which was added as an inhibitor of sulfate reduction, surprisingly enhanced methane oxidation, possibly related to the dominant abundance of Sulfurospirillum in those incubations. The presented data suggest the potential involvement of ANME-1b in AQDS-enhanced anaerobic methane oxidation, possibly via electron shuttling to AQDS or via interaction with other members of the microbial community.

The Effect of Bacterial Sulfate Reduction Inhibition on the Production and Stable Isotopic Composition of Methane in Hypersaline Environments

The aim of this research was to investigate the competition between methanogens and sulfate-reducing bacteria in hypersaline environments. Samples of photosynthetic microbial mats, both soft mats (salinities of 55–126 ppt) and gypsum-hosted endoevaporite mats (salinities of 77–320 ppt), were obtained from hypersaline environments in California, USA, Mexico and Chile. Methane production was determined from the increase in headspace methane concentration within incubation vials containing mat samples. At the end of the incubation period, the δ13C values of produced methane were measured. Soft microbial mat vials containing molybdate, a specific inhibitor of bacterial sulfate reduction, exhibited dramatically higher methane production rates and higher (enriched in 13C) methane δ13C values than the controls. This suggests that the inhibition of sulfate reduction allowed the methanogens at these sites to use the competitive substrates (H2 and/or acetate) made available. Further, the higher δ13C values of the produced methane suggest that substrates (both competitive and non-competitive) were used to near completion. At the endoevaporite sites, which have much higher salinities than the soft mat sites, methane production was not significantly different and the methane δ13C values either remained the same or decreased (depleted in 13C) with added molybdate. We suggest that substrate availability increased enough to allow for somewhat greater isotopic fractionation resulting in the lower methane δ13C values that were observed, but not enough to significantly increase measured production rates. Where no changes in either methane production rates or δ13C values occurred, we hypothesize that salinity itself was inhibiting sulfate reduction and thus controlling microbe populations and rates of metabolism.

H2 Kinetic Isotope Fractionation Superimposed by Equilibrium Isotope Fractionation During Hydrogenase Activity of D. vulgaris Strain Miyazaki.

We determined 2H stable isotope fractionation at natural abundances associated with hydrogenase activity by whole cells of Desulfovibrio vulgaris strain Miyazaki F expressing a NiFe(Se) hydrogenase. Inhibition of sulfate reduction by molybdate inhibited the overall oxidation of hydrogen but still facilitated an equilibrium isotope exchange reaction with water. The theoretical equilibrium isotope exchange δ2H-values of the chemical exchange reaction were identical to the hydrogenase reaction, as confirmed using three isotopically different waters with δ2H-values of – 62, +461, and + 1533‰. Expected kinetic isotope fractionation of hydrogen oxidation by non-inhibited cells was also superimposed by an equilibrium isotope exchange. The isotope effects were solely catalyzed biotically as hydrogen isotope signatures did not change in control experiments without cells of D. vulgaris Miyazaki.

Effects of pressure, methane concentration, sulfate reduction activity, and temperature on methane production in surface sediments of the Gulf of Mexico

AbstractMicrobial methanogenesis is a known source of methane in marine environment, but the factors affecting the production of methane from different pathways remain largely unconstrained. We investigated the effects of pressure, methane concentration, temperature, and sulfate reduction activity on the conversion of methanogenic substrates to methane using radiotracers in nearshore and offshore surface sediments in the northern Gulf of Mexico. The transformation of bicarbonate to methane increased with methane concentrations under quasi in situ conditions of deep‐sea sediment, potentially reflecting, to some degree, enhancement of the enzymatic back reaction under high‐methane conditions. In coastal sediments, a positive effect of both increased methane concentration and inhibition of sulfate reduction by molybdate on 14CH4 accumulation suggests that anaerobic oxidation of methane and methanogenesis occur concurrently. In contrast, methane production from methanol was suppressed by increased methane concentration likely due to the reduced energy yield of methanogenesis at elevated methane concentrations. Temperature could limit hydrogenotrophic and acetoclastic methanogenesis under in situ conditions, as higher rates were observed at 40°C in nearshore sediments. Inhibitor experiments with 2‐bromoethanesulfonate and molybdate indicated that sulfate‐reducing bacteria preferentially utilized acetate, as well as H2. Methanol was a noncompetitive substrate for methanogens in the deep‐sea sediments but was competitively cycled in coastal sediments. An authentic noncompetitive substrate, methylamine, was channeled predominantly to methane production under all conditions tested. The different response of methanogenic activity to substrate availability, metabolic competition, temperature, and pressure between sites suggest variable environmental controls on the methane production in coastal vs. deep‐sea surface sediments.

