Achieving deep autotrophic nitrogen removal in aerated biofilter driven by sponge iron: Performance and mechanism.

Different from anammox, the combination of Fe (III) reduction coupled to anaerobic ammonium oxidation (Feammox) and nitrate/nitrite dependent ferrous oxidation (NDFO) do not require to control nitrite accumulation. Furthermore, sponge iron can avoid continuous iron supplementation in practice and is a good iron source for the occurrence of Feammox and NDFO in wastewater treatment. Therefore, a biofilter using sponge iron as carrier treating low nitrogen wastewater was built. In this study, the performances of nitrogen removal were explored under different hydraulic retention times (HRT) and gas-water ratios in sponge iron biofilter. And the pathways of nitrogen removal were analyzed by activity tests. The results showed ammonia removal efficiency reached 94.1% and total inorganic nitrogen removal efficiency was up to 70.6% at HRT of 19 h and gas-water ratio of 18. Compared to nitrogen removal by adsorption under non-aeration, the activity tests showed that total inorganic nitrogen loss was caused by Feammox and NDFO after aeration. The results of microbial communities showed that appearances of nitrifier-Nitrosomonadaceae, Feammox bacteria-Clostridiaceae and NDFO bacteria-Gallionellaceae resulted in deep nitrogen removal after aeration, in which Nitrosomonadaceae and Clostridiaceae contributed to ammonia removal and Gallionellaceae contributed to nitrite/nitrate reduction to nitrogen gas. Therefore, it was feasible to achieve deep autotrophic nitrogen removal and Fe (II) and Fe (III) cycle in sponge iron biofilter.

Insights into Carbon Metabolism Provided by Fluorescence In Situ Hybridization-Secondary Ion Mass Spectrometry Imaging of an Autotrophic, Nitrate-Reducing, Fe(II)-Oxidizing Enrichment Culture.

The enrichment culture KS is one of the few existing autotrophic, nitrate-reducing, Fe(II)-oxidizing cultures that can be continuously transferred without an organic carbon source. We used a combination of catalyzed amplification reporter deposition fluorescence in situ hybridization (CARD-FISH) and nanoscale secondary ion mass spectrometry (NanoSIMS) to analyze community dynamics, single-cell activities, and interactions among the two most abundant microbial community members (i.e., Gallionellaceae sp. and Bradyrhizobium spp.) under autotrophic and heterotrophic growth conditions. CARD-FISH cell counts showed the dominance of the Fe(II) oxidizer Gallionellaceae sp. under autotrophic conditions as well as of Bradyrhizobium spp. under heterotrophic conditions. We used NanoSIMS to monitor the fate of 13C-labeled bicarbonate and acetate as well as 15N-labeled ammonium at the single-cell level for both taxa. Under autotrophic conditions, only the Gallionellaceae sp. was actively incorporating 13C-labeled bicarbonate and 15N-labeled ammonium. Interestingly, both Bradyrhizobium spp. and Gallionellaceae sp. became enriched in [13C]acetate and [15N]ammonium under heterotrophic conditions. Our experiments demonstrated that Gallionellaceae sp. was capable of assimilating [13C]acetate while Bradyrhizobium spp. were not able to fix CO2, although a metagenomics survey of culture KS recently revealed that Gallionellaceae sp. lacks genes for acetate uptake and that the Bradyrhizobium sp. carries the genetic potential to fix CO2 The study furthermore extends our understanding of the microbial reactions that interlink the nitrogen and Fe cycles in the environment.IMPORTANCE Microbial mechanisms by which Fe(II) is oxidized with nitrate as the terminal electron acceptor are generally referred to as "nitrate-dependent Fe(II) oxidation" (NDFO). NDFO has been demonstrated in laboratory cultures (such as the one studied in this work) and in a variety of marine and freshwater sediments. Recently, the importance of NDFO for the transport of sediment-derived Fe in aquatic ecosystems has been emphasized in a series of studies discussing the impact of NDFO for sedimentary nutrient cycling and redox dynamics in marine and freshwater environments. In this article, we report results from an isotope labeling study performed with the autotrophic, nitrate-reducing, Fe(II)-oxidizing enrichment culture KS, which was first described by Straub et al. (1) about 20 years ago. Our current study builds on the recently published metagenome of culture KS (2).

