Nitrogen loss through anaerobic ammonium oxidation coupled to Iron reduction from ecosystem habitats in the Taihu estuary region

Decoding the role of organic matter in groundwater Feammox processes: Insights from the Yangtze River paleochannel

Groundwater nitrogen (N) contamination is becoming increasingly severe worldwide. Anaerobic ammonia oxidation coupled with iron-reduction processes (Feammox) has great potential as an effective method for N removal in groundwater systems. However, previous studies on nitrogen removal by Feammox have generally focused on surface sediment soils, and the quantification of this process in groundwater remains inadequate. Moreover, the impact of native organic matter (OM) within the groundwater system on the Feammox process remains uncertain. The paleochannel of the middle reaches of the Yangtze River was selected as a representative study area for this research. The occurrence of Feammox and other N cycle (non-Feammox) processes in regional groundwater was identified and differentiated through the analysis of δ15N/δ56Fe isotopes and 16S rRNA functional gene quantification, along with hydrochemical characteristics. These findings indicate that the groundwater in the study area is characterized by anoxic conditions and slight acidity. The occurrence of Feammox is substantiated by an increase in δ15NNH4, which coincides with the concurrent increase of Fe(II) concentrations and δ56Fe values in the groundwater, alongside the predominance of Acidimicrobiaceae bacterium A6. 15N isotope-labeled incubation experiments demonstrated that the potential rate of N removal via the Feammox process in the groundwater system ranged from 0.09 to 0.16 mg N kg-1d-1. Correlation results suggested that the functional microorganisms facilitating the Feammox process are closely linked to environmental factors associated with organic matter activity. Terrestrial humic substances present in groundwater, characterized by a high degree of unsaturation, aromaticity, humification, elevated biological activity, and nitrogen-rich composition, may act as pivotal drivers of the Feammox process.

Iron-dependent autotrophic denitrification as a novel microbial driven and iron-mediated denitrification process: A critical review

Based on previous research results, iron-dependent autotrophic denitrification (IDAD) was evaluated in an all-around way to provide a theoretical basis for further research. First, this review systematically and comprehensively summarizes the development of IDAD technology and describes the physiological properties of relevant functional microorganisms and their potential mechanisms from different perspectives. Second, the possible Fe-N pathways involved in the reaction of different iron-based materials are discussed in detail. Then, the theoretical advantages of the IDAD process and potential problems are described, and the corresponding control strategies are summarized. The influence of key factors on denitrification is discussed in terms of operational and water quality parameters. In addition, the application and research direction of this technology in engineering are summarized. Finally, the latest development trends and prospects for future applications are discussed to promote an in-depth understanding of IDAD and its practical application in sewage treatment.

Genome-Resolved Metagenomic and Metatranscriptomics Reveal Feammox Metabolism of Anaerobic Ammonia Oxidation Bacteria in Microaerobic Granular Sludge

Anammox is an energy-efficient nitrogen removal process in which anammox bacteria (AnAOB) oxidize NH4+-N to N2 using NO2--N as the electron acceptor. Recent evidence suggests that AnAOB can perform extracellular electron transfer (EET), potentially coupling Fe(III) reduction with NH4+-N oxidation (Feammox). However, whether AnAOB directly participate in Feammox within complex wastewater treatment systems remains unclear. Here, we investigated the iron-mediated nitrogen metabolism pathways in a microaerobic granular sludge (MGS) reactor by integrating enzyme inhibition assays with analyses of gene dynamics and co-occurrence patterns of nitrogen- and iron-cycling genes. Results demonstrate that AnAOB contributed to Feammox activity. The iron reduction gene CT573071, coding a porin-cytochrome c protein complex associated with EET, co-occurred with hao, hzsABC, and hdh genes in Candidatus Kuenenia, suggesting its role in Feammox. Furthermore, four high-quality metagenome-assembled genomes (MAGs) affiliated with Kuenenia stuttgartiensis_A harbored CT573071, hao-like, hzsABC, and hdh genes, along with the hao-cluster, which catalyzes the oxidation of NH4+-N to hydroxylamine. This genomic evidence further supports their dual metabolic capacity. Metatranscriptomic analysis confirmed CT573071 upregulation and its coexpression with the hao, hzsABC, and hdh genes. These findings establish the potential role of K. stuttgartiensis_A in Feammox, providing novel insights into nitrogen removal in low-strength wastewater treatment systems.

