Dissimilatory Fe(III) and Mn(IV) reduction

Exogenous paths regulate electron transfer enhancing sediment phosphorus immobilization

The lack of electron acceptors in anaerobic sediments leads to endogenous phosphorus release and low removal efficiency of organic pollutants. This study introduced electrodes and iron oxides into sediments to construct electron network transport chains to supplement electron acceptors. The sediment total organic carbon (TOC) removal efficiencies of closed-circuit (CC) and closed-circuit with Fe addition (CC-Fe) were estimated to be 1.4 and 1.7 times of the control. Unlike the fluctuation of phosphorus in the overlying water of the controls, the CC-Fe was stabled at 0.04-0.08 mg/L during the 84-d operation. The phosphorus in interstitial water of CC-Fe was 30 % less than in control, whereas in sediment, the redox sensitive phosphorus was increased by 14 %, indicating phosphorus was preferred to fix into sediments rather than interstitial water. This is important to reduce the risk of endogenous phosphorus returning to the overlying water. Microbial community analysis showed that the multiplication of Fonticella in CC-Fe (20 %) was 1.8-fold of control (11 %) which improved the TOC removal efficiency. While electroactive microorganisms accumulated near the electrode reduced the abundance of Fe-reducing bacteria, such as Desulfitobacterium (2.4 %), leading to better phosphorus fixation. These findings suggest a strategy for the efficient bioremediation of endogenous pollution in water, with broader implications for regulating electron transport paths and element cycles in aquatic environments.

Contrasting organic carbon respiration pathways in coastal wetlands undergoing accelerated sea level changes

Carbon cycling in coastal wetland soil is controlled by a complex interplay between microbial processes and porewater chemistry that are often influenced by various external forcings like wind, river discharge, and sea-level changes, where most of the organic carbon is mineralized to its inorganic form by various aerobic and anaerobic respiration pathways. The export of this inorganic carbon (DIC) from coastal wetlands has long been recognized as a significant component of the global carbon cycle. The major objective of this work is to determine the relative contribution of various respiration pathways to seasonal DIC production in two contrasting marshes (brackish and salt). The DIC fluxes estimates for the brackish and salt marshes were found to range between 36.52 ± 5.81 and 33.98 ± 2.21 mmol m-2 d-1 in winter and 133.10 ± 102.60 and 82.37 ± 30.87 mmol m-2 d-1 during summer of 2020. For the brackish marsh, aerobic respiration and iron reduction were found to be the primary contributors to DIC production representing 17.91-35.21 % and 61.13-81.97 % of total measured organic matter (OM) respiration respectively. On the other hand, aerobic respiration and sulfate reduction were the primary contributors to DIC production in the salt marsh, accounting for 37.91-83.93 % and 15.87-62.04 % of the total measured OM respiration respectively. The Mississippi River Deltaic Plain experiences high relative sea level rise and expected to undergo rapid change in salinity regime in near future from additional changes in river discharge, proposed sediment diversion plans, and storm surge intensities. The current study represents the first attempt to concurrently estimate various respiration pathways in this region and more studies are needed to understand the trajectories of soil OM respiration pathways and its impact on coastal carbon cycling.

Crosstalk between 6-methyladenine and 4-methylcytosine in Geobacter sulfurreducens exposed to extremely low-frequency electromagnetic field

4-Methylcytosine (4mC) and 6-methyladenine (6mA) are the most prevalent types of DNA modifications in prokaryotes. However, whether there is crosstalk between 4mC and 6mA remain unknown. Here, methylomes and transcriptomes of Geobacter sulfurreducens exposed to different intensities of extremely low frequency electromagnetic fields (ELF-EMF) were investigated. Results showed that the second adenine of all the 5'-GTACAG-3' motif was modified to 6mA (M-6mA). For the other 6mA (O-6mA), the variation in their distance from the neighboring M-6mA increased with the intensity of ELF-EMF. Moreover, cytosine adjacent to O-6mA has a much higher probability of being modified to 4mC than cytosine adjacent to M-6mA, and the closer an unmodified cytosine is to 4mC, the higher the probability that the cytosine will be modified to 4mC. Furthermore, there was no significant correlation between DNA methylation and gene expression regulation. These results suggest a reference signal that goes from M-6mA to O-6mA to 4mC.

Emerging investigator series: Coprecipitation with glucuronic acid limits reductive dissolution and transformation of ferrihydrite in an anoxic soil

Ferrihydrite, a poorly crystalline Fe(III)-oxyhydroxide, is abundant in soils and is often found associated with organic matter. Model studies consistently show that in the presence of aqueous Fe(II), organic carbon (OC)-associated ferrihydrite undergoes less transformation than OC-free ferrihydrite. Yet, these findings contrast microbial reductive dissolution studies in which the OC promotes the reductive dissolution of Fe(III) in ferrihydrite and leads to the release of associated OC. To shed light on these complex processes, we quantified the extent of reductive dissolution and transformation of native Fe minerals and added ferrihydrite in anoxic soil incubations where pure 57Fe-ferrihydrite (57Fh), pure 57Fe-ferrihydrite plus dissolved glucuronic acid (57Fh + GluCaq), a 57Fe-ferrihydrite-13C-glucuronic acid coprecipitate (57Fh13GluC), or only dissolved glucuronic acid (13GluCaq) were added. By tracking the transformation of the 57Fe-ferrihydrite in the solid phase with Mössbauer spectroscopy together with analysis of the iron isotope composition of the aqueous phase and chemical extractions with inductively coupled plasma-mass spectrometry, we show that the pure 57Fe-ferrihydrite underwent more reductive dissolution and transformation than the coprecipitated 57Fe-ferrihydrite when identical amounts of glucuronic acid were provided (57Fh + GluCaqversus57Fh13GluC treatments). In the absence of glucuronic acid, the pure 57Fe-ferrihydrite underwent the least reductive dissolution and transformation (57Fh). Comparing all treatments, the overall extent of Fe(III) reduction, including the added and native Fe minerals, determined with X-ray absorption spectroscopy, was highest in the 57Fh + GluCaq treatment. Collectively, our results suggest that the limited bioavailability of the coprecipitated OC restricts not only the reductive dissolution of the coprecipitated mineral, but it also limits the enhanced reduction of native soil Fe(III) minerals.

Cryo-EM Structure of the Mnx Protein Complex Reveals a Tunnel Framework for the Mechanism of Manganese Biomineralization

The global manganese cycle relies on microbes to oxidize soluble Mn(II) to insoluble Mn(IV) oxides. Some microbes require peroxide or superoxide as oxidants, but others can use O2 directly, via multicopper oxidase (MCO) enzymes. One of these, MnxG from Bacillus sp. strain PL-12, was isolated in tight association with small accessory proteins, MnxE and MnxF. The protein complex, called Mnx, has eluded crystallization efforts, but we now report the 3D structure of a point mutant using cryo-EM single particle analysis, cross-linking mass spectrometry, and AlphaFold Multimer prediction. The β-sheet-rich complex features MnxG enzyme, capped by a heterohexameric ring of alternating MnxE and MnxF subunits, and a tunnel that runs through MnxG and its MnxE3F3 cap. The tunnel dimensions and charges can accommodate the mechanistically inferred binuclear manganese intermediates. Comparison with the Fe(II)-oxidizing MCO, ceruloplasmin, identifies likely coordinating groups for the Mn(II) substrate, at the entrance to the tunnel. Thus, the 3D structure provides a rationale for the established manganese oxidase mechanism, and a platform for further experiments to elucidate mechanistic details of manganese biomineralization.

Influence of anti-seasonal inundation on geochemical processes of arsenic speciation in the water-level-fluctuation zone soil of the Three Gorges Reservoir, China

Since the completion of Three Gorges Dam, the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) experiences the periodic anti-seasonal inundation. However, knowledge for mechanisms of mobilization and transformation of arsenic (As) in WLFZ soils of the TGR remains scarce. To address this gap, a combination of field observation and simulated flooding experiments attempts to illustrate the As mobilization, the transformation between As(V) and As(III), and the factors driving these processes. The study revealed that anti-seasonal inundation (with a temperature at 13 ℃) mitigated As release from submerged soils. Interestingly, the total As and ratio of As(III) (the more toxic form of As) concentrations in porewater at 13 ℃ was lower, and the prevalence of As(III) occurred later than those at 32 °C (imitate the seasonal inundation condition). The results indicated that the As reduction and the corresponding toxic risks in submerged soils were alleviated under anti-seasonal inundation. The study proposes the reduction of As-bearing manganese (Mn) mineral assemblages and competitive adsorption of dissolved organic carbon (DOC) as primary mechanisms for As mobilization. Furthermore, microorganism-mediated detoxification/reduction processes involving DOC, nitrogen, and Mn (oxyhydr)oxides were identified as central pathways for As(III) enrichment under anti-seasonal inundation. This study enhances understandings of the biogeochemical processes and fate of As in WLFZ soils influenced by artificial regulation of the reservoir.

