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Solid organic matter (SOM) is an important component of natural sediments and plays a crucial role in providing substrate for microbial reactions and the degradation of contaminants in soil and groundwater. Knowledge about its distribution in the subsurface is crucial for the delineation of potential hotspots of microbial activity. The subsurface is, however, difficult to access, limiting our ability to reliably delineate the spatially heterogeneous distribution of SOM. Recently, the geophysical method induced polarization (IP) has been shown to be a potentially promising mapping tool, able to detect the presence of SOM. However, the mechanisms controlling IP signals in the presence of SOM are not (yet) well understood, with a handful of studies highlighting inconclusive results (Katona et al., 2021; Mellage et al., 2022; Ponziani et al., 2012; Schwartz & Furman, 2014). Moreover, a non-negligible contribution of polarization from the organic matrix can yield signals that may cause misinterpretation of other petro-physical relationships in unconsolidated sediments.In this study, we measured the spectral IP (SIP) response of aquifer sediment cores (2 – 8 m depth) collected from an alluvial floodplain aquifer in southwest Germany. The total organic carbon (TOC) content in the cores and the cation exchange capacity (CEC) exhibit a positive correlation with the magnitude of polarization (i.e. imaginary conductivity). In addition, strong differences in the frequency dependence of the IP measurements as a function of TOC fraction were observed for the otherwise calcareous matrix devoid of other strongly polarizing mineral phases (e.g. pyrite or clay minerals). While the CEC at the site is strongly dominated by the amount of SOM, polarization is more strongly linked to SOM than CEC. We hypothesize that the weaker correlation between SOM and CEC highlights the contribution of poorly understood charge storage mechanisms within the polydisperse organic matrix that differ from polarization at mineral surfaces. Ongoing experiments with artificial soil mixtures of calcitic sand and varying fractions of peat, under controlled conditions (i.e. constant electrical conductivity of the pore fluid), will help to shed light on the controls behind our field-derived relationships. We expect that our combined field and laboratory investigations will provide insights into the petro-, or rather, organo-physical relationship between SOM and the imaginary conductivity, and thus contribute to a conceptualization of the underlying polarization mechanisms in organic matrices. ReferencesKatona, T., Gilfedder, B. S., Frei, S., Bücker, M., & Flores Orozco, A. (2021). High-resolution induced polarization imaging of biogeochemical carbon-turnover hot spots in a peatland. Biogeosciences, 18(13), 4039–4058.Mellage, A., Zakai, G., Efrati, B., Pagel, H., & Schwartz, N. (2022). Paraquat sorption- and organic matter-induced modifications of soil spectral induced polarization (SIP) signals. Geophysical Journal International, 229(2), 1422–1433. https://doi.org/10.1093/gji/ggab531Ponziani, M., Slob, E. C., Vanhala, H., & Ngan-Tillard, D. (2012). Influence of physical and chemical properties on the low-frequency complex conductivity of peat. Near Surface Geophysics, 10(6), 491–501. https://doi.org/10.3997/1873-0604.2011037Schwartz, N., & Furman, A. (2014). On the spectral induced polarization signature of soil organic matter. Geophysical Journal International, 200(1), 589–595. https://doi.org/10.1093/gji/ggu410

SUMMARY Quantifying the capacity of soils to immobilize sorbing contaminants of concern relies on batch sorption experiments, typically performed at skewed solid-to-liquid ratios. The geophysical method spectral induced polarization (SIP) provides a powerful non-invasive monitoring alternative that can capture changes in soil electrical properties driven by contaminant sorption, yielding an approach whereby immobilization can be monitored in situ. Here, we present SIP signals obtained from a series of columns packed with a water saturated natural sandy-loam soil, with and without solid organic matter (SOM) amendment, contaminated with increasing concentrations of the herbicide paraquat. Our results highlight that soil polarization drops proportional to increasing amounts of sorbed paraquat in the SOM-free soil, exhibiting a Langmuir-type leveling-off behaviour. The addition of 8 percent-SOM yielded an increase in both the real ($\sigma ^{\prime}$) and imaginary ($\sigma ^{\prime\prime}$) conductivity of the uncontaminated treatment, driven by the contribution of charged sites in the SOM. Further, SOM modified the dependence between $\sigma ^{\prime\prime}$ and sorbed paraquat, likely driven by continued polarization within the polydisperse SOM with continued paraquat addition. However, the time constant ($\tau $), derived using the Cole–Cole model, shed light on a saturation-type dependence governed by a drop in ion mobility with increasing sorption. Thus, aiding the interpretation of sorption-driven SIP signals.

