Fe (III) reduction strategies of dissimilatory iron reducing bacteria

High-level nitrogen removal achieved by Feammox-based autotrophic nitrogen conversion

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is an essential process in the geochemical iron and nitrogen cycling. This study explores Feammox-based nitrogen removal in a continuous laboratory up-flow bioreactor stimulated by intermittently adding 5 mM Fe(OH)3 at intervals of approximately two months. The feed was synthetic wastewater with a relatively low ammonium concentration (∼100 mg N/L), yet without organic carbon in order to test its autotrophic nitrogen removal performance. The operation of this system showed the achievement of high-level ammonium and total nitrogen removal efficiency (∼97% and ∼90% on average, respectively) within four months of operation, along with a relatively practical rate of ∼50 mg N/(L·d). The demand of Fe(Ⅲ) for ammonium removal during the whole bioreactor operation was estimated to be only 0.033, two orders of magnitude less than that calculated based on the Feammox reaction producing nitrogen gas. A series of assays on Fe(II) oxidation with different oxidants (O2, NO2 - and NO3 -) in abiotic and biotic batch tests further revealed an important role of Fe(II) oxidation processes, likely driven by microbial nitrate reduction and chemical oxygen reduction, in assisting the regeneration of Fe(III) for continuous Feammox-based nitrogen removal. This work demonstrates that Feammox-based autotrophic nitrogen conversion is a potential option for future wastewater treatment.

Microbially mediated iron redox processes for carbon and nitrogen removal from wastewater: Recent advances

Iron is the most abundant redox-active metal on Earth. The microbially mediated iron redox processes, including dissimilatory iron reduction (DIR), ammonium oxidation coupled with Fe(III) reduction (Feammox), Fe(III) dependent anaerobic oxidation of methane (Fe(III)-AOM), nitrate-reducing Fe(II) oxidation (NDFO), and Fe(II) dependent dissimilatory nitrate reduction to ammonium (Fe(II)-DNRA), play important parts in carbon and nitrogen biogeochemical cycling globally. In this review, the reaction mechanisms, electron transfer pathways, functional microorganisms, and characteristics of these processes are summarized; the prospective applications for carbon and nitrogen removal from wastewater are reviewed and discussed; and the research gaps and future directions of these processes for the treatment of wastewater are also underlined. This review is expected to give new insights into the development of economic and environmentally friendly iron-based wastewater treatment procedures.

Metagenomic Insights into the Abundance of Iron-Reducing Microorganisms in a Petroleum-Contaminated Iron-Rich Aquifer

In petroleum-contaminated aquifers, iron (III) serves as an electron acceptor, enabling microbial degradation of organic matter. While previous studies have focused on iron reduction and organic matter degradation under laboratory conditions, research on iron-associated microorganisms in petroleum-contaminated aquifers is limited. To explore the diversity and distribution of such microorganisms in natural settings, this study used metagenomic analysis of an iron-rich, petroleum-contaminated aquifer. Sixteen groundwater samples from both pollution source and background areas were collected for species annotation and functional gene identification. Results show more than 7000 species were identified as iron-reducing microorganisms (IRMs), including several previously well-characterized iron-reducing species (e.g., Geobacter luticola and Geobacter sulfurreducens). However, the majority of IRMs were not found in existing iron-reducing microbial databases. Some of them, such as Sulfurospirillum sp. and Extensimonas perlucida, could be taxonomically classified at the species level, while most were only annotated as unclassified bacteria. In the contamination source zone, these microorganisms proliferated extensively, which led to hydrocarbon degradation predominantly driven by iron reduction in the aquifer. This study enhances our understanding of hydrocarbon-degrading microorganisms and supports the management of petroleum-contaminated sites.

Metagenomic and isotopic insights into carbon fixation by autotrophic microorganisms in a petroleum hydrocarbon impacted red clay aquifer

Autotrophic microorganisms, the pivotal carbon fixers, exhibit a broad distribution across diverse environments, playing critical roles in the process of carbon sequestration. However, insights into their distribution characteristics in aquifers, particularly in those petroleum-hydrocarbon-contaminated (PHC) aquifers that were known for rich in heterotrophs, have been limited. In the study, groundwater samples were collected from red clay aquifers in the storage tank leakage area of a PHC site, a prevalent aquifer type in southern China and other regions. Metagenomics combined with hydrochemical and inorganic carbon isotope analyses were employed to elucidate the presence of microbial carbon fixation and its driving forces. Results showed that there were hundreds of autotrophic microorganisms participating in distinct carbon fixation processes in the red clay PHC aquifers. Reductive tricarboxylic acid (rTCA) and dicarboxylate/4-hydroxybutyrate (DC/4HB), as well as 3-hydroxypropionate (3HP or/and 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB)) were the predominant carbon fixation pathways. The abundances of carbon fixation genes and autotrophic microorganisms were significantly and positively correlated with hydrocarbon concentrations and δ13C of dissolved inorganic carbon (δ13C-DIC) values. This finding indicated that the petroleum hydrocarbon significantly promoted the proliferation of carbon fixation microorganisms, leading to a substantial uptake of inorganic carbon. Therefore, we deduce that this process holds considerable potential for carbon sequestration in PHC-contaminated aquifers.

Shewanella oneidensis MR-1 dissimilatory reduction of ferrihydrite to highly enhance mineral transformation and reactive oxygen species production in redox-fluctuating environments

Diverse paths generated by reactive oxygen species (ROS) can mediate contaminant transformation and fate in the soil/aquatic environments. However, the pathways for ROS production upon the oxygenation of redox-active ferrous iron minerals are underappreciated. Ferrihydrite (Fh) can be reduced to produce Fe(II) by Shewanella oneidensis MR-1, a representative strain of dissimilatory iron-reducing bacteria (DIRB). The microbial reaction formed a spent Fh product named mr-Fh that contained Fe(II). Material properties of mr-Fh were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Magnetite could be observed in all mr-Fh samples produced over 1-day incubation, which might greatly favor the Fe(II) oxygenation process to produce hydroxyl radical (•OH). The maximum amount of dissolved Fe(II) can reach 1.1 mM derived from added 1 g/L Fh together with glucose as a carbon source, much higher than the 0.5 mM generated in the case of the Luria-Bertani carbon source. This may confirm that MR-1 can effectively reduce Fh and produce biogenetic Fe(II). Furthermore, the oxygenation of Fe(II) on the mr-Fh surface can produce abundant ROS, wherein the maximum cumulative •OH content is raised to about 120 μM within 48 h at pH 5, but it is decreased to about 100 μM at pH 7 for the case of MR-1/Fh system after a 7-day incubation. Thus, MR-1-mediated Fh reduction is a critical link to enhance ROS production, and the •OH species is among them the predominant form. XPS analysis proves that a conservable amount of Fe(II) species is subject to adsorption onto mr-Fh. Here, MR-1-mediated ROS production is highly dependent on the redox activity of the form Fe(II), which should be the counterpart presented as the adsorbed Fe(II) on surfaces. Hence, our study provides new insights into understanding the mechanisms that can significantly govern ROS generation in the redox-oscillation environment.

