Energetic constraints on H2-dependent terminal electron accepting processes in anoxic environments: a review of observations and model approaches

Inside the microbial black box: a redox-centric framework for deciphering microbial metabolism

Microbial metabolism influences the global climate and human health and is governed by the balance between NADH and NAD+ through redox reactions. Historically, oxidative (i.e., catabolism) and reductive (i.e., fermentation) pathways have been studied in isolation, obscuring the complete metabolic picture. However, new omics technologies and biotechnological tools now allow an integrated system-level understanding of the drivers of microbial metabolism through observation and manipulation of redox reactions. Here we present perspectives on the importance of viewing microbial metabolism as the dynamic interplay between oxidative and reductive processes and apply this framework to diverse microbial systems. Additionally, we highlight novel biotechnologies to monitor and manipulate microbial redox status to control metabolism in unprecedented ways. This redox-focused systems biology framework enables a more mechanistic understanding of microbial metabolism.

Microbiological hydrogen (H2 ) thresholds in anaerobic continuous-flow systems: Effects of system characteristics

Hydrogen (H₂ ) concentrations that were associated with microbiological respiratory processes (RPs) such as sulfate reduction and methanogenesis were quantified in continuous-flow systems (CFSs) (e.g., bioreactors, sediments). Gibbs free energy yield (ΔǴ ~ 0) of the relevant RP has been proposed to control the observed H₂ concentrations, but most of the reported values do not align with the proposed energetic trends. Alternatively, we postulate that system characteristics of each experimental design influence all system components including H₂ concentrations. To analyze this proposal, a Monod-based mathematical model was developed and used to design a gas-liquid bioreactor for hydrogenotrophic methanogenesis with Methanobacterium bryantii M.o.H. Gas-to-liquid H₂ mass transfer, microbiological H₂ consumption, biomass growth, methane formation, and Gibbs free energy yields were evaluated systematically. Combining model predictions and experimental results revealed that an initially large biomass concentration created transients during which biomass consumed [H₂ ]_L rapidly to the thermodynamic H₂ -threshold (≤1 nM) that triggerred the microorganisms to stop H₂ oxidation. With no H₂ oxidation, continuous gas-to-liquid H₂ transfer increased [H₂ ]_L to a level that signaled the methanogens to resume H₂ oxidation. Thus, an oscillatory H₂ -concentration profile developed between the thermodynamic H₂ -threshold (≤1 nM) and a low [H₂ ]_L (~10 nM) that relied on the rate of gas-to-liquid H₂ -transfer. The transient [H₂ ]_L values were too low to support biomass synthesis that could balance biomass losses through endogenous oxidation and advection; thus, biomass declined continuously and disappeared. A stable [H₂ ]_L (1807 nM) emerged as a result of abiotic H₂ -balance between gas-to-liquid H₂ transfer and H₂ removal via advection of liquid-phase.

Spatiotemporal Variability of Groundwater Iodine in the Northern Arid Basins: Significance for Safe Water Supply

The genesis of geogenic iodine (I)-contaminated groundwater poses a significant threat to long-term water exploitation. Safe and sustainable water supply, particularly in the northern arid basins, demands a quantitative prediction of the high variability of I distribution over hydrogeological timescales. Here, bioenergetics-informed reactive transport modeling was combined with high-resolution molecular characterization of fueling organic matter to decipher the time-controlled interactions between vertical flow and (bio)geochemical processes in I transport within the Datong aquifers. The declining reactivities of I-bearing organic matter and Fe oxides in the 15-40 m depth decreased the rate of I release, while a growing number of pore volumes flushed through the aquifers to leach out I^- and organic I. This removal effect is compensated by the desorption of I^- from Fe oxides and secondary FeS generated from the concurrent reduction of Fe oxides and SO₄^2-. Consequently, peak concentrations of groundwater I^- may have appeared, depending upon the vertical recharge rate, at the first several pore volumes flushed through the aquifers. The current vertical distributions of the various I species likely represent a quasi-steady state between I mobilization and leaching. These new mechanistic insights into the dynamic hydrogeological-(bio)geochemical processes support secure groundwater use in the I-affected northern arid basins.

Solar-driven methanogenesis with ultrahigh selectivity by turning down H2 production at biotic-abiotic interface

Integration of methanogens with semiconductors is an effective approach to sustainable solar-driven methanogenesis. However, the H₂ production rate by semiconductors largely exceeds that of methanogen metabolism, resulting in abundant H₂ as side product. Here, we report that binary metallic active sites (namely, NiCu alloys) are incorporated into the interface between CdS semiconductors and Methanosarcina barkeri. The self-assembled Methanosarcina barkeri-NiCu@CdS exhibits nearly 100% CH₄ selectivity with a quantum yield of 12.41 ± 0.16% under light illumination, which not only exceeds the reported biotic-abiotic hybrid systems but also is superior to most photocatalytic systems. Further investigation reveal that the Ni-Cu-Cu hollow sites in NiCu alloys can directly supply hydrogen atoms and electrons through photocatalysis to the Methanosarcina barkeri for methanogenesis via both extracellular and intracellular hydrogen cycles, effectively turning down the H₂ production. This work provides important insights into the biotic-abiotic hybrid interface, and offers an avenue for engineering the methanogenesis process.

Bacterially mediated release and mobilization of As/Fe coupled to nitrate reduction in a sediment environment

Metal-reducing bacteria play an important role in the release and mobilization of arsenic from sediments into groundwater. This study aimed to investigate the influence of nitrate on arsenic bio-release. Microcosm experiments consisting of high arsenic sediments and indigenous bacterium Bacillus sp. D2201 were conducted and the effects of nitrate on the mobilization of As/Fe determined. The results show arsenic release is triggered by iron reduction, which is regulated by nitrate. Increasing the nitrate concentration from 0 to 1 and 3 mM decreased Fe(III) reduction by 62.5% and 16.9% and decreased As(V) bio-release by 41.5% and 85.5%, respectively. Moreover, the results of step-wise Wenzel sequential extractions indicate nitrate addition prevents the transformation of poorly crystalline iron oxides to well crystalline iron oxides. Overall, nitrate appears to have a dual effect, inhibiting both iron reduction and arsenic release by incubation strain D2201. This study offers new insights regarding the biogeochemistry of arsenic in groundwater systems.

