The interplay of microbially mediated and abiotic reactions in the biogeochemical Fe cycle

Agent-based modelling of iron cycling bacteria provides a framework for testing alternative environmental conditions and modes of action

Iron-reducing and iron-oxidizing bacteria are of interest in a variety of environmental and industrial applications. Such bacteria often co-occur at oxic-anoxic gradients in aquatic and terrestrial habitats. In this paper, we present the first computational agent-based model of microbial iron cycling, between the anaerobic ferric iron (Fe³⁺)-reducing bacteria Shewanella spp. and the microaerophilic ferrous iron (Fe²⁺)-oxidizing bacteria Sideroxydans spp. By including the key processes of reduction/oxidation, movement, adhesion, Fe²⁺-equilibration and nanoparticle formation, we derive a core model which enables hypothesis testing and prediction for different environmental conditions including temporal cycles of oxic and anoxic conditions. We compared (i) combinations of different Fe³⁺-reducing/Fe²⁺-oxidizing modes of action of the bacteria and (ii) system behaviour for different pH values. We predicted that the beneficial effect of a high number of iron-nanoparticles on the total Fe³⁺ reduction rate of the system is not only due to the faster reduction of these iron-nanoparticles, but also to the nanoparticles' additional capacity to bind Fe²⁺ on their surfaces. Efficient iron-nanoparticle reduction is confined to pH around 6, being twice as high than at pH 7, whereas at pH 5 negligible reduction takes place. Furthermore, in accordance with experimental evidence our model showed that shorter oxic/anoxic periods exhibit a faster increase of total Fe³⁺ reduction rate than longer periods.

Sulfidation of ferric (hydr)oxides and its implication on contaminants transformation: a review

Rapid industrialization and urbanization have resulted in elevated concentrations of contaminants in the groundwaters and subsurface soils, posing a growing hazard to humans and ecosystems. The transformation of most contaminants is closely linked to the mineralogy of ferric (hydr)oxides. Sulfidation of ferric (hydr)oxides is one of the most significant biogeochemical reactions in the anoxic environments, causing reductive dissolution and recrystallization of ferric (hydr)oxides and further affecting the transformation of iron-associated contaminants. This paper provides a comprehensive review on the sulfidation process of ferric (hydr)oxides and the transformation of relevant contaminants. This review presents detailed reaction mechanisms between ferric (hydr)oxides and dissolved sulfide, and elucidates the factors (e.g. crystallinity of ferric (hydr)oxides, the ratio of sulfide concentration to the surface area concentration of ferric (hydr)oxides) that control the formation of surface associated Fe(II), iron sulfide minerals, as well as transformation of secondary minerals. Then, we summarized the transformation mechanisms of a variety of typical environmentally relevant contaminants existing in groundwater and subsurface soils, including heavy metals, metal(loid) oxyanions (arsenic, antimony, chromium), radionuclides (uranium, technetium), organic contaminants and phosphate/nitrate species. The general mechanisms of contaminant transformation involve a combination of release, reduction and re-adsorption/incorporation processes, the specific pathway of which is highly dependent on the properties of the contaminant itself and the extent of sulfidation. Moreover, the challenge of extending our knowledge towards in situ remediation, as well as further research needs are identified.

Co-response of Fe-reducing/oxidizing bacteria and Fe species to the dynamic redox cycles of natural sediment

Fe(III)-reducing bacteria (FRB) and Fe(II)-oxidizing bacteria (FOB) play essential roles in the biogeochemical cycling of iron (Fe). Although the redox transformation of Fe species mediated by FRB/FOB has been extensively studied, the co-responses of FRB and FOB and Fe species transformation in natural sediment under dynamic redox conditions are poorly known. This study explored the variations of potential FRB and FOB abundances and Fe species transformation in natural sediment during successive anoxic-oxic-anoxic-oxic-anoxic cycles. Compared with the pristine sediment sample, the FRB abundance increased 121-793% (initial: (2.6 ± 0.6) × 10⁷ copies/g) in the anoxic stages, while it decreased by 38-64% in the oxic stages. The increase in FRB abundance was ascribed to energy gain of FRB from the reduction of the amorphous Fe(III) (Fe(III)_am) and the crystalline Fe(III) (Fe(III)_cry) to the aqueous Fe(II) (Fe(II)_aq), the adsorbed Fe(II) (Fe(II)_ad) and the amorphous Fe(II) (Fe(II)_am), while the decrease was attributed to the oxidative stress caused by the reactive oxidant produced from the abiotic oxidation of Fe(II)_aq, Fe(II)_ad and Fe(II)_am to Fe(III)_am and Fe(III)_cry. The FOB abundance decreased 38-44% (initial: (5 ± 1.8) × 10⁷ copies/g) in the second and third anoxic stages, while slightly fluctuated in the oxic periods. This observation was contrary to the variation of FRB, which might be attributed to the strong resistance to oxidative stress of FOB and its ability to obtain energy under oxic conditions. Although the functions of FRB and FOB were impaired during anoxic-oxic cycles, the transformation of Fe(II)/Fe(III) was not immediately affected, which may be related to the residual reactivity of dead bacteria and the bio-availability of Fe(II)/Fe(III) species. In the anoxic-oxic alternation process, the iron cycle is mainly the mutual transformation between Fe(II)_aq, Fe(II)_ad, Fe(II)_am and Fe(III)_am, Fe(III)_cry. This finding deepens our understanding of the biogeochemical cycling of Fe in the redox-dynamic environments.