Magnetite-equipped algal-rich sediments for microbial fuel cells: Remediation of sediment organic matter pollution and mechanisms of remote electron transfer

Using the bio-electrochemical methods for the restoration of high algae sediments is full of potential and challenges. How to promote extracellular electron transfer (EET) process in microbial fuel cells (MFC) is the key bottleneck. The study had explored the potential application of magnetite on accelerating electron transfer for improving the output of MFC and sediment pollution remediation. The results indicated that the organic matter degradation rate showed a remarkable increase of 27.45 %, and the voltage output was approximately 1.68 times higher compared to the MFC configured with regular sediment. Abundant electroactive bacteria (EABs), such as Geobacter and Burkholderiaceae, and fermentative bacteria were responsible for these results, accompanied by the enhanced fluorescence of humic substances (HS), increased concentration and activity of cytochrome C (25.05 % and 21.12 %), as well as elevated extracellular polymeric substance content. Moreover, the intrinsic EET mechanisms among Fe-oxides, HS, and EABs were explored. According to the electrochemical analysis and substance transformation, the EET process involved four stages: magnetite-enhanced direct electron transfer via strong conductivity, iron respiration mediating electron transfer to the electrode, the model quinone substance acting as an electron shuttle facilitating EET and iron reduction, and iron cycling mediating electron transfer. This study provides an effective strategy for pollution remediation in algal-rich sediment, which was beneficial for the harmless treatment and resource utilization of both algae and sediment, simultaneously.

Microbe interactions drive the formation of floating iron films in circumneutral wetlands

Floating iron (Fe) films are widely found in wetlands that can form oxic-anoxic boundaries under circumneutral conditions. These films play a crucial role in the redox transformations and bioavailability of nutrients and trace metals. Current studies mainly focus on chemical oxidation during Fe film formation under circumneutral conditions. The functional microorganisms and associated microbial processes involved in Fe film formation have yet to be investigated in detail. Here, we investigated the microbial communities and involved microbial processes for the formation of floating Fe films in wetlands. Ferrihydrite was the dominant Fe(III) phase in films, accompanied by moderate levels of carbon and silicon. The Fe species and microbial analysis indicated that Fe films contain mixed-valent Fe and can form biotically. Microbial community analysis showed that the dominant genera in these Fe films were Fe-oxidizing and reducing bacteria and methanotrophs, including Leptothrix, Ferriphasclus, Gallionella, Geobacter and Methylococcales. Leptothrix, Ferriphasclus and Gallionella, as classical Fe(II)-oxidizing bacteria (FeOB), can oxidize Fe(II) with limited oxygen and form special structures that are consistent with Fe film morphology. Geobacter can provide a source of Fe(II) for FeOB growth, and Methylococcales can perform methane oxidation to provide energy for Fe cycling. The high ratios of Gallionella- and Geobacter-related microorganisms and carbon fixation genes proved the contribution of potential of Fe cycling and autotrophic microbial communities to the formation of Fe films. The diversity of microbial community suggested that Fe(II) oxidation could trigger carbon fixation, while Fe(III) reduction accelerated Fe and carbon cycling through anaerobic respiration and autotrophic chemosynthesis. These results highlight the contribution of these multiple microbial processes to Fe and carbon cycling during the formation of floating Fe films in wetlands. However, further studies are required to fully elucidate the interaction of functional microorganisms involved in floating film formation and their biogeochemical role in wetlands.

Ferrihydrite-mediated methanotrophic nitrogen fixation in paddy soil under hypoxia

Biological nitrogen fixation (BNF) by methanotrophic bacteria has been shown to play an important role in maintaining fertility. However, this process is still limited to aerobic methane oxidation with sufficient oxygen. It has remained unknown whether and how methanotrophic BNF proceeds in hypoxic environments. Herein, we incubated paddy soils with a ferrihydrite-containing mineral salt medium to enrich methanotrophic bacteria in the presence of methane (20%, v/v) under oxygen constraints (0.27%, v/v). The resulting microcosms showed that ferrihydrite-dependent aerobic methane oxidation significantly contributed (81%) to total BNF, increasing the 15N fixation rate by 13-fold from 0.02 to 0.28 μmol 15N2 (g dry weight soil) -1 d-1. BNF was reduced by 97% when ferrihydrite was omitted, demonstrating the involvement of ferrihydrite in methanotrophic BNF. DNA stable-isotope probing indicated that Methylocystis, Methylophilaceae, and Methylomicrobium were the dominant methanotrophs/methylotrophs that assimilated labeled isotopes (13C or 15N) into biomass. Metagenomic binning combined with electrochemical analysis suggested that Methylocystis and Methylophilaceae had the potential to perform methane-induced BNF and likely utilized riboflavin and c-type cytochromes as electron carriers for ferrihydrite reduction. It was concluded that ferrihydrite mediated methanotrophic BNF by methanotrophs/methylotrophs solely or in conjunction with iron-reducing bacteria. Overall, this study revealed a previously overlooked yet pronounced coupling of iron-dependent aerobic methane oxidation to BNF and improves our understanding of methanotrophic BNF in hypoxic zones.

How iron-bearing minerals affect the biological reduction of Sb(V): A newly discovered function of nitrate reductase

As a toxic element of global concern, the elevated concentration of antimony (Sb) in the environment has attracted increasing attention. Microorganisms have been reported as important driving forces for Sb transformation. Iron (Fe) is the most important metal associated element of Sb, however, how Fe-bearing minerals affect the biological transformation of Sb is still unclear. In this study, the effects of Fe-bearing minerals on biological Sb(V) reduction were investigated by employing a marine Shewanella sp. CNZ-1 (CNZ-1). Our results showed that the presence of hematite, magnetite and ferrihydrite (1 g/L) resulted in a decrease in Sb(III) concentration of ~19-31 % compared to the Fe(III)-minerals free system. The calculated Sb(V) reduction rates are 0.0256 (R2 0.71), 0.0389 (R2 0.87), 0.0299 (R2 0.96) and 0.0428 (R2 0.95) h-1 in the hematite-, magnetite-, ferrihydrite-supplemented and Fe(III)-minerals free systems, respectively. The cube-shaped Sb2O3 was characterized as a reductive product by using XRD, XPS, FTIR, TG and SEM approaches. Differential proteomic analysis showed that flagellar protein, cytochrome c, electron transfer flavoprotein, nitrate reductase and polysulfide reductase (up-regulation >1.5-fold, p value <0.05) were supposed to be included in the electron transport pathway of Sb(V) reduction by strain CNZ-1, and the key role of nitrate reductases was further highlighted during this reaction process based on the RT-qPCR and confirmatory experiments. Overall, these findings are beneficial to understand the environmental fate of Sb in the presence of Fe-bearing minerals and provide guidance in developing the bacteria/enzyme-mediated control strategy for Sb pollution.

Dithionite promoted microbial dechlorination of hexachlorobenzene while goethite further accelerated abiotic degradation by sulfidation in paddy soil

It is of great scientific and practical importance to explore the mechanisms of accelerated degradation of Hexachlorobenzene (HCB) in soil. Both iron oxide and dithionite may promote the reductive dechlorination of HCB, but their effects on the microbial community and the biotic and abiotic mechanisms behind it remain unclear. This study investigated the effects of goethite, dithionite, and their interaction on microbial community composition and structure, and their potential contribution to HCB dechlorination in a paddy soil to reveal the underlying mechanism. The results showed that goethite addition alone did not significantly affect HCB dechlorination because the studied soil lacked iron-reducing bacteria. In contrast, dithionite addition significantly decreased the HCB contents by 44.0-54.9%, while the coexistence of dithionite and goethite further decreased the HCB content by 57.9-69.3%. Random Forest analysis suggested that indicator taxa (Paenibacillus, Acidothermus, Haliagium, G12-WMSP1, and Frankia), Pseudomonas, richness and Shannon's index of microbial community, and immobilized Fe content were dominant driving factors for HCB dechlorination. The dithionite addition, either with or without goethite, accelerated HCB anaerobic dechlorination by increasing microbial diversity and richness as well as the relative abundance of the above specific bacterial genera. When goethite and dithionite coexist, sulfidation of goethite with dithionite could remarkably increase FeS formation and then further promote HCB dechlorination rates. Overall, our results suggested that the combined application of goethite and dithionite could be a practicable strategy for the remediation of HCB contaminated soil.