The interactive application and impacts of iron/nitrogen biogeochemical cycling in distributed ponds for non-point source pollution control in a watershed

The linkages of distributed ponds are utilized in conjunction with one another to remediate non-point source (NPS) pollution in a water-scarce basin. This study provides an overview of a state-of-the-art thorough evaluation of ponds, which offers insight into the majority of topics covered by the ongoing scientific studies, including their various functions and factors affecting their functioning on the hydrological, physicochemical, and biological processes, such as environmental climate factors and basin-specific landscape configuration parameters, as well as process parameters for design, operation and management aspects. The linkages of ponds provide a variety of sustainable services (6R functions), such as resources, restoration, reduction, reuse, recycling, and recovery. The significance of regional environmental geochemical substrates in the ponds, such as red soil, as a hotspot for microbial reaction is emphasized to demonstrate the significant contribution of the migration and transformation of Fe/N cycles to the pollution removal process. In this review, 178 original research publications were thoroughly analyzed to improve our knowledge of the iron-nitrogen cycle in wetlands. From a molecular biology standpoint, the identification of functional microbe species and genes linked to microbially driven iron-nitrogen cycle activities is delved. Reliable data and homogeneous datasets from 42 studies were collected. The correlation analysis results demonstrated Feammox rates contributed to the N loss amount (r = 0.871; p < 0.01), and they had a positive correlation with Fe(III) concentration (r = 0.965; p < 0.01). The proposal for the treatment of NPS pollution by large-scale linkages of ponds in a basin involves optimizing Fe/N microbial processes to promote iron crystallization and efficient circulation of Fe(II) and Fe(III). The co-benefits of geochemistry, biotechnology, and environmental science should be considered when managing contamination in engineering applications. The linkages framework for integrated ponds, which incorporates macro (watershed management) and micro (biogeochemical cycle mechanism) investigations, provides a systematic approach to the application of integrated ponds and sustainable water management for NPS pollution control.

Process-based modeling to reveal spatio-temporal variations of coastal wetland-mediated nutrient removal

Nutrient pollution intensifies the strain on coastal ecosystems globally. Despite wetlands' significant nutrient removal potential, process-based assessments of nutrient removal in large-scale coastal wetlands face limitations that hinder accurate quantification of water quality improvements. This study proposes a novel quantitative approach by developing a coupled hydrodynamic-water quality-wetland biogeochemical model. The spatio-temporal distributions of water-sediment-vegetation nutrients in a coastal wetland and bay were simulated over an annual cycle, with model parameters and results calibrated and validated through field investigations and laboratory experiments. The findings show distinct spatio-temporal characteristics of nutrient pollutant reduction in coastal wetlands. In the Liaohe estuarine tidal wetland, a large-scale coastal wetland in China, nitrogen and phosphorus reduction fluxes reached 1184 t·yr-1 and 53 t·yr-1, accounting for 19 % and 12 % of the river's fluxes to the sea, respectively. These results underscore the critical function of coastal wetlands in mitigating coastal nutrient pollution. This study presents a novel framework for quantifying coastal water quality improvements by wetlands on a larger scale.

Efficient manganese ammonia oxidation (Mnammox) and its influencing factors at low temperature: Metal oxide-mediated denitrification process in water bodies

This study explores the metal oxide-mediated NH4+-N reduction process: manganese ammonia oxidation efficiency, influencing factors and its resistance to low-temperature environments in water bodies. After 177d of stabilized startup of an up-flow reactor, NH4+-N removal efficiency was 63.51 %, total nitrogen (TN) removal rate was 0.021 kg/(m3.d), and effluent Mn2+ concentration was 1.503 mg/L, which was in dynamic equilibrium. X-ray photoelectron spectroscopy exhibited manganese valence state 3.29, similar to biological manganese oxidation. High-throughput sequencing revealed that phyla's denitrification function increased relative abundance, and manganese-reducing bacterial genera appeared. The batch test showed that 5 mg MnO2 had NH4+-N removal at 85.01 %. After 44 days, NH4+-N removal efficiency was 77.47 %, effluent Mn2+ concentration was 3.280 mg/L, TN removal rate was 0.063 kg/(m3.d). The long-term effect of the influent load change on the denitrification and Mnammox efficiency at 25 ∼ 15 °C was examined. Effluent Mn2+ concentration was 1.811 mg/L was relatively stable. Manganese valence decreased from 3.29 to 3.20, Mn4+ decreased by 9.58 %, while Mn3+ and Mn2+ increased by 10.94 % and 1.37 %, respectively. A new phylum Thermotogota and genus SBR1031 appeared in the microbial community.

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.