“Green” ZnO-Interlinked Chitosan Nanoparticles for the Efficient Inhibition of Sulfate-Reducing Bacteria in Inject Seawater

Antimicrobial agents and corrosion inhibitors are widely used as biocides in the oil and gas industry to disinfect water and inhibit excessive biofilm formation caused mainly by sulfate reducing bacteria (SRBs). However, traditional biocides may induce bacterial resistance and/or be detrimental to environment by forming harmful disinfection byproducts. In this first systematic study, we synthesized a “green” and highly stable biocide formulations composed of ZnO-interlinked chitosan (Ch) nanoparticles (CZNCs) and evaluated their antimicrobial activity against mixed SRBs culture isolated from real oil field sludge. SEM, TEM, X-ray diffraction (XRD) and FTIR suggested the formation of stable nanocomposites with strong interaction between ZnO and Ch nanoparticles. Synthesized nanocomposites showed highly stable behaviors in the high salt concentrations of inject seawater. The inhibition of SRBs activity was concentration-dependent and more than 73% and 43% inhibition of sulfate reduction and total organic ca...

CORRELATION BETWEEN BIOMASS CONCENTRATION AND EXTENT OF SULPHATE REDUCTION IN AN ACIDOGENIC REACTOR

Two stage high rate anaerobic treatment systems comprising of an acidogenic reactor (or equalisation/ buffer tank) followed by a methanogenic reactor are becoming increasingly popular to treat high strength wastewater from industries. In these systems, sulphate present in the wastewater is reduced to sulphide either partially or completely in the acidogenic reactor and completely in the methanogenic reactor. The effect of fermentation products on the extent of sulphate reduction in the acidogenic reactor was investigated in a continuously-fed, well mixed laboratory-scale 3 L fermenter operating at a temperature of 35oC and pH of 6. The feed was based on either glucose or molasses as the carbon source. It was observed that as the carbohydrate concentration in feed was increased sulphate reduction was suppressed. It was confirmed that volatile organic acids like acetic, propionic and butyric acids, hydrogen and residual glucose did not cause inhibition of sulphate reduction. However, biomass concentration correlated negatively with extent of sulphate reduction. This correlation was expressed mathematically and the same expression with the same parameters adequately predicted the effect of biomass concentration on extent of sulphate removal for both steady state and transient data irrespective of glucose or molasses feed. It was seen from the best fit of this expression that a biomass concentration of 3300 mg-COD.L-1 would completely repress sulphate reduction in the acidogenic reactor. Even when sulphate removal was suppressed the presence of sulphate reducing bacteria (SRB) in the reactor was confirmed through Fluorescent In Situ Hybridisation (FISH) visualisation. Moreover, the numbers of SRB seemed to increase with carbohydrate concentration in feed. It was speculated the suppression of sulphate removal might be due to a switch in function of SRB from sulphate reducing to fermentation or acetogenesis. Keywords: sulphate reducing bacteria, anaerobic digestion, inhibition, sulphate reduction, acidogenic reactor, high rate anaerobic treatment

Nitrogen Fixation in Thermophilic Chemosynthetic Microbial Communities Depending on Hydrogen, Sulfate, and Carbon Dioxide.