Transcriptome Analysis Reveals the Inhibitory Effect of Cu2+ on Polyferric Sulfate Floc Reduction by Shewanella putrefaciens CN32

Polyferric sulfate (PFS), an economical coagulant widely used for removing heavy metal contaminants from water, is susceptible to reduction and transformation by iron-reducing bacteria prevalent in sediments. However, the effect of heavy metal ions adsorbed in PFS flocs on this biological process remains unclear. According to our results, compared with other heavy metal cations (e.g., Cu2+, Cd2+, Zn2+, Ni2+, Pb2+, and Co2+), Cu2+ had a stronger inhibitory effect on PFS floc reduction by Shewanella putrefaciens CN32, a typical dissimilatory iron-reducing bacterium. The presence of Cu2+ remarkably influenced the global transcription of CN32, resulting in 782 upregulated genes and 713 downregulated genes that are mainly annotated in energy production, amino acid metabolism, protein biosynthesis, and oxidation‒reduction processes. The anaerobic TCA cycle for energy (electron) production was significantly activated in the presence of Cu2+, while the transcription of many genes related to the extracellular electron transfer pathway was downregulated, which is responsible for the decreased Fe3+ reduction. Moreover, the pathways of assimilatory sulfate reduction and subsequent cysteine biosynthesis were significantly enriched, which is hypothesized to result in the consumption of abundant energy produced from the enhanced anaerobic TCA cycle, revealing a strategy to address the oxidative stress caused by Cu2+. This work elucidates the unusual suppressive effects of Cu2+ on the microbial reduction of PFS flocs, which reveals the high resistance of PFS flocs to microbial destruction when used to treat Cu2+ pollution in water, thus demonstrating their tremendous practical prospects.

A critical review of approaches to enhance the performance of bio-electro-Fenton and photo-bio-electro-Fenton systems

The development of sustainable advanced energy conversion technologies and efficient pollutant treatment processes is a viable solution to the two global crises of the lack of non-renewable energy resources and environmental harm. In recent years, the interaction of biological and chemical oxidation units to utilize biomass has been extensively studied. Among these systems, bio-electro-Fenton (BEF) and photo-bio-electro-Fenton (PBEF) systems have shown prospects for application due to making rational and practical conversion and use of energy. This review compared and analyzed the electron transfer mechanisms in BEF and PBEF systems, and systematically summarized the techniques for enhancing system performance based on the generation, transfer, and utilization of electrons, including increasing the anode electron recovery efficiency, enhancing the generation of reactive oxygen species, and optimizing operational modes. This review compared the effects of different methods on the electron flow process and fully evaluated the benefits and drawbacks. This review may provide straightforward suggestions and methods to enhance the performance of BEF and PBEF systems and inspire the reader to explore the generation and utilization of sustainable energy more deeply.

Microbial electrosynthesis: opportunities for microbial pure cultures

Microbial electrosynthesis (MES) is an emerging technology that couples renewable electricity to microbial production processes. Although advances in MES performance have been driven largely by microbial mixed cultures, we see a great limitation in the diversity, and hence value, of products that can be achieved in undefined mixed cultures. By contrast, metabolic control of pure cultures and genetic engineering could greatly expand the scope of MES, and even of broader electrobiotechnology, to include targeted high-value products. To leverage this potential, we advocate for more efforts and activities to develop engineered electroactive microbes for synthesis, and we highlight the need for a standardized electrobioreactor infrastructure that allows the establishment and engineering of electrobioprocesses with these novel biocatalysts.

Manganese reduction and associated microbial communities in Antarctic surface sediments

The polar regions are the fastest warming places on earth. Accelerated glacial melting causes increased supply of nutrients such as metal oxides (i.e., iron and manganese oxides) into the surrounding environment, such as the marine sediments of Potter Cove, King George Island/Isla 25 de Mayo (West Antarctic Peninsula). Microbial manganese oxide reduction and the associated microbial communities are poorly understood in Antarctic sediments. Here, we investigated this process by geochemical measurements of in situ sediment pore water and by slurry incubation experiments which were accompanied by 16S rRNA sequencing. Members of the genus Desulfuromusa were the main responder to manganese oxide and acetate amendment in the incubations. Other organisms identified in relation to manganese and/or acetate utilization included Desulfuromonas, Sva1033 (family of Desulfuromonadales) and unclassified Arcobacteraceae. Our data show that distinct members of Desulfuromonadales are most active in organotrophic manganese reduction, thus providing strong evidence of their relevance in manganese reduction in permanently cold Antarctic sediments.

Metagenomes from microbial populations beneath a chromium waste tip give insight into the mechanism of Cr (VI) reduction

Dumped Chromium Ore Processing Residue (COPR) at legacy sites poses a threat to health through leaching of toxic Cr(VI) into groundwater. Previous work implicates microbial activity in reducing Cr(VI) to less mobile and toxic Cr(III), but the mechanism has not been explored. To address this question a combined metagenomic and geochemical study was undertaken. Soil samples from below the COPR waste were used to establish anaerobic microcosms which were challenged with Cr(VI), with or without acetate as an electron donor, and incubated for 70 days. Cr was rapidly reduced in both systems, which also reduced nitrate, nitrite then sulfate, but this sequence was accelerated in the acetate amended microcosms. 16S rRNA gene sequencing revealed that the original soil sample was diverse but both microcosm systems became less diverse by the end of the experiment. A high proportion of 16S rRNA gene reads and metagenome-assembled genomes (MAGs) with high completeness could not be taxonomically classified, highlighting the distinctiveness of these alkaline Cr impacted systems. Examination of the coding capacity revealed widespread capability for metal tolerance and Fe uptake and storage, and both populations possessed metabolic capability to degrade a wide range of organic molecules. The relative abundance of genes for fatty acid degradation was 4× higher in the unamended compared to the acetate amended system, whereas the capacity for dissimilatory sulfate metabolism was 3× higher in the acetate amended system. We demonstrate that naturally occurring in situ bacterial populations have the metabolic capability to couple acetate oxidation to sequential reduction of electron acceptors which can reduce Cr(VI) to less mobile and toxic Cr(III), and that microbially produced sulfide may be important in reductive precipitation of chromate. This capability could be harnessed to create a Cr(VI) trap-zone beneath COPR tips without the need to disturb the waste.

Enhancing iron biogeochemical cycling for canga ecosystem restoration: insights from microbial stimuli

Introduction

The microbial-induced restoration of ferruginous crusts (canga), which partially cover iron deposits and host unique ecosystems, is a promising alternative for reducing the environmental impacts of the iron mining industry.

Methods

To investigate the potential of microbial action to accelerate the reduction and oxidation of iron in substrates rich in hematite and goethite, four different microbial treatments (water only as a control - W; culture medium only - MO; medium + microbial consortium - MI; medium + microbial consortium + soluble iron - MIC) were periodically applied to induce iron dissolution and subsequent precipitation. Except for W, all the treatments resulted in the formation of biocemented blocks.

Results

MO and MI treatments resulted in significant goethite dissolution, followed by precipitation of iron oxyhydroxides and an iron sulfate phase, due to iron oxidation, in addition to the preservation of microfossils. In the MIC treatment, biofilms were identified, but with few mineralogical changes in the iron-rich particles, indicating less iron cycling compared to the MO or MI treatment. Regarding microbial diversity, iron-reducing families, such as Enterobacteriaceae, were found in all microbially treated substrates.

Discussion

However, the presence of Bacillaceae indicates the importance of fermentative bacteria in accelerating the dissolution of iron minerals. The acceleration of iron cycling was also promoted by microorganisms that couple nitrate reduction with Fe(II) oxidation. These findings demonstrate a sustainable and streamlined opportunity for restoration in mining areas.