Asynchronous characteristics of Feammox and iron reduction from paddy soils in Southern China

Recently, the newly discovered anaerobic ammonium oxidation coupled with iron reduction (i.e., Feammox) has been proven to be a widespread nitrogen (N) loss pathway in ecosystems and has an essential contribution to gaseous N loss in paddy soil. However, the mechanism of iron-nitrogen coupling transformation and the role of iron-reducing bacteria (IRB) in Feammox were poorly understood. This study investigated the Feammox and iron reduction changes and microbial community evolution in a long-term anaerobic incubation by 15N isotope labeling combined with molecular biological techniques. The average rates of Feammox and iron reduction during the whole incubation were 0.25 ± 0.04 μg N g-1 d-1 and 40.58 ± 3.28 μg Fe g-1 d-1, respectively. High iron oxide content increased the Feammox rate, but decreased the proportion of Feammox-N2 in three Feammox pathways. RBG-13-54-9, Brevundimonas, and Pelomonas played a vital role in the evolution of microbial communities. The characteristics of asynchronous changes between Feammox and iron reduction were found through long-term incubation. IRB might not be the key species directly driving Feammox, and it is necessary to reevaluate the role of IRB in Feammox process.

Unraveling nitrogen loss in paddy soils: A study of anaerobic nitrogen transformation in response to various irrigation practice

Soil nitrogen (N) transformation processes, encompassing denitrification, anaerobic ammonium oxidation (anammox), and anaerobic ammonium oxidation coupled with iron reduction (Feammox), constitute the primary mechanisms of soil dinitrogen (N2) loss. Despite the significance of these processes, there is a notable gap in research regarding the assessment of managed fertilization and irrigation impacts on anaerobic N transformations in paddy soil, crucial for achieving sustainable soil fertility management. This study addressed the gap by investigating the contributions of soil denitrification, anammox, and Feammox to N2 loss in paddy soil across varying soil depths, employing different fertilization and irrigation practices by utilizing N stable isotope technique for comprehensive insights. The results showed that anaerobic N transformation processes decreased with increasing soil depth under alternate wetting and drying (AWD) irrigation, but increased with the increasing soil depth under conventional continuous flooding (CF) irrigation. The denitrification and anammox rates varied from 0.41 to 2.12 mg N kg-1 d-1 and 0.062-0.394 mg N kg-1 d-1, respectively, which accounted for 84.3-88.1% and 11.8-15.7% of the total soil N2 loss. Significant correlations were found among denitrification rate and anammox rate (r = 0.986, p < 0.01), Fe (Ⅲ) reduction rate and denitrification rate (r = 0.527, p < 0.05), and Fe(Ⅲ) reduction rate and anammox rate (r = 0.622, p < 0.05). Moreover, nitrogen loss was more pronounced in the surface layer of the paddy soil compared to the deep layer. The study revealed that denitrification predominantly contributed to N loss in the surface soil, while Feammox emerged as a significant N loss pathway at depths ranging from 20 to 40 cm, accounting for up to 26.1% of the N loss. It was concluded that fertilization, irrigation, and soil depth significantly influenced anaerobic N transformation processes. In addition, the CF irrigation practice is best option to reduce N loss under managed fertilization. Furthermore, the role of microbial communities and their response to varying soil depths, fertilization practices, and irrigation methods could enhance our understanding on nitrogen loss pathways should be explored in future study.

Soil carbon and nitrogen cycles driven by iron redox: A review

Soil carbon and nitrogen cycles affect agricultural production, environmental quality, and global climate. Iron (Fe), regarded as the most abundant redox-active metal element in the Earth's crust, is involved in a biogeochemical cycle that includes Fe(III) reduction and Fe(II) oxidation. The redox reactions of Fe can be linked to the carbon and nitrogen cycles in soil in various ways. Investigating the transformation processes and mechanisms of soil carbon and nitrogen species driven by Fe redox can provide theoretical guidance for improving soil fertility, and addressing global environmental pollution as well as climate change. Although the widespread occurrence of these coupling processes in soils has been revealed, explorations of the effects of Fe redox on soil carbon and nitrogen cycles remain in the early stages, particularly when considering the broader context of global climate and environmental changes. The key functional microorganisms, mechanisms, and contributions of these coupling processes to soil carbon and nitrogen cycles have not been fully elucidated. Here, we present a systematic review of the research progress on soil carbon and nitrogen cycles mediated by Fe redox, including the underlying reaction processes, the key microorganisms involved, the influencing factors, and their environmental significance. Finally, some unresolved issues and future perspectives are addressed. This knowledge expands our understanding of the interconnected cycles of Fe, carbon and nitrogen in soils.