Glyphosate, an ionizable organic herbicide, is frequently detected in soils and groundwater globally despite its strong retention via sorption. Understanding its apparent mobility hinges on our ability to quantify its system-specific sorption behavior, hindered by its affinity to adsorb onto sediments, yielding very low aqueous concentrations. Here, we present findings from a saturated flow-through column experiment in which we monitored glyphosate sorption onto a natural calcareous aquifer sediment, using the noninvasive geophysical method spectral induced polarization (SIP). Our kinetic sorption reactive transport model predicted the strong nonlinear reversible retention of glyphosate and reproduced the spatial profile of retained glyphosate in the sediment, with a measured maximum of 0.06 mg g

Compound-specific isotope analysis (CSIA) can reveal mass-transfer limitations during biodegradation of organic pollutants by enabling the detection of masked isotope fractionation. Here, we applied CSIA to monitor the adaptive response of bacterial degradation in inoculated sediment to low contaminant concentrations over time. We characterized Aminobacter sp. MSH1 activity in a flow-through sediment tank in response to a transient supply of elevated 2,6-dichlorobenzamide (BAM) concentrations as a priming strategy and took advantage of an inadvertent intermittence to investigate the effect of short-term flow fluctuations. Priming and flow fluctuations yielded improved biodegradation performance and increased biodegradation capacity, as evaluated from bacterial activity and residual concentration time series. However, changes in isotope ratios in space and over time evidenced that mass transfer became increasingly limiting for degradation of BAM at low concentrations under such stimulated conditions, and that activity decreased further due to bacterial adaptation at low BAM (μg/L) levels. Isotope ratios, in conjunction with residual substrate concentrations, therefore helped identifying underlying limitations of biodegradation in such a stimulated system, offering important insight for future optimization of remediation schemes.

This datasheet on Alosa aestivalis covers Identity, Overview, Distribution, Dispersal, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Uses, Prevention/Control, Further Information.

This datasheet on Lepomis microlophus covers Identity, Overview, Distribution, Dispersal, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Uses, Prevention/Control, Further Information.

This datasheet on Bythotrephes longimanus covers Identity, Overview, Distribution, Dispersal, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Prevention/Control, Further Information.

This datasheet on Perca flavescens covers Identity, Overview, Distribution, Dispersal, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Uses, Prevention/Control, Further Information.

The interface between rivers and groundwater is a key driver for the turnover of reactive nitrogen compounds, that cause eutrophication of rivers and endanger drinking-water production from groundw...

Anoxygenic phototrophic Fe(II)-oxidizers (photoferrotrophs) are thought to have thrived in Earth’s ancient ferruginous oceans and played a primary role in the precipitation of Archean and Paleoproterozoic (3.8-1.85 Ga) banded iron formations (BIF). The end of BIF deposition by photoferrotrophs has often been interpreted as being the result a deepening of water column oxygenation below the photic zone concomitant with the proliferation of cyanobacteria. We suggest here that a potentially overlooked aspect influencing BIF precipitation by photoferrotrophs is competition with another anaerobic Fe(II)-oxidizing metabolism. It is speculated that microorganisms capable of coupling Fe(II) oxidation to the reduction of nitrate were also present early in Earth history when BIF were being deposited, but the extent to which they could compete with photoferrotrophs when favourable geochemical conditions overlapped is unknown. Utilizing microbial incubations and numerical modelling, we show that nitrate-reducing Fe(II)-oxidizers metabolically outcompete photoferrotrophs for dissolved Fe(II). Moreover, the nitrate-reducing Fe(II)-oxidizers inhibit photoferrotrophy via the production of toxic nitric oxide (NO). Four different photoferrotrophs, representing both green sulfur and purple non-sulfur bacteria, are susceptible to this toxic effect despite having genomic capabilities for NO detoxification. Indeed, despite NO detoxification mechanisms being ubiquitous in some groups of phototrophs at the genomic level (e.g. Chlorobi and Cyanobacteria) it is likely they would still be influenced by NO stress. We suggest that the production of NO during nitrate-reducing Fe(II) oxidation in ferruginous environments represents an as yet unreported control on the activity of photoferrotrophs in the ancient oceans and thus the mechanisms driving precipitation of BIF.