Organohalide-Respiring Bacteria at the Heart of Anaerobic Metabolism in Arctic Wet Tundra Soils

Recent work revealed an active biological chlorine cycle in coastal Arctic tundra of northern Alaska. This raised the question of whether chlorine cycling was restricted to coastal areas or if these processes extended to inland tundra. The anaerobic process of organohalide respiration, carried out by specialized bacteria like Dehalococcoides, consumes hydrogen gas and acetate using halogenated organic compounds as terminal electron acceptors, potentially competing with methanogens that produce the greenhouse gas methane. We measured microbial community composition and soil chemistry along an ∼262-km coastal-inland transect to test for the potential of organohalide respiration across the Arctic Coastal Plain and studied the microbial community associated with Dehalococcoides to explore the ecology of this group and its potential to impact C cycling in the Arctic. Concentrations of brominated organic compounds declined sharply with distance from the coast, but the decrease in organic chlorine pools was more subtle. The relative abundances of Dehalococcoides were similar across the transect, except for being lower at the most inland site. Dehalococcoides correlated with other strictly anaerobic genera, plus some facultative ones, that had the genetic potential to provide essential resources (hydrogen, acetate, corrinoids, or organic chlorine). This community included iron reducers, sulfate reducers, syntrophic bacteria, acetogens, and methanogens, some of which might also compete with Dehalococcoides for hydrogen and acetate. Throughout the Arctic Coastal Plain, Dehalococcoides is associated with the dominant anaerobes that control fluxes of hydrogen, acetate, methane, and carbon dioxide. Depending on seasonal electron acceptor availability, organohalide-respiring bacteria could impact carbon cycling in Arctic wet tundra soils.IMPORTANCE Once considered relevant only in contaminated sites, it is now recognized that biological chlorine cycling is widespread in natural environments. However, linkages between chlorine cycling and other ecosystem processes are not well established. Species in the genus Dehalococcoides are highly specialized, using hydrogen, acetate, vitamin B₁₂-like compounds, and organic chlorine produced by the surrounding community. We studied which neighbors might produce these essential resources for Dehalococcoides species. We found that Dehalococcoides species are ubiquitous across the Arctic Coastal Plain and are closely associated with a network of microbes that produce or consume hydrogen or acetate, including the most abundant anaerobic bacteria and methanogenic archaea. We also found organic chlorine and microbes that can produce these compounds throughout the study area. Therefore, Dehalococcoides could control the balance between carbon dioxide and methane (a more potent greenhouse gas) when suitable organic chlorine compounds are available to drive hydrogen and acetate uptake.

Electron competition and electron selectivity in abiotic, biotic, and coupled systems for dechlorinating chlorinated aliphatic hydrocarbons in groundwater: A review

Chlorinated aliphatic hydrocarbons (CAHs) have been frequently detected in aquifers in recent years. Owing to the bioaccumulation and toxicity of CAHs, it is essential to explore high-efficiency technologies for their complete dechlorination in groundwater. At present, the most widely used abiotic and biotic remediation technologies are based on zero-valent iron (ZVI) and functional anaerobic bacteria (FAB), respectively. However, the main obstacles to the full potential of both technologies in the field include their lowered efficiencies and increased economic costs due to the co-existence of a variety of natural electron acceptors in the environment, such as dissolved oxygen (DO), nitrate (NO₃^-), sulfate (SO₄^2-), ferric iron (Fe (III)), bicarbonate (HCO₃^-), and even water, which compete for electrons with the target contaminants. Therefore, a clear understanding of the mechanisms governing electron competition and electron selectivity is significant for the accurate evaluation of the effectiveness of both technologies under natural hydrochemical conditions. We collected data from both abiotic and biotic CAH-remediation systems, summarized the dechlorination and undesired reactions in groundwater, discussed the characterization methods and general principles of electron competition, and described strategies to improve electron selectivity in both systems. Furthermore, we reviewed the emerging ZVI-FAB coupled system, which integrates abiotic and biotic processes to enhance dechlorination performance and electron utilization efficiency. Lastly, we propose future research needs to quantitatively understand the electron competition in abiotic, biotic, and coupled systems in more detail and to promote improved electron selectivity in groundwater remediation.

Growth inhibition of sulfate-reducing bacteria for trichloroethylene dechlorination enhancement

Trichloroethylene (TCE) is a frequently found organic contaminant in polluted-groundwater. In this microcosm study, effects of hydrogen-producing bacteria [Clostridium butyricum (Clostridium sp.)] and inhibitor of sulfate-reducing bacteria (SRB) addition on the enhancement of TCE dechlorination were evaluated. Results indicate that Clostridium sp. supplement could effectively enhance TCE reductive dechlorination (97.4% of TCE removal) due to increased hydrogen concentration and Dehalococcoides (DHC) populations (increased to 1 × 10⁴ gene copies/L). However, addition of Clostridium sp. also caused the increase in dsrA (dissimilatory sulfide reductase subunit A) (increased to 2 × 10⁸ gene copies/L), and thus, part of the hydrogen was consumed by SRB, which would limit the effective application of hydrogen by DHC. Control of Clostridium sp. addition is a necessity to minimize the adverse impact of Clostridium sp. on DHC growth. Ferric citrate caused the slight raise of the oxidation-reduction state, which resulted in growth inhibition of SRB. Molybdate addition inhibited the growth of SRB, and thus, the dsrA concentrations (dropped from 4 × 10⁷ to 9 × 10⁵ gene copies/L) and sulfate reduction efficiency were decreased. Increased DHC populations (increased from 8 × 10³ to 1 × 10⁵ gene copies/L) were due to increased available hydrogen (increased from 0 to 2 mg/L), which enhanced TCE dechlorination (99.3% TCE removal). Metagenomic analyses show that a significant microbial diversity was detected in microcosms with different treatments. Clostridium sp., ferric citrate, and molybdate addition caused a decreased SRB communities and increased fatty acid production microbial communities (increased from 4.9% to 20.2%), which would be beneficial to the hydrogen production and TCE dechlorination processes.