High diversity of potential nitrate-reducing Fe(II)-oxidizing bacteria enriched from activated sludge.

Nitrate-dependent Fe(II) oxidation (NDFO) has been discovered in various environments including activated sludge and can potentially be used to remove nitrate from wastewater. In this study, NDFO sludge was successfully enriched from activated sludge under high Fe(II) concentrations over 100 days and the denitrification rate achieved 1.37 mmol N/(gVSS day). High-throughput sequencing of the bacterial 16S rRNA gene was used to investigate the microbial community structure dynamics during the enrichment process. The results showed that the microbial community changed significantly and high diversity of potential Fe(II)-oxidizing bacteria (FeOB) was observed in the enriched sludge. Thermomonas and Gallionella were the dominant bacterial genera in the enriched sludge and their relative abundances accounted for 9.49 and 4.08%, respectively. Furthermore, it was found that potential FeOB were also abundantly present in activated sludge samples of common municipal wastewater treatment plants. Collectively, this study demonstrated that NDFO could be successfully performed by enriched activated sludge and high diversity of bacteria is involved in this process, and the results also provide baseline information for future research and engineering application of NDFO process.

"Candidatus Siderophilus nitratireducens": a putative nap-dependent nitrate-reducing iron oxidizer within the new order Siderophiliales.

Nitrate leaching from agricultural soils is increasingly found in groundwater, a primary source of drinking water worldwide. This nitrate influx can potentially stimulate the biological oxidation of iron in anoxic groundwater reservoirs. Nitrate-dependent iron-oxidizing (NDFO) bacteria have been extensively studied in laboratory settings, yet their ecophysiology in natural environments remains largely unknown. To this end, we established a pilot-scale filter on nitrate-rich groundwater to elucidate the structure and metabolism of nitrate-reducing iron-oxidizing microbiomes under oligotrophic conditions mimicking natural groundwaters. The enriched community stoichiometrically removed iron and nitrate consistently with the NDFO metabolism. Genome-resolved metagenomics revealed the underlying metabolic network between the dominant iron-dependent denitrifying autotrophs and the less abundant organoheterotrophs. The most abundant genome belonged to a new Candidate order, named Siderophiliales. This new species, "Candidatus Siderophilus nitratireducens," carries genes central genes to iron oxidation (cytochrome c cyc2), carbon fixation (rbc), and for the sole periplasmic nitrate reductase (nap). Using thermodynamics, we demonstrate that iron oxidation coupled to nap based dissimilatory reduction of nitrate to nitrite is energetically favorable under realistic Fe3+/Fe2+ and NO3-/NO2- concentration ratios. Ultimately, by bridging the gap between laboratory investigations and nitrate real-world conditions, this study provides insights into the intricate interplay between nitrate and iron in groundwater ecosystems, and expands our understanding of NDFOs taxonomic diversity and ecological role.

Zeolite enhanced iron-modified biocarrier drives Fe(II)/Fe(III) cycle to achieve nitrogen removal from eutrophic water.

The problem of nitrogen removal in eutrophic water needs to be solved. Two new autotrophic nitrogen removal technologies, ammonia oxidation coupled with Fe(III) reduction (Feammox) and Nitrate-dependent Fe(II) oxidation (NDFO), have been shown to have the potential to treat eutrophic water. However, the continuous addition of iron sources not only costs more, but also leads to sludge mineralization. In this study, nano-sized iron powder was loaded on the surface of K3 filler as a solid iron source for the extracellular metabolism of iron-trophic bacteria. At the same time, due to the high selective adsorption of zeolite for ammonia can improve the low nitrogen metabolism rate caused by low nitrogen concentrations in eutrophic water, three kinds of modified functional biological carriers were prepared by mixing zeolite powder and iron powder in different proportions (Z1, Zeolite:iron = 1; Z2, Zeolite:iron = 2; Z3, Zeolite:iron = 3). Z3 exhibited the best performance, with removal efficiencies of 54.8% for total nitrogen during 70 days of cultivation. The chemical structure and state of iron compounds changed under microorganism activity. The ex-situ test detected high NDFO and Feammox activities, with values of 1.02 ± 0.23 and 0.16 ± 0.04 mgN/gVSS/h. The enrichment of NDFO bacteria (Gallionellaceae, 0.73%-1.43%-0.74%) and Feammox bacteria (Alicycliphilus, 1.51%-0.88%-2.30%) indicated that collaboration between various functional microorganisms led to autotrophic nitrogen removal. Hence, zeolite/iron-modified biocarrier could drive the Fe(II)/Fe(III) cycle to remove nitrogen autotrophically from eutrophic water without carbon and Fe resource addition.