Neutrophilic microbial pyrite (FeS2) oxidation coupled to denitrification is thought to be an important natural nitrate attenuation pathway in nitrate-contaminated aquifers. However, the poor solubility of pyrite raises questions about its bioavailability and the mechanisms underlying its oxidation. Here, we investigated direct microbial pyrite oxidation by a neutrophilic chemolithoautotrophic nitrate-reducing Fe(II)-oxidizing culture enriched from a pyrite-rich aquifer. We used pyrite with natural abundance (NA) of Fe isotopes (NAFe-pyrite) and 57Fe-labeled siderite to evaluate whether the oxidation of the more soluble Fe(II)-carbonate (FeCO3) can indirectly drive abiotic pyrite oxidation. Our results showed that in setups where only pyrite was incubated with bacteria, direct microbial pyrite oxidation contributed ca. 26% to overall nitrate reduction. The rest was attributed to the oxidation of elemental sulfur (S0), present as a residue from pyrite synthesis. Pyrite oxidation was evidenced in the NAFe-pyrite/57Fe-siderite setups by maps of 56FeO and 32S obtained using a combination of SEM with nanoscale secondary ion MS (NanoSIMS), which showed the presence of 56Fe(III) (oxyhydr)oxides that could solely originate from 56FeS2. Based on the fit of a reaction model to the geochemical data and the Fe-isotope distributions from NanoSIMS, we conclude that anaerobic oxidation of pyrite by our neutrophilic enrichment culture was mainly driven by direct enzymatic activity of the cells. The contribution of abiotic pyrite oxidation by Fe3+ appeared to be negligible in our experimental setup.

Biomolecular quantities like gene, transcript or enzyme concentrations related to a specific reaction promise to provide information about the turnover of nutrients or contaminants in the environment. Particularly transcript-to-gene ratios have been suggested to provide a measure for reaction rates but a relationship with rates currently lacks validation.We applied an enzyme-based reactive transport model for denitrification and aerobic respiration at the river-groundwater interface to simulate the temporal and spatial patterns of transcripts, enzymes and biomass under diurnal dissolved oxygen fluctuations.Our analysis showed that transcript concentrations of denitrification genes exhibit considerable diurnal fluctuations, whereas enzyme concentrations and biomass are stable over time. The daily fluctuations in denitrification rates yielded a poor correlation between rates and transcript and enzyme concentrations. Daily averaged reaction rates, however, show a close-to-linear relationship with enzyme concentrations and mean transcript concentrations.Our findings suggest that, under dynamic environmental conditions, single-event sampling may result in the misinterpretation of biomelucular quantities as these relate to reaction rates. A better representation of rates can be achieved via sampling that captures the temporal variability of a particular system.

Organic contaminant degradation by suspended bacteria in chemostats has shown that isotope fractionation decreases dramatically when pollutant concentrations fall below the (half-saturation) Monod constant. This masked isotope fractionation implies that membrane transfer is slow relative to the enzyme turnover at μg L–1 substrate levels. Analogous evidence of mass transfer as a bottleneck for biodegradation in aquifer settings, where microbes are attached to the sediment, is lacking. A quasi-two-dimensional flow-through sediment microcosm/tank system enabled us to study the aerobic degradation of 2,6-dichlorobenzamide (BAM), while collecting sufficient samples at the outlet for compound-specific isotope analysis. By feeding an anoxic BAM solution through the center inlet port and dissolved oxygen (DO) above and below, strong transverse concentration cross-gradients of BAM and DO yielded zones of low (μg L–1) steady-state concentrations. We were able to simulate the profiles of concentrations and isotope ratios of the contaminant plume using a reactive transport model that accounted for a mass-transfer limitation into bacterial cells, where apparent isotope enrichment factors *ε decreased strongly below concentrations around 600 μg/L BAM. For the biodegradation of organic micropollutants, mass transfer into the cell emerges as a bottleneck, specifically at low (μg L–1) concentrations. Neglecting this effect when interpreting isotope ratios at field sites may lead to a significant underestimation of biodegradation., Please provide a synopsis

