Terminal Electron-accepting Processes Research Articles

Naphthalene and Benzene Degradation under Fe(III)-Reducing Conditions in Petroleum-Contaminated Aquifers

Naphthalene was oxidized anaerobically to CO2 in sediments collected from a petroleum-contaminated aquifer in Bemidji, Minnesota in which Fe(III) reduction was the terminal electron-accepting process. Naphthalene was not oxidized in sediments from the methanogenic zone at Bemidji or in sediments from the Fe(III)-reducing zone of other petroleum-contaminated aquifers studied. In a profile across the Fe(III)-reducing zone of the Bemidji aquifer, rates of naphthalene oxidation were fastest in sediments with the highest proportion of Fe(III), which was also the zone of the most rapid degradation of benzene, toluene, and acetate. The comparative studies attempted to elucidate factors that might account for the fact that unsubstituted aromatic hydrocarbons such as benzene and naphthalene were degraded under Fe(III)-reducing conditions at Bemidji, but not at the other aquifers examined. These studies indicated that the ability of Fe(III)-reducing microorganisms to degrade benzene and naphthalene at the Bemidji site cannot be attributed to groundwater components that make Fe(III) more available for reduction or other potential factors that were evaluated. However, unlike the other aquifers evaluated, uncontaminated sediments at the Bemidji site could be adapted for anaerobic benzene degradation merely with the addition of benzene. These findings indicate that Bemidji sediments naturally contain Fe(III) reducers capable of degradation of unsubstituted aromatic hydrocarbons.

Effects of dynamic redox zonation on the potential for natural attenuation of trichloroethylene at a fire-training-impacted aquifer

Hydrogeochemical and microbiological methods were used to characterize temporal changes along a transect of an aquifer contaminated by mixed hydrocarbon and solvent wastes from fire training activities at Wurtsmith Air Force Base (Oscoda, MI). Predominant terminal electron accepting processes (TEAPs) as measured by dissolved hydrogen indicated reoxygenation along the transect between October 1995 and October 1996, possibly because of recharge, fluctuations in water table elevation, or microbial activity. Microbiological analyses using universal and archaeal probes revealed a relationship between groundwater hydrogen concentration, TEAP, and predominant bacterial phylogeny. Specifically, a raised water table level and evidence of methanogenesis corresponded to an order of magnitude increase in archaeal 16S rRNA relative to when this zone was unsaturated.Spatial microbial and geochemical dynamics did not result in measurable differences in trichloroethylene (TCE) mineralization potential in vadose, capillary fringe, and saturated zone soils during a 500-day microcosm experiment using unprocessed contaminated soil and groundwater. Aerobic systems indicated that methane, but not toluene, may serve as a cosubstrate for TCE cometabolism. Anaerobic microcosms demonstrated evidence for methanogenesis, CO2 production and hydrogen consumption, yet dechlorination activity was only observed in a microcosm with sulfate-reduction as the dominant TEAP. Mass balance calculations indicated less than 5% mineralization, regardless of redox zone or degree of saturation, at maximum rates of 0.01–0.03 μmol/g soil·d. The general lack of dechlorination activity under laboratory conditions corroborates the limited evidence for natural dechlorination at this site, despite abundant electron donor material and accumulated organic acids from microbial degradation of alkylbenzenes. Thus, the short-term temporal dynamics in redox conditions is unlikely to have measurable effects on the long-term natural remediation potential of the aquifer.

Redox zoning, rates of sulfate reduction and interactions with Fe-reduction and methanogenesis in a shallow sandy aquifer, Rømø, Denmark