Fe3+/Fe2+ cycling drove novel ammonia oxidation and simultaneously removed lead, cadmium, and copper.

The discharge of several pollutants, such as ammonia (NH4+-N), nitrate (NO3--N), and heavy metals, from aquaculture wastewater into the aquatic environment can cause severe pollution issues. In this work, microbial techniques were employed to enable concurrent elimination of NH4+-N and NO3--N by Fe3+/Fe2+ cycling. The greatest NH4+-N and NO3--N removal efficiencies of 96.1 % and 97.6 % were gained by Aquabacterium sp. XL4 at NH4+/NO3- ratio of 1:1, carbon to nitrogen ratio of 4.0, pH of 6.5, and Fe3+ dosage of 20.0 mg L-1. Inhibitor and nitrogen balance assays suggested that nitrogen removal process of strain XL4 was a coupled function of anaerobic ammonia oxidation, ferric reduction driven ammonia oxidation, and iron-based denitrification. Furthermore, under the compound influence of strain XL4 metabolic processes and microbial iron oxide adsorption, the removal efficiencies of Pb2+, Cd2+, and Cu2+ reached above 90 %. This work contributes to theoretical grounding for microbial removal of multiple pollutants.

Novel insights into Feammox coupled with the NDFO: A critical review.

Ammonium oxidation coupled with Fe(III) reduction, known as Feammox, and nitrate-dependent ferrous oxidation (NDFO) are two processes that can be synergistically achieved through the Fe(III)/Fe(II) cycle. This integrated approach enables the simultaneous removal of ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) from wastewater, representing a novel method for complete nitrogen removal. This study presents a systematic and exhaustive examination of the Feammox-NDFO coupled process. An initial thorough exploration of the underlying mechanisms behind the coupling process is conducted, highlighting how the Fe(III)/Fe(II) cycle enables the concurrent occurrence of these reactions. Further, the functional microorganisms associated with and playing a crucial role in the Feammox-NDFO process are summarized. Next, the key influencing factors that govern the efficiency of the Feammox-NDFO process are explored. These include parameters such as pH, temperature, carbon source, iron source, nitrogen source, and various electron shuttles that may mediate electron transfer. Understanding the impact of these factors is essential for optimizing the process. The most recent trends and endeavors on the Feammox-NDFO coupling technology in wastewater treatment applications are also examined. This includes examining both laboratory-scale studies and field trials, highlighting their successes and challenges. Finally, an outlook is presented regarding the future advancement of the Feammox-NDFO technology. Areas of improvement and novel strategies that could further enhance the efficiency of simultaneous nitrogen removal from the iron cycle are discussed. In summary, this study aspires to offer a thorough comprehension of the Feammox-NDFO coupled process, with a focus on its mechanisms, influencing factors, applications, and prospects. It is anticipated to yield invaluable insights for the advancement of process optimization, thus sparking fresh ideas and strategies aimed at accomplishing the thorough elimination of nitrogen from wastewater via the iron cycle.

New insights and enhancement mechanisms of activated carbon in autotrophic denitrification system utilizing zero-valent iron as indirect electron donors.

Zero-valent iron acts as an indirect electron donor, supplying ferrous iron for the nitrate-dependent ferrous oxidation (NDFO) process. The addition of activated carbon (AC) increased the specific NDFO activity in situ and ex situ by 0.4 mg-N/(d·g VSS) and 2.2 mg-N/(d·g VSS), respectively, due to the enrichment of NDFO bacteria. Furthermore, AC reduced the nitrous oxide emission potential of the sludge, a mechanism that metagenomic analysis suggests may act as a cellular energy storage strategy. During a 196-day experiment, a total nitrogen removal efficiency of 53.7 % was achieved, which may be attributed to the upregulation of key genes involved in iron oxidation and denitrification. Based on these findings, a model involving pilin, 'nanowires,' and a cyc2/?→/(FoxE→FoxY)/?→cymA/Complex III/?-mediated pathway for extracellular electron uptake was proposed. Overall, this work provides a feasible strategy for enhancing the nitrogen removal performance of the ZVI-NDFO process.