Biochar-derived dissolved black carbon accelerates ferrihydrite microbial transformation and subsequent imidacloprid degradation

The effects of an electron shuttle (dissolved black carbon (DBC) derived from biochar) on the microbial reduction of ferrihydrite and subsequent imidacloprid (IMI) degradation were studied. The results showed that DBC addition enhanced the microbial reduction of Fe(III) in ferrihydrite and increased the quantity of Fe(II) released into the liquid phase. The electron transfer capacity of DBC was significantly influenced by the content of redox-active oxygen-containing functional groups (e.g., quinone, hydroquinone, and polyphenol groups), which was dependent on the pyrolysis temperature. The electrochemical characteristics of DBC resulted in enhanced electron transfer, which promoted Fe(III) reduction and mediated the microbial transformation of ferrihydrite. The microbial transformation of ferrihydrite resulted in the formation of secondary minerals such as siderite and vivianite. The IMI degradation efficiency was related to the Fe(III) reduction rate and the pyrolysis temperature used in DBC production, and the degradation pathways were nitrate reduction and imino hydrolysis induced by the Fe(II) generated from the reduction of Fe(III) in ferrihydrite. The results obtained in this study provide new data for understanding the multifunctional roles of biochar-derived DBC in the redox and transformation processes of iron minerals induced by iron-reducing bacteria, the related biogeochemical cycles of iron and the fate of pollutants.

Estrogen toxicity reduction of industrial biochemical tailwater by electrolysis biofilters with ceramsite‑sulfur-siderite fillers

Biochemical tailwater of the industrial park wastewater treatment plant is facing the growing demand of advanced treatment and toxicity reduction. However, existing information on toxicity reduction of real industrial biochemical tailwater is still limited so far. Herein, the water quality of biochemical tailwater from an integrated industrial park in Taihu Lake Basin, China, was systematically investigated, and typical endocrine disrupting chemicals (EDCs) and estrogen toxicity were detected. Estrogen toxicity reduction by previously proposed electrolysis biofilters with ceramsite‑sulfur-siderite fillers was further verified. Results showed that total nitrogen (TN) and estrogen toxicity removal increased by 41.0 % and 30.3 % respectively under the optimal voltage of 4 V and electric loading of 24 h/d, and significantly positive correlation between estrogen toxicity reduction and TN removal (p < 0.05) was observed. The lowest effluent estrogen toxicity was 0.79-0.95 ng_E2/L (in estradiol equivalent concentration) by the biofilter with ceramsite‑sulfur-siderite fillers, which was lower than the estrogen disruption effects mass concentration threshold of 1 ng/L. Electrical stimulation promoted the increase of the abundance of denitrifying bacteria Thauera and electroactive bacteria Hydrogenophaga, thus enhancing the removal of TN, furthermore, the abundance of Thiobacillus and Sulfuritalea were significantly correlated with the reduction of estrogen toxicity. The study highlights the potential of electrolysis biofilter in realizing the synergy of estrogen toxicity reduction and autotrophic denitrification of industrial biochemical tailwater, and paves the way for the application of electrolysis biofilter in the advanced purification of industrial biochemical tailwater.

Biostimulation of jarosite and iron oxide-bearing mine waste enhances subsequent metal recovery

Novel resource recovery technologies are required for metals-bearing hazardous wastes in order to achieve circular economy outcomes and industrial symbiosis. Iron oxide and co-occurring hydroxysulphate-bearing wastes are globally abundant and often contain other elements of value. This work addresses the biostimulation of indigenous microbial communities within an iron oxide/ hydroxysulphate-bearing waste and its effect on the subsequent recoverability of metals by hydrochloric, sulphuric, citric acids, and EDTA. Laboratory-scale flow-through column reactors were used to examine the effect of using glycerol (10% w/w) to stimulate the in situ microbial community in an iron oxide/ hydroxysulphate-bearing mine waste. The effects on the evolution of leachate chemistry, changes in microbiological community, and subsequent hydrometallurgical extractability of metals were studied. Results demonstrated increased leachability and selectivity of Pb, Cu, and Zn relative to iron after biostimulation with a total of 0.027 kg of glycerol per kg of waste. Biostimulation, which can be readily applied in situ, potentially opens new routes to metal recovery from globally abundant waste streams that contain jarosite and iron oxides.

A review on the bioleaching of toxic metal(loid)s from contaminated soil: Insight into the mechanism of action and the role of influencing factors

The anthropogenic activities in agriculture, industrialization, mining, and metallurgy combined with the natural weathering of rocks, have led to severe contamination of soils by toxic metal(loid)s. In an attempt to remediate these polluted sites, a plethora of conventional approaches such as Solidification/Stabilization (S/S), soil washing, electrokinetic remediation, and chemical oxidation/reduction have been used for the immobilization and removal of toxic metal(loid)s in the soil. However, these conventional methods are associated with certain limitations. These limitations include high operational costs, high energy demands, post-waste disposal difficulties, and secondary pollution. Bioleaching has proven to be a promising alternative to these conventional approaches in removing toxic metal(loid)s from contaminated soil as it is cost-effective, environmentally friendly, and esthetically pleasing. The bioleaching process is influenced by factors including pH, temperature, oxygen, and carbon dioxide supply, as well as nutrients in the medium. It is crucial to monitor these parameters before and throughout the reaction since a change in any, for instance, pH during the reaction, can alter the microbial activity and, therefore, the rate of metal leaching. However, research on these influencing factors and recent innovations has brought significant progress in bioleaching over the years. This critical review, therefore, presents the current approaches to bioleaching and the mechanisms involved in removing toxic metal(loid)s from contaminated soil. We further examined and discussed the fundamental principles of various influencing factors that necessitate optimization in the bioleaching process. Additionally, the future perspectives on adding omics for bioleaching as an emerging technology are discussed.

Iron-based passivator mitigates the coupling process of anaerobic methane oxidation and arsenate reduction in paddy soils

Arsenic (As) is a toxic metalloid that is ubiquitous in paddy soils, where passivation is the most widely used method for remediating As contamination. Recently, anaerobic methane oxidation coupled with arsenate (As(V)) reduction (AOM-AsR) has been shown to act as a critical driver for As release in paddy fields. However, the effect and mechanism of the passivators on the AOM-AsR process remain unclear. In this study, we incubated arsenate-contaminated paddy soils under anaerobic conditions. Using isotopically labelled methane and different passivators, we found that an iron-based passivator containing calcium sulfate and iron oxide (9:1, m/m) named IBP showed a much better performance than the other passivators. Adding IBP decreased the arsenite (As(III)) concentration in the soil solution by 78% and increased the AOM rate by 55%. Furthermore, we employed high-throughput sequencing and real-time quantitative polymerase chain reaction (qPCR) to investigate the ability of IBP to control As release mediated by AOM-AsR in paddy fields, as well as its underlying mechanism. Our results showed that IBP addition significantly increased anaerobic methanotrophic (ANME) archaea (ANME-2a-c, ANME-2d, and ANME-3) by 91%, and increased the methane-oxidizing bacterium Methylobacter by 262%. Similarly, IBP addition significantly increased the Fe(III) concentration in soil solution by 39% and increased the absolute abundance of Fe(III)-reducing bacteria (Geobacteraceae) by 21 times in soil. Adding IBP may significantly promote AOM coupled with Fe(III) reduction, significantly reducing electron transfer from AOM to As(V) reduction. Hence, IBP may be used as an efficient passivator to remediate As-contaminated soil using an active AOM-AsR process. These results provide a novel insight into controlling soil As release by regulating an active and critical As mobilization pathway in the environment.