Iron cycle-enhanced anaerobic ammonium oxidation in microaerobic granular sludge

Granule-based partial nitritation and anaerobic ammonium oxidation (PN/A) is an energy-efficient approach for treating ammonia wastewater. When treating low-strength ammonia wastewater, the stable synergy between PN and anammox is however difficult to establish due to unstable dissolved oxygen control. Here, we proposed, the PN/A granular sludge formed by a micro-oxygen-driven iron redox cycle with continuous aeration (0.42 ± 0.10 mg-O2/L) as a novel strategy to achieve stable and efficient nitrogen (N) removal. 240-day bioreactor operation showed that the iron-involved reactor had 37 % higher N removal efficiency than the iron-free reactor. Due to the formation of the microaerobic granular sludge (MGS), the bio(chemistry)-driven iron cycle could be formed with the support of anaerobic ammonium oxidation coupled to Fe3+ reduction. Both ammonia-oxidizing bacteria and generated Fe2+ could scavenge the oxygen as a defensive shield for oxygen-sensitive anammox bacteria in the MGS. Moreover, the iron minerals derived from iron oxidation and Fe-P precipitates were also deposited on the MGS surface and/or embedded in the internal channels, thus reducing the size of the channels that could limit oxygen mass transfer inside the MGS. The spatiotemporal assembly of diverse functional microorganisms in the MGS for the realization of stable PN/A could be achieved with the support of the iron redox cycle. In contrast, the iron-free MGS could not optimize oxygen mass transfer, which led to an unstable and inefficient PN/A. This work provides an alternative iron-related autotrophic N removal for low-strength ammonia wastewater.

Ammonium loss microbiologically mediated by Fe(III) and Mn(IV) reduction along a coastal lagoon system

Anaerobic ammonium oxidation, associated with both iron (Feammox) and manganese (Mnammox) reduction, is a microbial nitrogen (N) removal mechanism recently identified in natural ecosystems. Nevertheless, the spatial distributions of these non-canonical Anammox (NC-Anammox) pathways and their environmental drivers in subtidal coastal sediments are still unknown. Here, we determined the potential NC-Anammox rates and abundance of dissimilatory metal-reducing bacteria (Acidomicrobiaceae A6 and Geobacteraceae) at different horizons (0-20 cm at 5 cm intervals) of subtidal coastal sediments using the 15N isotope-tracing technique and molecular analyses. Sediments were collected across three sectors (inlet, transition, and inner) in a coastal lagoon system (Bahia de San Quintin, Mexico) dominated by seagrass meadows. The positive relationship between 30N2 production rates and dissimilatory Fe and Mn reduction provided evidence for Feammox's and Mnammox's co-occurrence. N loss through NC-Anammox was detected in subtidal sediments, with potential rates of 0.07-0.62 μg N g-1 day-1. NC-Anammox process in vegetated sediments tended to be higher than those in adjacent unvegetated ones. NC-Anammox rates showed a subsurface peak (between 5 and 15 cm) in the vegetated sediments but decreased consistently with depth in the adjacent bare bottoms. Thus, the presence/absence of seagrasses and sediment characteristics, particularly the availability of organic carbon and microbiologically reducible Fe(III) and Mn(IV), affected the abundance of dissimilatory metal-reducing bacteria, which mediated NC-Anammox activity and the associated N removal. An annual loss of 32.31 ± 3.57 t N was estimated to be associated with Feammox and Mnammox within the investigated area, accounting for 2.8-4.7% of the gross total import of reactive N from the ocean into the Bahia de San Quintin. Taken as a whole, this study reveals the distribution patterns and controlling factors of the NC-Anammox pathways along a coastal lagoon system. It improves our understanding of the coupling between N and trace metal cycles in coastal environments.

Non-rhizosphere reinforces the contributions of Feammox and anammox to nitrogen loss than rhizosphere in riparian zones

The emergence of anaerobic ammonium oxidation (anammox) coupled to iron reduction (named Feammox) refreshes the microbial pathways for nitrogen (N) loss. However, the ecological role of Feammox, compared with conventional denitrification and anammox, in microbial N attenuation in ecosystems remains unclear. Here, the specific contribution of Feammox to N loss and the underlying microbiome interactive characteristics in a riparian ecosystem were investigated through 15N isotope tracing and molecular analysis. Feammox was highlighted in the riparian interface soils and maximally contributed 14.2% of N loss. Denitrification remained the dominant contributor to N loss (68.0%-95.3%), followed by anammox (5.7%-19.1%) and Feammox (0-14.2%). The rates of Feammox and anammox significantly decreased in rhizosphere soils (0.15 ± 0.08 μg N g-1 d -1 for Feammox, 0.80 ± 0.39 μg N g-1 d -1 for anammox) compared with those in non-rhizosphere soils; however, the activities of denitrification remarkably increased in the rhizosphere (13.17 ± 3.71 μg N g-1 d -1). In rhizosphere soils, the competition between bioavailable organic matter (e.g., amino acids and carbohydrates) and ammonium for electron acceptor [i.e., Fe(III)] was the vital inducement for restricted Feammox, while the nitrite consumption boosted by heterotrophic denitrifiers was responsible for weakened anammox. The functional gene of autotrophic Acidimicrobiaceae bacterium A6, instead of heterotrophic Geobacteraceae spp., was significantly positively correlated with Feammox activity. Rare iron-reducing bacteria showed higher node degrees in the non-rhizosphere network than in the rhizosphere network. A syntrophic relationship was found between iron-reducing bacteria (e.g., Anaeromyxobacter, Geobacter) and iron-oxidizing bacteria (e.g., Sideroxydans) in the non-rhizosphere network and facilitated the Feammox pathway. This study provides an in-depth exploration of microbial driven N loss in a riparian ecosystem and introduces new insights into riparian management practices toward high-efficient N pollution alleviation.