Extracellular proteins of Desulfovibrio vulgaris as adsorbents and redox shuttles promote biomineralization of antimony

Biomineralization is the key process governing the biogeochemical cycling of multivalent metals in the environment. Although some sulfate-reducing bacteria (SRB) are recently recognized to respire metal ions, the role of their extracellular proteins in the immobilization and redox transformation of antimony (Sb) remains elusive. Here, a model strain Desulfovibrio vulgaris Hildenborough (DvH) was used to study microbial extracellular proteins of functions and possible mechanisms in Sb(V) biomineralization. We found that the functional groups (N-H, CO, O-CO, NH₂-R and RCOH/RCNH₂) of extracellular proteins could adsorb and fix Sb(V) through electrostatic attraction and chelation. DvH could rapidly reduce Sb(V) adsorbed on the cell surface and form amorphous nanometer-sized stibnite and/or antimony trioxide, respectively with sulfur and oxygen. Proteomic analysis indicated that some extracellular proteins involved in electron transfer increased significantly (p < 0.05) at 1.8 mM Sb(V). The upregulated flavoproteins could serve as a redox shuttle to transfer electrons from c-type cytochrome networks to reduce Sb(V). Also, the upregulated extracellular proteins involved in sulfur reduction, amino acid transport and protein synthesis processes, and the downregulated flagellar proteins would contribute to a better adaption under 1.8 mM Sb(V). This study advances our understanding of how microbial extracellular proteins promote Sb biomineralization in DvH.

Activated carbon as an insoluble electron shuttle to enhance the anaerobic ammonium oxidation coupled with Fe(III) reduction process

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is an autotrophic biological nitrogen removal (BNR) technique in treating low-C/N wastewater. However, the nitrogen removal rate of Feammox is limited by the extracellular electron transfer. In this study, wood activated carbon (AC) was chosen as electron shuttle to enhance the start-up of the Feammox process. Within an operational period of 150 days, the NH₄⁺-N removal efficiency reached 97.9-99.5% with a volumetric loading rate (VLR) of 0.04-0.06 kg N m^-3 d^-1. Batch experiments indicated that compared with Fe₂O₃-AQDS and Fe₂O₃ groups, Fe₂O₃-AC group showed higher catalytic performance and TN removal efficiency reached 85.7%. Quinone (CO) and phenolic (-OH) chemical groups of AC were equipped with electron transfer capacity (76.51 ± 9.27 μmol e^- g^-1). Moreover, Fe(II)/Fe(III) species and the secondary iron minerals were found in our system. Microbial analysis showed that Proteobacteria and Acidobacteriota, which observed with relatively high abundance, were played an important role in the integrated Feammox system. This study demonstrates the significant influence of AC on Feammox process and provides an enhanced biological nitrogen removal strategy for practice engineering application.

Effects of riboflavin and desferrioxamine B on Fe(II) oxidation by O2

Flavins and siderophores secreted by various plants, fungi and bacteria under iron (Fe) deficient conditions play important roles in the biogeochemical cycling of Fe in the environment. Although the mechanisms of flavin and siderophore mediated Fe(III) reduction and dissolution under anoxic conditions have been widely studied, the influence of these compounds on Fe(II) oxidation under oxic conditions is still unclear. In this study, we investigated the kinetics of aqueous Fe(II) (17.8 μM) oxidation by O2 at pH 5‒7 in the presence of riboflavin (oxidized (RBF) and reduced (RBFH2)) and desferrioxamine B (DFOB) as representative flavins and siderophores, respectively. Results showed that the addition of RBF/RBFH2 or DFOB markedly accelerates the oxidation of aqueous Fe(II) by O2. For instance, at pH 6, the rate of Fe(II) oxidation was enhanced 20‒70 times when 10 μM RBFH2 was added. The mechanisms responsible for the accelerated Fe(II) oxidation are related to the redox reactivity and complexation ability of RBFH2, RBF and DFOB. While RBFH2 does not readily complex Fe(II)/Fe(III), it can activate O2 and generate reactive oxygen species, which then rapidly oxidize Fe(II). In contrast, both RBF and DFOB do not reduce O2 but react with Fe(II) to form RBF/DFOB-complexed Fe(II), which in turn accelerates Fe(II) oxidation. Furthermore, the lower standard reduction potential of the Fe(II)-DFOB complex, compared to the Fe(II)-RBF complex, correlates with a higher oxidation rate constant for the Fe(II)-DFOB complex. Our study reveals an overlooked catalytic role of flavins and siderophores that may contribute to Fe(II)/Fe(III) cycling at oxic-anoxic interfaces.