The activity of nitrogen fixation measured by acetylene reduction was examined in chemosynthetic microbial mats at 72–75°C in slightly-alkaline sulfidic hot springs in Nakabusa, Japan. Nitrogenase activity markedly varied from sampling to sampling. Nitrogenase activity did not correlate with methane production, but was detected in samples showing methane production levels less than the maximum amount, indicating a possible redox dependency of nitrogenase activity. Nitrogenase activity was not affected by 2-bromo-ethane sulfonate, an inhibitor of methanogenesis. However, it was inhibited by the addition of molybdate, an inhibitor of sulfate reduction and sulfur disproportionation, suggesting the involvement of sulfate-reducing or sulfur-disproportionating organisms. Nitrogenase activity was affected by different O2 concentrations in the gas phase, again supporting the hypothesis of a redox potential dependency, and was decreased by the dispersion of mats with a homogenizer. The loss of activity that occurred from dispersion was partially recovered by the addition of H2, sulfate, and carbon dioxide. These results suggested that the observed activity of nitrogen fixation was related to chemoautotrophic sulfate reducers, and fixation may be active in a limited range of ambient redox potential. Since thermophilic chemosynthetic communities may resemble ancient microbial communities before the appearance of photosynthesis, the present results may be useful when considering the ancient nitrogen cycle on earth.

Effects of phenol on sulfate reduction by mixed microbial culture: kinetics and bio-kinetics analysis.

Mixed microbial culture collected from the wastewater treatment plant of Indian Institute of Technology Guwahati (IITG) was further grown in anaerobic condition in presence of sulfate where lactate was added as a carbon source. Sulfate addition was increased stepwise up to 1,000 mg l-1 before phenol was added at increasing concentrations from 10 mg l-1 to 300 mg l-1. Kinetics of sulfate, phenol and chemical oxygen demand reduction were studied and experimental findings were analyzed using various bio-models to estimate the bio-kinetic coefficients. This is the first detailed report on kinetics and bio-kinetic studies of sulfate reduction in presence of phenol. Experimental results showed that there was no inhibition of sulfate reduction and microbial growth up to 100 mg l-1 phenol addition. However, inhibition to different degrees was observed at higher phenol addition. The experimental data of microbial growth and substrate consumption in presence of phenol fitted well to the Edward model (R2 = 0.85, root mean square error = 0.001011) with maximum specific growth rate = 0.052 h-1, substrate inhibition constant = 88.05 mg l-1 and half saturation constant = 58.22 mg l-1. The characteristics of the cultured microbes were determined through a series of analysis and microbial tests.

Arsenic Methylation Dynamics in a Rice Paddy Soil Anaerobic Enrichment Culture.

Methylated arsenic (As) species represent a significant fraction of the As accumulating in rice grains, and there are geographic patterns in the abundance of methylated arsenic in rice that are not understood. The microorganisms driving As biomethylation in paddy environments, and thus the soil conditions conducive to the accumulation of methylated arsenic, are unknown. We tested the hypothesis that sulfate-reducing bacteria (SRB) are key drivers of arsenic methylation in metabolically versatile mixed anaerobic enrichments from a Mekong Delta paddy soil. We used molybdate and monofluorophosphate as inhibitors of sulfate reduction to evaluate the contribution of SRB to arsenic biomethylation, and developed degenerate primers for the amplification of arsM genes to identify methylating organisms. Enrichment cultures converted 63% of arsenite into methylated products, with dimethylarsinic acid as the major product. While molybdate inhibited As biomethylation, this effect was unrelated to its inhibition of sulfate reduction and instead inhibited the methylation pathway. Based on arsM sequences and the physiological response of cultures to media conditions, we propose that amino acid fermenting organisms are potential drivers of As methylation in the enrichments. The lack of a demethylating capacity may have contributed to the robust methylation efficiencies in this mixed culture.