Mechanisms governing the dissolution of phosphorus and iron in sewage sludge by the bioacidification process and its correlation with iron phosphate speciation

In the last two decades, phosphorus (P) recovery from sewage sludge liquors gained much interest for its high-quality product potential. However, the consistently reported constraints are the low phosphorus availability and the technical-economical difficulties to increase it through chemical acidification. This article discusses the mechanisms of phosphorus dissolution by the biological acidification process (Biological acidification or acidic fermentation) as an alternative to chemical acidification. In addition, we investigate the potential correlation between the phosphorus dissolution and iron phosphate speciation of several types of sludge from different sewage treatment plants and P removal technologies. The results show that the percentage of P dissolution by bioacidification is always higher than the P dissolution by chemical acidification at equal pH for all types of sludge except for the settled primary sludge. The highest P dissolution was recorded for the sludge from the Enhanced Biological P Removal process assisted with Chemical P Removal process (EBPR-CPR) with around 65% of P dissolution. Three mechanisms were identified as contributing to the increased P dissolution by bioacidification: P release by the Polyphosphate Accumulating Organisms (PAO), P dissolution by pH decrease, and P dissolution by a biological activity at acidic pH (3.7-4) that includes iron reduction and aluminum dissolution. The principal component analysis and Pearson's correlation indicate that P dissolution by bioacidification is negatively correlated with the P-bound to ferric iron, hence positively correlated with the P-bound to ferrous iron, which characterizes the sludge from the EBPR-CPR process. This study suggests that the choice of the P removal technology significantly influences the P recovery from sewage sludge liquors.

Iron reducing sludge as a source of electroactive bacteria: assessing iron reduction in biofilm bacteria, planktonic cells and isolates from a microbial fuel cell

In this study, bacteria from a microbial fuel cell (MFC) and isolates were evaluated on their Fe³⁺ reduction capability at different concentrations of iron using acetate as the sole source of carbon. The results demonstrated that the planktonic cells can reach an iron reduction up to 60% at 27 mmol Fe³⁺. Azospira oryzae (µ 0.89 ± 0.27 d^-1) and Cupriavidus metallidurans CH34 (µ 2.34 ± 0.81 d^-1) presented 55 and 62% of Fe³⁺ reduction, respectively, at 16 mmol l^-1. Enterobacter bugandensis (µ 0.4 ± 0.01 d^-1) 40% Fe³⁺ at 27 mmol l^-1, Citrobacter freundii ATCC 8090 (µ 0.23 ± 0.05 d^-1) and Citrobacter murliniae CDC2970-59 (µ 0.34 ± 0.02 d^-1) reduced Fe³⁺ in ~ 50%, at 55 mmol l^-1. This is the first report on these bacteria on a percentage of iron reduction. These results may be useful for anode design to contribute to a higher energy generation in MFCs.

Performance and mechanisms of enhanced hydrolysis acidification by adding different iron scraps: Microbial characteristics and fate of iron scraps

HA, as one of low-carbon pre-treatment technology could be enhanced by packing of iron or iron oxide powder for enhancing the transformation of large molecular weight to generate volatile fatty acids (VFAs) for fuel production. However, the controversy of iron strengthening the HA and inherent drawbacks of iron oxide, such as poor mass transfer, and difficult recovery, limit this pretreatment technology. Clean and rusty iron scraps were packed into an HA system to address these issues while focusing on the system performance and the response of core bacterial and fungal microbiomes to iron scrap exposure. Results showed that clean and rusty iron scraps can significantly improve the HA performance while considering hydrolysis efficiency (HE), acidification efficiency (AE) and VFAs production, given that VFAs ratios (C_acetate: C_propionate: C_butyrate) were changed from the 14:5:1 to 14:2:1 and 29:4:1, respectively, and the obtained VFAs ratios in iron scraps addition systems were more closely to the optimal VFAs ratio for lipids production. Redundant and molecular ecological network analyses indicated that iron scraps promote the system stability and acidogenesis capacity by boosting the complexity of microbes’ networks and enriching core functional microbes that show a positive response to HA performance, among which the relative abundance of related bacterial genera was promoted by 19.71 and 17.25% for R_Rusty and R_Clean systems. Moreover, except for the differences between the control and iron scraps addition systems, the findings confirmed that the R_Rusty system is slightly different from the R_Clean one, which was perhaps driven by the behavior of 6.20% of DIRB in R_Rusty system and only 1.16% of homoacetogens in R_Clean system when considering the microbial community and fate of iron scraps. Totally, the observed results highlight the application potential of the iron scrap-coupled HA process for the generation of VFAs and provide new insights into the response of different iron scraps in microbes communities.

Understanding roles of humic substance and protein on iron phosphate transformation during anaerobic fermentation of waste activated sludge

Effects of fulvic acid (FA) and bovine serum albumin (BSA) on the transformation of ferric phosphate (FePO₄) during anaerobic fermentation of waste activated sludge were investigated. Both FA and BSA promoted phosphorus (P) release from FePO₄. A higher P release efficiency was achieved with FA addition compared with BSA at the same dose although BSA promoted iron (Fe) reduction more effectively. Both FA and BSA contributed to the enrichment of vivianite but hindered P re-precipitation with other ions, and FA affected more significantly. Microbial analysis revealed that FA contributed to the enrichment of iron-reducing bacteria (IRB) transporting electrons indirectly and increased the bioavailable Fe(III) via siderophores; BSA provided more electron donors, thereby enriched IRB transferring electrons directly to Fe(III). This study provides an in-depth understanding of Fe and P transformations in sludge bearing iron-phosphorus compounds and it is of practical value for P recovery as vivianite.

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.

Deposition of Lead Phosphate by Lead-Tolerant Bacteria Isolated from Fresh Water near an Abandoned Mine

Specialist bacteria can synthesize nanoparticles from various metal ions in solution. Metal recovery with high efficiency can be achieved by metal-tolerant microorganisms that proliferate in a concentrated metal solution. In this study, we isolated bacteria (Pseudomonas sp. strain KKY-29) from a bacterial library collected from water near an abandoned mine in Komatsu City, Ishikawa Prefecture, Japan. KKY-29 was maintained in nutrient medium with lead acetate and synthesized hydrocerussite and pyromorphite nanoparticles inside the cell; KKY-29 also survived nanoparticle synthesis. Quantitative PCR analysis of genes related to phosphate metabolism showed that KKY-29 decomposed organic phosphorus to synthesize lead phosphate. KKY-29 also deposited various metal ions and synthesized metal nanoparticles when incubated in various metal salt solutions other than lead. The present study considers the development of biotechnology to recover lead as an economically valuable material.