Development of bioanodes rich in exoelectrogenic bacteria using iron-rich palaeomarine sediment inoculum

Microbial Fuel Cells (MFC) convert energy stored in chemicals into electrical energy thanks to exoelectrogenic microorganisms who also play a crucial role in geochemical cycles in their natural environment, including that of iron. In this study, we investigated paleomarine sediments as inoculum for bioanode development in MFCs. These sediments were formed under anoxic conditions ca. 113 million years ago and are rich in clay minerals, organic matter, and iron. The marlstone inoculum was incubated in the anolyte of an MFC using acetate as the added electron donor and ferricyanide as the electron acceptor in the catholyte. After seven weeks of incubation, the current density increased to 0.15 mA.cm-2 and a stable + 700 mV open circuit potential was reached. Community analysis revealed the presence of two exoelectrogenic bacterial genera, Geovibrio and Geobacter. Development of electroactive biofilms was correlated to bulk chemical transformations of the sediment inoculum with an increase in the Fe(II) to Fetotal ratio. Comparisons to sediments sterilized prior to inoculation confirmed that bioanode development derives from the native microbiota of these paleomarine sediments. This study illustrates the feasibility of developing exoelectrogenic biofilms from iron-rich marlstone and has implications for the role of such bacteria in broader paleoenvironmental phenomena.

Ammonium-Enhanced Arsenic Mobilization from Aquifer Sediments

Ammonium-related pathways are important for groundwater arsenic (As) enrichment, especially via microbial Fe(III) reduction coupled with anaerobic ammonium oxidation; however, the key pathways (and microorganisms) underpinning ammonium-induced Fe(III) reduction and their contributions to As mobilization in groundwater are still unknown. To address this gap, aquifer sediments hosting high As groundwater from the western Hetao Basin were incubated with 15N-labeled ammonium and external organic carbon sources (including glucose, lactate, and lactate/acetate). Decreases in ammonium concentrations were positively correlated with increases in the total produced Fe(II) (Fe(II)tot) and released As. The molar ratios of Fe(II)tot to oxidized ammonium ranged from 3.1 to 3.7 for all incubations, and the δ15N values of N2 from the headspace increased in 15N-labeled ammonium-treated series, suggesting N2 as the key end product of ammonium oxidation. The addition of ammonium increased the As release by 16.1% to 49.6%, which was more pronounced when copresented with organic electron donors. Genome-resolved metagenomic analyses (326 good-quality MAGs) suggested that ammonium-induced Fe(III) reduction in this system required syntrophic metabolic interactions between bacterial Fe(III) reduction and archaeal ammonium oxidation. The current results highlight the significance of syntrophic ammonium-stimulated Fe(III) reduction in driving As mobilization, which is underestimated in high As groundwater.

Unveil of the role of fungal taxa in iron(III) reduction in paddy soil

Hitherto, research on iron(III)-reduction has mainly focused on bacteria rather than fungal communities. To acquire insight into fungi involved in iron(III) reduction, typical organic matters (containing cellulose, glucose, lactate, and acetate) and ferrihydrite were used as electron donors and acceptors, respectively, in the presence of antibiotics. After antibiotic addition, microbial iron(III) reduction was still detected at quite high rates. In comparison, rates of iron(III) reduction were significantly lower in cellulose-amended groups than those with glucose, lactate, and acetate under the antibiotic-added condition. Patterns of intermediate (e.g., acetate, pyruvate, glucose) turnover were markedly different between treatments with and without antibiotics during organic degradation. A total of 20 genera of potential respiratory and fermentative iron(III)-reducing fungi were discovered based on ITS sequencing and genome annotation. This study provided an insight into the diversity of iron(III)-reducing fungi, indicating the underestimated contribution of fungi to iron and the coupled carbon biogeochemical cycling in environments.

Oxidation of Biogenic U(IV) in the Presence of Bioreduced Clay Minerals and Organic Ligands

Bioreduction of soluble U(VI) to sparingly soluble U(IV) is proposed as an effective approach to remediating uranium contamination. However, the stability of biogenic U(IV) in natural environments remains unclear. We conducted U(IV) reoxidation experiments following U(VI) bioreduction in the presence of ubiquitous clay minerals and organic ligands. Bioreduced Fe-rich nontronite (rNAu-2) and Fe-poor montmorillonite (rSWy-2) enhanced U(IV) oxidation through shuttling electrons between oxygen and U(IV). Ethylenediaminetetraacetic acid (EDTA), citrate, and siderophore desferrioxamine B (DFOB) promoted U(IV) oxidation via complexation with U(IV). In the presence of both rNAu-2 and EDTA, the rate of U(IV) oxidation was between those in the presence of rNAu-2 and EDTA, due to a clay/ligand-induced change of U(IV) speciation. However, the rate of U(IV) oxidation in other combinations of reduced clay and ligands was higher than their individual ones because both promoted U(IV) oxidation. Unexpectedly, the copresence of rNAu-2/rSWy-2 and DFOB inhibited U(IV) oxidation, possibly due to (1) blockage of the electron transport pathway by DFOB, (2) inability of DFOB-complexed Fe(III) to oxidize U(IV), and (3) stability of the U(IV)-DFOB complex in the clay interlayers. These findings provide novel insights into the stability of U(IV) in the environment and have important implications for the remediation of uranium contamination.

Multiple microbial guilds mediate soil methane cycling along a wetland salinity gradient

Estuarine wetlands harbor considerable carbon stocks, but rising sea levels could affect their ability to sequester soil carbon as well as their potential to emit methane (CH4). While sulfate loading from seawater intrusion may reduce CH4 production due to the higher energy yield of microbial sulfate reduction, existing studies suggest other factors are likely at play. Our study of 11 wetland complexes spanning a natural salinity and productivity gradient across the San Francisco Bay and Delta found that while CH4 fluxes generally declined with salinity, they were highest in oligohaline wetlands (ca. 3-ppt salinity). Methanogens and methanogenesis genes were weakly correlated with CH4 fluxes but alone did not explain the highest rates observed. Taxonomic and functional gene data suggested that other microbial guilds that influence carbon and nitrogen cycling need to be accounted for to better predict CH4 fluxes at landscape scales. Higher methane production occurring near the freshwater boundary with slight salinization (and sulfate incursion) might result from increased sulfate-reducing fermenter and syntrophic populations, which can produce substrates used by methanogens. Moreover, higher salinities can solubilize ionically bound ammonium abundant in the lower salinity wetland soils examined here, which could inhibit methanotrophs and potentially contribute to greater CH4 fluxes observed in oligohaline sediments.IMPORTANCELow-level salinity intrusion could increase CH4 flux in tidal freshwater wetlands, while higher levels of salinization might instead decrease CH4 fluxes. High CH4 emissions in oligohaline sites are concerning because seawater intrusion will cause tidal freshwater wetlands to become oligohaline. Methanogenesis genes alone did not account for landscape patterns of CH4 fluxes, suggesting mechanisms altering methanogenesis, methanotrophy, nitrogen cycling, and ammonium release, and increasing decomposition and syntrophic bacterial populations could contribute to increases in net CH4 flux at oligohaline salinities. Improved understanding of these influences on net CH4 emissions could improve restoration efforts and accounting of carbon sequestration in estuarine wetlands. More pristine reference sites may have older and more abundant organic matter with higher carbon:nitrogen compared to wetlands impacted by agricultural activity and may present different interactions between salinity and CH4. This distinction might be critical for modeling efforts to scale up biogeochemical process interactions in estuarine wetlands.

Arsenic Contamination of Groundwater Is Determined by Complex Interactions between Various Chemical and Biological Processes

At a great many locations worldwide, the safety of drinking water is not assured due to pollution with arsenic. Arsenic toxicity is a matter of both systems chemistry and systems biology: it is determined by complex and intertwined networks of chemical reactions in the inanimate environment, in microbes in that environment, and in the human body. We here review what is known about these networks and their interconnections. We then discuss how consideration of the systems aspects of arsenic levels in groundwater may open up new avenues towards the realization of safer drinking water. Along such avenues, both geochemical and microbiological conditions can optimize groundwater microbial ecology vis-à-vis reduced arsenic toxicity.