The biochar/Fe-modified biocarrier driven simultaneous NDFO and Feammox to remove nitrogen from eutrophic water.

Novelty techniques of Fe(III) reduction coupled to anaerobic ammonium oxidation (i.e. Feammox) and nitrate-dependent Fe(II) oxidation (i.e. NDFO) provide new insights into autotrophic nitrogen removal from eutrophic waters. Given that Feammox and NDFO can theoretically complete the simultaneous NH+ 4-N and NO- 3-N removal via Fe(III)/Fe(II) cycle, this study introduces iron powder to the surface of the biocarrier as a solid-phase source of Fe, and biochar was used as an electron shuttle to mix with the iron powder to improve the bioavailability of iron. Batch experiments was carried out for 70 days using simulated eutrophic water as the medium to investigate the effects of the modified biocarrier for enhanced nitrogen removal. The results showed that BC1 (Fe:BC=1:1) with the highest relative Fe content exhibited the highest nitrogen removal efficiency of 66.74%. XPS and XRD results showed both Fe(III) and Fe(II) compounds on the biocarrier surface, confirming the occurrence of Fe(III)/Fe(II) cycle. The ex-situ activity test indicated that functional activity was positively correlated with the iron content of the biocarrier. The in-situ experiments with different substrates showed the occurrence of Feammox and NDFO. NDFO bacteria (Gallionellaceae), Feammox bacteria (Alicycliphilus), denitrifying and digesting bacteria were enriched, suggesting that the coupled nitrogen removal of NDFO and Feammox is the result of cooperation between different functional microorganisms. Thus, the Fe-modified biocarrier showed superior performance and application potential in catalyzing autotrophic nitrogen removal from eutrophic water by functional microorganisms.

Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process.

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFOchem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Simultaneous removal of COD and NH4+-N from domestic sewage by a single-stage up-flow anaerobic biological filter based on Feammox.

In recent years, Feammox has made it possible to remove NH4+-N under anaerobic conditions; however, its application in practical wastewater treatment processes has not been extensively reported. In this study, an up-flow anaerobic biological filter based on limonite (Lim-UAF) was developed to facilitate long-term and stable treatment of domestic sewage. Lim-UAF achieved the highest removal efficiency of chemical oxygen demand (COD) and NH4+-N at a hydraulic retention time (HRT) of 24 h (Stage II). Specifically, the COD and NH4+-N content decreased from 240.8 and 30.0 mg/L to about 7.5 and 0.35 mg/L, respectively. To analyze the potential nitrogen removal mechanism, the Lim-UAF was divided into three layers according to the height of the reactor. The results showed that COD and NH4+-N removal had remarkable characteristics in Lim-UAF. More than 55.0% of influent COD was removed in the lower layer (0-30 cm) of Lim-UAF, while 60.2% of NH4+-N was removed in the middle layer (30-60 cm). Microbial community analysis showed that the community structure in the middle and upper layers (60-90 cm) was relatively similar, but quite different from that of the lower layer. Heterotrophic bacteria were dominant in the lower layer, whereas iron-reducing and iron-oxidizing bacteria were enriched in the upper and middle layers. The formation of secondary minerals (siderite and Fe(OH)3) indicated that the Fe(III)/Fe(II) redox cycle occurred in Lim-UAF, which was triggered by the Feammox and NDFO processes. In summary, limonite was used to develop a single-stage wastewater treatment process for simultaneously removing organic matter and NH4+-N, which has excellent application prospects in domestic sewage treatment.

Oligotrophic Growth of Nitrate-Dependent Fe2+-Oxidising Microorganisms Under Simulated Early Martian Conditions.