Anodic ammonium oxidation in microbial electrolysis cell: Towards nitrogen removal in low C/N environment

Biological nitrogen removal in low C/N environment is challenging in wastewater treatment for a long time. Autotrophic ammonium oxidation is promising due to the no need of carbon source addition, but alternative electron acceptors other than oxygen has to be widely investigated. Recently, microbial electrolysis cell (MEC), which applies a polarized inert electrode as the electron harvester, has been proved effective to oxidize ammonium with electroactive biofilm. That is, anodic microbes stimulated by exogenous low power can extract electron from ammonium and transfer electron to electrodes. This review aims to consolidate the recent advances in anodic ammonium oxidation in MEC. Various technologies based on different functional microbes and mechanisms of these processes are reviewed. Thereafter, the crucial factors influencing the ammonium oxidation technology are discussed. Challenges and prospects of anodic ammonium oxidation in ammonium-containing wastewater treatment are also proposed to provide valuable insights on the technologic reference and potential value of MEC in ammonium-containing wastewater treatment.

New insight and enhancement mechanisms for Feammox process by electron shuttles in wastewater treatment - A systematic review

Ammonium oxidation coupled to Fe(III) reduction (Feammox) is a newly discovered iron-nitrogen cycle process of microbial catalyzed NH₄⁺ oxidation coupled with iron reduction. Fe(III) often exists in the form of insoluble iron minerals resulting in reduced microbial availability and low efficiency of Feammox. Electron shuttles(ESs) can be reversibly oxidized and reduced which has the potential to improve Feammox efficiency. This review summarizes the discovery process, electron transfer mechanism, influencing factors and driven microorganisms of Feammox, ang expounds the possibility and potential mechanism of ESs to enhance Feammox efficiency. Based on an in-depth analysis of the current research situation of Feammox for nitrogen removal, the knowledge gaps and future research directions including how to apply ESs enhanced Feammox to promote nitrogen removal in practical wastewater treatment have been highlighted. This review can provide new ideas for the engineering application research of Feammox and strong theoretical support for its development.

Volatile fatty acids changed the microbial community during feammox in coastal saline-alkaline paddy soil

In order to indicate the effect of volatile fatty acids (VFAs) on the characteristics of feammox and dissimilatory iron reducing bacteria (DIRB) in paddy soils, different VFAs were selected with paddy soils for anaerobic cultivation. Five treatments were set up, respectively, only adding N and both adding N and C (formate + NH₄⁺ (Fo-N), acetate + NH₄⁺ (Ac-N), propionate + NH₄⁺ (Pr-N), and butyrate + NH₄⁺ (Bu-N)) treatments. The concentration of Fe(II), Fe(III), NH₄⁺, and VFAs was assessed within 45 d, and the bacterial community was determined after cultivation. The oxidation rates of NH₄⁺ were the highest in N treatment, while it was the lowest in Fo-N treatment. Under the four C treatments, the consumption of NH₄⁺ and Fe(III) was the fastest in Pr-N treatment, which was consumed by 31.2% and 76.3%, respectively. Different VFAs selected for distinct DIRB. Compared with N treatment, Ac-N and Bu-N treatment increased the relative abundance of DIRB, such as Geobacter and Clostridia, which increased the consumption of VFAs during incubation. Overall, VFAs, especially formate, could promote Fe(III) reduction and compete with the feammox process for the electron acceptors to decrease the feammox reaction, and prohibited soil NH₄⁺ loss. Therefore, VFAs, which was released from organic fertilizer, could reduce NH₄⁺ loss in feammox process of saline-alkaline paddy soils.