Nitrogen Removal by an Anaerobic Iron-Dependent Ammonium Oxidation (Feammox) Enrichment: Potential for Wastewater Treatment

Nitrogen pollution in water is a growing concern. Anthropogenic activities have increased the amount of nitrogen released into watercourses, which harms human health and the environment, and causes serious problems, such as eutrophication. Feammox is a recently discovered biological pathway associated with the nitrogen cycle that has gained scientific interest. This process couples anaerobic ammonium oxidation with iron reduction. This work presents a study on the Feammox mechanism from the enrichment of an activated sludge obtained from a sewage treatment plant. The enrichment was carried out at neutral pH to study the N2 pathway, that is, the Feammox process with the oxidation of ammonium (NH4+) directly to N2. In addition, different sources of iron were studied: iron chloride (FeCl3); ferrihydrite; and goethite. The characterization of the sludge showed the genes associated with ammonia monooxygenase, nitrate and nitrite reductases processes, along with relevant microbial species. The enrichment, carried out for 42 days and monitored every 14 days, showed that FeCl3 as a source of Fe was more effective for the coupled process of oxidation of NH4+ and the reduction of Fe(III) to Fe(II). At the end of the enrichment period, a removal of 31% and 32.2% of NH4+, and an increase in Fe(II) concentration by 52.4 and 63.9 times regarding the initial value were achieved in aerobic and anaerobic sludge, respectively. This study provides information on the potential of Feammox in the removal of N from wastewater, and the oxidation/reduction yields in the initial enrichment phase.

Original implementation of low-temperature SPS for bioactive glass used as a bone biomaterial

Alkali borated bioactive glasses powders with compositions based on the SiO₂-Na₂O-CaO-P₂O₅-x B₂O₃ system (0 < x < 20 wt%); have been consolidated at low temperature using Spark Plasma Sintering (SPS). Through SPS technique under 50 MPa, it was possible to achieve fully dense and completely amorphous borated glasses at temperatures as low as 420 °C. By increasing the sintering temperature up to 430 °C, the dense samples crystallized which is mostly achieved at higher temperatures. This study reveals that the mechanical properties of these new borated biomaterials are suitable to be used as a promising candidate for repairing defects in non-load-bearing bones as well as for coating on the metallic surface implants to improve the bioactivity process bone/implant. The pressure had a weak effect on the crystallization and densification of the glass compared to the temperature during the powder consolidation by SPS. Moreover, by increasing the boron content, the compressive strength and the elastic modulus of the elaborated glasses decreased for being close to those of the natural.

Differential transformation mechanisms of exotic Cr(VI) in agricultural soils with contrasting physio-chemical and biological properties

The transformation mechanisms of Cr(VI) in agricultural soils at the molecular level remain largely unknown due to the multitude of abiotic and biotic factors. In this study, the different speciation and distribution of Cr in two types of agricultural soil (Ultisol and Fluvo-aquic soils) after two weeks of aging was investigated using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy, microfocused X-ray fluorescence (μ-XRF) and X-ray transmission microscopy (STXM). The microbial community structure of the two soils was also analyzed via high-throughput sequencing of 16S rRNA. Cr(VI) availability was relatively lower in the Ultisol than in the Fluvo-aquic soil after aging. Cr K-edge bulk XANES and STXM analysis indicated that Cr(VI) was reduced to Cr(III) in both soils. μ-XRF analysis and STXM analysis indicated the predominant association of Cr with Mn/Fe oxides and/or organo-Fe oxides in both soils. Additionally, STXM-coupled imaging and multiedge XANES analyses demonstrated that carboxylic groups were involved in the reduction of Cr(VI) and subsequent retention of Cr(III). 16S rRNA analysis showed considerably different bacterial communities across the two soils. Redundancy analysis (RDA) suggested that soil properties, including the total carbon content, Fe oxide component and pH, were closely linked to Cr(VI)-reducing functional bacteria in the Ultisol, including chromium-reducing bacteria (CRB) (e.g., Bacillus sp.) and dissimilatory iron-reducing (DIRB) (e.g., Shewanella sp.) bacteria, which possibly promoted Cr(VI) reduction. These findings shed light on the molecular-level transformation mechanisms of Cr(VI) in agricultural soils, which facilitates the effective management of Cr-enriched farmland.

Iron and total organic carbon shape the spatial distribution pattern of sediment Fe(III) reducing bacteria in a volcanic lake, NE China

Fe(III) reducing bacteria (FeRB) play a vital role in the biogeochemical cycle of Fe, C and N in nature. The volcanic lake can be considered as an ideal habitat for FeRB. Here, we investigated the diversity and spatial distribution of FeRB in sediments of Wenbo lake in Wudalianchi volcano based on culture-dependent and independent methods. A total of 28 isolates affiliated with the genera of Enterobacter, Bacillus, Pseudomonas and Clostridium were obtained from 18 sediment samples. We detected 783 operational taxonomic units (OTUs) belonged to FeRB using high high-throughput sequencing, and the dominant phyla were Proteobacteria (3.65%), Acidobacteria (0.29%), Firmicutes (10.78%). The representative FeRB genera such as Geobacter, Pseudomonas, Thiobacillus and Acinetobacter distributed widely in Wenbo lake. Results showed that the diversity and abundance of FeRB declined along the water-flow direction from Libo to Jingbo. In contrast, the FeRB diversity decreased and the FeRB abundance increased along with depth transect of sediments. It was found that the dominant phylum changed from Firmicutes to Proteobacteria along the water-flow direction, while changed from Proteobacteria to Firmicutes along with the depth of sediments. RDA indicated that the FeRB distribution were driven by soluble total iron, total organic carbon, Fe(II) and Fe(III). These will provide information for understanding the role of FeRB in the elements geochemical cycles in the volcanic environment.

Optimisation of a gold nanoparticle-based aptasensor integrated with image processing for the colorimetric detection of acephate using response surface methodology

Acephate (Ac) is an organophosphate (OP) compound, which is able to inhibit the activity of acetylcholinesterase. Thus, the aim of this study was to optimize the detection of Ac using a thiolated acephate binding aptamer-citrate capped gold nanoparticle (TABA-Cit-AuNP) sensor that also incorporated an image processing technique. The effects of independent variables, such as the incubation period of TABA-Cit-AuNPs (3-24 h) for binding TABA to Cit-AuNPs, the concentration of phosphate buffer saline (PBS) (0.001-0.01 M), the concentration of thiolated acephate binding aptamer (TABA) (50-200 nM), and the concentration of magnesium sulphate (MgSO4) (1-300 mM) were investigated. A quadratic model was developed using a central composite design (CCD) from response surface methodology (RSM) to predict the sensing response to Ac. The optimum conditions such as the concentration of PBS (0.01 M), the concentration of TABA (200 nM), the incubation period of TABA-Cit-AuNPs (3 h), and the concentration of MgSO4 (1 mM) were used to produce a TABA-Cit-AuNPs sensor for the detection of Ac. Under optimal conditions, this sensor showed a detection ranging from 0.01 to 2.73 μM and a limit of detection (LOD) of 0.06 μM. Real sample analysis demonstrated this aptasensor as a good analytical method to detect Ac.