Nitrate-dependent Fe2+ oxidation (NDFO) is a microbially mediated process observed in many anaerobic, low-nutrient (oligotrophic) neutral-alkaline environments on Earth, which describes oxidation of Fe2+ to Fe3+ in tandem with microbial nitrate reduction. Evidence suggests that similar environments existed on Mars during the Noachian epoch (4.1-3.7 Ga) and in periodic, localised environments more recently, indicating that NDFO metabolism could have played a role in a potential early martian biosphere. In this paper, three NDFO microorganisms, Acidovorax sp. strain BoFeN1, Pseudogulbenkiania sp. strain 2002 and Paracoccus sp. strain KS1, were assessed for their ability to grow oligotrophically in simulated martian brines and in a minimal medium with olivine as a solid Fe2+ source. These simulant-derived media were developed from modelled fluids based on the geochemistry of Mars sample locations at Rocknest (contemporary Mars soil), Paso Robles (sulphur-rich soil), Haematite Slope (haematite-rich soil) and a Shergottite meteorite (common basalt). The Shergottite medium was able to support growth of all three organisms, while the contemporary Mars medium supported growth of Acidovorax sp. strain BoFeN1 and Pseudogulbenkiania sp. strain 2002; however, growth was not accompanied by significant Fe2+ oxidation. Each of the strains was also able to grow in oligotrophic minimal media with olivine as the sole Fe2+ source. Biomineralised cells of Pseudogulbenkiania sp. strain 2002 were identified on the surface of the olivine, representing a potential biosignature for NDFO microorganisms in martian samples. The results suggest that NDFO microorganisms could have thrived in early martian groundwaters under oligotrophic conditions, depending on the local lithology. This can guide missions in identifying palaeoenvironments of interest for biosignature detection. Indeed, biomineralised cells identified on the olivine surface provide a previously unexplored mechanism for the preservation of morphological biosignatures in the martian geological record.

Simultaneous nitrogen and phosphorus removal from municipal wastewater by Fe(III)/Fe(II) cycling mediated partial-denitrification/anammox.

The efficient removal of nitrogen and phosphorus remains challenging for traditional wastewater treatment. In this study, the feasibility for enhancing the partial-denitrification and anammox process by Fe (III) reduction coupled to anammox and nitrate-dependent Fe (II) oxidation was explored using municipal wastewater. The nitrogen removal efficiency increased from 75.5 % to 83.0 % by adding Fe (III). Batch tests showed that NH4+-N was first oxidized to N2 or NO2--N by Fe (III), then NO3--N was reduced to NO2--N and N2 by Fe (II), and finally, NO2--N was utilized by anammox. Furthermore, the performance of phosphorus removal improved by Fe addition and the removal efficiency increased to 78.7 %. High-throughput sequencing showed that the Fe-reducing bacteria Pseudomonas and Thiobacillus were successfully enriched. The abundance of anammox bacterial increased from 0.03 % to 0.22 % by multiple nitrite supply pathways. Fe addition presents a promising pathway for application in the anammox process.

Shewanella sp. T2.3D-1.1 a Novel Microorganism Sustaining the Iron Cycle in the Deep Subsurface of the Iberian Pyrite Belt.

The Iberian Pyrite Belt (IPB) is one of the largest deposits of sulphidic minerals on Earth. Río Tinto raises from its core, presenting low a pH and high metal concentration. Several drilling cores were extracted from the IPB's subsurface, and strain T2.3D-1.1 was isolated from a core at 121.8 m depth. We aimed to characterize this subterranean microorganism, revealing its phylogenomic affiliation (Average Nucleotide Identity, digital DNA-DNA Hybridization) and inferring its physiology through genome annotation, backed with physiological experiments to explore its relationship with the Fe biogeochemical cycle. Results determined that the isolate belongs to the Shewanella putrefaciens (with ANI 99.25 with S. putrefaciens CN-32). Its genome harbours the necessary genes, including omcA mtrCAB, to perform the Extracellular Electron Transfer (EET) and reduce acceptors such as Fe3+, napAB to reduce NO3- to NO2-, hydAB to produce H2 and genes sirA, phsABC and ttrABC to reduce SO32-, S2O32- and S4O62-, respectively. A full CRISPR-Cas 1F type system was found as well. S. putrefaciens T2.3D-1.1 can reduce Fe3+ and promote the oxidation of Fe2+ in the presence of NO3- under anaerobic conditions. Production of H2 has been observed under anaerobic conditions with lactate or pyruvate as the electron donor and fumarate as the electron acceptor. Besides Fe3+ and NO3-, the isolate also grows with Dimethyl Sulfoxide and Trimethyl N-oxide, S4O62- and S2O32- as electron acceptors. It tolerates different concentrations of heavy metals such as 7.5 mM of Pb, 5 mM of Cr and Cu and 1 mM of Cd, Co, Ni and Zn. This array of traits suggests that S. putrefaciens T2.3D-1.1 could have an important role within the Iberian Pyrite Belt subsurface participating in the iron cycle, through the dissolution of iron minerals and therefore contributing to generate the extreme conditions detected in the Río Tinto basin.