Rising industrial interest in the application of nanomaterials for the remediation of contaminated sites has led to concern over the environmental fate of the nanoremediation agents used. A critical requirement in evaluating and understanding nanoparticle (NP) behaviour in porous media is the development of analytical methods capable of in situ monitoring of complex NP transport dynamics. Spectral induced polarization (SIP), a non-invasive geo-electrical technique, offers a promising tool for detecting and quantifying NPs in soil and aquifer media. However, its application for monitoring the spatial migration and attachment behaviour of NPs remains uninvestigated. Here, we present results from flow-through experiments where we monitored the transport of cobalt ferrite nanoparticles (CoFe-NPs) coated with Pluronic, an amphiphilic polymer, in natural aquifer sand columns. We coupled concentration breakthrough curve analysis with SIP monitoring and reactive transport modeling to relate spatiotemporal NP concentration distributions to geo-electrical signals. Changes in the real (σ′) conductivity at three different locations along the columns closely correlated with model-computed total (solid plus aqueous phase) NP concentrations during the propagation of a NP slug. The imaginary conductivity (σ″) correlated closely with the arrival of the NP-slug. However, during the receding front, bimodal σ″-signal peak behaviour was observed propagating through the columns, indicating the existence of complex in situ NP transport dynamics, potentially revealing the rupture of nanoclusters upon straining and their effect on bulk charge storage that may not be obvious from breakthrough curve data alone. Fitting of a double Cole-Cole relaxation model yielded distinct shifts in relaxation time (τ) associated with the polarization of smaller length-scale particles. Post-NP pulse τ and σ″ did not return to pre-injection values; these lingering signals were caused by retained NP concentrations as low as 8.8 mg kg−1. Our results support the applicability of SIP for spatial and temporal monitoring of NP distributions, with implications for the investigation of NP transport and nanoremediation strategies.

Redox-active organic molecules such as anthraquinone-2,6-disulfonate (AQDS) and natural organic matter (NOM) can act as electron shuttles thus facilitating electron transfer from Fe(III)-reducing bacteria (FeRB) to terminal electron acceptors such as Fe(III) minerals. In this research, we examined the length scale over which this electron shuttling can occur. We present results from agar-solidified experimental incubations, containing either AQDS or NOM, where FeRB were physically separated from ferrihydrite or goethite by 2 cm. Iron speciation and concentration measurements coupled to a diffusion-reaction model highlighted clearly Fe(III) reduction in the presence of electron shuttles, independent of the type of FeRB. Based on our fitted model, the rate of ferrihydrite reduction increased from 0.07 to 0.19 μmol d-1 with a 10-fold increase in the AQDS concentration, highlighting a dependence of the reduction rate on the electron-shuttle concentration. To capture the kinetics of Fe(II) production, the effective AQDS diffusion coefficient had to be increased by a factor of 9.4. Thus, we postulate that the 2 cm electron transfer was enabled by a combination of AQDS molecular diffusion and an electron hopping contribution from reduced to oxidized AQDS molecules. Our results demonstrate that AQDS and NOM can drive microbial Fe(III) reduction across 2 cm distances and shed light on the electron transfer process in natural anoxic environments.

The development of nanoparticle-based soil remediation techniques is hindered by the lack of accurate in situ nanoparticle (NP) monitoring and characterization methods. Spectral induced polarization (SIP), a noninvasive geophysical technique, offers a promising approach to detect and quantify NPs in porous media. However, its successful implementation as a monitoring tool requires an understanding of the polarization mechanisms, the governing NP-associated SIP responses and their dependence on the stabilizing coatings that are typically used for NPs deployed in environmental applications. Herein, we present SIP responses (0.1-10 000 Hz) measured during injection of a poloxamer-coated superparamagnetic iron-oxide nanoparticle (SPION) suspension in flow-through columns packed with natural sand from the Borden aquifer. An advective-dispersive transport model is fitted to outflow SPION concentration measurements to compute average concentrations over the SIP spatial response domain (within the columns). The average SPION concentrations are compared with the real and imaginary components of the complex conductivity. Excellent correspondence is found between the average SPION concentrations in the columns and the imaginary conductivity values, suggesting that NP-mediated polarization (that is, charge storage) increases proportionally with increasing SPION concentration. Our results support the possibility of SIP monitoring of spatial and temporal NP distributions, which can be immediately deployed in bench-scale studies with the prospect of future real-world field applications.