Fermentation, methanotrophy and methanogenesis influence sedimentary Fe and As dynamics in As-affected aquifers in Vietnam

High arsenic (As) concentrations in groundwater are a worldwide problem threatening the health of millions of people. Microbial processes are central in the (trans)formation of the As-bearing ferric and ferrous minerals, and thus regulate dissolved As levels in many aquifers. Mineralogy, microbiology and dissolved As levels can vary sharply within aquifers, making high-resolution measurements particularly valuable in understanding the linkages between them. We conducted a high spatial resolution geomicrobiological study in combination with analysis of sediment chemistry and mineralogy in an alluvial aquifer system affected by geogenic As in the Red River delta in Vietnam. Microbial community analysis revealed a dominance of fermenters, methanogens and methanotrophs whereas sediment mineralogy along a 46 m deep core showed a diversity of Fe minerals including poorly crystalline Fe (II/III) and Fe(III) (oxyhydr)oxides such as goethite, hematite, and magnetite, but also the presence of Fe(II)-bearing carbonates and sulfides which likely formed as a result of microbially driven organic carbon (OC) degradation. A potential important role of methane (CH₄) as electron donor for reductive Fe mineral (trans)formation was supported by the high abundance of Candidatus Methanoperedens, a known Fe(III)-reducing methanotroph. Overall, these results imply that OC turnover including fermentation, methanogenesis and CH₄ oxidation are important mechanisms leading to Fe mineral (trans)formation, dissolution and precipitation, and thus indirectly affecting As mobility by changing the Fe-mineral inventory.

Microbial transformation of biogenic and abiogenic Fe minerals followed by in-situ incubations in an As-contaminated vs. non-contaminated aquifer

Fe(III) minerals play a crucial role for arsenic (As) mobility in aquifers as they usually represent the main As-bearing phases. Microbial reductive dissolution of As-bearing Fe(III) minerals is responsible for the release of As and the resulting groundwater contamination in many sites worldwide. So far, in most studies mainly abiogenic iron minerals have been considered. Yet, biogenic minerals that possess different properties to their abiogenic counterparts are also present in the environment. In some environments they dominate the iron mineral inventory but so far, it is unclear what this means for the As mobility. We, therefore, performed an in-situ aquifer Fe(III) minerals exposure experiment i) to evaluate how different biogenic and abiogenic Fe(III) minerals are transformed in a strongly reducing, As-contaminated aquifer (25 m) compared to As-free moderately reducing aquifer (32 m) and ii) to assess which microbial taxa are involved in these Fe(III) minerals transformations. We found that higher numbers of bacteria and archaea were associated with the minerals incubated in the As-contaminated compared to the non-contaminated aquifer and that all Fe(III) minerals were mainly colonized by Fe(III)-reducing bacteria, with Geobacter being the most abundant taxon. Additionally, fermenting microorganisms were abundant on minerals incubated in the As-contaminated aquifer, while methanotrophs were identified on the minerals incubated in the As-free moderately reducing aquifer, implying involvement of these microorganisms in Fe(III) reduction. We observed that biogenic Fe(III) minerals generally tend to become more reduced and when incubated in the As-contaminated aquifer sorbed more As than the abiogenic ones. Most of abiogenic and biogenic Fe(III) minerals were transformed into magnetite while biogenic more crystalline mixed phases were not subjected to visible transformation. This in-situ Fe(III) minerals incubation approach shows that biogenic minerals are more prone to be colonized by (Fe(III)-reducing) microorganisms and bind more As, although ultimately produce similar minerals during Fe(III) reduction.

Functional Interrelationships of Microorganisms in Iron-Based Anaerobic Wastewater Treatment

This study explicated the functional activities of microorganisms and their interrelationships under four previously reported iron reducing conditions to identify critical factors that governed the performance of these novel iron-dosed anaerobic biological wastewater treatment processes. Various iron-reducing bacteria (FeRB) and sulfate reducing bacteria (SRB) were identified as the predominant species that concurrently facilitated organics oxidation and the main contributors to removal of organics. The high organic contents of wastewater provided sufficient electron donors for active growth of both FeRB and SRB. In addition to the organic content, Fe (III) and sulfate concentrations (expressed by Fe/S ratio) were found to play a significant role in regulating the microbial abundance and functional activities. Various fermentative bacteria contributed to this FeRB-SRB synergy by fermenting larger organic compounds to smaller compounds, which were subsequently used by FeRB and SRB. Feammox (ferric reduction coupled to ammonium oxidation) bacterium was identified in the bioreactor fed with wastewater containing ammonium. Organic substrate level was a critical factor that regulated the competitive relationship between heterotrophic FeRB and Feammox bacteria. There were evidences that suggested a synergistic relationship between FeRB and nitrogen-fixing bacteria (NFB), where ferric iron and organics concentrations both promoted microbial activities of FeRB and NFB. A concept model was developed to illustrate the identified functional interrelationships and their governing factors for further development of the iron-based wastewater treatment systems.

Reductive release of Fe mineral-associated organic matter accelerated by oxalic acid

The properties and composition of soil dissolved organic matter (DOM) are highly affected by the adsorption and desorption of organic matter (OM) on soil minerals and heterotrophic microbial respiration. Organic acids (e.g., oxalic acid), components of root exudates, have been revealed to liberate organic matter (OM) by the dissolution of protective mineral phases and stimulate microbial degradation of OM. However, the effects of organic acids on the properties and composition of soil DOM molecules and the related mechanisms are still poorly understood. In this study, we conducted microcosm incubation experiments with and without oxalic acid addition, and aimed to elucidate the variations of DOM properties and composition, employing a combination of Fourier transform ion cyclotron resonance mass spectrometry, optical spectroscopy, and bacterial community composition analysis. Our results indicated that the released OM from the direct dissolution of protective mineral phases by oxalic acid further stimulated the microbial reductive release of Fe mineral-associated OM under anoxic conditions. Furthermore, the addition of oxalic acid enhanced the degradation of aliphatic compounds and lignins with low O/C ratios, and increased the accumulation of lignins with high O/C ratios, tannins, and condensed aromatics. Linking the bacterial community composition to DOM molecular properties and composition further suggested that the enhanced reductive release of Fe mineral-associated OM was highly related to the increased abundances of Proteobacteria and Actinobacteria. Overall, oxalic acid induced long-lasting impacts on soil DOM properties and composition under anoxic soil conditions in our study. We expect that our results will contribute to understanding the dynamics of soil DOM molecules in the environment.

Green synthesis of metal nanoparticles using microorganisms and their application in the agrifood sector

The agricultural sector is currently facing many global challenges, such as climate change, and environmental problems such as the release of pesticides and fertilizers, which will be exacerbated in the face of population growth and food shortages. Therefore, the need to change traditional farming methods and replace them with new technologies is essential, and the application of nanotechnology, especially green technology offers considerable promise in alleviating these problems. Nanotechnology has led to changes and advances in many technologies and has the potential to transform various fields of the agricultural sector, including biosensors, pesticides, fertilizers, food packaging and other areas of the agricultural industry. Due to their unique properties, nanomaterials are considered as suitable carriers for stabilizing fertilizers and pesticides, as well as facilitating controlled nutrient transfer and increasing crop protection. The production of nanoparticles by physical and chemical methods requires the use of hazardous materials, advanced equipment, and has a negative impact on the environment. Thus, over the last decade, research activities in the context of nanotechnology have shifted towards environmentally friendly and economically viable 'green' synthesis to support the increasing use of nanoparticles in various industries. Green synthesis, as part of bio-inspired protocols, provides reliable and sustainable methods for the biosynthesis of nanoparticles by a wide range of microorganisms rather than current synthetic processes. Therefore, this field is developing rapidly and new methods in this field are constantly being invented to improve the properties of nanoparticles. In this review, we consider the latest advances and innovations in the production of metal nanoparticles using green synthesis by different groups of microorganisms and the application of these nanoparticles in various agricultural sectors to achieve food security, improve crop production and reduce the use of pesticides. In addition, the mechanism of synthesis of metal nanoparticles by different microorganisms and their advantages and disadvantages compared to other common methods are presented.