Silicon fertilization influences microbial assemblages in rice roots and decreases arsenic concentration in grain: A five-season in-situ remediation field study

Microbial mechanism of in-situ remediation of arsenic (As) in As-contaminated paddy fields by silicon (Si) fertilization has been rarely reported, especially under continuous rice cultivation and Si applications. In this study, two Si fertilizers were applied for three phases in five consecutive rice seasons to investigate the long-lasting impacts on in-situ remediation of As, and the underpinning microbial mechanism of root-associated compartments (bulk soil, rhizosphere and endosphere) was explored using the last double-cropping rice. Repeated application of Si fertilizers as base manure had a long-lasting effect on reducing As concentrations in rice grains. Application of Si fertilizer at an adequate amount resulted in an extended in-situ remediation effect from endosphere to rhizosphere. The microbial diversity and richness in rhizosphere soil and endosphere were significantly impacted by Si fertilization, the effects depending on application doses and prolonged seasons. Si fertilization can immobilize As in the root or rhizosphere, and Fe concentrations and the As- and Fe-transforming microorganisms (i.e. Geobacteraceae) are the determinants of As uptake in rice. We recommend more extensive supplementation of Si fertilizer at a higher rate to decrease grain As concentration for in-situ remediation. This study sheds light on the microbial-mediated mechanism underlying Si fertilization effect on decreased As uptake in paddy fields.

Advances in interfacial engineering for enhanced microbial extracellular electron transfer

The extracellular electron transfer (EET) efficiency between electroactive microbes (EAMs) and electrode is a key factor determining the development of microbial electrochemical technology (MET). Currently, the low EET efficiency of EAMs limits the application of MET in the fields of organic matter degradation, electric energy production, seawater desalination, bioremediation and biosensing. Enhancement of the interaction between EAMs and electrode by interfacial engineering methods brings bright prospects for the improvement of the EET efficiency of EAMs. In view of the research in recent years, this mini-review systematically summarizes various interfacial engineering strategies ranging from electrode surface modification to hybrid biofilm formation, then to single cell interfacial engineering and intracellular reformation for promoting the electron transfer between EAMs and electrode, focusing on the applicability and limitations of these methodologies. Finally, the possible key directions, challenges and opportunities for future interfacial engineering to strengthen the microbial EET are proposed in this mini-review.

Understanding Interaction Patterns within Deep-Sea Microbial Communities and Their Potential Applications

Environmental microbes living in communities engage in complex interspecies interactions that are challenging to decipher. Nevertheless, the interactions provide the basis for shaping community structure and functioning, which is crucial for ecosystem service. In addition, microbial interactions facilitate specific adaptation and ecological evolution processes particularly essential for microbial communities dwelling in resource-limiting habitats, such as the deep oceans. Recent technological and knowledge advancements provide an opportunity for the study of interactions within complex microbial communities, such as those inhabiting deep-sea waters and sediments. The microbial interaction studies provide insights into developing new strategies for biotechnical applications. For example, cooperative microbial interactions drive the degradation of complex organic matter such as chitins and celluloses. Such microbiologically-driven biogeochemical processes stimulate creative designs in many applied sciences. Understanding the interaction processes and mechanisms provides the basis for the development of synthetic communities and consequently the achievement of specific community functions. Microbial community engineering has many application potentials, including the production of novel antibiotics, biofuels, and other valuable chemicals and biomaterials. It can also be developed into biotechniques for waste processing and environmental contaminant bioremediation. This review summarizes our current understanding of the microbial interaction mechanisms and emerging techniques for inferring interactions in deep-sea microbial communities, aiding in future biotechnological and therapeutic applications.

Effectiveness of Exogenous Fe2+ on Nutrient Removal in Gravel-Based Constructed Wetlands