Magnetite nanoparticles are promising materials for treating toxic Cr(VI), but safe handling is challenging due to their small size. We prepared flow-through columns containing 10% or 100% (v/v) magnetite-coated sand. Cr(VI) removal efficiency was determined for different Cr(VI) concentrations (0.1 or 1.0 mM), neutral or alkaline pH, and oxic/anoxic conditions. We formulated a reactive-transport model that accurately predicted total Cr removal, accounting for reversible and irreversible (chemi)sorption reactions. Our results show that the material removes and irreversibly sequesters Cr(VI). For the concentration range used 10% and 100% (v/v) -packed columns removed > 99% and 72% of influent Cr(VI), respectively. Two distinct parameter sets were necessary to fit the identical model formulation to the 10 or 100% (v/v) columns (e.g., maximum sorption capacities (qmax) of 1.37 µmol Cr/g sand and 2.48 µmol Cr/g, respectively), which we attributed to abrasion-driven magnetite micro-particle detachment during packing yielding an increase in reactive surface area. Furthermore, experiments under oxic conditions showed that, even when handled in the presence of O2, the magnetite-coated sand maintained a high removal capacity (47%). Our coupled experimental and modelling analyses indicates that magnetite-coated sand is a promising and suitable medium for treating Cr(VI)-contaminated water in fixed-bed reactors or permeable reactive barriers.

This paper provides an update on the fast‐evolving field of the induced polarization method applied to biogeophysics. It emphasizes recent advances in the understanding of the induced polarization signals stemming from biological materials and their activity, points out new developments and applications, and identifies existing knowledge gaps. The focus of this review is on the application of induced polarization to study living organisms: soil microorganisms and plants (both roots and stems). We first discuss observed links between the induced polarization signal and microbial cell structure, activity and biofilm formation. We provide an up‐to‐date conceptual model of the electrical behaviour of the microbial cells and biofilms under the influence of an external electrical field. We also review the latest biogeophysical studies, including work on hydrocarbon biodegradation, contaminant sequestration, soil strengthening and peatland characterization. We then elaborate on the induced polarization signature of the plant‐root zone, relying on a conceptual model for the generation of biogeophysical signals from a plant‐root cell. First laboratory experiments show that single roots and root system are highly polarizable. They also present encouraging results for imaging root systems embedded in a medium, and gaining information on the mass density distribution, the structure or the physiological characteristics of root systems. In addition, we highlight the application of induced polarization to characterize wood and tree structures through tomography of the stem. Finally, we discuss up‐ and down‐scaling between laboratory and field studies, as well as joint interpretation of induced polarization and other environmental data. We emphasize the need for intermediate‐scale studies and the benefits of using induced polarization as a time‐lapse monitoring method. We conclude with the promising integration of induced polarization in interdisciplinary mechanistic models to better understand and quantify subsurface biogeochemical processes.

Spectral induced polarization signatures have been used as proxies for microbial abundance in subsurface environments, by taking advantage of the charged properties of microbial cell membranes. The method's applicability, however, remains qualitative, and signal interpretation ambiguous. The adoption of spectral induced polarization as a robust geo‐microbiological tool for monitoring microbial dynamics in porous media requires the development of quantitative relationships between biogeochemical targets and spectral induced polarization parameters, such as biomass density and imaginary conductivity (σ″). Furthermore, deriving cell density information from electrical signals in porous media necessitates a detailed understanding of the nature of the cell membrane surface charge dynamics. We present results from a fully saturated sand‐filled column reactor experiment where Shewanella oneidensis growth during nitrate reduction to ammonium was monitored using spectral induced polarization. While our results further confirm the direct dependence of σ″ on changing cell density, Cole–Cole derived relaxation times also record the changing surface charging properties of the cells, ascribed to toxic stress due to nitrite accumulation. Concurrent estimates of cell size yield the first measurement‐derived estimation of the apparent surface ion diffusion coefficient for cells (Ds = 5.4 ±1.3 µm2 s−1), strengthening the link between spectral induced polarization and electrochemical cell polarization. Our analysis provides a theoretical framework on which to build σ″–cell density relations using bench‐scale experiments, leading to eventual robust non‐destructive monitoring of in situ microbial growth dynamics., Canada Excellence Research Chair programme, Waterloo‐Technion University Cooperation Programme