Unveiling the impact of flooding and salinity on iron oxides-mediated binding of organic carbon in the rhizosphere of Scirpus mariqueter

The abundant Fe (hydr-) oxides present in wetland sediments can form stable iron (Fe)-organic carbon (OC) complexes (Fe-OC), which are key mechanisms contributing to the stability of sedimentary OC stocks in coastal wetland ecosystems. However, the effects of increased flooding and salinity stress, resulting from global change, on the Fe-OC complexes in sediments remain unclear. In this study, we conducted controlled experiments in a climate chamber to quantify the impacts of flooding and salinity on the different forms of Fe (hydr-) oxides binding to OC in the rhizosphere sediments of S. mariqueter as well as the influence on Fe redox cycling bacteria in the rhizosphere. The results of this study demonstrated that prolonged flooding and high salinity treatments significantly reduced the content of organo-metal complexes (FePP) in the rhizosphere. Under high salinity conditions, the content of FePP-OC increased significantly, while flooding led to a decrease in FePP-OC content, inhibiting co-precipitation processes. The association of amorphous Fe (hydr-) oxides (FeHH) with OC showed no significant differences under different flooding and salinity treatments. Prolonged flooding significantly increased the relative abundance of Fe-reducing bacteria (FeRB) Deferrisoma and Geothermobacter and decreased polyphenol oxidase in the rhizosphere, while the relative abundance of Fe-oxidizing bacteria (FeOB) Paracoccus and Pseudomonas decreased with increasing salinity and duration of flooding. Overall, short-term water and salinity stress promoted the binding of FeDH to OC in the rhizosphere of S. mariqueter, leading to a reduction in the OC content held by FePP. However, there were no significant differences observed in the OC stocks or the total Fe-OC content in the rhizosphere sediments. The findings suggest a degree of consistency in the Fe-OC of the "plant-soil" complex system within tidal flat wetlands, showing resilience to abrupt shifts in flooding and salinity over short periods.

Unexpected carbon utilization activity of sulfate-reducing microorganisms in temperate and permanently cold marine sediments

Significant amounts of organic carbon in marine sediments are degraded, coupled with sulfate reduction. However, the actual carbon and energy sources used in situ have not been assigned to each group of diverse sulfate-reducing microorganisms (SRM) owing to the microbial and environmental complexity in sediments. Here, we probed microbial activity in temperate and permanently cold marine sediments by using potential SRM substrates, organic fermentation products at very low concentrations (15-30 μM), with RNA-based stable isotope probing. Unexpectedly, SRM were involved only to a minor degree in organic fermentation product mineralization, whereas metal-reducing microbes were dominant. Contrastingly, distinct SRM strongly assimilated 13C-DIC (dissolved inorganic carbon) with H2 as the electron donor. Our study suggests that canonical SRM prefer autotrophic lifestyle, with hydrogen as the electron donor, while metal-reducing microorganisms are involved in heterotrophic organic matter turnover, and thus regulate carbon fluxes in an unexpected way in marine sediments.

Impact of Eucalyptus residue leaching on iron distribution in reservoir sediments assessed by high-resolution DGT technique

Blackwater occurs every winter in reservoirs with Eucalyptus plantations. The complexation reaction between ferric iron (Fe3+) and Eucalyptus leachate tannic acid from logging residues (especially leaves) is the vital cause of water blackness. However, the effect of Eucalyptus leaf leaching on the dynamic of iron in sediments and its contribution to reservoir blackwater remain unclear. In this study, two experiments were conducted to simulate the early decomposition processes of exotic Eucalyptus and native Pinus massoniana leaves in water (LW) and water-sediment (LWS) systems. In LW, high concentrations of tannic acid (>45.25 mg/L) rapidly leached from the Eucalyptus leaves to the water column, exceeding those of Pinus massoniana leaves (<1.80 mg/L). The chrominance increased from 5~10 to 80~140, and the water body finally appeared brown instead of black after the leaching of Eucalyptus leaves. The chrominance positively correlated with tannic acid concentrations (R=0.970, p<0.01), indicating that tannic acid was vital for the water column's brown color. Different in LWS, blackwater initially emerged near the sediment-water interface (SWI) and extended upward to the entire water column as Eucalyptus leaves leached. Dissolved oxygen (DO) and transmission values in the overlying water declined simultaneously (R>0.77, p<0.05) and were finally below 2.29 mg/L and 10%, respectively. During the leaching of Eucalyptus leaves, the DGT-labile Fe2+ in sediments migrated from deep to surface layers, and the diffusive fluxes of Fe2+ at the SWI increased from 12.42~19.93 to 18.98~26.28 mg/(m2·day), suggesting that sediment released abundant Fe3+ into the aerobic overlying water. Fe3+ was exposed to high concentrations of tannic acid at the SWI and immediately generated the black Fe-tannic acid complex. The results indicated that the supplement of dissolved Fe3+ from sediments is a critical factor for the periodic blackwater in the reservoirs with Eucalyptus plantations. Reducing the cultivation of Eucalyptus in the reservoir catchment is one of the effective ways to alleviate the reservoir blackwater.

Mineral transformation of poorly crystalline ferrihydrite to hematite and goethite facilitated by an acclimated microbial consortium in electrodes of soil microbial fuel cells

In this study, we investigated the biogenic mineral transformation of poorly crystalline ferrihydrite in the presence of an acclimated microbial consortium after confirming successful soil microbial fuel cell optimization. The acclimated microbial consortia in the electrodes distinctly transformed amorphous ferrihydrite into crystallized hematite (cathode) and goethite (anode) under ambient culture conditions (30 °C). Serial analysis, including transmission/scanning electron microscopy and X-ray/selected area electron diffraction, confirmed that the biogenically synthesized nanostructures were iron nanospheres (~100 nm) for hematite and nanostars (~300 nm) for goethite. Fe(II) ion production with acetate oxidation via anaerobic respiration was much higher in the anode electrode sample (3.2- to 17.8-fold) than for the cathode electrode or soil samples. Regarding the culturable bacteria from the acclimated microbial consortium, the microbial isolates were more abundant and diverse at the anode. These results provide new insights into the biogeochemistry of iron minerals and microbial fuel cells in a soil environment, along with physiological characters of microbes (i.e., iron-reducing bacteria), for in situ applications in sustainable energy research.

Upcycling spent palladium-based catalysts into high value-added catalysts via electronic regulation of Escherichia coli to high-efficiently reduce hexavalent chromium

Upgrading and recycling Palladium (Pd) from spent catalysts may address Pd resource shortages and environmental problems. In this paper, Escherichia coli (E. coli) was used as an electron transfer intermediate to upcycle spent Pd-based catalysts into high-perform hexavalent chromium bio-catalysts. The results showed that Pd (0) nanoparticles (NPs) combined with the bacterial surface changed the electron transfer by enhancing the cell conductivity, thus promoting the removal rate of Pd(II). The recovery efficiency of Pd exceeded 98.6%. Notably, E. coli heightened the adsorption of H• and HCOO• via electron transfer of the Pd NPs electron-rich centre, resulting in a higher catalytic performance of the recycled spent catalysed the reduction of 20 ppm Cr(VI) under mild conditions within 18 min, in which maintained above 98% catalytic activity after recycling five times. This efficiency was found to be higher than that of the reported Pd-based catalysts. Hence, an electron transfer mechanism for E. coli recovery Pd-based catalyst under electron donor adjusting is proposed. These findings provide an important method for recovering Pd NPs from spent catalysts and are crucial to effectively reuse Pd resources.

Iron-bound organic carbon dynamics in peatland profiles: The preservation equivalence of deep and surface soil

More than half of the carbon pools in peatlands are stored in the soil layers below 30 cm, yet little is known about the carbon stabilizing factors at these depths. Although iron oxide minerals are considered to be important for stabilizing organic carbon (OC), their role in the preservation of OC in peatlands, especially in the deep layers, is poorly understood. Here, we collected 1 m soil profiles from six peatlands in Central and West China to quantitatively study the vertical distribution characteristics of iron-bound OC (Fe-bound OC), and the influencing physicochemical properties of the soil. The results showed that the content of reactive iron (FeR) was enriched in the top layer and decreased gradually with depth. While Fe-bound OC was positively correlated with FeR, its concentration did not decrease with depth in the peat profile. There were no obvious trends in the distributions of FeR and Fe-bound OC with water level fluctuations in the peat profile. In addition, the proportion of Fe-bound OC to soil organic carbon in the deep peat (31 to 100 cm) was equivalent to that in the surface peat (0 to 30 cm), indicating that iron oxide mineral provides comparable protection of OC in both layers. According to upper estimates of global peatland carbon storage (612 Pg), it could be predicted that 23.81 ± 11.75 Pg of OC is protected by association with FeR. These results indicated that iron oxide minerals are the effective "rusty sink" of OC sequestration in peatland, and a key factor for its long-term preservation. The results from this study make a valuable contribution to the carbon dynamics knowledgebase for peatlands, and provide a basis for improved predictive simulations.