Fe-oxide reduction, sulfate reduction and methanogenesis, have been studied in a shallow aquifer with the main focus on sulfate reduction. Direct measurements of sulfate reduction rates have for the first time been applied in an aquifer system. Rates were much lower than reported in other anoxic environments - three orders of magnitude lower than in marine settings and one order of magnitude lower than in lacustrine environments, and varied substantially mainly due to differences in the reactivity of the organic matter. At the extremely low substrate levels in the aquifer, sulfate reduction rates are not primarily limited by low sulfate concentrations. The produced sulfide forms framboidal pyrite via a FeS precursor, with elemental sulfur as an intermediate. Fe-oxide reduction rates were comparable to sulfate reduction rates, but appeared to depend more on Fe-oxide reactivity than organic matter reactivity. Low sulfate concentrations, combined with low-reactivity Fe(III), in the aquifer sediment, has led to an increased appreciation of the existence of concomitant redox processes. This indicates that competitive exclusion is not always effective, and raises questions as to what H 2 data reflect in such a system. Calculations of the in situ energy yield for Fe- and sulfate reduction via H 2 oxidation are ∼2.5 kcal/mol H 2, indicating that thermodynamic equilibrium is approached. The calculated available energy yield for methanogenesis was very low indicating that CH 4 production must occur in micro-environments where higher H 2 concentrations prevail. The system may be described as being in a state of partial equilibrium, where the overall rate of organic matter oxidation is controlled by the rate of fermentation of the organic matter, and terminal electron acceptor processes occur at close to equilibrium conditions . This partial equilibrium depends on other processes in the system, in this case an increase in pH due to calcite dissolution appears to induce a shift from predominantly Fe-reducing to predominantly sulfate reducing conditions by changing the energy available to Fe-oxide reduction. Numerical modeling using the partial equilibrium approach was successful in modeling this complex of interacting processes.

H2 Concentrations in a Landfill Leachate Plume (Grindsted, Denmark): In Situ Energetics of Terminal Electron Acceptor Processes

Empirical H2 concentration ranges are currently related to specific redox processes, assuming steady-state conditions at which only one microbiologically mediated redox process occurs due to competetive exclusion of others. Here the first H2 data from a landfill leachate plume are presented, and an alternative partial equilibrium approach is used. The approach implies that TEAPs (terminal electron-accepting processes) occur at negative ΔGr values, close to thermodynamic equilibrium, and that the fermentative H2 production is overall rate limiting. It eliminates the steady-state prerequisite and may explain the occurrence of concomitant TEAPs. Concentrations of H2 and redox process reactants and products were measured in 52 sampling points, downgradient of the Grindsted Landfill (Denmark), and used to calculate in situ ΔGr values of TEAPs, assuming partial equilibrium. H2 generally ranged from 0.004 to 0.88 nM, with most values around 0.2 nM. Fe reduction was, according to the empirically defined ranges, the most prominent TEAP, but concomitant methanogenesis and sulfate reduction occurred as well. This indicated a need for an alternative approach to explaining the H2 distribution, and the measured H2 concentrations are viewed as being controlled by a partial equilibrium. A derived theoretical relation between H2 concentrations and temperature indicates temperature effects to be more important than currently appreciated. Calculated in situ ΔGr values can, combined with a threshold value, predict which TEAPs can occur via H2 oxidation. For our samples, ΔGr for methanogenesis was always >−7 kJ/mol, and CO2 reduction should only occur in stagnant porewater at higher H2 concentrations or by direct interspecies transfer. In contrast, sulfate and Fe reduction occur close to or slightly below a threshold of −7 kJ/mol H2 and may occur concomitantly at partial equilibrium.

Thermodynamic control on hydrogen concentrations in anoxic sediments

Molecular hydrogen plays a central role in bacterially mediated anoxic sediment chemistry, as both an important electron transfer agent and a key thermodynamic control. We studied the response of hydrogen concentrations to changes in temperature, specific electron acceptor, sulfate concentration, and pH in a series of laboratory experiments using sediments from Cape Lookout Bight, North Carolina. Hydrogen concentrations were found to depend significantly on each of these factors in a fashion that suggests thermodynamic control. In general, the change in hydrogen concentrations was apparently driven by a necessity to maintain a constant ΔG for the predominant terminal electron-accepting process. We hypothesize this situation derives from highly competetive conditions that force terminal metabolic bacteria to operate right at their thermodynamic limits. The response of hydrogen to individual controls in the laboratory experiments was reflected in relatively quantitiative fashion in down-core, seasonal, and inter-environmental variations observed in sediment cores from Cape Lookout Bight and the White Oak River, NC.