Oxygen Is the High-Energy Molecule Powering Complex Multicellular Life: Fundamental Corrections to Traditional Bioenergetics

A fundamental re-assessment of the overall energetics of biochemical electron transfer chains and cycles is presented, highlighting the crucial role of the highest-energy molecule involved, O₂. The chemical energy utilized by most complex multicellular organisms is not predominantly stored in glucose or fat, but rather in O₂ with its relatively weak (i.e., high-energy) double bond. Accordingly, reactions of O₂ with organic molecules are highly exergonic, while other reactions of glucose, fat, NAD(P)H, or ubiquinol (QH₂) are not, as demonstrated in anaerobic respiration with its meager energy output. The notion that "reduced molecules" such as alkanes or fatty acids are energy-rich is shown to be incorrect; they only unlock the energy of more O₂, compared to O-containing molecules of similar mass. Glucose contains a moderate amount of chemical energy per bond (<20% compared to O₂), as confirmed by the relatively small energy output in glycolysis and the Krebs cycle converting glucose to CO₂ and NADH. Only in the "terminal" aerobic respiration reaction with O₂ does a large free energy change occur due to the release of oxygen's stored chemical energy. The actual reaction of O₂ in complex IV of the inner mitochondrial membrane does not even involve any organic fuel molecule and yet releases >1 MJ when 6 mol of O₂ reacts. The traditional presentation that relegated O₂ to the role of a low-energy terminal acceptor for depleted electrons has not explained these salient observations and must be abandoned. Its central notion that electrons release energy because they move from a high-energy donor to a low-energy acceptor is demonstrably false. The energies of (at least) two donor and two acceptor species come into play, and the low "terminal" negative reduction potential in aerobic respiration can be attributed to the unusually high energy of O₂, the crucial reactant. This is confirmed by comparison with the corresponding half-reaction without O₂, which is endergonic. In addition, the electrons are mostly not accepted by oxygen but by hydrogen. Redox energy transfer and release diagrams are introduced to provide a superior representation of the energetics of the various species in coupled half-reactions. Electron transport by movement of reduced molecules in the electron transfer chain is shown to run counter to the energy flow, which is carried by oxidized species. O₂, rather than glucose, NAD(P)H, or ATP, is the molecule that provides the most energy to animals and plants and is crucial for sustaining large complex life forms. The analysis also highlights a significant discrepancy in the proposed energetics of reactions of aerobic respiration, which should be re-evaluated.

Advanced redox zonation of the San Pedro Sula alluvial aquifer (Honduras) using data fusion and multivariate geostatistics

The incorrect wastewater management and the land use distribution lead to severe environmental problems, creating heavy eutrophication condition in surface-water; when surface-water/groundwater relationships exist, the organic matter transferred to the aquifer oxidizes and triggers redox processes (i.e. Terminal Electron Accepting Processes, TEAPs), that provoke severe groundwater quality modifications and complicate its exploitation and management. For this reason, the definition of the redox zonation within an aquifer is an effective tool for the identification of the contamination sources and for the conceptual model refinement, when remediation strategies need to be planned. Although the redox processes are dynamic reactions, the aquifer redox zonation is generally aimed to identify homogenous zones, characterized by a predominant TEAP. To overcome this methodological approach, the Multi-Collocated Factorial Kriging (MCFK) was applied to redox-related physico-chemical parameters, which allowed identifying their spatial relationships at different scales, transferring this method from precision agriculture and soil science to hydrogeochemistry. The selected study area is the San Pedro Sula aquifer (Honduras), a multi-layer alluvial aquifer characterized by well-known surface-water/groundwater interactions and heavy eutrophicated streams. Here, high concentrations of Mn and Fe were found in the aquifer. The MCFK results identified a short-range (2300 m) factor, highlighting a strong relation between Mn concentrations and anoxic conditions, due to the organic matter transfer from eutrophicated surface-water into the aquifer. Simultaneously, the relationship between Fe and turbidity is related to a fine Fe(III) oxi-hydroxide colloidal phase, developed when different redox conditions of groundwater mix up in the wells. The long-range (6000 m) factor points out that Fe is related to redox processes at a wider scale, especially in the northern San Pedro Sula alluvial plain. These results are supported by both the Principal Component Analysis and the hydrogeochemical numerical modeling. As a result, different TEAPs occur simultaneously in contaminated areas, acting at multiple scales.

Influence of Hydrogen Electron Donor, Alkaline pH, and High Nitrate Concentrations on Microbial Denitrification: A Review

Bacterial respiration of nitrate is a natural process of nitrate reduction, which has been industrialized to treat anthropic nitrate pollution. This process, also known as “microbial denitrification”, is widely documented from the fundamental and engineering points of view for the enhancement of the removal of nitrate in wastewater. For this purpose, experiments are generally conducted with heterotrophic microbial metabolism, neutral pH and moderate nitrate concentrations (<50 mM). The present review focuses on a different approach as it aims to understand the effects of hydrogenotrophy, alkaline pH and high nitrate concentration on microbial denitrification. Hydrogen has a high energy content but its low solubility, 0.74 mM (1 atm, 30 °C), in aqueous medium limits its bioavailability, putting it at a kinetic disadvantage compared to more soluble organic compounds. For most bacteria, the optimal pH varies between 7.5 and 9.5. Outside this range, denitrification is slowed down and nitrite (NO₂⁻) accumulates. Some alkaliphilic bacteria are able to express denitrifying activity at pH levels close to 12 thanks to specific adaptation and resistance mechanisms detailed in this manuscript, and some bacterial populations support nitrate concentrations in the range of several hundred mM to 1 M. A high concentration of nitrate generally leads to an accumulation of nitrite. Nitrite accumulation can inhibit bacterial activity and may be a cause of cell death.

Syngas-aided anaerobic fermentation for medium-chain carboxylate and alcohol production: the case for microbial communities

Syngas fermentation has been successfully implemented in commercial-scale plants and can enable the biochemical conversion of the driest fractions of biomass through synthesis gas (H₂, CO₂, and CO). The process relies on optimized acetogenic strains able to reach and maintain high productivity of ethanol and acetate. In parallel, microbial communities have shown to be the best choice for the production of valuable medium-chain carboxylates through anaerobic fermentation of biomass, demanding low technical complexity and being able to realize simultaneous hydrolysis of the substrate. Each of the two technologies benefits from different strong points and has different challenges to overcome. This review discusses the rationales for merging these two seemingly disparate technologies by analyzing previous studies and drawing opinions based on the lessons learned from such studies. For keeping the technical demands of the resulting process low, a case is built for using microbial communities instead of pure strains. For that to occur, a shift from conventional syngas-based to "syngas-aided" anaerobic fermentation is suggested. Strategies for tackling the intricacies of working simultaneously with communities and syngas, such as competing pathways, and thermodynamic aspects are discussed as well as the stoichiometry and economic feasibility of the concept. Overall, syngas-aided anaerobic fermentation seems to be a promising concept for the biorefinery of the future. However, the effects of process parameters on microbial interactions have to be understood in greater detail, in order to achieve and sustain feasible medium-chain carboxylate and alcohol productivity.