A group of microcosm-scale unplanted constructed wetlands (CWs) were established to evaluate the effectiveness of exogenous Fe²⁺ addition on ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total phosphorus (TP) removal. The addition of Fe²⁺ concentrations were 5 mg/L (CW-Fe5), 10 mg/L (CW-Fe10), 20 mg/L (CW-Fe20), 30 mg/L (CW-Fe30), and 0 mg/L (CW-CK). The microbial community in CWs was also analyzed to reveal the enhancement mechanism of pollutant removal. The results showed that the addition of Fe²⁺ could significantly (p < 0.05) reduce the NO₃^--N concentration in the CWs. When 10 mg/L Fe²⁺ was added and the hydraulic retention time (HRT) was 8 h, the highest removal rate of NO₃^--N was 88.66%. For NH₄⁺-N, when the HRT was 8-24 h, the removal rate of CW-Fe5 was the highest (35.23% at 8 h and 59.24% at 24 h). When the HRT was 48-72 h, the removal rate of NH₄⁺-N in CWs with 10 mg/L Fe²⁺ addition was the highest (85.19% at 48 h and 88.66% and 72 h). The removal rate of TP in all CWs was higher than 57.06%, compared with CW-CK, it increased 0.63-31.62% in CWs with Fe²⁺ addition; the final effluent TP concentration in CW-Fe5 (0.13 mg/L) and CW-Fe10 (0.16 mg/L) met the class III water standards in Surface Water Environmental Quality Standards of China (GB3838-2002). Microbical diversity indexes, including Shannon and Chao1, were significantly lower (p < 0.05) in Fe²⁺ amended treatment than that in CW-CK treatment. Furthermore, phylum Firmicutes, family Carnobacteriaceae, and genus Trichococcus in Fe²⁺ amended treatments was significantly (p < 0.05) higher than that in CW-CK treatment. Fe³⁺ reducing bacteria, such as Trichococcus genus, belonging to the Carnobacteriaceae in family-level, and Lactobacillales order affiliated to Firmicutes in the phylum-level, can reduce the oxidized Fe³⁺ to Fe²⁺ and continue to provide electrons for nitrate. It is recommended to consider adding an appropriate amount of iron into the water to strengthen its purifying capacity effect for constructed artificial wetlands in the future.

Elevated temperatures drive abiotic and biotic degradation of organic matter in a peat bog under oxic conditions

Understanding the effects of elevated temperatures on soil organic matter (SOM) decomposition pathways in northern peatlands is central to predicting their fate under future warming. Peatlands role as carbon (C) sink is dependent on both anoxic conditions and low temperatures that limit SOM decomposition. Previous studies have shown that elevated temperatures due to climate change can disrupt peatland's C balance by enhancing SOM decomposition and increasing CO₂ emissions. However, little is known about how SOM decomposition pathways change at higher temperatures. Here, we used an integrated research approach to investigate the mechanisms behind enhanced CO₂ emissions and SOM decomposition under elevated temperatures of surface peat soil collected from a raised and Sphagnum dominated mid-continental bog (S1 bog) peatland at the Marcel Experimental Forest in Minnesota, USA, incubated under oxic conditions at three different temperatures (4, 21, and 35 °C). Our results indicated that elevated temperatures could destabilize peatland's C pool via a combination of abiotic and biotic processes. In particular, temperature-driven changes in redox conditions can lead to abiotic destabilization of Fe-organic matter (phenol) complexes, previously an underestimated decomposition pathway in peatlands, leading to increased CO₂ production and accumulation of polyphenol-like compounds that could further inhibit extracellular enzyme activities and/or fuel the microbial communities with labile compounds. Further, increased temperatures can alter strategies of microbial communities for nutrient acquisition via changes in the activities of extracellular enzymes by priming SOM decomposition, leading to enhanced CO₂ emission from peatlands. Therefore, coupled biotic and abiotic processes need to be incorporated into process-based climate models to predict the fate of SOM under elevated temperatures and to project the likely impacts of environmental change on northern peatlands and CO₂ emissions.

Anoxic oxidation of As(III) during Fe(II)-induced goethite recrystallization: Evidence and importance of Fe(IV) intermediate

Under anoxic conditions, aqueous Fe(II) (Fe(II)_aq)-induced recrystallization of iron (oxyhydr)oxides changes the speciation and geochemical cycle of trace elements in environments. Oxidation of trace element, i.e., As(III), driven by Fe(II)_aq-iron (oxyhydr)oxides interactions under anoxic condition was observed previously, but the oxidative species and involved mechanisms are remained unknown. In the present study, we explored the formed oxidative intermediates during Fe(II)_aq-induced recrystallization of goethite under anoxic conditions. The methyl phenyl sulfoxide-based probe experiment suggested the featured oxidation by Fe(IV) species in Fe(II)_aq-goethite system. Both the Mössbauer spectra and X-ray absorption near edge structure spectroscopic evidenced the generation and quenching of Fe(IV) intermediate. It was proved that the interfacial electron exchange between Fe(II)_aq and Fe(III) of goethite initiated the generation of Fe(IV). After transferring electrons to goethite, Fe(II)_aq was transformed to labile Fe(III), which was then transformed to Fe(IV) via a proton-coupled electron transfer process. This highly reactive transient Fe(IV) could quickly react with reductive species, i.e. Fe(II) or As(III). Considering the ubiquitous occurrence of Fe(II)-iron (oxyhydr)oxides reactions under anoxic conditions, our findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.