Insights into microbial interactive mechanism regulating dissimilatory nitrate reduction processes in riparian freshwater aquaculture sediments

Aquaculture can substantially alter the accumulation and cycling of nutrients in sediments. However, the microbial mechanisms mediating sediment dissimilatory nitrate (NO₃^-) reduction in freshwater aquaculture ponds are still unclear, which rule the removal and retention of N element. In the present study, three microbial NO₃^- reduction processes in riparian aquaculture pond sediments (i.e., crab, shrimp and fish ponds) and natural freshwater sediments (i.e., lakes and rivers) were investigated via isotopic tracing and molecular analyses. The potential rates of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) significantly increased in the aquaculture ponds compared with the natural freshwaters. Denitrification contributed 90.40-94.22% to the total NO₃^- reduction (product as N₂), followed by 2.49-5.82% of anammox (product as N₂) and 2.09-5.18% of DRNA (product as NH₄⁺). The availability of C and N substrates, rather than functional gene abundance, regulated the activities of NO₃^- reductions and microbiome composition. Microbial mechanism based on network analysis indicated that heterotrophic denitrifiers and DNRA bacteria (e.g., Bacillus, Micromonospora, Mycobacterium and Brachybacterium) determined the community structure and function for N conversions in aquaculture ponds, whereas the such microbial network in natural freshwater sediments was manipulated by autotrophic denitrifiers (e.g., Desulfuromonas, Polaromonas, Solitalea). Collectively, this study provides an in-depth exploration of microbial nitrogen removal in freshwater aquaculture areas and supports management strategies for N pollution caused by reclamation for aquaculture in riparian zones.

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.

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.

Recent progress in applications of Feammox technology for nitrogen removal from wastewaters: A review

Feammox process is crucial for the global nitrogen cycle and has great potentials for the treatment of low COD/NH₄⁺-N wastewaters. This work provides a systematic and comprehensive overview of the Feammox process. Specifically, underlying mechanisms and functional microbes mediating the Feammox process are summarized in detail. And key influencing factors including pH, temperature, dissolved oxygen, organic carbon, source of Fe(III) as well as various electron shuttles are discussed. Additionally, recent development trends and attempts of the Feammox technology in wastewater treatment applications are reviewed, and perspectives for future development are presented. A thorough review of the recent progress in Feammox process is expected to provide valuable information for further process optimization, which is helpful to achieve a more economical operation and better nitrogen removal performance in future field applications.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Anaerobic ammonium oxidation coupled to Fe(III) reduction: Discovery, mechanism and application prospects in wastewater treatment

Fe(III) reduction coupled with anaerobic ammonium oxidation is known as Feammox. Feammox, which was first discovered in wetland ecosystems, has the potential to be used in wastewater treatment systems due to its ability to remove ammonium. Feammox can produce N₂, NO₂^- or NO₃^- through the reduction of Fe(III) and oxidation of ammonium, which is a potential process to nitrogen loss from aquatic ecosystems and terrestrial ecosystems. The Acidimicrobiaceae sp. A6 was the first Feammox functional bacteria that was successfully isolated from wetlands. The nitrogen removal effect of Feammox can be influenced by many environmental factors, such as pH, organic matter, and different sources of Fe(III). Feammox has broad application prospects, but more exploration is needed to apply this principle to wastewater treatment. This review introduces the development, mechanism, functional microbes and factors affecting the Feammox process, and discusses its potential applications.

A promising destiny for Feammox: From biogeochemical ammonium oxidation to wastewater treatment

Ammonium is one of the most common forms of nitrogen that exists in wastewater, and it can cause severe pollution when it is discharged without treatment. New technologies must be developed to effectively remove ammonium because conventional nitrification-denitrification methods are limited by the lack of organic carbon. Anaerobic ammonium oxidation coupled to Fe(III) reduction is known as Feammox, and is a recently discovered nitrogen cycling process. Feammox can proceed under autotrophic or anaerobic conditions and effectively transforms ammonium to stable, innocuous dinitrogen gas, using the ferric iron as an electron acceptor. This method is cost-effective, environmentally friendly, and conducive to joint application with other nitrogen removal reactions in low-C/N municipal wastewater treatments. This review provides a comprehensive survey of Feammox mechanistic investigations and presents studies regarding the functional microorganism colonies. The potential for Feammox to be applied for the removal of nitrogen from various polluted water sources and the combination of the Feammox process with other frontier environmental technologies are also discussed. In addition, future perspectives for removing ammonium using Feammox are presented.

Anammox Bacteria Are Potentially Involved in Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction in the Wastewater Treatment System

Anaerobic ammonium oxidation coupled to nitrite reduction (termed as Anammox) was demonstrated as an efficient pathway to remove nitrogen from a wastewater treatment system. Recently, anaerobic ammonium oxidation was also identified to be linked to iron(III) reduction (termed Feammox) with dinitrogen, nitrite, or nitrate as end-product, reporting to enhance nitrogen removal from the wastewater treatment system. However, little is known about the role of Anammox bacteria in the Feammox process. Here, slurry from wastewater reactor amended with ferrihydrite was employed to investigate activity of Anammox bacteria in the Feammox process using the ¹⁵N isotopic tracing technique combined with 16S rRNA gene amplicon sequencing. A significantly positive relationship between rates of ¹⁵N₂ production and iron(III) reduction indicated the occurrence of Feammox during incubation. Relative abundances of Anammox bacteria including Brocadia, Kuenenia, Jettenia, and unclassified Brocadiaceae were detected with low relative abundances, whereas Geobacteraceae dominated in the treatment throughout the incubation. ¹⁵N₂ production rates significantly positively correlated with relative abundances of Geobacter, unclassified Geobacteraceae, and Anammox bacteria, revealing their contribution to nitrogen generation via Feammox. Overall, these findings suggested Anammox bacteria or cooperation between Anammox bacteria and iron(III) reducers serves a potential role in Feammox process.