Microbial Abundance and Activity in a Low-Conductivity Aquifer System in East-Central Texas

aquifer sands (28‐31 m), even though the total organic carbon content was very low. Rates of anaerobic H2, lactate, and formate consumption were also high in aquifer sands, relative to the other strata. The higher microorganism numbers and activities in the aquifer sediments likely reflect the importance of increased electron donor and acceptor transport in higher hydraulic conductivity sands, relative to other strata. The presence of sulfate and near absence of other electron acceptors (O2 ,N O 3 , and Fe 3+ ), absence of methanogens, high numbers of SRB, and relatively high sulfate reduction activity suggest that sulfate reduction is the dominant terminal electron-accepting process in the deep aquifer. In contrast to aerobic Atlantic Coastal Plain sediments, aquifer sediments at 30 m in this east-central Texas aquifer contain greater numbers of anaerobes than aerobes. Low pH and high sulfate contents argue for oxidation of pyrite in or near the shallow aquifer. This pyrite oxidation supplies sulfate to deeper aquifers. Pyrite from the deep aquifer sediments, with the greatest abundance and activity of SRB, is too enriched in 34 S (up to 58‰) to have formed from modern or typical groundwater sulfate. Overall, our results suggest that the resident microbiota are capable of cycling carbon and sulfur in the aquifer, but probably without substantial remineralization of pyrite in the formation.

Metabolic Adaptation and in Situ Attenuation of Chlorinated Ethenes by Naturally Occurring Microorganisms in a Fractured Dolomite Aquifer near Niagara Falls, New York

A combination of hydrogeological, geochemical, and microbiological methods was used to document the biotransformation of trichloroethene (TCE) to ethene, a completely dechlorinated and environmentally benign compound, by naturally occurring microorganisms within a fractured dolomite aquifer. Analyses of groundwater samples showed that three microbially produced TCE breakdown products (cis-1,2-dichloroethene, vinyl chloride, and ethene) were present in the contaminant plume. Hydrogen (H2) concentra tions in groundwater indicated that iron reduction was the predominant terminal electron-accepting process in the most contaminated geologic zone of the site. Laboratory microcosms prepared with groundwater demonstrated complete sequential dechlorination of TCE to ethene. Microcosm assays also revealed that reductive dechlorination activity was present in waters from the center but not from the periphery of the contaminant plume. This dechlorination activity indicated that naturally occurring microorganisms have adapted to utilize chlorinated ethenes and suggested that dehalorespiring rather than cometabolic, microbial processes were the cause of the dechlorination. The addition of pulverized dolomite to microcosms enhanced the rate of reductive dechlorination, suggesting that hydrocarbons in the dolomite aquifer may serve as electron donors to drive microbially mediated reductive dechlorination reactions. Biodegradation of the chlorinated ethenes appears to contribute significantly to decontamination of the site.

Distribution of terminal electron-accepting processes in an aquifer having multiple contaminant sources

Concentrations of electron acceptors, electron donors, and H 2 in groundwater were measured to determine the distribution of terminal electron-accepting processes (TEAPs) in an alluvial aquifer having multiple contaminant sources. Upgradient contaminant sources included two separate hydrocarbon point sources, one of which contained the fuel oxygenate methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO 3, SO 4 and organic carbon (DOC) to the downgradient part of the aquifer. Groundwater downgradient from the MTBE source had larger concentrations of electron acceptors (dissolved O 2 and SO 4) and smaller concentrations of TEAP end products (dissolved inorganic C, Fe 2+ and CH 4) than groundwater downgradient from the other hydrocarbon source, suggesting that MTBE was not as suitable for supporting TEAPs as the other hydrocarbons. Measurements of dissolved H 2 indicated that SO 4 reduction predominated in the aquifer during a period of high water levels in the aquifer and river. The predominant TEAP shifted to Fe 3+ reduction in upgradient areas after water levels receded but remained SO 4 reducing downgradient near the river. This distribution of TEAPs is the opposite of what is commonly observed in aquifers having a single contaminant point source and probably reflects the input of DOC and SO 4 to the aquifer from the river. Results of this study indicate that the distribution of TEAPs in aquifers having multiple contaminant sources depends on the composition and location of the contaminants and on the availability of electron acceptors.

Lack of Correlation between Organic Acid Concentrations and Predominant Electron-Accepting Processes in a Contaminated Aquifer

Long-term (1992−1995) monitoring data from a petroleum hydrocarbon-contaminated aquifer were used to examine the hypothesis that concentrations of low molecular weight (LMW) aliphatic organic acids reflect terminal electron-accepting processes. During the period of study, concentrations of dissolved hydrogen (H2) indicated that methanogenic, sulfate-reducing, and iron(III)-reducing condi tions predominated at the site. However, there was no correlation between LMW organic acid concentrations and concentrations of dissolved H2. These results indicate that organic acid concentrations are not a reliable indicator of local redox conditions at this site.