Plant communities control long term carbon accumulation and biogeochemical gradients in a Patagonian bog

Peat carbon accumulation is controlled by both large scale factors, such as climate and hydrological setting, and small scale factors, such as microtopography and plant community. These small scale factors commonly vary within peatlands and can cause variation in biogeochemical traits and carbon accumulation within the same site. To understand these within-site variations, we investigated long term carbon accumulation, peat decomposition, biogeochemistry of pore water and plant macrofossils along a transect in an ombrotrophic bog in southern Patagonia. An additional question we addressed is how historical deposition of volcanic ash on the peatland has affected its carbon balance. Variability in plant community and water table led to differences in long term peat and carbon accumulation (peat moss > cushion plant), organic matter decomposition (cushion plant > peat moss), and methane production (peat moss > cushion plant). Macrofossil analysis and radiocarbon dating indicated a relationship between plant community and carbon accumulation or decomposition during the historical succession of vegetation in the peatland. C/N ratio and isotopic signatures reflected variability in plant community as litter source, and DOC concentrations were controlled by humification level. Volcanic ash deposition had only limited effect on plant composition, but it was associated with increased decomposition in overlying peat layers. This study highlights the importance of understanding how plant communities develop, as changes in communities could significantly affect the potential of ombrotrophic peatlands as C sink.

Revealing chlorinated ethene transformation hotspots in a nitrate-impacted hyporheic zone

Hyporheic zones are increasingly thought of as natural bioreactors, capable of transforming and attenuating groundwater pollutants present in diffuse baseflow. An underappreciated scenario in the understanding of contaminant fate in hyporheic zones is the interaction between point-source trichloroethene (TCE) plumes and ubiquitous, non-point source pollutants such as nitrate. This study aims to conceptualise critical biogeochemical gradients in the hyporheic zone which govern the export potential of these redox-sensitive pollutants from carbon-poor, oxic aquifers. Within the TCE plume discharge zone, discrete vertical profiling of the upper 100 cm of sediment pore water chemistry revealed an 80% increase in dissolved organic carbon (DOC) concentrations and 20-60 cm thick hypoxic zones (<2 mg O₂ L^-1) within which most reactive transport was observed. A 33% reduction of nitrate concentrations coincided with elevated pore water nitrous oxide concentrations as well as the appearance of manganese and the TCE metabolite cis-1,2-dichloroethene (cDCE). Elevated groundwater nitrate concentrations (>50 mg L^-1) create a large stoichiometric demand for bioavailable DOC in discharging groundwater. With the benefit of a high-resolution grid of pore water samplers investigating the shallowest 30 cm of hypoxic groundwater flow paths, we identified DOC-rich hotspots associated with submerged vegetation (Ranunculus spp.), where low-energy metabolic processes such as mineral dissolution/reduction, methanogenesis and ammonification dominate. Using a chlorine index metric, we show that enhanced TCE to cDCE transformation takes place within these biogeochemical hotspots, highlighting their relevance for natural plume attenuation.

Transcriptome analysis of activated sludge microbiomes reveals an unexpected role of minority nitrifiers in carbon metabolism

Although metagenomics researches have illuminated microbial diversity in numerous biospheres, understanding individual microbial functions is yet difficult due to the complexity of ecosystems. To address this issue, we applied a metagenome-independent, de novo assembly-based metatranscriptomics to a complex microbiome, activated sludge, which has been used for wastewater treatment for over a century. Even though two bioreactors were operated under the same conditions, their performances differed from each other with unknown causes. Metatranscriptome profiles in high- and low-performance reactors demonstrated that denitrifiers contributed to the anaerobic degradation of heavy oil; however, no marked difference in the gene expression was found. Instead, gene expression-based nitrification activities that fueled the denitrifiers by providing the respiratory substrate were notably high in the high-performance reactor only. Nitrifiers-small minorities with relative abundances of <0.25%-governed the heavy-oil degradation performances of the reactors, unveiling an unexpected linkage of carbon- and nitrogen-metabolisms of the complex microbiome.

Role of hydrogen (H2) mass transfer in microbiological H2-threshold studies

Gas-to-liquid mass transfer of hydrogen (H₂) was investigated in a gas-liquid reactor with a continuous gas phase, a batch liquid phase, and liquid mixing regimes relevant to assessing kinetics of microbial H₂ consumption. H₂ transfer was quantified in real-time with a H₂ microsensor for no mixing, moderate mixing [100 rotations per minute (rpm)], and rapid mixing (200 rpm). The experimental results were simulated by mathematical models to find best-fit values of volumetric mass transfer coefficients-k_La-for H₂, which were 1.6/day for no mixing, 7/day for 100 rpm, and 30/day for 200 rpm. Microbiological H₂-consumption experiments were conducted with Methanobacterium bryantii M.o.H. to assess effects of H₂ mass transfer on microbiological H₂-threshold studies. The results illustrate that slow mixing reduced the gas-to-liquid H₂ transfer rate, which fell behind the rate of microbiological H₂ consumption in the liquid phase. As a result, the liquid-phase H₂ concentration remained much lower than the liquid-phase H₂ concentration that would be in equilibrium with the gas-phase H₂ concentration. Direct measurements of the liquid-phase H₂ concentration by an in situ probe demonstrated the problems associated with slow H₂ transfer in past H₂ threshold studies. The findings indicate that some of the previously reported H₂-thresholds most likely were over-estimates due to slow gas-to-liquid H₂ transfer. Essential requirements to conduct microbiological H₂ threshold experiments are to have vigorous mixing, large gas-to-liquid volume, large interfacial area, and low initial biomass concentration.

Iron and sulfur cycling in acid sulfate soil wetlands under dynamic redox conditions: A review

Acid sulfate soils (ASS) contain substantial quantities of iron sulfide minerals or the oxidation reaction products of these sulfidic minerals. Transformation of iron (Fe) and sulfur (S) bearing minerals is an important process in ASS wetlands with fluctuating redox conditions. A range of potentially toxic metals and metalloids can either be adsorbed on or incorporated into the structure of Fe and S bearing minerals. Therefore, transformation of these minerals as affected by dynamic redox conditions may regulate the mobility and bioavailability of associated metals/metalloids. Better understanding of the interaction between Fe/S biogeochemistry and trace metal/metalloid mobility under fluctuating redox conditions is important for assessing contaminant risk to the environment. This review paper provides an overview of current knowledge regarding cycling of Fe, S and selected trace metal/metalloids in ASS wetlands under fluctuating redox conditions and outlines future research challenges and directions on this subject.

Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Thermodynamic and hydrochemical controls on CH4 in a coal seam gas and overlying alluvial aquifer: new insights into CH4 origins

Using a comprehensive data set (dissolved CH₄, δ¹³C-CH₄, δ²H-CH₄, δ¹³C-DIC, δ³⁷Cl, δ²H-H₂O, δ¹⁸O-H₂O, Na, K, Ca, Mg, HCO₃, Cl, Br, SO₄, NO₃ and DO), in combination with a novel application of isometric log ratios, this study describes hydrochemical and thermodynamic controls on dissolved CH₄ from a coal seam gas reservoir and an alluvial aquifer in the Condamine catchment, eastern Surat/north-western Clarence-Moreton basins, Australia. δ¹³C-CH₄ data in the gas reservoir (−58‰ to −49‰) and shallow coal measures underlying the alluvium (−80‰ to −65‰) are distinct. CO₂ reduction is the dominant methanogenic pathway in all aquifers, and it is controlled by SO₄ concentrations and competition for reactants such as H₂. At isolated, brackish sites in the shallow coal measures and alluvium, highly depleted δ²H-CH₄ (<310‰) indicate acetoclastic methanogenesis where SO₄ concentrations inhibit CO₂ reduction. Evidence of CH₄ migration from the deep gas reservoir (200–500 m) to the shallow coal measures (<200 m) or the alluvium was not observed. The study demonstrates the importance of understanding CH₄ at different depth profiles within and between aquifers. Further research, including culturing studies of microbial consortia, will improve our understanding of the occurrence of CH₄ within and between aquifers in these basins.

Aerobically respiring prokaryotic strains exhibit a broader temperature-pH-salinity space for cell division than anaerobically respiring and fermentative strains

Biological processes on the Earth operate within a parameter space that is constrained by physical and chemical extremes. Aerobic respiration can result in adenosine triphosphate yields up to over an order of magnitude higher than those attained anaerobically and, under certain conditions, may enable microbial multiplication over a broader range of extremes than other modes of catabolism. We employed growth data published for 241 prokaryotic strains to compare temperature, pH and salinity values for cell division between aerobically and anaerobically metabolizing taxa. Isolates employing oxygen as the terminal electron acceptor exhibited a considerably more extensive three-dimensional phase space for cell division (90% of the total volume) than taxa using other inorganic substrates or organic compounds as the electron acceptor (15% and 28% of the total volume, respectively), with all groups differing in their growth characteristics. Understanding the mechanistic basis of these differences will require integration of research into microbial ecology, physiology and energetics, with a focus on global-scale processes. Critical knowledge gaps include the combined impacts of diverse stress parameters on Gibbs energy yields and rates of microbial activity, interactions between cellular energetics and adaptations to extremes, and relating laboratory-based data to in situ limits for cell division.

Desulfovibrio vulgaris Growth Coupled to Formate-Driven H2 Production

Formate is recognized as a superior substrate for biological H2 production by several bacteria. However, the growth of a single organism coupled to this energetic pathway has not been shown in mesophilic conditions. In the present study, a bioreactor with gas sparging was used, where we observed for the first time that H2 production from formate can be coupled with growth of the model sulfate-reducing bacterium Desulfovibrio vulgaris in the absence of sulfate or a syntrophic partner. In these conditions, D. vulgaris had a maximum growth rate of 0.078 h(-1) and a doubling time of 9 h, and the ΔG of the reaction ranged between -21 and -18 kJ mol(-1). This is the first report of a single mesophilic organism that can grow while catalyzing the oxidation of formate to H2 and bicarbonate. Furthermore, high volumetric and specific H2 production rates (125 mL L(-1) h(-1) and 2500 mL gdcw(-1) h(-1)) were achieved in a new bioreactor designed and optimized for H2 production. This high H2 production demonstrates that the nonconventional H2-producing organism D. vulgaris is a good biocatalyst for converting formate to H2.

The geomicrobiology of CO2 geosequestration: a focused review on prokaryotic community responses to field-scale CO2 injection

Our primary research paper (Mu et al., 2014) demonstrated selective changes to a deep subsurface prokaryotic community as a result of CO2 stress. Analyzing geochemical and microbial 16S rRNA gene profiles, we evaluated how in situ prokaryotic communities responded to increased CO2 and the presence of trace organic compounds, and related temporal shifts in phylogeny to changes in metabolic potential. In this focused review, we extend upon our previous discussion to present analysis of taxonomic unit co-occurrence profiles from the same field experiment, to attempt to describe dynamic community behavior within the deep subsurface. Understanding the physiology of the subsurface microbial biosphere, including how key functional groups integrate into the community, will be critical to determining the fate of injected CO2. For example, community-wide network analyses may provide insights to whether microbes cooperatively produce biofilm biomass, and/or biomineralize the CO2, and hence, induce changes to formation porosity or changes in electron flow. Furthermore, we discuss potential impacts to the feasibility of subsurface CO2 storage of selectively enriching for particular metabolic functions (e.g., methanogenesis) as a result of CO2 injection.

Efficient metabolic exchange and electron transfer within a syntrophic trichloroethene-degrading coculture of Dehalococcoides mccartyi 195 and Syntrophomonas wolfei

Dehalococcoides mccartyi 195 (strain 195) and Syntrophomonas wolfei were grown in a sustainable syntrophic coculture using butyrate as an electron donor and carbon source and trichloroethene (TCE) as an electron acceptor. The maximum dechlorination rate (9.9 ± 0.1 μmol day(-1)) and cell yield [(1.1 ± 0.3) × 10(8) cells μmol(-1) Cl(-)] of strain 195 maintained in coculture were, respectively, 2.6 and 1.6 times higher than those measured in the pure culture. The strain 195 cell concentration was about 16 times higher than that of S. wolfei in the coculture. Aqueous H2 concentrations ranged from 24 to 180 nM during dechlorination and increased to 350 ± 20 nM when TCE was depleted, resulting in cessation of butyrate fermentation by S. wolfei with a theoretical Gibbs free energy of -13.7 ± 0.2 kJ mol(-1). Carbon monoxide in the coculture was around 0.06 μmol per bottle, which was lower than that observed for strain 195 in isolation. The minimum H2 threshold value for TCE dechlorination by strain 195 in the coculture was 0.6 ± 0.1 nM. Cell aggregates during syntrophic growth were observed by scanning electron microscopy. The interspecies distances to achieve H2 fluxes required to support the measured dechlorination rates were predicted using Fick's law and demonstrated the need for aggregation. Filamentous appendages and extracellular polymeric substance (EPS)-like structures were present in the intercellular spaces. The transcriptome of strain 195 during exponential growth in the coculture indicated increased ATP-binding cassette transporter activities compared to the pure culture, while the membrane-bound energy metabolism related genes were expressed at stable levels.