Nitrogen loss through denitrification, anammox and Feammox in a paddy soil

Since the process of anaerobic ammonium oxidation (anammox) coupled with ferric iron reduction (termed Feammox) was discovered, it has been observed in various natural environments. However, besides the vertical distribution of Feammox in paddy soils, its differences and relationships with traditional nitrogen loss processes, including denitrification and anammox, remain unclear. Here, we studied the distribution of nitrogen loss pathways in different layers (0-50 cm) of paddy soil in southeastern China using ¹⁵N isotope tracer technology and molecular analysis. Our study showed that denitrification had a rate of 2.19 ± 0.39 mg N·kg^-1·d^-1, which was the highest activity in the surface layer (0-10 cm). The activities of anammox and Feammox reached peak values in the 10-20 cm (1.13 ± 0.16 mg N·kg^-1·d^-1) and 20-30 cm (0.23 ± 0.02 mg N·kg^-1·d^-1) soil layer, respectively. The nitrogen loss in the surface layer was more serious than that in the deep layer under paddy cultivation. In this study, denitrification was the main nitrogen loss pathway in the surface soil, but Feammox became an important nitrogen loss pathway (up to 26.1%) in the 20-40 cm depth. Overall, our research could improve and perfect the nitrogen cycle pathways in paddy soil.

Soil Biogeochemical Cycle Couplings Inferred from a Function-Taxon Network

Soil biogeochemical cycles and their interconnections play a critical role in regulating functions and services of environmental systems. However, the coupling of soil biogeochemical processes with their mediating microbes remains poorly understood. Here, we identified key microbial taxa regulating soil biogeochemical processes by exploring biomarker genes and taxa of contigs assembled from metagenomes of forest soils collected along a latitudinal transect (18° N to 48° N) in eastern China. Among environmental and soil factors, soil pH was a sensitive indicator for functional gene composition and diversity. A function-taxon bipartite network inferred from metagenomic contigs identified the microbial taxa regulating coupled biogeochemical cycles between carbon and phosphorus, nitrogen and sulfur, and nitrogen and iron. Our results provide novel evidence for the coupling of soil biogeochemical cycles, identify key regulating microbes, and demonstrate the efficacy of a new approach to investigate the processes and microbial taxa regulating soil ecosystem functions.

Effects of the addition of nitrogen and phosphorus on anaerobic ammonium oxidation coupled with iron reduction (Feammox) in the farmland soils

Anaerobic ammonium oxidation coupled with iron reduction is termed as Feammox, and is a new nitrogen removal process. However, there is a paucity of studies on the response of nutrient additions on Feammox process in farmland ecosystems. In this study, we investigated the shifts of Feammox and iron-reducers under nitrogen (N) and phosphorus (P) applications via isotopic tracing and high-throughput sequencing technology. In the isotopic tracing experiment, Feammox rates was significantly greater in the N and/or P applications soil (0.184-0.239 μg N g^-1 day^-1) than in the no fertilizer soil (0.172 μg N g^-1 day^-1). The results indicated that N and P applications could favor the Feammox reaction. Molecular analysis showed that five predominant iron-reducing bacteria, including Geobacter, Anaeromyxobacter, Pseudomonas, Thiobacillus and Bacillus, were detected. Their abundance in the soil with no fertilizer, N, P and N combined with P was 0.93%, 1.11%-1.71%, 0.99%, and 1.40%-1.75%, respectively. This implied that iron-reducing bacteria can be stimulated under N and P applications. Overall, the results of this study demonstrated that N and/or P applications could alter the activity of Feammox, and modulate the potential of IRB in the farmland soils.