Acetogenic bacteria: what are the in situ consequences of their diverse metabolic versatilities?

The four decades of the now classic studies by Harland G. Wood and Lars G. Ljungdahl lead to the resolution of the autotrophic acetyl-CoA 'Wood/Ljungdahl' pathway of acetogenesis. This pathway is the hallmark of acetogens, but is also used by other bacteria, including methanogens and sulfate-reducing bacteria, for both catabolic and anabolic purposes. Thus, the pathway is wide spread in nature and plays an important role in the global turnover of carbon. Because most historical studies with acetogens focused on the biochemistry of the acetyl-CoA pathway, the metabolic diversity and ecology of acetogens remained largely unexplored for many years. Although acetogens were initially conceived to be a somewhat obscure bacteriological group with limited metabolic capabilities, it is now clear that acctogens are arguably the most metabolically diverse group of obligate anaerobes characterized to date. Their anaerobic metabolic arsenal includes the capacity to oxidize diverse substrates, including aromatic, C1, C2, and halogenated compounds, and engage a large number of alternative energy-conserving, terminal electron-accepting processes, including classic fermentations and the dissimilation of inorganic nitrogen. In this regard, one might consider acetogens on a collective basis as the pseudomonads of obligate anaerobes. By virtue of their diverse metabolic talents, acetogens can be found in essentially all habitats. This review evaluates the metabolic versatilities of acetogens relative to both the engagement (regulation) of the acetyl-CoA pathway and the ecological roles likely played by this bacteriogical group.

Anaerobic Hydrocarbon Degradation in Petroleum-Contaminated Harbor Sediments under Sulfate-Reducing and Artificially Imposed Iron-Reducing Conditions

The potential use of iron(III) oxide to stimulate in-situ hydrocarbon degradation in anaerobic petroleum-contaminated harbor sediments was investigated. Previous studies have indicated that Fe(III)-reducing bacteria (FeRB) can oxidize some electron donors more effectively than sulfate-reducing bacteria (SRB). In contrast to previous results in freshwater sediments, the addition of Fe(III) to marine sediments from San Diego Bay, CA did not switch the terminal electron-accepting process (TEAP) from sulfate reduction to Fe(III) reduction. Addition of Fe(III) also did not stimulate anaerobic hydrocarbon oxidation. Exposure of the sediment to air [to reoxidize Fe(II) to Fe(III)] followed by anaerobic incubation of the sediments, resulted in Fe(III) reduction as the TEAP, but contaminant degradation was not stimulated and in some instances was inhibited. The difference in the ability of FeRB to compete with the SRB in the different sediment treatments was related to relative population sizes. Although the addition of Fe(III) did not stimulate hydrocarbon degradation, the results presented here as well as other recent studies demonstrate that there may be significant anaerobic hydrocarbon degradation under sulfate-reducing conditions in harbor sediments.

Stable Carbon Isotope Evidence of Biodegradation Zonation in a Shallow Jet-Fuel Contaminated Aquifer

δ13C values in dissolved inorganic carbon (DIC) ranged from −28 to +11.9‰ in a sandy, noncarbonate shallow aquifer contaminated with jet-fuel petroleum hydrocarbons. This range was observed over a 4-year study in shallow and deep monitoring wells and comprised δ13C values representative of the aerobic and anaerobic microbial biodegradation of 13C-depleted jet fuel (δ13C ∼ −27‰). The δ13C DIC values were found to be influenced by the extent of rainwater infiltration of dissolved oxygen or sulfate or, conversely, by the absence of recharge, lack of dissolved oxygen or sulfate input to the aquifer, and the ensuing methanogenic conditions. After some recharge events delivered dissolved oxygen or sulfate to the shallow part of the aquifer, low to medium DIC δ13C values were measured, and reflected biodegradation of 13C-depleted jet fuel under aerobic (δ13C DIC ∼ −26‰) or sulfate-reducing (δ13C DIC ∼ −18‰) conditions; the deeper part of the aquifer isolated from recharge was methanogenic and had higher δ13C DIC values. Conversely, when rainfall was absent and dissolved oxygen and sulfate concentrations were low in the aquifer, higher DIC δ13C values were measured in both shallow and deep contaminated groundwater (δ13C DIC up to +11.9‰) where H2 concentrations indicated that the predominant terminal electron-accepting process was methanogenesis. The highest δ13C values (+2.6 to +11.9‰) were from contaminated groundwater that contained no dissolved oxygen and little sulfate, CH4 concentrations up to 1985 μmol/L, and acetate concentrations exceeding 12 000 μmol/L. These results suggest that stable carbon isotopes in DIC can be used to indicate the zonation of 13C-depleted hydrocarbon biodegradation processes under the influence of hydrologically controlled electron-acceptor availability.