The effect of biogeochemical redox oscillations on arsenic release from legacy mine tailings

Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.

Fe(III) (oxyhydr)oxide reduction by the thermophilic iron-reducing bacterium Desulfovulcanus ferrireducens

Some thermophilic bacteria from deep-sea hydrothermal vents grow by dissimilatory iron reduction, but our understanding of their biogenic mineral transformations is nascent. Mineral transformations catalyzed by the thermophilic iron-reducing bacterium Desulfovulcanus ferrireducens during growth at 55°C were examined using synthetic nanophase ferrihydrite, akaganeite, and lepidocrocite separately as terminal electron acceptors. Spectral analyses using visible-near infrared (VNIR), Fourier-transform infrared attenuated total reflectance (FTIR-ATR), and Mössbauer spectroscopies were complemented with x-ray diffraction (XRD) and transmission electron microscopy (TEM) using selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analyses. The most extensive biogenic mineral transformation occurred with ferrihydrite, which produced a magnetic, visibly dark mineral with spectral features matching cation-deficient magnetite. Desulfovulcanus ferrireducens also grew on akaganeite and lepidocrocite and produced non-magnetic, visibly dark minerals that were poorly soluble in the oxalate solution. Bioreduced mineral products from akaganeite and lepidocrocite reduction were almost entirely absorbed in the VNIR spectroscopy in contrast to both parent minerals and the abiotic controls. However, FTIR-ATR and Mössbauer spectra and XRD analyses of both biogenic minerals were almost identical to the parent and control minerals. The TEM of these biogenic minerals showed the presence of poorly crystalline iron nanospheres (50-200 nm in diameter) of unknown mineralogy that were likely coating the larger parent minerals and were absent from the controls. The study demonstrated that thermophilic bacteria transform different types of Fe(III) (oxyhydr)oxide minerals for growth with varying mineral products. These mineral products are likely formed through dissolution-reprecipitation reactions but are not easily predictable through chemical equilibrium reactions alone.

Co-digestion of sulfur-rich vegetable waste with waste activated sludge enhanced phosphorus release and hydrogenotrophic methanogenesis

Anaerobic co-digestion of sulfur-rich vegetable waste (SVW) with waste activated sludge (WAS) and the underlying mechanisms associated with methane production and phosphorus (P) release were investigated. Four types of SVW (Chinese cabbage, cabbage, rapeseed cake, and garlic) were utilized for co-digestion with WAS, and the methane yield increased by 7.3%-35.3%; in the meantime, the P release amount from WAS was enhanced by 9.8%-24.9%. The organic carbon in SVW promoted methane production, while organic sulfur and the formation of FeS facilitated P release. Among the four types of SVW, rapeseed cake was identified as the most suitable co-digestion substrate for enhancing both methane production and P release due to its balanced nutrients and relatively high sulfur content. Syntrophic bacteria working with hydrogenotrophic methanogens, iron-reducing bacteria, sulfate-reducing bacteria, and hydrogenotrophic methanogens were enriched. Metabolic pathways related to sulfate reduction and methanogenesis were facilitated, especially hydrogenotrophic methanogenesis. Enzymes involved in hydrogenotrophic methanogenesis were promoted by 76.05%-407.98% with the addition of Chinese cabbage, cabbage, or rapeseed cake. This study provides an eco-friendly technology for promoting P resource and energy recovery from WAS and an in-depth understanding of the corresponding microbial mechanisms.

Reductive transformation of birnessite by low-molecular-weight organic acids

Soil biogeochemistry is intrinsically coupled to the redox cycling of iron and manganese. Oxidized manganese forms various (hydr)oxides that may reductively transform and dissolve, thereby serving as electron acceptors for microbial metabolisms. Furthermore, manganese oxides might reduce purely abiotically by oxidation of dissolved Mn²⁺ in a specific route of transformation from birnessite (Mn^IVO₂) into metastable feitknechtite (β-Mn^IIIOOH) and stable manganite (γ-Mn^IIIOOH). In natural soil solutions, however, dissolved Mn²⁺ is not abundant and organic substances such as low-molecular-weight organic acids (LMWOA) may be oxidized and serve as an electron donor for manganese oxide reduction instead. We investigated whether LMWOA would impact the transformation of birnessite at a temperature of 290 ± 2 K under ambient pressure for up to 1200 d. We found that birnessite was reductively transformed into feitknechtite, which subsequently alters into the more stable manganite without releasing Mn²⁺ into the solution. Instead, LMWOA served as electron donors and were oxidized from lactate into pyruvate, acetate, oxalate, and finally, inorganic carbon. We conclude that the reductive transformation of short-range ordered minerals like birnessite by the abiotic oxidation of LMWOA is a critical process controlling the abundance of LMWOA in natural systems besides their microbial consumption. Our results further suggest that the reduction of Mn^IV oxides not necessarily results in their dissolution at neutral and alkaline pH but also forms more stable Mn^III oxyhydroxides with less oxidative degradation potential for organic contaminants.

Direct degradation of Bisphenol A from aqueous solution by active red mud in aerobic environment

As industrial waste from aluminum production, red mud (RM) poses a severe threat to the local environment that needs to be appropriately utilized. The activation of iron oxide, which is abundant in RM, improves its effectiveness as a catalytic material for the degradation of organic pollutants. This study developed a novel activation approach by adding dithionite citrate bicarbonate (DCB) for Bisphenol A (BPA) degradation under aeration conditions. Electrochemical experiments and reactive oxygen species (ROSs) trapping experiments showed that DCB treatment enhanced the redox cycle of Fe(II)/Fe(III), which promoted free radical generation. The optimized condition for the RM activation was achieved at 21 mmol/L dithionites, 84 mmol/L citrates, and 34 mmol/L bicarbonate, and the degradation of BPA by activated RM reached 410 µg BPA per gram of RM. This work provided a feasible way to utilize RM resources as an efficient, low-cost catalyst for organic pollutants treatment.

Iron plaque formation and its effect on key elements cycling in constructed wetlands: Functions and outlooks

Ecological restoration of wetland plants has emerged as an environmentally-friendly and less carbon footprint method for treating secondary effluent wastewater. Root iron plaque (IP) is located at the important ecological niches in constructed wetlands (CWs) ecosystem and is the critical micro-zone for pollutants migration and transformation. Root IP can affect the chemical behaviors and bioavailability of key elements (C, N, P) since its formation/dissolution is a dynamic equilibrium process jointly influenced by rhizosphere habitats. However, as an efficient approach to further explore the mechanism of pollutant removal in CWs, the dynamic formation of root IP and its function have not been fully studied, especially in substrate-enhanced CWs. This article concentrates on the biogeochemical processes between Fe cycling involved in root IP with carbon turnover, nitrogen transformation, and phosphorus availability in CWs rhizosphere. As IP has the potential to enhance pollutant removal by being regulated and managed, we summarized the critical factors affecting the IP formation from the perspective of wetland design and operation, as well as emphasizing the heterogeneity of rhizosphere redox and the role of key microbes in nutrient cycling. Subsequently, interactions between redox-controlled root IP and biogeochemical elements (C, N, P) are emphatically discussed. Additionally, the effects of IP on emerging contaminants and heavy metals in CWs rhizosphere are assessed. Finally, major challenges and outlooks for future research in regards to root IP are proposed. It is expected that this review can provide a new perspective for the efficient removal of target pollutants in CWs.