Mineralosphere Microbiome Leading to Changed Geochemical Properties of Sedimentary Rocks from Aiqigou Mud Volcano, Northwest China

The properties of rocks can be greatly affected by seepage hydrocarbons in petroleum-related mud volcanoes. Among them, the color of sedimentary rocks can reflect the changes of sedimentary environment and weathering history. However, little is known about the microbial communities and their biogeochemical significance in these environments. In this study, contrasting rock samples were collected from the Aiqigou mud volcano on the southern margin of the Junggar Basin in Northwest China as guided by rock colors indicative of redox conditions. The physicochemical properties and mineral composition are similar under the same redox conditions. For example, the content of chlorite, muscovite, quartz, and total carbon were higher, and the total iron was lower under reduced conditions compared with oxidized environments. High-throughput sequencing of 16S rRNA gene amplicons revealed that different functional microorganisms may exist under different redox conditions; microbes in oxidized conditions have higher diversity. Statistical analysis and incubation experiments indicated that the microbial community structure is closely related to the content of iron which may be an important factor for color stratification of continental sedimentary rocks in the Aiqigou mud volcano. The interactions between organics and iron-bearing minerals mediated by microorganisms have also been hypothesized.

Heavy metal reduction coupled to methane oxidation：Mechanisms, recent advances and future perspectives

Methane emission has contributed greatly to the global warming and climate change, and the pollution of heavy metals is an important concern due to their toxicity and environmental persistence. Recently, multiple heavy metals have been demonstrated to be electron acceptors for methane oxidation, which offers a potential for simultaneous methane emission mitigation and heavy metal detoxification. This review provides a comprehensive discussion of heavy metals reduction coupled to methane oxidation, and identifies knowledge gaps and opportunities for future research. The functional microorganisms and possible mechanisms are detailed in groups under aerobic, hypoxic and anaerobic conditions. The potential application and major environmental significances for global methane mitigation, the elements cycle and heavy metals detoxification are also discussed. The future research opportunities are also discussed to provide insights for further research and efficient practical application.

Effects of ferric-phosphate forms on phosphorus release and the performance of anaerobic fermentation of waste activated sludge

Five ferric-phosphate (Fe(III)Ps) with amorphous or crystalline structures were added to waste activated sludge (WAS) for anaerobic fermentation, aiming to investigate effects of Fe(III)Ps forms on phosphorus (P) release and the performance of WAS fermentation. The results revealed that the Fe(III) reduction rate of hexagonal-FePO₄ was faster than that of monoclinic-FePO₄·2H₂O, thanks to its lower crystal field stabilization energy. FePO₄·nH₂O was reduced to vivianite and part of the phosphate was released as orthophosphate (PO₄-P). Giniite (Fe₅(PO₄)₄(OH)₃·2H₂O) as an iron hydroxyphosphate was transformed to βFe(III)Fe(II)(PO₄)O-like compounds without PO₄-P release. In addition, Fe(III)Ps had an adverse effect on the anaerobic fermentation of WAS. The specific hydrolysis rate constant and volatile fatty acids (VFAs) yield decreased by 38.4% and 41.9%, respectively, for the sludge sample with amorphous-FePO₄·3H₂O, which dropped the most. This study provides new insights into various forms of Fe(III)Ps performance during anaerobic fermentation and is beneficial to enhancing P recovery efficiency.

Microbe-Mediated Extracellular and Intracellular Mineralization: Environmental, Industrial, and Biotechnological Applications

Microbe-mediated mineralization is ubiquitous in nature, involving bacteria, fungi, viruses, and algae. These mineralization processes comprise calcification, silicification, and iron mineralization. The mechanisms for mineral formation include extracellular and intracellular biomineralization. The mineral precipitating capability of microbes is often harnessed for green synthesis of metal nanoparticles, which are relatively less toxic compared with those synthesized through physical or chemical methods. Microbe-mediated mineralization has important applications ranging from pollutant removal and nonreactive carriers, to other industrial and biomedical applications. Herein, the different types of microbe-mediated biomineralization that occur in nature, their mechanisms, as well as their applications are elucidated to create a backdrop for future research.

A Novel "Microbial Bait" Technique for Capturing Fe(III)-Reducing Bacteria

Microbial reduction of Fe(III) is a key geochemical process in anoxic environments, controlling the degradation of organics and the mobility of metals and radionuclides. To further understand these processes, it is vital to develop a reliable means of capturing Fe(III)-reducing microorganisms from the field for analysis and lab-based investigations. In this study, a novel method of capturing Fe(III)-reducing bacteria using Fe(III)-coated pumice "microbe-baits" was demonstrated. The methodology involved the coating of pumice (approximately diameter 4 to 6 mm) with a bioavailable Fe(III) mineral (akaganeite), and was verified by deployment into a freshwater spring for 2 months. On retrieval, the coated pumice baits were incubated in a series of lab-based microcosms, amended with and without electron donors (lactate and acetate), and incubated at 20°C for 8 weeks. 16S rRNA gene sequencing using the Illumina MiSeq platform showed that the Fe(III)-coated pumice baits, when incubated in the presence of lactate and acetate, enriched for Deltaproteobacteria (relative abundance of 52% of the sequences detected corresponded to Geobacter species and 24% to Desulfovibrio species). In the absence of added electron donors, Betaproteobacteria were the most abundant class detected, most heavily represented by a close relative to Rhodoferax ferrireducens (15% of species detected), that most likely used organic matter sequestered from the spring waters to support Fe(III) reduction. In addition, TEM-EDS analysis of the Fe(III)-coated pumice slurries amended with electron donors revealed that a biogenic Fe(II) mineral, magnetite, was formed at the end of the incubation period. These results demonstrate that Fe(III)-coated pumice "microbe baits" can potentially help target metal-reducing bacteria for culture-dependent studies, to further our understanding of the nano-scale microbe-mineral interactions in aquifers.

Water managements limit heavy metal accumulation in rice: Dual effects of iron-plaque formation and microbial communities

Understanding the mechanisms on how water management can minimize the concentrations of heavy metals in rice grains is important. Two water managements were concerned in our studies, including continuously flooding and alternate wetting and drying (AWD). Compared to AWD, a continuously flooded culture reduces the concentration of cadmium and other metals in the rice grains by reducing the root-to-shoot translocation and the availability of metals in rhizosphere. In a flooded environment, the rice rhizosphere was characterized by an increased soil pH, reduced fluorescein diacetate (FDA) activity, and lower metal bioavailability. In addition, flooding significantly decreased the iron plaque on the root surface and reduced the affinity for metals in rhizosphere. Water managements significantly changed soil microbial diversity, especially the proportion of anaerobic bacteria, including the iron-reducing bacteria Latescibacteria, Desulfuromonadales, and Geobacteraceae. Interestingly, these bacteria exhibited a significant correlation with cadmium that was adsorbed on the root. This study revealed that continuously flooded culture is a valuable strategy for minimizing heavy metal accumulation in rice grains. By increasing the abundance of unique bacterial community, iron plaque formation and the affinity of metals in rhizosphere were reduced, and the uptake and accumulation of heavy metals in rice plants was finally mitigated.