Bioassessment of heavy metal toxicity and enhancement of heavy metal removal by sulfate-reducing bacteria in the presence of zero valent iron

A simple and valid toxicity evaluation of Zn2+, Mn2+ and Cr6+ on sulfate-reducing bacteria (SRB) and heavy metal removal were investigated using the SRB system and SRB+Fe0 system. The heavy metal toxicity coefficient (β) and the heavy metal concentration resulting in 50% inhibition of sulfate reduction (I) from a modeling process were proposed to evaluate the heavy metal toxicity and nonlinear regression was applied to search for evaluation indices β and I. The heavy metal toxicity order was Cr6+ > Mn2+ > Zn2+. Compared with the SRB system, the SRB+Fe0 system exhibited a better capability for sulfate reduction and heavy metal removal. The heavy metal removal was above 99% in the SRB+Fe0 system, except for Mn2+. The energy-dispersive spectroscopy (EDS) analysis showed that the precipitates were removed primarily as sulfide for Zn2+ and hydroxide for Mn2+ and Cr6+.The method of evaluating the heavy metal toxicity on SRB was of great significance to understand the fundamentals of the heavy metal toxicity and inhibition effects on the microorganism and regulate the process of microbial sulfate reduction.

Emission and oxidation of methane in a meromictic, eutrophic and temperate lake (Dendre, Belgium)

We sampled the water column of the Dendre stone pit lake (Belgium) in spring, summer, autumn and winter. Depth profiles of several physico-chemical variables, nutrients, dissolved gases (CO2, CH4, N2O), sulfate, sulfide, iron and manganese concentrations and δ13C-CH4 were determined. We performed incubation experiments to quantify CH4 oxidation rates, with a focus on anaerobic CH4 oxidation (AOM), without and with an inhibitor of sulfate reduction (molybdate). The evolution of nitrate and sulfate concentrations during the incubations was monitored. The water column was anoxic below 20 m throughout the year, and was thermally stratified in summer and autumn. High partial pressure of CO2 and CH4 and high concentrations of ammonium and phosphate were observed in anoxic waters. Important nitrous oxide and nitrate concentration maxima were also observed (up to 440 nmol L−1 and 80 μmol L−1, respectively). Vertical profiles of δ13C-CH4 unambiguously showed the occurrence of AOM. Important AOM rates (up to 14 μmol L−1 d−1) were observed and often co-occurred with nitrate consumption peaks, suggesting the occurrence of AOM coupled with nitrate reduction. AOM coupled with sulfate reduction also occurred, since AOM rates tended to be lower when molybdate was added. CH4 oxidation was mostly aerobic (∼80% of total oxidation) in spring and winter, and almost exclusively anaerobic in summer and autumn. Despite important CH4 oxidation rates, the estimated CH4 fluxes from the water surface to the atmosphere were high (mean of 732 μmol m−2 d−1 in spring, summer and autumn, and up to 12,482 μmol m−2 d−1 in winter).

Electron-acceptor utilization and methanogenesis in brackish aquaculture sludge

Anaerobic digestion (AD) of sulfate-rich brackish aquaculture sludge (BAS) is an effective method to reduce negative environmental impact and produce biogas. However, the mechanisms of electron-acceptor utilization during AD of BAS, and the effects of elevated sulfate, nitrate and nitrite levels on biogas production, have never been investigated. The current study is the first to demonstrate the mechanisms of electron-acceptor utilization in BAS under methanogenic conditions. Nitrate, nitrite and iron were almost completely depleted during the first day of AD, and methane production began only when these electron acceptors were removed. In contrast, sulfate reduction also started immediately, but took 15–28d, dependent on the treatment, and largely coincided with methane production. The volume of produced biogas, the lag phase for methane production and methane content in the biogas were affected by the inhibition of sulfate reduction and by the increase in nitrate and nitrite concentrations. However, the average methane-production rates in all treatments were similar. Our data suggest that methanogenesis in BAS under naturally high sulfate levels results in the highest possible methane-producing capability of the system; no additional manipulations are required to further increase methane production, suggesting the robustness of AD of BAS under the currently reported conditions.