Use of Dissolved H2 Concentrations To Determine Distribution of Microbially Catalyzed Redox Reactions in Anoxic Groundwater

The potential for using concentrations of dissolved H 2 to determine the distribution of redox processes in anoxic groundwaters was evaluated. In pristine aquifers in which standard geochemical measurements indicated that Fe(III) reduction, sulfate reduction, or methanogenesis was the terminal electron accepting process (TEAP), the H 2 concentrations were similar to the H 2 concentrations that have previously been reported for aquatic sediments with the same TEAPs. In two aquifers containated with petroleum products, it was impossible with standard geochemical analyses to determine which TEAPs predominated in specific locations. However, the TEAPs predicted from measurements of dissolved H 2 were the same as those determined directly through measurements of microbial processes in incubated aquifer material

Temporal and spatial changes of terminal electron‐accepting processes in a petroleum hydrocarbon‐contaminated aquifer and the significance for contaminant biodegradation

Measurements of dissolved hydrogen and other biologically active solutes in groundwater from a shallow petroleum hydrocarbon‐contaminated aquifer indicate that the distribution of microbial terminal electron‐accepting processes (TEAPs), such as methanogenesis, sulfate reduction, and ferric iron (Fe (III)) reduction, is highly dynamic in both time and space. Delivery of sulfate to methanogenic zones by infiltrating recharge or lateral transport can result in a TEAP shift from methanogenesis to sulfate reduction. Conversely, lack of recharge and consumption of available sulfate can result in a shift from sulfate reduction to methanogenesis. Temporal shifts between sulfate and Fe (III) reduction were also observed. Time lags associated with TEAP shifts ranged from less than 10 days to about months. The relation between TEAP and biodegradation rates of a variety of organic compounds indicate that biodegradation rates of petroleum hydrocarbons probably vary temporally and spatially in a contaminated aquifer.

Geochemistry of dissolved inorganic carbon in a Coastal Plain aquifer. 1. Sulfate from confining beds as an oxidant in microbial CO 2 production

A primary source of dissolved inorganic carbon (DIC) in the Black Creek aquifer of South Carolina is carbon dioxide produced by microbially mediated oxidation of sedimentary organic matter. Groundwater chemistry data indicate, however, that the available mass of inorganic electron acceptors (oxygen, Fe(III), and sulfate) and observed methane production is inadequate to account for observed CO 2 production. Although sulfate concentrations are low (approximately 0.05–0.10 mM) in aquifer water throughout the flow system, sulfate concentrations are greater in confining-bed pore water (0.4–20 mM). The distribution of culturable sulfate-reducing bacteria in these sediments suggests that this concentration gradient is maintained by greater sulfate-reducing activity in sands than in clays. Calculations based on Fick's Law indicate that possible rates of sulfate diffusion to aquifer sediments are sufficient to explain observed rates of CO 2 production (about 10 −5 mmoll −1 year −1), thus eliminating the apparent electron-acceptor deficit. Furthermore, concentrations of dissolved hydrogen in aquifer water are in the range characteristic of sulfate reduction (2–6 nM), which provides independent evidence that sulfate reduction is the predominant terminal electron-accepting process in this system. The observed accumulation of pyrite- and calcite-cemented sandstones at sand-clay interfaces is direct physical evidence that these processes have been continuing over the history of these sediments.