Fe(III) enhances Cr(VI) bioreduction in a MFC-granular sludge coupling system: Experimental evidence and metagenomics analysis

The influence of Fe(III) on the bioreduction efficiency of Cr(VI) in a microbial fuel cell (MFC)-granular sludge coupling system using dissolved methane as an electron donor and carbon source was explored, and the mechanism of Fe(III) mediating enhancement in the bioreduction process of Cr(VI) in the coupling system was also investigated. Results showed that the presence of Fe(III) enhanced the ability of the coupling system to reduce Cr(VI). The average removal efficiencies of Cr(VI) in the anaerobic zone in response to 0, 5, and 20 mg/L of Fe(III) were 16.53±2.12%, 24.17±2.10%, and 46.33±4.41%, respectively. Fe(III) improved the reducing ability and output power of the system. In addition, Fe(III) enhanced the electron transport systems activity of the sludge, the polysaccharide and protein content in the anaerobic sludge. Meanwhile, X-ray photoelectron spectrometer (XPS) spectra demonstrated that Cr(VI) was reduced to Cr(III), while Fe2p participated in reducing Cr(VI) in the form of Fe(III) and Fe(II). Proteobacteria, Chloroflexi, and Bacteroidetes were the dominant phylum in the Fe(III)-enhanced MFC-granular sludge coupling system, accounting for 49.7%-81.83% of the microbial community. The relative abundance of Syntrophobacter and Geobacter increased after adding Fe(III), indicating that Fe(III) contributed to the microbial mediated anaerobic oxidation of methane (AOM) and bioreduction of Cr(VI). The genes mcr, hdr, and mtr were highly expressed in the coupling system after the Fe(III) concentration increased. Meanwhile, the relative abundances of coo and aacs genes were up-regulated by 0.014% and 0.075%, respectively. Overall, these findings deepen understanding of the mechanism of the Cr(VI) bioreduction in the MFC-granular sludge coupling system driven by methane under the influence of Fe(III).

Electroactive Bacteria in Natural Ecosystems and Their Applications in Microbial Fuel Cells for Bioremediation: A Review

Electroactive bacteria (EAB) are natural microorganisms (mainly Bacteria and Archaea) living in various habitats (e.g., water, soil, sediment), including extreme ones, which can interact electrically each other and/or with their extracellular environments. There has been an increased interest in recent years in EAB because they can generate an electrical current in microbial fuel cells (MFCs). MFCs rely on microorganisms able to oxidize organic matter and transfer electrons to an anode. The latter electrons flow, through an external circuit, to a cathode where they react with protons and oxygen. Any source of biodegradable organic matter can be used by EAB for power generation. The plasticity of electroactive bacteria in exploiting different carbon sources makes MFCs a green technology for renewable bioelectricity generation from wastewater rich in organic carbon. This paper reports the most recent applications of this promising technology for water, wastewater, soil, and sediment recovery. The performance of MFCs in terms of electrical measurements (e.g., electric power), the extracellular electron transfer mechanisms by EAB, and MFC studies aimed at heavy metal and organic contaminant bioremediationF are all described and discussed.

Short-chain fatty acid production and phosphorous recovery from waste activated sludge via anaerobic fermentation: A comparison of in-situ and ex-situ thiosulfate-assisted Fe2+/persulfate pretreatment

Recently, increasing attention is given on the resource and energy recovery (e.g. short-chain fatty acids (SCFAs) and phosphorus (P)) from waste active sludge (WAS) under the "Dual carbon goals". This study compared four thiosulfate-assisted Fe²⁺/persulfate (TAFP) pretreatments of WAS, i.e. in-situ TAFP pretreatment (R1), ex-situ TAFP pretreatment (R2), in-situ TAFP pretreatment + pH adjustment (R3) and ex-situ TAFP pretreatment + pH adjustment (R4), followed by anaerobic fermentation over 20 days for SCFA production and P recovery. The results showed that the maximal SCFA yields in R1-4 were 730.2 ± 7.0, 1017.4 ± 13.9, 860.1 ± 40.8, and 1072.0 ± 33.2 mg COD/L, respectively, significantly higher than Control (365.2 ± 17.8 mg COD/L). The findings indicated that TAFP pretreatments (particularly ex-situ TAFP pretreatment) enhanced WAS disintegration and provided more soluble organics and subsequently promoted SCFA production. The P fractionation results showed the non-apatite inorganic P increased from 11.6 ± 0.2 mg P/g TSS in Control to 11.8 ± 0.5 (R1), 12.4 ± 0.3 (R2), 13.2 ± 0.7 (R3) and 12.7 ± 0.7 mg P/g TSS (R4), suggesting TAFP pretreatments improved P bioavailability due to formation of Fe-P mineral (Fe(H₂PO₄)₂·2H₂O), which could be recycled through magnetic separators. These findings were further strengthened by the analysis of microbial community and related marker genes that fermentative bacteria containing SCFA biosynthesis genes (e.g. pyk, pdhA, accA and accB) and iron-reducing bacteria containing iron-related proteins (e.g. feoA and feoB) were enriched in R1-4 (dominant in ex-situ pretreatment systems, R2 and R4). Economic evaluation further verified ex-situ TAFP pretreatment was cost-effective and a better strategy over other operations to treat WAS for SCFA production and P recovery.

Unraveling the Mechanisms of Fe Oxidation and Mn Reduction on Mn Indicators of Reduction in Soil (IRIS) Films

Indicators of reduction in soil (IRIS) devices are low-cost soil redox sensors coated with Fe or Mn oxides, which can be reductively dissolved from the device under suitable redox conditions. Removal of the metal oxide coating from the surface, leaving behind the white film, can be quantified and used as an indicator of reducing conditions in soils. Manganese IRIS, coated with birnessite, can also oxidize Fe(II), resulting in a color change from brown to orange that complicates the interpretation of coating removal. Here, we studied field-deployed Mn IRIS films where Fe oxidation was present to unravel the mechanisms of Mn oxidation of Fe(II) and the resulting minerals on the IRIS film surface. We observed reductions in the Mn average oxidation state when Fe precipitation was evident. Fe precipitation was primarily ferrihydrite (30-90%), but lepidocrocite and goethite were also detected, notably when the Mn average oxidation state decreased. The decrease in the average oxidation state of Mn was due to the adsorption of Mn(II) to the oxidized Fe and the precipitation of rhodochrosite (MnCO₃) on the film. The results were variable on small spatial scales (<1 mm), highlighting the suitability of IRIS in studying heterogeneous redox reactions in soil. Mn IRIS also provides a tool to bridge lab and field studies of the interactions between Mn oxides and reduced constituents.

Charging-discharging cycles of geobattery activated carbon enhance iron reduction and vivianite recovery from wastewater

Vivianite as a significant secondary mineral of dissimilatory iron reduction (DIR) exhibits marvelous potential to solve eutrophication as well as phosphorus shortage. Geobattery represents by natural organic matters (NOM) with rich functional groups influences bioreduction of natural iron mineral. Activated carbon (AC) which contains abundant functional groups is expected to serve as geobattery, but there remains insufficient understanding on its geobattery mechanism and how it benefits the vivianite formation. In this study, the charging and discharging cycle of "geobattery" AC enhanced extracellular electron transfer (EET) and vivianite recovery was demonstrated. Feeding with ferric citrate, AC addition increased vivianite formation efficiency by 141 %. The enhancement was attributed to the electron shuttle capacity of storage battery AC, which was contributed by the redox cycle between CO and O-H. Feeding with iron oxides, huge gap of redox potential between AC and Fe(III) minerals broke through the reduction energy barrier. Therefore the iron reduction efficiency of four Fe(III) minerals was accelerated to the same high level around 80 %, and the vivianite formation efficiency were increased by 104 %-256 % in pure culture batches. Except acting as storage battery, AC as a dry cell contributed 80 % to the whole enhancement towards iron reduction, in which O-H groups were the dominant driver. Due to the rechargeable nature and considerable electron exchange capacity, AC served as geobattery playing the role of both storage battery and dry cell on electron storaging and transferring to influence biogeochemical Fe cycle and vivianite recovery.

A Biochemical Deconstruction-Based Strategy to Assist the Characterization of Bacterial Electric Conductive Filaments

Periplasmic nanowires and electric conductive filaments made of the polymeric assembly of c-type cytochromes from Geobacter sulfurreducens bacterium are crucial for electron storage and/or extracellular electron transfer. The elucidation of the redox properties of each heme is fundamental to the understanding of the electron transfer mechanisms in these systems, which first requires the specific assignment of the heme NMR signals. The high number of hemes and the molecular weight of the nanowires dramatically decrease the spectral resolution and make this assignment extremely complex or unattainable. The nanowire cytochrome GSU1996 (~42 kDa) is composed of four domains (A to D) each containing three c-type heme groups. In this work, the individual domains (A to D), bi-domains (AB, CD) and full-length nanowire were separately produced at natural abundance. Sufficient protein expression was obtained for domains C (~11 kDa/three hemes) and D (~10 kDa/three hemes), as well as for bi-domain CD (~21 kDa/six hemes). Using 2D-NMR experiments, the assignment of the heme proton NMR signals for domains C and D was obtained and then used to guide the assignment of the corresponding signals in the hexaheme bi-domain CD. This new biochemical deconstruction-based procedure, using nanowire GSU1996 as a model, establishes a new strategy to functionally characterize large multiheme cytochromes.