Tu1871 Excretion of Breath Hydrogen Is Reduced by Inhibition of Sulfate Reduction by Molybdate in Rats

G A A b st ra ct s increased in the DRG during colitis contains CREB binding sequence in its promoter thus is responsive to CREB activity. AIMS: 1) To examine the phosphorylation of CREB and expression of CGRP by BDNF in vivo and in vitro; and 2) To examine whether the PLC/ calcium pathway is involved in BDNF-induced CREB phosphorylation.METHODS: Colonic inflammation was induced in rats by intracolonic instillation of trinitrobenzene sulfonic acid (TNBS: 1.5 mL/kg of 60 mg/mL solution in 50% EtOH). Control animals received 50% EtOH. Rats were killed on day 3 or 7 following the induction of colitis. CREB phosphorylation was examined by western blot and immunohistochemistry with a specific antibody recognizing CREB phospho-serine 133. The pair-matched DRG explants culture were incubated with BDNF (10 ng/mL) for various time points to examine the effects of BDNF signaling on CREB phosphorylation. Endogenous BDNF was blocked by BDNF neutralizing antibody (36 μg/ kg, i.v.). CGRP expression was examined by real-time PCR. RESULTS: Colitis increased CREB phosphorylation level in the L1 DRG at 3 days and 7 days post TNBS treatment examined by both western blot and immunohistochemistry. CREB phosphorylation in the DRG was expressed in small-to-medium sized sensory neurons. Anti-BDNF treatment of the colitic animals reduced colitis-induced CREB phosphorylation in the L1 DRG. Incubation of DRG explants with BDNF also increased CREB phosphorylation level which was blocked by inhibition of the phospholipase C (PLC) pathway with U-73122. Blockade of endogenous BDNF with BDNF neutralizing antibody also reversed colitis-induced up-regulation of CGRP transcripts in the L1 DRG. CONCLUSION: BDNF regulates CREB activity and CGRP expression through the PLC/calcium pathway in the DRG during colitis. The BDNF-PLC/ calcium-CREB-CGRP axis is an important component in visceral hypersensitivity during colitis.

Identification of Desulfobacterales as primary hydrogenotrophs in a complex microbial mat community.

Hypersaline microbial mats have been shown to produce significant quantities of H2 under dark, anoxic conditions via cyanobacterial fermentation. This flux of a widely accessible microbial substrate has potential to significantly influence the ecology of the mat, and any consumption will affect the net efflux of H2 that might otherwise be captured as a resource. Here, we focus on H2 consumption in a microbial mat from Elkhorn Slough, California, USA, for which H2 production has been previously characterized. Active biologic H2 consumption in this mat is indicated by a significant time-dependent decrease in added H2 compared with a killed control. Inhibition of sulfate reduction, as indicated by a decrease in hydrogen sulfide production relative to controls, resulted in a significant increase in H2 efflux, suggesting that sulfate-reducing bacteria (SRB) are important hydrogenotrophs. Low methane efflux under these same conditions indicated that methanogens are likely not important hydrogenotrophs. Analyses of genes and transcripts that encode for rRNA or dissimilatory sulfite reductase, using both PCR-dependent and PCR-independent metatranscriptomic sequencing methods, demonstrated that Desulfobacterales are the dominant, active SRB in the upper, H2-producing layer of the mat (0-2 mm). This hypothesis was further supported by the identification of transcripts encoding hydrogenases derived from Desulfobacterales capable of H2 oxidation. Analysis of molecular data provided no evidence for the activity of hydrogenotrophic methanogens. The combined biogeochemical and molecular data strongly indicate that SRB belonging to the Desulfobacterales are the quantitatively important hydrogenotrophs in the Elkhorn Slough mat.

Enrichment and isolation of acidophilic sulfate‐reducing bacteria from Tinto River sediments

Although some acidophilic and alkaliphilic species have been described recently, most of the known sulfate-reducing bacteria (SRB) grow optimally at neutral pH. In this study, sulfate reduction was studied with sediment samples from the extremely acidic Tinto River basin. Stable enrichments of SRB were obtained at pH 4 with glycerol, methanol and hydrogen; at pH 4.5 with lactate and at pH 5.5 with succinate as substrates. Inhibition of sulfate reduction by organic acids below their pKa was observed. Cloning and sequencing of 16S rRNA gene showed that fermentative bacteria (Paludibacter spp., Oscillibacter spp.) and SRB (Thermodesulfobium spp., Desulfosporosinus spp., Desulfitobacterium spp., Desulfotomaculum spp.) were co-enriched. By repeated serial dilutions and streaking on agar plates, four strains of SRB belonging to the Firmicutes phylum were obtained. Two of them show 96% 16S rRNA gene sequence similarity with Desulfosporosinus acidophilus, and a third one with Desulfosporosinus orientis. Another isolate has just 93% rRNA gene sequence similarity with the Desulfosporosinus/Desulfitobacterium cluster and might represent a novel species within a novel genus. One of the Desulfosporosinus strains was further investigated showing maximum growth at pH 5.5, and a pH-dependent inhibitory effect of organic acids and sulfide.