Enhanced Microbial Ferrihydrite Reduction by Pyrogenic Carbon: Impact of Graphitic Structures

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial (Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H₂ atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H₂ treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Fungal-Mineral Interactions Modulating Intrinsic Peroxidase-like Activity of Iron Nanoparticles: Implications for the Biogeochemical Cycles of Nutrient Elements and Attenuation of Contaminants

Fungal-mediated extracellular reactive oxygen species (ROS) are essential for biogeochemical cycles of carbon, nitrogen, and contaminants in terrestrial environments. These ROS levels may be modulated by iron nanoparticles that possess intrinsic peroxidase (POD)-like activity (nanozymes). However, it remains largely undescribed how fungi modulate the POD-like activity of the iron nanoparticles with various crystallinities and crystal facets. Using well-controlled fungal-mineral cultivation experiments, here, we showed that fungi possessed a robust defect engineering strategy to modulate the POD-like activity of the attached iron minerals by decreasing the catalytic activity of poorly ordered ferrihydrite but enhancing that of well-crystallized hematite. The dynamics of POD-like activity were found to reside in molecular trade-offs between lattice oxygen and oxygen vacancies in the iron nanoparticles, which may be located in a cytoprotective fungal exoskeleton. Together, our findings unveil coupled POD-like activity and oxygen redox dynamics during fungal-mineral interactions, which increase the understanding of the catalytic mechanisms of POD-like nanozymes and microbial-mediated biogeochemical cycles of nutrient elements as well as the attenuation of contaminants in terrestrial environments.

Effects of Magnetic Minerals Exposure and Microbial Responses in Surface Sediment across the Bohai Sea

Extensive production and application of magnetic minerals introduces significant amounts of magnetic wastes into the environment. Exposure to magnetic minerals could affect microbial community composition and geographic distribution. Here, we report that magnetic susceptibility is involved in determining bacterial α-diversity and community composition in surface sediment across the Bohai Sea by high-throughput sequencing analysis of the 16S rRNA gene. The results showed that environmental factors (explained 9.80%) played a larger role than spatial variables (explained 6.72%) in conditioning the bacterial community composition. Exposure to a magnetite center may shape the geographical distribution of five dissimilatory iron reducing bacteria. The microbial iron reduction ability and electroactive activity in sediment close to a magnetite center are stronger than those far away. Our study provides a novel understanding for the response of DIRB and electroactive bacteria to magnetic minerals exposure.

Dielectric constants of organic pollutants determine their strength for enhancing microbial iron reduction

Physicochemical properties are essential characteristics of organic compounds, which not only impact the fate of organic pollutants but also determine their application in biological processes. Here, we first found that the dielectric constants (ɛ) of organic pollutants negatively correlated to their strength for enhancing microbial Fe(III) reduction. Those with lower ɛ values than 2.61 potentially promoted the above process following the sequence carbon tetrachloride (CT) > benzene > toluene > tetrachloroethylene (PCE) due to their different ability to deprotonate the phosphorus-related groups on the outer cell membrane of iron-reducing bacteria Shewanella oneidensis MR-1 (MR-1). The stronger deprotonation of phosphorus-related groups induced more negative charge of cell surface and more strongly increased cell membrane permeability and consequently stimulated faster release of flavin mononucleotide (FMN) as an electron shuttle/cofactor for Fe(III) reduction. These findings are significant for understanding the biogeochemistry in multi-organic contaminated subsurface and providing knowledge for remediation strategies and current production.

The effects of Fe(III) and Fe(II) on anammox process and the Fe-N metabolism

This study aims to compare the effects of different Fe stress on anammox (anaerobic ammonium oxidation) process, therefore seven identical reactors were operated under different Fe(II)/Fe(III) concentrations. After 38 days of operation, the anammox activity was highest (10.49 ± 0.41 mg-TN/(g-VSS·h)) under conditions of 5 mg/L-Fe(II), while under 30 mg/L-Fe(III) displayed severe inhibition. The results showed that continuous addition of 30 mg/L-Fe(III) would damage the composition of EPS (extracellular polymeric substances) and make anammox bacteria more sensitive to environmental stress. While high Fe(II) concentrations could result in precipitates encasing granular sludge, affecting substrate utilization. Moreover, the results of ΔNO₃^--N/ΔNH₄⁺-N indicated that Fe(II)-dependent nitrate reduction was induced in reactors added with Fe(II). OM27_clade and norank_f__Burkholderiaceae might be candidates for this process according to the correlation of genera and functional genes (based on the PICRUSt 2 functional prediction). Overall, this research is expected to provide new ideas to the effects of Fe(II)/Fe(III) on anammox and to the practical application of coupled system based on anammox in wastewater treatment.

Capturing the signal of weak electricigens: a worthy endeavour

Recently several non-traditional electroactive microorganisms have been discovered. These can be considered weak electricigens; microorganisms that typically rely on soluble electron acceptors and donors in their lifecycle but are also capable of extracellular electron transfer (EET), resulting in either a low, unreliable, or otherwise unexpected current. These unanticipated electroactive microorganisms represent a new chapter in electromicrobiology and have important medical, environmental, and biotechnological relevance. As such, it is essential to continue the momentum of their discovery. However, their study poses unique challenges due to their low current output. Capturing their signal necessitates novel approaches including unconventional electrode choice, the use of sensitive electrochemical techniques, and modifications of conventional experiments that use bioelectrochemical systems (BES).