Use of sponge iron as an indirect electron donor to provide ferrous iron for nitrate-dependent ferrous oxidation processes: Denitrification performance and mechanism.

Sponge iron (SI) can serve as an indirect electron donor to provide Fe(II) for the nitrate-dependent ferrous oxidation (NDFO) process, producing OH- and magnetite. The SI-NDFO system mainly uses Fe(OH)2 as an electron donor, achieving a TN reduction rate of 0.42 mg-TN/(gVSS·h) for a period of at least 90 days. The enrichment of iron-oxidizing bacteria and the competition of iron-carbon micro-electrolysis for reaction sites on the surface of SI are the main reasons for the improvement of total nitrogen removal efficiency (TNRE). With an influent NO3--N concentration of 50 mg/L and a SI concentration of 50 g/L (at pH 5.0 and 30 °C), the TNRE reached a maximum level of 38.28%. In addition, reducing the pH environment was found to improve the denitrification efficiency of the SI-NDFO system, although denitrification stability was also reduced as a result. Overall, the SI-mediated NDFO process is a promising technique.

Nitrate-Dependent Iron Oxidation: A Potential Mars Metabolism.

This work considers the hypothetical viability of microbial nitrate-dependent Fe2+ oxidation (NDFO) for supporting simple life in the context of the early Mars environment. This draws on knowledge built up over several decades of remote and in situ observation, as well as recent discoveries that have shaped current understanding of early Mars. Our current understanding is that certain early martian environments fulfill several of the key requirements for microbes with NDFO metabolism. First, abundant Fe2+ has been identified on Mars and provides evidence of an accessible electron donor; evidence of anoxia suggests that abiotic Fe2+ oxidation by molecular oxygen would not have interfered and competed with microbial iron metabolism in these environments. Second, nitrate, which can be used by some iron oxidizing microorganisms as an electron acceptor, has also been confirmed in modern aeolian and ancient sediment deposits on Mars. In addition to redox substrates, reservoirs of both organic and inorganic carbon are available for biosynthesis, and geochemical evidence suggests that lacustrine systems during the hydrologically active Noachian period (4.1-3.7 Ga) match the circumneutral pH requirements of nitrate-dependent iron-oxidizing microorganisms. As well as potentially acting as a primary producer in early martian lakes and fluvial systems, the light-independent nature of NDFO suggests that such microbes could have persisted in sub-surface aquifers long after the desiccation of the surface, provided that adequate carbon and nitrates sources were prevalent. Traces of NDFO microorganisms may be preserved in the rock record by biomineralization and cellular encrustation in zones of high Fe2+ concentrations. These processes could produce morphological biosignatures, preserve distinctive Fe-isotope variation patterns, and enhance preservation of biological organic compounds. Such biosignatures could be detectable by future missions to Mars with appropriate instrumentation.

Toward a mechanistic understanding of anaerobic nitrate-dependent iron oxidation: balancing electron uptake and detoxification.

The anaerobic oxidation of Fe(II) by subsurface microorganisms is an important part of biogeochemical cycling in the environment, but the biochemical mechanisms used to couple iron oxidation to nitrate respiration are not well understood. Based on our own work and the evidence available in the literature, we propose a mechanistic model for anaerobic nitrate-dependent iron oxidation. We suggest that anaerobic iron-oxidizing microorganisms likely exist along a continuum including: (1) bacteria that inadvertently oxidize Fe(II) by abiotic or biotic reactions with enzymes or chemical intermediates in their metabolic pathways (e.g., denitrification) and suffer from toxicity or energetic penalty, (2) Fe(II) tolerant bacteria that gain little or no growth benefit from iron oxidation but can manage the toxic reactions, and (3) bacteria that efficiently accept electrons from Fe(II) to gain a growth advantage while preventing or mitigating the toxic reactions. Predictions of the proposed model are highlighted and experimental approaches are discussed.