Incidence of invasive macrophytes on methylmercury budget in temperate lakes: Central role of bacterial periphytic communities

Several studies demonstrated high mercury (Hg) methylation and demethylation in the periphyton associated with floating roots in tropical ecosystems. The importance of aquatic plants on methylmercury production in three temperate ecosystems from south-western France was evaluated through Hg species concentrations, and Hg methylation/demethylation activities by using stable isotopic tracers (199Hg(II), Me201Hg). Hg accumulation and high methylation and demethylation yields were detected in plant roots and periphyton, whereas results for sediment and water were low to insignificant. The presence of sulfate reducing prokaryotes was detected in all compartments (T-RFLP based on dsrAB amplified through nested PCR) and their main role in Hg methylation could be demonstrated. In turn, sulfate reduction inhibition did not affect demethylation activities. The estimation of net MeHg budgets in these ecosystems suggested that aquatic rhizosphere is the principal location for methylmercury production and may represent an important source for the contamination of the aquatic food chain.

Inhibition of sulfate reduction by iron, cadmium and sulfide in granular sludge

This study investigated the inhibition effect of iron, cadmium and sulfide on the substrate utilization rate of sulfate reducing granular sludge. A series of batch experiments in a UASB reactor were conducted with different concentrations of iron (Fe 2+, 4.0–8.5 mM), cadmium (Cd 2+, 0.53–3.0 mM) and sulfide (4.2–10.6 mM), the reactor was fed with ethanol at 1 g chemical oxygen demand (COD)/L and sulfate to yield a COD/SO 4 2− (g/g) ratio of 0.5. The addition of iron, up to a concentration of 8.1 mM, had a positive effect on the substrate utilization rate which increased 40% compared to the rate obtained without metal addition (0.25 g COD/g VSS-d). Nonetheless, iron concentration of 8.5 mM inhibited the specific substrate utilization rate by 57% compared to the substrate utilization rate obtained in the batch amended with 4.0 mM Fe 2+ (0.44 g COD/g VSS-d). Cadmium had a negative effect on the specific substrate utilization rate at the concentrations tested; at 3.0 mM Cd 2+ the substrate utilization rate was inhibited by 44% compared with the substrate utilization rate without metal addition. Cadmium precipitation with sulfide did not decrease the inhibition of cadmium on sulfate reduction. These results could have important practical implications mainly when considering the application of the sulfate reducing process to treat effluents with high concentrations of sulfate and dissolved metals such as iron and cadmium.

Nitrogen Fixation in Microbial Mat and Stromatolite Communities from Cuatro Cienegas, Mexico

Nitrogen fixation (nitrogenase activity, NA) of a microbial mat and a living stromatolite from Cuatro Cienegas, Mexico, was examined over spring, summer, and winter of 2004. The goal of the study was to characterize the diazotrophic community through molecular analysis of the nifH gene and using inhibitors of sulfate reduction and oxygenic and anoxygenic photosynthesis. We also evaluated the role of ultraviolet radiation on the diazotrophic activity of the microbial communities. Both microbial communities showed patterns of NA with maximum rates during the day that decreased significantly with 3-3,4-dichlorophenyl-1',1'-dimethylurea, suggesting the potential importance of heterocystous cyanobacteria. There is also evidence of NA by sulfur-reducing bacteria in both microbial communities suggested by the negative effect exerted by the addition of sodium molybdate. Elimination of infrared and ultraviolet radiation had no effect on NA. Both microbial communities had nifH sequences that related to group I, including cyanobacteria and purple sulfur and nonsulfur bacteria, as well as group II nitrogenases, including sulfur reducing and green sulfur bacteria.