Active Iron Phases Regulate the Abiotic Transformation of Organic Carbon during Redox Fluctuation Cycles of Paddy Soil

Iron (Fe) phases are tightly linked to the preservation rather than the loss of organic carbon (OC) in soil; however, during redox fluctuations, OC may be lost due to Fe phase-mediated abiotic processes. This study examined the role of Fe phases in driving hydroxyl radical (^•OH) formation and OC transformation during redox cycles in paddy soils. Chemical probes, sequential extraction, and Mössbauer analyses showed that the active Fe species, such as exchangeable and surface-bound Fe and Fe in low-crystalline minerals (e.g., green rust-like Fe phases), predominantly regulated ^•OH formation during redox cycles. The ^•OH oxidation strongly induced the oxidative transformation of OC, which accounted for 15.1-30.8% of CO₂ production during oxygenation. Microbial processes contributed 7.3-12.1% of CO₂ production, as estimated by chemical quenching and γ-irradiation experiments. After five redox cycles, 30.1-71.9% of the OC associated with active Fe species was released, whereas 5.2-7.1% was stabilized by high-crystalline Fe phases due to the irreversible transformation of these active Fe species during redox cycles. Collectively, our findings might unveil the under-appreciated role of active Fe phases in driving more loss than conservation of OC in soil redox fluctuation events.

Enhancing the retention of phosphorus through bacterial oxidation of iron or sulfide in the eutrophic sediments of Lake Taihu

Microbial activity can enhance the sequestration of phosphorus (P) in sediments, but little is known about the mechanisms behind it. In this study, sediment cores were sampled from the most eutrophic Meiliang Bay of Lake Taihu, and three treatments were set up in a laboratory incubation experiment, involving (a) the non-treated sediment cores, (b) inoculation, and (c) sterilization. The dissolved and labile iron (Fe) and P were obtained by high-resolution dialysis and the diffusive gradients in thin films (DGT) technique, respectively. AgI-based DGT was used for measuring the 2D distribution of labile sulfide. The bacterial community was investigated using a scanning electron microscope and 16S rRNA high throughput sequencing technique. The results showed that sterilization reduced the capacity of sediment to immobilize P, and that the critical sediment depth layer for microbial P sequestration was 0-10 mm. In addition, sterilization or inoculation significantly changes the structure of bacterial communities. Fe or S oxidation under micro-aerobic or anaerobic conditions played an important role in bacterial retention of P in the sediments. Nitrate-reducing coupling Fe(II)-oxidizing bacteria (Acidovorax) in the inoculated sediment and electrogenic sulfur-oxidizing bacteria (Candidatus Electronema) in the non-treated sediment were identified as the key bacterial genera responsible for the retention of P in sediments. This implies that bacterial communities could quickly establish the ability for negative feedback regulation by inoculation once the function and structure of indigenous sediment bacteria are seriously impaired, although this needs further validation in the field.

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

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

Conductive property of secondary minerals triggered Cr(VI) bioreduction by dissimilatory iron reducing bacteria

Although secondary minerals have great potential for heavy metal removal, their impact on chromium biogeochemistry in subsurface environments associated with dissimilatory iron reducing bacteria (DIRB) remains poorly characterized. Here, we have investigated the mechanisms of biogenic secondary minerals on the rate of Cr(VI) bioreduction with shewanella oneidensis MR-1. Batch results showed that the biogenic secondary minerals, schwertmannite and jarosite, appreciably increased the Cr(VI) bioreduction rate. UV-vis diffuse reflection spectra showed that schwertmannite and jarosite are semiconductive minerals, which can be activated by MR-1, followed by transferred conduction electrons toward Cr(VI). Cyclic voltammetry and Tafel analysis suggested that the resistance of secondary minerals is a dominant factor controlling Cr(VI) bioreduction. In addition, Cr(VI) adsorption on secondary minerals through ligand exchange promoted Cr(VI) bioreduction by decreasing the electron transfer distance between MR-1 and chromate. Fe(III)/Fe(II) cycling in schwertmannite and jarosite also contributed to Cr(VI) bioreduction as reflected by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer. Complementary characterizations further verified the contributions of Fe(III)/Fe(II) cycling, Cr(VI) adsorption, and conduction band electron transfer to enhanced Cr(VI) bioreduction. This study provides new insights on the understanding of Cr(VI) bioreduction by semiconductor minerals containing sulfate in subsurface environments.