Assessment of microbial communities associated with fermentative-methanogenic biodegradation of aromatic hydrocarbons in groundwater contaminated with a biodiesel blend (B20)

A controlled field experiment was conducted to assess the potential for fermentative-methanogenic biostimulation (by ammonium-acetate injection) to enhance biodegradation of benzene, toluene, ethylbenzene and xylenes (BTEX) as well as polycyclic aromatic hydrocarbons (PAHs) in groundwater contaminated with biodiesel B20 (20:80 v/v soybean biodiesel and diesel). Changes in microbial community structure were assessed by pyrosequencing 16S rRNA analyses. BTEX and PAH removal began 0.7 year following the release, concomitantly with the increase in the relative abundance of Desulfitobacterium and Geobacter spp. (from 5 to 52.7 % and 15.8 to 37.3 % of total Bacteria 16S rRNA, respectively), which are known to anaerobically degrade hydrocarbons. The accumulation of anaerobic metabolites acetate and hydrogen that could hinder the thermodynamic feasibility of BTEX and PAH biotransformations under fermentative/methanogenic conditions was apparently alleviated by the growing predominance of Methanosarcina. This suggests the importance of microbial population shifts that enrich microorganisms capable of interacting syntrophically to enhance the feasibility of fermentative-methanogenic bioremediation of biodiesel blend releases.

Investigating dominant processes in ZVI permeable reactive barriers using reactive transport modeling

The reactive and hydraulic efficacy of zero valent iron permeable reactive barriers (ZVI PRBs) is strongly affected by geochemical composition of the groundwater treated. An enhanced version of the geochemical simulation code MIN3P was applied to simulate dominating processes in chlorinated hydrocarbons (CHCs) treating ZVI PRBs including geochemical dependency of ZVI reactivity, gas phase formation and a basic formulation of degassing. Results of target oriented column experiments with distinct chemical conditions (carbonate, calcium, sulfate, CHCs) were simulated to parameterize the model. The simulations demonstrate the initial enhancement of anaerobic iron corrosion due to carbonate and long term inhibition by precipitates (chukanovite, siderite, iron sulfide). Calcium was shown to enhance long term corrosion due to competition for carbonate between siderite, chukanovite, and aragonite, with less inhibition of iron corrosion by the needle like aragonite crystals. Application of the parameterized model to a field site (Bernau, Germany) demonstrated that temporarily enhanced groundwater carbonate concentrations caused an increase in gas phase formation due to the acceleration of anaerobic iron corrosion.

Correlations between environmental variables and bacterial community structures suggest Fe(III) and vinyl chloride reduction as antagonistic terminal electron-accepting processes

Natural attenuation of anaerobic aquifers contaminated with tetrachloroethene (PCE) often results in the accumulation of the intermediates cis-dichloroethene and vinyl chloride (VC) which are even more toxic than the parent compound. Reasons for this accumulation were investigated in a PCE-contaminated aquifer in which VC accumulation has previously been shown to occur using stable isotope techniques. Multifactorial analysis of bacterial community structure data and environmental variables showed that in general terminal electron-accepting processes were shaping the bacterial community structures. Both VC and Fe(III) reduction were key but antagonistic terminal electron-accepting processes. The phylogenetic affiliation of terminal restriction fragments (T-RFs), together with correlation analyses, showed that T-RFs having significant correlation with VC reduction were closely affiliated to the genus Dehalococcoides and to uncultured bacteria belonging to the "Lahn Cluster" within the class Dehalococcoidetes. A T-RF that negatively correlated with a "Lahn Cluster" T-RF was affiliated to the genus Rhodoferax that contains members identified as iron-reducing bacteria. The higher affinity of Fe(III)-reducing bacteria for hydrogen compared with VC-reducing bacteria might explain why VC accumulated locally at the studied site. In conclusion, the combination of molecular and numerical ecology approaches was helpful to identify reasons for the accumulation of toxic dechlorination intermediates and could become a useful tool for characterizing contaminated sites in general.

Functional microbial diversity explains groundwater chemistry in a pristine aquifer

BACKGROUND

The diverse microbial populations that inhabit pristine aquifers are known to catalyze critical in situ biogeochemical reactions, yet little is known about how the structure and diversity of this subsurface community correlates with and impacts upon groundwater chemistry. Herein we examine 8,786 bacterial and 8,166 archaeal 16S rRNA gene sequences from an array of monitoring wells in the Mahomet aquifer of east-central Illinois. Using multivariate statistical analyses we provide a comparative analysis of the relationship between groundwater chemistry and the microbial communities attached to aquifer sediment along with those suspended in groundwater.

RESULTS

Statistical analyses of 16S rRNA gene sequences showed a clear distinction between attached and suspended communities; with iron-reducing bacteria far more abundant in attached samples than suspended, while archaeal clones related to groups associated with anaerobic methane oxidation and deep subsurface gold mines (ANME-2D and SAGMEG-1, respectively) distinguished the suspended community from the attached. Within the attached bacterial community, cloned sequences most closely related to the sulfate-reducing Desulfobacter and Desulfobulbus genera represented 20% of the bacterial community in wells where the concentration of sulfate in groundwater was high (> 0.2 mM), compared to only 3% in wells with less sulfate. Sequences related to the genus Geobacter, a genus containing ferric-iron reducers, were of nearly equal abundance (15%) to the sulfate reducers under high sulfate conditions, however their relative abundance increased to 34% when sulfate concentrations were < 0.03 mM. Also, in areas where sulfate concentrations were <0.03 mM, archaeal 16S rRNA gene sequences similar to those found in methanogens such as Methanosarcina and Methanosaeta comprised 73-80% of the community, and dissolved CH4 ranged between 220 and 1240 μM in these groundwaters. In contrast, methanogens (and their product, CH4) were nearly absent in samples collected from groundwater samples with > 0.2 mM sulfate. In the suspended fraction of wells where the concentration of sulfate was between 0.03 and 0.2 mM, the archaeal community was dominated by sequences most closely related to the ANME-2D, a group of archaea known for anaerobically oxidizing methane. Based on available energy (∆GA) estimations, results varied little for both sulfate reduction and methanogenesis throughout all wells studied, but could favor anaerobic oxidation of methane (AOM) in wells containing minimal sulfate and dihydrogen, suggesting AOM coupled with H2-oxidizing organisms such as sulfate or iron reducers could be an important pathway occurring in the Mahomet aquifer.