Impact of sand mining on the carbon sequestration and nitrogen removal ability of soil in the riparian area of Lijiang River, China

Riparian areas are widely recognized as the main areas for carbon sequestration and nitrogen pollution removal, while little is known about the effects of the respective sand mining activities on riparian zones. In this study, the effects of sand mining activities on the soil organic carbon (SOC) storage, different N-removal processes (Feammox, anammox, and denitrification), and composition of the relative bacterial community at a depth of 0-40 cm were determined based on investigations in riparian sand mining areas and adjacent forestlands. The SOC density of the sand mining areas (2.59 t ha^-1, depth of 0-40 cm) was lower than that of the riparian forestlands (80.42 t ha^-1). Compared with those of the riparian forestland, the sand mining area exhibited a dramatic reduction in the CO₂-fixed gene abundances (cbbL) and a significant change in the composition of cbbL-containing bacteria. The rates of the Feammox (0.038 ± 0.014 mg N kg^-1 d^-1), anammox (0.017 ± 0.017 mg N kg^-1 d^-1), and denitrification (0.090 ± 0.1 mg N kg^-1 d^-1) processes at a depth of 0-20 cm in the soil layer of the sand mining area were reduced by 70.17%, 91.5%, and 93.62% compared with those of the riparian forestland, respectively. The riparian areas in the study area (approximately 12 ha, depth of 0-40 cm) destroyed by sand mining activities released approximately 933.96 t stored soil carbon, which reduce the annual carbon sequestration potential by 28.8-40.8 t. Moreover, the potential N-removal rates in the riparian forestlands (depth of 0-20 cm) by the Feammox, anammox, and denitrification processes were 1514.21-1530.95 kg N ha^-1 year^-1, whereas the potential N-removal rates in the sand mining area were only 121.2-126.19 kg N ha^-1 year^-1. Therefore, more investigations are necessary for comparing the benefits and damage of sand mining activities in riparian areas before more sand mining activities are approved.

Silver nanoparticles and Fe(III) co-regulate microbial community and N2O emission in river sediments

The effects of environmental concentration silver nanoparticles (ecAgNPs) on microbial communities and the nitrogen cycling in river sediments remain largely uncharacterized. As a fundamental component of sediments, Fe(III) can interact with AgNPs and participate in nitrogen transformation processes. N₂O is an important intermediate in nitrogen transformation processes and can be a potent greenhouse gas with significant environmental effects. However, the impacts of the co-existence of AgNPs and Fe(III) on microbial communities and N₂O emission in river sediments are still unclear. In the present study, mesocosm experiments were conducted to assess the changes of microbial communities and N₂O emission in response to the co-existence of AgNPs and environmental concentration Fe(III). Our results revealed that the microbial community diversity and N₂O emission in river sediments responded differently to ecAgNPs (0.05 mg/kg) and high-polluting concentration AgNPs (hcAgNPs, 5 mg/kg), which was further regulated by the environmental concentration Fe(III) (1 mg/g and 10 mg/g). After ecAgNPs treatments, a marked increase was observed in microbial diversity compared to hcAgNPs treatments, regardless of the Fe(III) concentration in the sediment. The β-NTI index indicated that AgNPs had stronger impacts on phylogenetic distance of bacterial communities in sediments containing 1 mg/g Fe(III) than that containing 10 mg/g Fe(III). In sediments containing 1 mg/g Fe(III), ecAgNPs did not affect N₂O emission, but hcAgNPs significantly inhibited the emission of N₂O. However, in sediments containing 10 mg/g Fe(III), N₂O emission was significantly stimulated upon exposure to ecAgNPs, but the inhibition effect of hcAgNPs was barely observed. Functional prediction and real-time PCR analyses indicated that AgNPs and Fe(III) predominantly affected N₂O emissions by affecting the abundance of the nirK gene. Our results provide new insights into the ecological impacts of the co-existence of environmental concentration AgNPs and Fe(III) in altering microbial communities and nitrogen transformation functions in river sediments.

Spatial and seasonal distributions of Feammox from ecosystem habitats in the Wanshan region of the Taihu watershed, China

Anaerobic ammonium oxidation coupled to Fe(III) reduction, termed Feammox, is a newly identified microbial process that occurs in nitrogen and iron cycles. As the seasonal distribution of Feammox in different ecosystem habitats has not been fully explored, this study investigated the potential Feammox rates and the diversity and abundance of iron reducing bacteria (IRB) in three habitats during two seasons by using isotope tracing technique and molecular analysis, respectively. Results showed that potential Feammox rates vary both seasonally and spatially, having relatively higher rates in summer (0.05-0.19 mg N kg^-1 d^-1) and lower rates in winter (0.02-0.09 mg N kg^-1 d^-1). In addition, relatively higher and lower rates were observed in farmland soils (0.09-0.19 mg N kg^-1 d^-1) and river sediments (0.02-0.05 mg N kg^-1 d^-1), respectively. The abundance and diversity of IRB were also found to vary both spatially and seasonally. Furthermore, the results show that Feammox may transform nitrogen at a rate of approximately 2.4-22.5 kg N ha^-1 yr^-1 within the investigated area. It is considered that the soil moisture, the Fe(III) content, and the total organic carbon are important factors controlling Feammox and IRB. Overall, these results extend current scientific knowledge about nitrogen and iron cycles in ecosystem habitats.