Temporal responses of hydrochemical variables and dissolved Fe(II) to flooding at a lake riparian wetland under different vegetation revealing by high resolution DGT

The interplay between hydrological and biogeochemical processes in riparian wetland was recognized to lead directly to the temporal variations of surface water quality. However, the effects of flooding and vegetation on the release and entrapment of heavy metals and nutrients in riparian wetland remain poorly understood. The study aimed at investigating the influences of flooding and vegetation on the hydrochemical and Fe-redox change in the soil porewater and shallow groundwater, in Poyang lake riparian wetland through hydrochemical monitoring and diffusive gradient technology (DGT). The hydrochemical profiles and results of PCA analysis on the temporal datasets both demonstrated that vegetation had significant influences on the hydrochemistry of rhizosphere depth zone (RDZ) and shallow groundwater depth zone (SGZ). The Ca, K, Na, Mg, Mn and DOC at RDZ of both plants showed significant increasing trends from pre-to post-flooding while were observed minor change at the SGZ. The extracted PC1-PC3 from PCA analysis suggested that mineral dissolution and fermentation were dominating processes that explained 64.1% of the hydrochemical variability under the wetting-drying cycle. The synchronous changes of Fe(II), SO₄^2-, DOC and ORP were found to occur at the SGZ of Carex, implying the likely occurrence of Fe- and S- redox reactions. The Fe(II) DGT profiles evidenced the temporal iron reduction and oxidation occurring at the rhizosphere following the wetting-drying cycle, as also reflected by the high opposite Fe²⁺ and DO association through PCA analysis. The high resolution temporal-spatial Fe(II) distribution suggested also the interface between lake water and groundwater was relatively stable under flooding. These results highlight that the release of dissolved Fe(II) from the wetland rhizosphere driven by flood may result in the release of Fe-associated heavy metals from riparian wetland to surface water, and hence pose potential threats to the surface water quality. Thereby, the flow and flood should be properly controlled and vegetation effects need to be carefully considered in the water resources management of lake-floodplain system.

The short-term and long-term effects of Fe(II) on the performance of anammox granules

Fe(II) is one of the commonly used additives in wastewater treatment and proved to be beneficial for promoting microbial activity. In this study, the effects of Fe(II) on the specific anammox activity (SAA) and reactor performance were proved to be concentration-dependent. In the short-term experiment, low concentration of Fe(II) (5-80 mg/L) significantly enhanced the SAA, while high concentration of Fe(II) (120-300 mg/L) inhibited the SAA. It was confirmed that anammox can be domesticated after long-term exposure to low Fe(II) concentration, and the SAA could be further enhanced by higher Fe(II) concentration in the following phases. In addition, as an important factor for anammox granulation and maintaining the SAA, the extracellular polymeric substance (EPS) was also affected by Fe(II) addition. In spite of the effects on SAA and EPS, Fe(II) was proved to be the key factor that enhances the N₂ O emission via abiotic pathway in the anammox reactor. PRACTITIONER POINTS: Low Fe(II) concentrations enhanced SAA, while high concentrations inhibited SAA. Long-term acclimatization by Fe(II) improved the tolerance of anammox to Fe(II). Fe(II) affects the amount and constituent of EPS and the performance of anammox granules. Accumulation of Fe(II) in the AAFEB reactor promoted the N₂ O emission.

Extracellular polymeric substances from Shewanella oneidensis MR-1 biofilms mediate the transformation of Ferrihydrite

Extracellular polymeric substances (EPS) of dissimilatory iron-reducing bacteria (DIRB) such as Shewanella oneidensis MR-1 play a crucial role in the biotransformation of iron-containing minerals, but the mechanism has not been fully deciphered. Herein, abiotic and biotic transformation of ferrihydrite (Fh) were compared to clarify the contributions of MR-1, EPS-free MR-1 (MR-1-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). The results of abiotic Fh transformation indicated that EPS did not block the Fh surfaces and thus has an insignificant effect on the adsorbed Fe(II)-Fh interaction. The complexation of the Fe(III) intermediate (Fe(III)_active) with EPS, especially LB-EPS, however, inhibited the nucleation of secondary Fe minerals and changed the crystallization pathway. For biotic Fh transformation, on the other hand, EPS had dual effects that accelerated Fh bioreduction due to the enhanced extracellular electron transfer (EET) and constrained the following Fh mineralization by cutting of the chain reactions leading to mineral crystallization. Our finding also suggested that the effects of EPS on Fh biotransformation largely depend on the chemical properties of EPS, especially the polar functional groups such as carboxyl and phosphate, because of their important abilities for the cell attachment and Fe(II)/Fe(III) binding.