The Effect of Dissimilatory Manganese Reduction on Lactate Fermentation and Microbial Community Assembly

Fermentation and dissimilatory manganese (Mn) reduction are inter-related metabolic processes that microbes can perform in anoxic environments. Fermentation is less energetically favorable and is often not considered to compete for organic carbon with dissimilatory metal reduction. Therefore, the aim of our study was to investigate the outcome of the competition for lactate between fermentation and Mn oxide (birnessite) reduction in a mixed microbial community. A birnessite reducing enrichment culture was obtained from activated sludge with lactate and birnessite as the substrates. This enrichment was further used to test how various birnessite activities (0, 10, 20, and 40 mM) affected the rates of fermentation and metal reduction, as well as community composition. Increased birnessite activity led to a decrease of lactate consumption rate. Acetate and propionate were the main products. With increasing birnessite activity, the propionate/acetate ratio decreased from 1.4 to 0.47. Significant CO₂ production was detected only in the absence of birnessite. In its presence, CO₂ concentrations remained close to the background since most of the CO₂ produced in these experiments was recovered as MnCO₃. The Mn reduction efficiency (Mn(II) produced divided by birnessite added) was the highest at 10 mM birnessite added, where about 50% of added birnessite was reduced to Mn(II), whereas at 20 and 40 mM approximately 21 and 16% was reduced. The decreased birnessite reduction efficiency at higher birnessite activities points to inhibition by terminal electron acceptors and/or its toxicity which was also indicated by retarded lactate oxidation and decreased concentrations of microbial metabolites. Birnessite activity strongly affected microbial community structure. Firmicutes and Bacteroidetes were the most abundant phyla at 0 mM of birnessite. Their abundance was inversely correlated with birnessite concentration. The relative sequence abundance of Proteobacteria correlated with birnessite concentrations. Most of the enriched populations were involved in lactate/acetate or amino acid fermentation and the only previously known metal reducing genus detected was related to Shewanella sp. The sequencing data confirmed that lactate consumption coupled to metal reduction was only one of the processes occurring and did not outcompete fermentation processes.

RNA Stable Isotope Probing of Potential Feammox Population in Paddy Soil

Anaerobic ammonium oxidation coupled to iron reduction (Feammox) is a recently discovered pathway contributing to nitrogen loss in various ecosystems such as paddy soils and sediments. However, little is known about the microbes driving Feammox in an agricultural ecosystem. Here, we demonstrated the occurrence of Feammox in paddy soils of Southern China using a ¹⁵N isotopic tracing technique, and examined the microbial communities associated with Feammox using RNA based stable isotope probing (RNA-SIP) combined with Illumina sequencing. Feammox was detected in all collected soils with direct N₂ production as the dominant Feammox pathway. It was estimated that approximately 6.91% of the applied nitrogen fertilizers were lost through Feammox in the paddy soils. RNA-SIP results showed that the composition of enriched active microbial communities were dependent on soil properties, especially the soil pH and grain size. Geobacter were enriched in most soils across various properties. The abundance of enriched GOUTA19 were significantly higher in soils with low pH than those in soils with medium pH and high pH, and the relative abundance of active Nitrososphaeraceae and Pseudomonas only increased in soils with medium and high pH during 4-day of incubation. These results suggested Feammox is a ubiquitous and important process for N loss. Geobacter, GOUTA19, Nitrososphaeraceae and Pseudomonas were active during the incubation that favored Feammox and the growth of Feammox microbes, suggesting these microbes were potentially associated with Feammox in natural agricultural soils.

Effects of Fe/S ratio on the kinetics and microbial ecology of an Fe(III)-dosed anaerobic wastewater treatment system

Effects of Fe(III)/sulfate (Fe/S) ratio on organic carbon oxidation kinetics and microbial ecology of a novel Fe(III)-dosed anaerobic wastewater treatment system were investigated in this study. Fixed-film batch bioreactors under three Fe/S molar ratios (1, 2, and 3) yielded COD oxidation rates that increased with the Fe/S ratio, and estimated Michaelis-Menten model parameters V_max ranging in 0.47-1.09 mg/L⋅min and K_m in 2503-3267 mg/L. Both iron and sulfate reducing bacteria contributed to the organics oxidation, and the produced sludge materials contained both biomass (32-45 wt.%) and inorganic precipitates from biogenic ferrous iron and sulfide (68-55 wt.%). Spectroscopic and chemical elemental analyses indicated that the inorganic fraction of the sludge materials contained both FeS and FeS₂, and had Fe/S stoichiometric ratios close to 1. Microbiological analyses of the biofilm samples revealed that the major putative iron- and sulfate reducers were Geobacter sp. and Desulfovibrio sp. along with noticeable N-fixing and fermentative bacteria. The COD oxidation rate had a positive correlation with the relative abundance of iron reducers, and both increased with the Fe/S ratio. A conceptual framework was proposed to illustrate the effects of Fe/S ratio on organics oxidation rate, microbial ecology and their interplays.

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

Anaerobic ammonium oxidation coupled to iron reduction, termed Feammox, is a new microbial process linked the nitrogen cycles. However, the nitrogen losses through Feammox from different ecosystem habitats remain unclear. In this study, isotope tracing technology and molecular microbial analysis were used to investigate the Feammox and its contribution to the nitrogen loss in the farmland and riparian soils, and river sediments. The potential Feammox rates were detected, which varied from 0.07 to 0.15mgNkg^-1d^-1 among the three ecosystem habitats. Feammox rates were significantly higher in the farmlands or riparian soils than in the river sediments. Feammox, denitrification and anaerobic ammonium oxidation (anammox) were estimated to account for approximately 3.5-4.2%, 92.6-93.1% and 2.8-3.9% of the total nitrogen losses respectively, while a significant correlation was observed between the Feammox rates and the denitrification rates (r=0.72, P<0.05). In addition, a nitrogen loss at 8.3-17.8kgNha^-1yr^-1 was linked with Feammox in the examined soils. This study demonstrated that Feammox could be a potential pathway of nitrogen loss from ecosystem habitats.

The influence of liming on cadmium accumulation in rice grains via iron-reducing bacteria

Cadmium (Cd) in soil is attracting worldwide attention, and many valuable measures and suggestions of minimizing the rice grain Cd are available. Among these methods, liming can increase the soil pH and decrease the rice grain Cd content. Here, we report that soil pH was negatively and significantly correlated with the concentration of soil extractable Fe and Cd. In addition, the iron concentration on root surface was significantly and positively associated with the available metals in soil and the rice grain Cd. However, the return of contaminated rice straw significantly increased the Cd accumulation in the rice grain, although the returned straw did not significantly influence the concentration of extracted soil Cd. Furthermore, an analysis of the functional microbe community was performed, and the response of iron-reducing bacteria (IRB) under the six treatments provides valuable insights for reducing the available Cd concentration in soil. A LEfSe (LDA coupled with effect size measurement for significant differences) analysis showed that the application of liming reduced the abundance of IRB. The results of a redundancy analysis (RDA) indicated that soil pH was significantly and negatively associated with the abundance of Proteobacteria and Geobacter and the concentration of bioavailable Fe and Cd in the soil, which could explain the reduced accumulation of bioavailable Cd in rice grain. Collectively, our results demonstrated that liming in Cd-polluted paddy soil is a reasonable strategy for minimizing rice grain Cd by increasing the soil pH, which reduces the soil available iron and Cd concentration by shifting the diversity and composition of IRB, thus ultimately resulting in decreased Cd in rice.