Vertical Hydrologic Exchange Flows Control Methane Emissions from Riverbed Sediments

CH₄ emissions from inland waters are highly uncertain in the current global CH₄ budget, especially for streams, rivers, and other lotic systems. Previous studies have attributed the strong spatiotemporal heterogeneity of riverine CH₄ to environmental factors such as sediment type, water level, temperature, or particulate organic carbon abundance through correlation analysis. However, a mechanistic understanding of the basis for such heterogeneity is lacking. Here, we combine sediment CH₄ data from the Hanford reach of the Columbia River with a biogeochemical-transport model to show that vertical hydrologic exchange flows (VHEFs), driven by the difference between river stage and groundwater level, determine CH₄ flux at the sediment-water interface. CH₄ fluxes show a nonlinear relationship with the magnitude of VHEFs, where high VHEFs introduce O₂ into riverbed sediments, which inhibit CH₄ production and induce CH₄ oxidation, and low VHEFs cause transient reduction in CH₄ flux (relative to production) due to reduced advective CH₄ transport. In addition, VHEFs lead to the hysteresis of temperature rise and CH₄ emissions because high river discharge caused by snowmelt in spring leads to strong downwelling flow that offsets increasing CH₄ production with temperature rise. Our findings reveal how the interplay between in-stream hydrologic flux besides fluvial-wetland connectivity and microbial metabolic pathways that compete with methanogenic pathways can produce complex patterns in CH₄ production and emission in riverbed alluvial sediments.

Role of indigenous microorganisms and organics in the release of iron and trace elements from sediments impacted by iron mine tailings from failed Fundão dam

After Fundão Dam failure in 2015, most of Gualaxo do Norte River in Doce River Basin in Brazil became silted by iron mining tailings consisting mainly of fine-grained quartz, hematite, and goethite. Previous work pointed to the possibility of reductive dissolution of iron and manganese from tailings, leading to mobilization of iron, manganese and trace elements. Several microorganisms were shown to reduce Fe(III) to Fe(II) and Mn(III, IV) to Mn(II) "in vitro", but their roles in mobilization of Fe and trace elements from freshwater sediments are poorly understood. In this work, bottom sediments and water collected in Gualaxo do Norte River were used to build anoxic microcosms amended with acetate, glucose or yeast extract, in order to access if heterotrophic microorganisms, either fermenters or dissimilatory Fe reducers, could reduce Fe(III) from minerals in the sediments to soluble Fe(II), releasing trace elements. The Fe(II) concentrations were measured over time, and trace elements concentrations were evaluated at the end of the experiment. In addition, minerals and biopolymers in bottom sediments were quantified. Results showed that organic substrates, notably glucose, fuelled microbial reduction of iron minerals and release of Fe(II), Mn, Ba, Al and/or Zn from sediments. In general, higher concentrations of organic substrates elicited mobilization of larger amounts of Fe(II) and trace elements from sediments. The results point to the possibility of mobilization of huge amounts of iron and trace elements from sediments to water if excess biodegradable organic matter is released in rivers affected by iron mine tailings.

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.

Iron Reduction in Profundal Sediments of Ultraoligotrophic Lake Tahoe under Oxygen-Limited Conditions

Increased periods of bottom water anoxia in deep temperate lakes due to decreasing frequency and depth of water column mixing in a warming climate may result in the reductive dissolution of iron minerals and increased flux of nutrients from the sediment into the water column. Here, we assessed the sediment properties and reactivities under depleted oxygen concentrations of Lake Tahoe, a deep ultraoligotrophic lake in the Sierra Nevada mountain range. Using whole-core incubation experiments, we found that a decrease in dissolved oxygen concentration in the top 2 cm of the sediment resulted in an extension of the microbial iron reduction zone from below 4.5 to below 1.5 cm depth. Concentrations of reactive iron generally decreased with sediment depth, and microbial iron reduction seemingly ceased as concentrations of Fe(II) approximated concentrations of reactive iron. These findings suggest that microorganisms preferentially utilized reactive iron and/or iron minerals became less reactive due to mineral transformation and surface passivation. The estimated release of iron mineral-associated phosphorus is not expected to change Lake Tahoe's trophic state but will likely contribute to increased phytoplankton productivity if mixed into surface waters.

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 (oxyhydr)oxides shift the methanogenic community in deep sea methanic sediment - insights from long-term high-pressure incubations

Intermittent increases of dissolved ferrous iron concentrations have been observed in deep marine methanic sediments which is different from the traditional diagenetic electron acceptor cascade, where iron reduction precedes methanogenesis. Here we aimed to gain insight into the mechanism of iron reduction and the associated microbial processes in deep sea methanic sediment by setting up long-term high-pressure incubation experiments supplemented with ferrihydrite and methane. Continuous iron reduction was observed during the entire incubation period. Intriguingly, ferrihydrite addition shifted the archaeal community from the dominance of hydrogenotrophic methanogens (Methanogenium) to methylotrophic methanogens (Methanococcoides). The enriched samples were then amended with ¹³C-labeled methane and different iron (oxyhydr)oxides in batch slurries to test the mechanism of iron reduction. Intensive iron reduction was observed, the highest rates with ferrihydrite, followed by hematite and then magnetite, however, no anaerobic oxidation of methane (AOM) was observed in any treatment. Further tests on the enriched slurry showed that the addition of molybdate decreased iron reduction, suggesting a link between iron reduction with sulfur cycling. This was accompanied by the enrichment of microbes capable of dissimilatory sulfate reduction and sulfur/thiosulfate oxidation, which indicates the presence of a cryptic sulfur cycle in the incubation system with the addition of iron (oxyhydr)oxides. Our work suggests that under low sulfate conditions, the presence of iron (oxyhydr)oxides would trigger a cascade of microbial reactions, and iron reduction could link with the microbial sulfur cycle, changing the kinetics of the methanogenesis process in methanic sediment.

Carbonate Minerals and Dissimilatory Iron-Reducing Organisms Trigger Synergistic Abiotic and Biotic Chain Reactions under Elevated CO2 Concentration

Increasing CO₂ emission has resulted in pressing climate and environmental issues. While abiotic and biotic processes mediating the fate of CO₂ have been studied separately, their interactions and combined effects have been poorly understood. To explore this knowledge gap, an iron-reducing organism, Orenia metallireducens, was cultured under 18 conditions that systematically varied in headspace CO₂ concentrations, ferric oxide loading, and dolomite (CaMg(CO₃)₂) availability. The results showed that abiotic and biotic processes interactively mediate CO₂ acidification and sequestration through "chain reactions", with pH being the dominant variable. Specifically, dolomite alleviated CO₂ stress on microbial activity, possibly via pH control that transforms the inhibitory CO₂ to the more benign bicarbonate species. The microbial iron reduction further impacted pH via the competition between proton (H⁺) consumption during iron reduction and H⁺ generation from oxidization of the organic substrate. Under Fe(III)-rich conditions, microbial iron reduction increased pH, driving dissolved CO₂ to form bicarbonate. Spectroscopic and microscopic analyses showed enhanced formation of siderite (FeCO₃) under elevated CO₂, supporting its incorporation into solids. The results of these CO₂-microbe-mineral experiments provide insights into the synergistic abiotic and biotic processes that alleviate CO₂ acidification and favor its sequestration, which can be instructive for practical applications (e.g., acidification remediation, CO₂ sequestration, and modeling of carbon flux).