The microbially mediated reactions, that are responsible for field-scale natural attenuation of organic pollutants, are governed by the concurrent presence of a degrading microbial community, suitable energy and carbon sources, electron acceptors, as well as nutrients. The temporal lack of one of these essential components for microbial activity, arising from transient environmental conditions, might potentially impair in situ biodegradation. This study presents results of small scale flow-through experiments aimed at ascertaining the effects of substrate-starvation periods on the aerobic degradation of toluene by Pseudomonas putida F1. During the course of the experiments, concentrations of attached and mobile bacteria, as well as toluene and oxygen were monitored. Results from a fitted reactive-transport model, along with the observed profiles, show the ability of attached cells to survive substrate-starvation periods of up to four months and suggest a highly dynamic exchange between attached and mobile cells under growth conditions and negligible cell detachment under substrate-starvation conditions. Upon reinstatement of toluene, it was readily degraded without a significant lag period, even after a starvation period of 130 days. Our experimental and modeling results strongly suggest that aerobic biodegradation of BTEX-hydrocarbons at contaminated field sites is not hampered by intermittent starvation periods of up to four months.

Autotrophic nitrate-reducing Fe(II)-oxidizing (NRFeOx) microorganisms fix CO2 and oxidize Fe(II) coupled to denitrification, influencing carbon, iron, and nitrogen cycles in pH-neutral, anoxic environments. However, the distribution of electrons from Fe(II) oxidation to either biomass production (CO2 fixation) or energy generation (nitrate reduction) in autotrophic NRFeOx microorganisms has not been quantified. We therefore cultivated the autotrophic NRFeOx culture KS at different initial Fe/N ratios, followed geochemical parameters, identified minerals, analyzed N isotopes, and applied numerical modeling. We found that at all initial Fe/N ratios, the ratios of Fe(II)(oxidized) to nitrate(reduced) were slightly higher (5.11 to 5.94 at Fe/N ratios of 10:1 and 10:0.5) or lower (4.27 to 4.59 at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1) than the theoretical ratio for 100% Fe(II) oxidation being coupled to nitrate reduction (5:1). The main N denitrification product was N2O (71.88 to 96.29% at Fe/N-15 ratios of 10:4 and 5:1; 43.13 to 66.26% at an Fe/N-15 ratio of 10:1), implying that denitrification during NRFeOx was incomplete in culture KS. Based on the reaction model, on average 12% of electrons from Fe(II) oxidation were used for CO2 fixation while 88% of electrons were used for reduction of NO3- to N2O at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1. With 10 mM Fe(II) (and 4, 2, 1, or 0.5 mM nitrate), most cells were closely associated with and partially encrusted by the Fe(III) (oxyhydr)oxide minerals, whereas at 5 mM Fe(II), most cells were free of cell surface mineral precipitates. The genus Gallionella (>80%) dominated culture KS regardless of the initial Fe/N ratios. Our results showed that Fe/N ratios play a key role in regulating N2O emissions, for distributing electrons between nitrate reduction and CO2 fixation, and for the degree of cell-mineral interactions in the autotrophic NRFeOx culture KS.IMPORTANCE Autotrophic NRFeOx microorganisms that oxidize Fe(II), reduce nitrate, and produce biomass play a key role in carbon, iron, and nitrogen cycles in pH-neutral, anoxic environments. Electrons from Fe(II) oxidation are used for the reduction of both carbon dioxide and nitrate. However, the question is how many electrons go into biomass production versus energy generation during autotrophic growth. Here, we demonstrated that in the autotrophic NRFeOx culture KS cultivated at Fe/N ratios of 10:4, 10:2, 5:2, and 5:1, ca. 12% of electrons went into biomass formation, while 88% of electrons were used for reduction of NO3- to N2O. Isotope analysis also showed that denitrification during NRFeOx was incomplete in culture KS and the main N denitrification product was N2O. Therefore, most electrons stemming from Fe(II) oxidation seemed to be used for N2O formation in culture KS. This is environmentally important for the greenhouse gas budget., Autotrophic NRFeOx microorganisms that oxidize Fe(II), reduce nitrate, and produce biomass play a key role in carbon, iron, and nitrogen cycles in pH-neutral, anoxic environments. Electrons from Fe(II) oxidation are used for the reduction of both carbon dioxide and nitrate.