CONCLUSIONS

Overall, the results show several distinct factors control the composition of microbial communities in the Mahomet aquifer. Bacteria that respire insoluble substrates such as iron oxides, i.e. Geobacter, comprise a greater abundance of the attached community than the suspended regardless of groundwater chemistry. Differences in community structure driven by the concentration of sulfate point to a clear link between the availability of substrate and the abundance of certain functional groups, particularly iron reducers, sulfate reducers, methanogens, and methanotrophs. Integrating both geochemical and microbiological observations suggest that the relationships between these functional groups could be driven in part by mutualism, especially between ferric-iron and sulfate reducers.

Biostimulation of anaerobic BTEX biodegradation under fermentative methanogenic conditions at source-zone groundwater contaminated with a biodiesel blend (B20)

Field experiments were conducted to assess the potential for anaerobic biostimulation to enhance BTEX biodegradation under fermentative methanogenic conditions in groundwater impacted by a biodiesel blend (B20, consisting of 20 % v/v biodiesel and 80 % v/v diesel). B20 (100 L) was released at each of two plots through an area of 1 m(2) that was excavated down to the water table, 1.6 m below ground surface. One release was biostimulated with ammonium acetate, which was added weekly through injection wells near the source zone over 15 months. The other release was not biostimulated and served as a baseline control simulating natural attenuation. Ammonium acetate addition stimulated the development of strongly anaerobic conditions, as indicated by near-saturation methane concentrations. BTEX removal began within 8 months in the biostimulated source zone, but not in the natural attenuation control, where BTEX concentrations were still increasing (due to source dissolution) 2 years after the release. Phylogenetic analysis using quantitative PCR indicated an increase in concentration and relative abundance of Archaea (Crenarchaeota and Euryarchaeota), Geobacteraceae (Geobacter and Pelobacter spp.) and sulfate-reducing bacteria (Desulfovibrio, Desulfomicrobium, Desulfuromusa, and Desulfuromonas) in the biostimulated plot relative to the control. Apparently, biostimulation fortuitously enhanced the growth of putative anaerobic BTEX degraders and associated commensal microorganisms that consume acetate and H2, and enhance the thermodynamic feasibility of BTEX fermentation. This is the first field study to suggest that anaerobic-methanogenic biostimulation could enhance source zone bioremediation of groundwater aquifers impacted by biodiesel blends.

Acetogenesis in the energy-starved deep biosphere - a paradox?

Under anoxic conditions in sediments, acetogens are often thought to be outcompeted by microorganisms performing energetically more favorable metabolic pathways, such as sulfate reduction or methanogenesis. Recent evidence from deep subseafloor sediments suggesting acetogenesis in the presence of sulfate reduction and methanogenesis has called this notion into question, however. Here I argue that acetogens can successfully coexist with sulfate reducers and methanogens for multiple reasons. These include (1) substantial energy yields from most acetogenesis reactions across the wide range of conditions encountered in the subseafloor, (2) wide substrate spectra that enable niche differentiation by use of different substrates and/or pooling of energy from a broad range of energy substrates, (3) reduced energetic cost of biosynthesis among acetogens due to use of the reductive acetyl CoA pathway for both energy production and biosynthesis coupled with the ability to use many organic precursors to produce the key intermediate acetyl CoA. This leads to the general conclusion that, beside Gibbs free energy yields, variables such as metabolic strategy and energetic cost of biosynthesis need to be taken into account to understand microbial survival in the energy-depleted deep biosphere.

Experimental burial inhibits methanogenesis and anaerobic decomposition in water-saturated peats

A mechanistic understanding of carbon (C) sequestration and methane (CH(4)) production is of great interest due to the importance of these processes for the global C budget. Here we demonstrate experimentally, by means of column experiments, that burial of water saturated, anoxic bog peat leads to inactivation of anaerobic respiration and methanogenesis. This effect can be related to the slowness of diffusive transport of solutes and evolving energetic constraints on anaerobic respiration. Burial lowered decomposition constants in homogenized peat sand mixtures from about 10(-5) to 10(-7) yr(-1), which is considerably slower than previously assumed, and methanogenesis slowed down in a similar manner. The latter effect could be related to acetoclastic methanogenesis approaching a minimum energy quantum of -25 kJ mol(-1) (CH(4)). Given the robustness of hydraulic properties that locate the oxic-anoxic boundary near the peatland surface and constrain solute transport deeper into the peat, this effect has likely been critical for building the peatland C store and will continue supporting long-term C sequestration in northern peatlands even under moderately changing climatic conditions.

Model to couple anaerobic process kinetics with biological growth equilibrium thermodynamics

Monod kinetics indicates a substrate concentration limit (S(min)) at biological growth equilibrium where growth is just balanced by decay. A relationship between S(min) and the Gibbs free energy available at growth equilibrium (ΔG(E)) was introduced into the Monod model and applied directly to chemostat cultures. Results from four anaerobic mixed-culture chemostat studies yielded ΔG(E) of -17.7 ± 2.2 kJ/mol acetate converted to methane. ΔG(E) for propionate syntrophs in propionate-fed cultures was -8.0 ± 3.1 kJ/mol propionate, compared with that of -3.0 ± 0.9 kJ/mol H(2) for the hydrogenotrophs present. With ethanol present, however, ΔG(E) for the hydrogenotrophs became more favorable, -6.1 ± 1.6 kJ/mol H(2), while ΔG(E) for propionate became positive even though propionate was consumed, suggesting an alternative interspecies electron transport route. The results suggest that S(min), normally considered a function of an organism's intrinsic rate characteristics, is also a function of solution characteristics, and this is likely the case for the substrate affinity coefficient, K, as well. A comparison between ΔG(E) and S(min) and reported threshold thermodynamic and concentration limits, leads to the conclusion that ΔG(E) and S(min) represent lower and upper bounds, respectively, on such values. This study indicates that knowledge gained from pure-culture studies applies well to more complex natural anaerobic systems.