Elucidation of Iron(III) Bioleaching Properties of Gram-Positive Bacteria

Microbial-based iron reduction is an emerging technology used as an alternative to conventional chemical-based iron reduction. The iron reduction in kaolin refinement is vital for enhancing its commercial value. Extensive studies on microbial-based iron reduction mainly focus on Gram-negative bacteria, whereas little is understood about Gram-positive bacteria's mechanism and potential application. This study aims to investigate the iron-reducing mechanism of two Gram-positive bacterial isolates, Bacillus cereus (B. cereus) and Staphylococcus aureus (S. aureus). By varying the growth environment of bacteria and monitoring the biochemical changes during the process of iron reduction, the results show that Gram-positive bacterial iron reduction performance depends on the medium composition, differing from Gram-negative bacteria-based reduction processes. Nitrogen-rich medium facilitates the microbial basification of the medium, where the alkaline conditions impact the microbial iron reduction process by altering the gene expression involved in intracellular pH homeostasis and microbial growth. This discovery will contribute to the mineral refining processes and promote the development of microbial-based bioprocesses for ore purification, while also laying the foundation for investigating other Gram-positive bacterial iron-reducing ability.

Enrichment of sulfur-oxidizing bacteria using S-doped NiFe2O4 nanosheets as the anode in microbial fuel cell enhances power production and sulfur recovery

Microbial fuel cells (MFCs) have great promise for power generation by oxidizing organic wastewater, yet the challenge to realize high efficiency in simultaneous energy production and resource recovery remains. In this study, we designed a novel MFC anode by synthesizing S-doped NiFe₂O₄ nanosheet arrays on carbon cloth (S10-NiFe₂O₄@CC) to build a three-dimensional (3D) hierarchically porous structure, with the aim to regulate the microbial community of sulfur-cycling microbes in order to enhance power production and elemental sulfur (S⁰) recovery. The S10-NiFe₂O₄@CC anode obtained a faster start-up time of 2 d and the highest power density of 4.5 W/m² in acetate-fed and mixed bacteria-based MFCs. More importantly, sulfide removal efficiency (98.3 %) (initial concentration of 50 mg/L S^2-) could be achieved within 3 d and sulfur (S₈) could be produced. Microbial community analysis revealed that the S10-NiFe₂O₄@CC anode markedly enriched sulfur-oxidizing bacteria (SOB) and promoted enrichment of SOB and sulfate-reducing bacteria (SRB) in the bulk solution as well, leading to the enhancement of power generation and S⁰ recovery. This study shows how carefully designing and optimizing the composition and structure of the anode can lead to the enrichment of a multifunctional microbiota with excellent potential for sulfide removal and resource recovery.

A process-based model for describing redox kinetics of Cr(VI) in natural sediments containing variable reactive Fe(II) species

Sediment-associated Fe(II) is a critical reductant for immobilizing groundwater contaminants, such as Cr(VI). The reduction reactivity of sediment-associated Fe(II) is dependent on its binding environment and influenced by the biogeochemical transformation of other elements (i.e., C, N and Mn), challenging the description and prediction of the reactivity of Fe(II) in natural sediments. Here, anaerobic batch experiments were conducted to study the variation in sediment-associated Fe(II) reactivity toward Cr(VI) in natural sediments collected from an intensive agricultural area located in Guangxi, China, where nitrate is a common surface water and groundwater contaminant. Then, a process-based model was developed to describe the coupled biogeochemical processes of C, N, Mn, Fe, and Cr. In the process-based model, Cr(VI) reduction by sediment-associated Fe(II) was described using a previously developed multirate model, which categorized the reactive Fe(II) into three fractions based on their extractabilities in sodium acetate and HCl solutions. The experimental results showed that Fe(II) generation was inhibited by NO₃^- and/or NO₂^-. After NO₃^- and NO₂^- were exhausted, the Fe(II) content and its reduction rate toward Cr(VI) increased rapidly. As the Fe(II) content increased, the three reactive Fe(II) fractions exhibited approximately linear correlations with aqueous Fe(II) concentrations ( [Formula: see text] ), which was probably driven by sorptive equilibrium and redox equilibrium between aqueous and solid phases. The model results indicated that the reaction rate constants of the three Fe(II) fractions (k_n) significantly increased with incubation time, and log(k_n) correlated well with [Formula: see text] [ [Formula: see text] , [Formula: see text] and [Formula: see text] ]. The numerical model developed in this study provides an applicable method to describe and predict Cr(VI) removal from groundwater under dynamic redox conditions.

Arsenic speciation transformation in soils with high geological background: New insights from the governing role of Fe

In soils, the speciation transformation of As were inherently related to the behaviors of iron (oxyhydr) oxides. It is poorly understood that the effects of the transformation of iron (oxyhydr) oxides coupled with As speciation transformation during dissimilatory Fe(III) reduction (DIR) involving with humic substances (HS) as electron donor or shuttle in soils with high arsenic geological background. In this study, the relationships between the transformation of iron (oxyhydr)oxides and As speciation transformation were investigated according to the response between continuously As speciation monitoring and iron (oxyhydr) oxides identification during DIR in the soils. The results showed that F4 (arsenic incorporated with amorphous iron (oxyhydr)oxides including ferrihydrite and schwertmannite) and F5 (arsenic incorporated with crystalline iron (oxyhydr)oxides including hematite and magnetite) were the main source and sink for As(III)_Dissolved during DIR. During the incubation period, Fe(II) was the dominant driving force for the reduction of As(V) in the water-soil system. The XRD analysis indicated the changes of iron oxides such as ferrihydrite, schwertmannite, hematite and magnetite were closely related to the release and reduction of As, and those iron oxides could play governing roles for As speciation transformation during DIR in soils. Different from the known mechanism in low As concentrations, a limiting effect of As concentration on iron oxides transformation was found in our incubation experiments using soils with high As geological background (∼1000 mg/kg). This work provides new insights for Fe as governing role in As speciation transformation in soils with high arsenic geological background by firstly identifying the corresponding iron (oxyhydr)oxides in operationally defined arsenic speciation incorporated with iron oxides.

Limitations of microbial iron reduction under extreme conditions

Microbial iron reduction is a widespread and ancient metabolism on Earth, and may plausibly support microbial life on Mars and beyond. Yet, the extreme limits of this metabolism are yet to be defined. To investigate this, we surveyed the recorded limits to microbial iron reduction in a wide range of characterized iron-reducing microorganisms (n = 141), with a focus on pH and temperature. We then calculated Gibbs free energy of common microbially mediated iron reduction reactions across the pH-temperature habitability space to identify thermodynamic limits. Comparing predicted and observed limits, we show that microbial iron reduction is generally reported at extremes of pH or temperature alone, but not when these extremes are combined (with the exception of a small number of acidophilic hyperthermophiles). These patterns leave thermodynamically favourable combinations of pH and temperature apparently unoccupied. The empty spaces could be explained by experimental bias, but they could also be explained by energetic and biochemical limits to iron reduction at combined extremes. Our data allow for a review of our current understanding of the limits to microbial iron reduction at extremes and provide a basis to test more general hypotheses about the extent to which biochemistry establishes the limits to life.

Long-term metal pollution shifts microbial functional profiles of nitrification and denitrification in agricultural soils

The increasing contamination of heavy metals in agricultural soils and its impact on the nitrogen (N) cycle and N use efficiency have attracted considerable attention in recent years. In this study, agricultural soils neighboring the Dabaoshan copper mining area (DBS) and Qingyuan electronic-waste recycling area (QY), in Guangdong, China, were sampled to study the interaction between heavy metals and nitrification/denitrification processes, especially the related microbial functional profiles. Results showed that the contamination of heavy metals affected nitrifiers and denitrifiers differently. The potential nitrification activity was about four times lower in metal-polluted soils compared with the unpolluted ones, with a significant decrease in the abundance of amoA and nxrB (p < 0.05) in the polluted samples. On the other hand, the potential denitrification activity was more metal-resistant, which attributed to its complex species composition as shown by a slightly higher α-diversity index, and was slightly higher (p > 0.05) in the polluted samples. Among the five denitrifying genes tested, nosZ gene had the highest increase and the nirK gene the most decrease in numbers and in the polluted soils. The metal-polluted soils had fewer correlations among N functional genes based on the co-occurrence network analysis. In addition, the core taxa of the whole bacterial community changed from copiotrophic to oligotrophic bacteria in the presence of heavy metals. Mantel test indicated that heavy metals were the dominant factors determining N-related genes while the bacterial community composition was due to a combination of heavy metal presence and soil properties such as TOC, NO₂^-, and pH. It is concluded that long-term heavy metals pollution potentially affected nitrifiers and denitrifiers differently as indicated by the shift in N functional genes and the change in nitrification/denitrification processes.