Fe(III)-reducing bacteria in deeply buried sediments of the Atlantic Coastal Plain

The possibility that microorganisms are catalyzing the ongoing reduction of Fe(III) in the sediments of deep (20-250 m) aquifers was investigated. Acetate-oxidizing, Fe(III)-reducing bacteria were recovered from deep subsurface sediments, but only from sediments in which it appeared that Fe(III) reduction was the terminal electron-accepting process for oxidation of organic matter. The Fe(III)-reducing microorganisms were capable of reducing ferric oxides present in deep subsurface sediments. Although Fe(III) reduction in subsurface sediments is frequently regarded as an abiological reaction, the enzymatic reduction of Fe(III) by microorganisms reported here is the first mechanism of any kind actually shown to have the potential to couple the oxidation of organic matter to carbon dioxide with the reduction of Fe(III) under the environmental conditions typically found in deep aquifers. We propose that microbially catalyzed Fe(III) reduction is responsible for such late postdepositional phenomena as the formation of variegated red beds and the release of high concentrations of dissolved iron into anaerobic ground waters.

Requirement for a Microbial Consortium To Completely Oxidize Glucose in Fe(III)-Reducing Sediments

In various sediments in which Fe(III) reduction was the terminal electron-accepting process, [C]glucose was fermented to C-fatty acids in a manner similar to that observed in methanogenic sediments. These results are consistent with the hypothesis that, in Fe(III)-reducing sediments, fermentable substrates are oxidized to carbon dioxide by the combined activity of fermentative bacteria and fatty acid-oxidizing, Fe(III)-reducing bacteria.

Hydrogen concentrations as an indicator of the predominant terminal electron-accepting reactions in aquatic sediments

Factors controlling the concentration of dissolved hydrogen gas in anaerobic sedimentary environments were investigated. Results, presented here or previously, demonstrated that, in sediments, only microorganisms catalyze the oxidation of H 2 coupled to the reduction of nitrate, Mn(IV), Fe(III), sulfate, or carbon dioxide. Theoretical considerations suggested that, at steady-state conditions, H 2 concentrations are primarily dependent upon the physiological characteristics of the microorganism(s) consuming the H 2 and that organisms catalyzing H 2 oxidation, with the reduction of a more electrochemically positive electron acceptor, can maintain lower H 2 concentrations than organisms using electron acceptors which yield less energy from H 2 oxidation. The H 2 concentrations associated with the specified predominant terminal electron-accepting reactions in bottom sediments of a variety of surface water environments were: methanogenesis, 7–10 nM; sulfate reduction, 1–1.5 nM; Fe(III) reduction, 0.2 nM; Mn(IV) or nitrate reduction, less than 0.05 nM. Sediments with the same terminal electron acceptor for organic matter oxidation had comparable H 2 concentrations, despite variations in the rate of organic matter decomposition, pH, and salinity. Thus, each terminal electron-accepting reaction had a unique range of steady-state H 2 concentrations associated with it. Preliminary studies in a coastal plain aquifer indicated that H 2 concentrations also vary in response to changes in the predominant terminal electron-accepting process in deep subsurface environments. These studies suggest that H 2 measurements may aid in determining which terminal electron-accepting reactions are taking place in surface and subsurface sedimentary environments.

Competitive Mechanisms for Inhibition of Sulfate Reduction and Methane Production in the Zone of Ferric Iron Reduction in Sediments

Mechanisms for inhibition of sulfate reduction and methane production in the zone of Fe(III) reduction in sediments were investigated. Addition of amorphic iron(III) oxyhydroxide to sediments in which sulfate reduction was the predominant terminal electron-accepting process inhibited sulfate reduction 86 to 100%. The decrease in electron flow to sulfate reduction was accompanied by a corresponding increase in electron flow to Fe(III) reduction. In a similar manner, Fe(III) additions also inhibited methane production in sulfate-depleted sediments. The inhibition of sulfate reduction and methane production was the result of substrate limitation, because the sediments retained the potential for sulfate reduction and methane production in the presence of excess hydrogen and acetate. Sediments in which Fe(III) reduction was the predominant terminal electron-accepting process had much lower concentrations of hydrogen and acetate than sediments in which sulfate reduction or methane production was the predominant terminal process. The low concentrations of hydrogen and acetate in the Fe(III)-reducing sediments were the result of metabolism by Fe(III)-reducing organisms of hydrogen and acetate at concentrations lower than sulfate reducers or methanogens could metabolize them. The results indicate that when Fe(III) is in a form that Fe(III)-reducing organisms can readily reduce, Fe(III)-reducing organisms can inhibit sulfate reduction and methane production by outcompeting sulfate reducers and methanogens for electron donors.