Detection of environmentally persistent free radicals at a superfund wood treating site

Environmentally persistent free radicals (EPFRs) have previously been observed in association with combustion-generated particles and airborne PM(2.5) (particulate matter, d < 2.5um). The purpose of this study was to determine if similar radicals were present in soils and sediments at Superfund sites. The site was a former wood treating facility containing pentachlorophenol (PCP) as a major contaminant. Both contaminated and noncontaminated (just outside the contaminated area) soil samples were collected. The samples were subjected to the conventional humic substances (HS) extraction procedure. Electron paramagnetic resonance (EPR) spectroscopy was used to measure the EPFR concentrations and determine their structure for each sample fraction. Analyses revealed a ∼30× higher EPFR concentration in the PCP contaminated soils (20.2 × 10(17) spins/g) than in the noncontaminated soil (0.7 × 10(17) spins/g). Almost 90% of the EPFR signal originated from the minerals/clays/humins fraction. GC-MS analyses revealed ∼6500 ppm of PCP in the contaminated soil samples and none detected in the background samples. Inductively coupled plasma-atomic emission spectrophotometry (ICP-AES) analyses revealed ∼7× higher concentrations of redox-active transition metals, in the contaminated soils than the noncontaminated soil. Vapor phase and liquid phase dosing of the clays/minerals/humins fraction of the soil with PCP resulted in an EPR signal identical to that observed in the contaminated soil, strongly suggesting the observed EPFR is pentachlorophenoxyl radical. Chemisorption and electron transfer from PCP to transition metals and other electron sinks in the soil are proposed to be responsible for EPFR formation.

Metabolic basis for the self-referential genetic code

An investigation of the biosynthesis pathways producing glycine and serine was necessary to clarify an apparent inconsistency between the self-referential model (SRM) for the formation of the genetic code and the model of coevolution of encodings and of amino acid biosynthesis routes. According to the SRM proposal, glycine was the first amino acid encoded, followed by serine. The coevolution model does not state precisely which the first encodings were, only presenting a list of about ten early assignments including the derivation of glycine from serine-this being derived from the glycolysis intermediate glycerate, which reverses the order proposed by the self-referential model. Our search identified the glycine-serine pathway of syntheses based on one-carbon sources, involving activities of the glycine decarboxylase complex and its associated serine hydroxymethyltransferase, which is consistent with the order proposed by the self-referential model and supports its rationale for the origin of the genetic code: protein synthesis was developed inside an early metabolic system, serving the function of a sink of amino acids; the first peptides were glycine-rich and fit for the function of building the early ribonucleoproteins; glycine consumption in proteins drove the fixation of the glycine-serine pathway.

BTEX plume dynamics following an ethanol blend release: geochemical footprint and thermodynamic constraints on natural attenuation

In this 10 year study, Brazilian gasoline (100 L, containing 24% ethanol by volume) was released to a sandy aquifer to evaluate the natural attenuation of benzene, toluene, ethylbenzene, and total xylenes (BTEX) in the presence of ethanol. Groundwater concentrations of BTEX, ethanol, and degradation products (e.g., acetate and methane) were measured over the entire plume using an array of monitoring well clusters, to quantify changes in plume mass and region of influence. Ethanol biodegradation coincided with the development of methanogenic conditions while acetate (a common anaerobic metabolite) accumulated. The benzene plume expanded beyond the 30 m long monitored area and began to recede after 2.7 years, when ethanol had disappeared. Theoretical calculations suggest that the transient accumulation of acetate (up to 166 mg L(-1)) may have hindered the thermodynamic feasibility of benzene degradation under methanogenic conditions. Yet, benzene removal proceeded relatively fast compared to literature values (and faster than the alkylbenzenes present at this site) after acetate concentrations had decreased below inhibitory levels. Thus, site investigations of ethanol blend releases should consider monitoring acetate concentrations. Overall, this study shows that inhibitory effects of ethanol and acetate are relatively short-lived, and demonstrates that monitored natural attenuation can be a viable option to deal with ethanol blend releases.

Predominant contribution of syntrophic acetate oxidation to thermophilic methane formation at high acetate concentrations

To quantify the contribution of syntrophic acetate oxidation to thermophilic anaerobic methanogenesis under the stressed condition induced by acidification, the methanogenic conversion process of 100 mmol/L acetate was monitored simultaneously by using isotopic tracing and selective inhibition techniques, supplemented with the analysis of unculturable microorganisms. Both quantitative methods demonstrated that, in the presence of aceticlastic and hydrogenotrophic methanogens, a large percentage of methane (up to 89%) was initially derived from CO(2) reduction, indicating the predominant contribution of the syntrophic acetate oxidation pathway to acetate degradation at high acid concentrations. A temporal decrease of the fraction of hydrogenotrophic methanogenesis from more than 60% to less than 40% reflected the gradual prevalence of the aceticlastic methanogenesis pathway along with the reduction of acetate. This apparent discrimination of acetate methanization pathways highlighted the importance of the syntrophic acetate-oxidizing bacteria to initialize methanogenesis from high organic loadings.

Fe(II)-Fe(III)-bearing phases as a mineralogical control on the heterogeneity of arsenic in Southeast Asian groundwater

Although groundwater arsenic constitutes a major hazard to the health of the people of Southeast Asia, the exact mineralogical origin of the arsenic in these fluvial aquifers is still under debate. Fe(III) oxides are the dominant hosts of mobilizable arsenic in the sediments, with the role of secondary Fe(II)-bearing phases like mackinawite, siderite, vivianite, magnetite, and carbonate green rust (fougerite) still unclear. Based on published field data from Chakdaha (India), the importance of the phases for arsenic mobility is evaluated quantitatively using models of growing complexity. Arsenic heterogeneity can be explained by the presence of two contrasted redox zones in the aquifers, with Fe(III) oxides being the dominant sorbent for arsenic in the less reduced zones and Fe(II) sulfides and/or Fe(II) carbonates being the solid-phase hosts for arsenic under more reduced conditions below impermeable soils or close to rivers where sulfate is reduced. A 1D reactive transport model which simulates the transition between the two environments has been developed and compared to field data. The results show that microbial sulfate reduction followed by abiotic and/or biotic reduction of As(III)-bearing iron oxides accounts for the spatial heterogeneity of arsenic in such reduced aquifers.