Occurrence of arsenite in surface and groundwater associated with a perennial stream located in Western Nebraska, USA

Dissolved arsenic typically results from chemical weathering of arsenic rich sediments and is most often found in oxidized forms in surface water. The mobility of arsenic is controlled by its valence state and also by its association with iron oxides minerals, the forms of which are both influenced by abiotic and biotic processes in aqueous environment. In this study, speciation methods were used to measure and confirm the presence of reduced arsenic species in the surface water of Frenchman creek, a gaining stream that crosses the Colorado-Nebraska border. Selective extraction analysis of aquifer and stream bed sediments shows that the bulk of the arsenic occurs with labile iron-rich oxy(hydroxide) minerals. Total dissolved arsenic in surface and groundwater ranged from ~3-18 µg L^-1, and reduced arsenic species comprise about 41% of the total dissolved arsenic (16.0 µg L^-1) in Frenchman creek. Leachable arsenic in the aquifer sediment samples ranged up to 1553 µg kg^-1, while samples from Frenchman creek bed sediments contained 4218 µg kg^-1. Dynamic surface and groundwater interaction sustains arsenite in iron-rich surface headwaters, and the implied toxicity of reduced arsenic in this hydrogeological setting, which can be important in surface water environments around the globe.

Inhibition effects of long-term calcium-magnesia phosphate fertilizer application on Cd uptake in rice: Regulation of the iron-nitrogen coupling cycle driven by the soil microbial community

Cadmium (Cd) pollution in paddy soil seriously endangers food safety production. To investigate the effects and microbiological mechanisms of calcium-magnesium-phosphate (CMP) fertilizer application on Cd reduction in rice, field experiments were conducted in Cd-contaminated paddy soil. Compared with conventional compound fertilizer, CMP fertilizer treatments inhibited Cd uptake through plant roots, significantly decreasing Cd content in rice grains from 0.340 to 0.062 mg/kg. Soil pH and total Ca, Mg and P contents increased after CMP fertilizer application, resulting in a further decrease in soil available Cd content from 0.246 to 0.181 mg/kg. Specific extraction analysis recorded a decrease in both available Fe content and the ratio of nitrate to ammonium nitrogen, indicating that the soil Fe-N cycle was affected by the addition of CMP fertilizer. This finding was also recorded using soil bacterial community sequencing, with CMP fertilizer promoting the progress of nitrate-dependent Fe-oxidation driven by Thiobacillus (1.60-2.83%) and subsequent dissimilatory nitrate reduction to ammonium (DNRA) driven by Ignavibacteriae (1.01-1.92%); Fe-reduction driven by Anaeromyxobacter (3.09-2.23%) was also inhibited. Our results indicate that CMP fertilizer application regulates the Fe-N coupling cycle driven by the soil microbial community to benefit remediation of Cd contaminated paddy soil.

Co-existing siderite alleviates the Fe(II) oxidation-induced inactivation of Fe(III)-reducing bacteria

Abiotic Fe (II) oxidation widely occurs in the natural subsurface environment and engineered dynamic processes, which possibly impacts the growth of indigenous microbes. As previously discovered, the oxidation of aqueous Fe²⁺ at neutral pH effectively inactivates iron-reducing bacteria Shewanella oneidensis strain MR-1 (MR-1). Herein, the impacts of co-existing iron mineral on the oxidation of aqueous Fe²⁺ and the subsequent disinfection activity on MR-1 were investigated with siderite selected as a representative iron mineral in the subsurface environment. The oxidation rate of aqueous Fe²⁺ and the amount of generated OH radical increased as the content of siderite increased, while the MR-1 inactivation was alleviated. An initial concentration of 2.0 × 10⁶ CFU/mL MR-1 was inactivated by about 2.7 orders of magnitude during oxidation of 0.2 mM FeSO₄ alone for 30 min, which was reduced to only about 0.6 orders of magnitude in the presence of 4.3 mM co-existing siderite. ROS scavenging results confirmed that the OH radical generated in the bulk solution was not the leading role for the inactivation of MR-1. Morphological changes of the cells observed by SEM demonstrated that the disruption of the cell membrane was alleviated by siderite, which was further supported by the XRD and FTIR spectra. The underlying mechanism was proposed to be the reduced contact time of Fe²⁺ and MR-1 cells due to the accelerated oxidation. This work provides new insights into the disinfection behavior of heterogeneous Fe (II) oxidation on iron cycling bacterial in the natural environment.

Iron Cycle Tuned by Outer-Membrane Cytochromes of Dissimilatory Metal-Reducing Bacteria: Interfacial Dynamics and Mechanisms In Vitro

The biogeochemical cycle of iron is of great importance to living organisms on Earth, and dissimilatory metal-reducing bacteria (DMRB) with the capability of reducing hematite (α-Fe2O3) by outer-membrane (OM) cytochromes play a great role in the iron cycle. However, the dynamic binding of cytochromes to α-Fe2O3 at the molecular level and the resulting impact on the photon-to-electron conversion of α-Fe2O3 for the iron cycle are not fully understood. To address these issues, two-dimensional IR correlation analysis coupled with molecular dynamics (MD) simulations was conducted for an OmcA-Fe2O3 system as OmcA bonds stronger with hematite in a typical DMRB,Shewanella. The photoelectric response of α-Fe2O3 with the OmcA coating was evaluated at three different potentials. Specifically, the binding groups from OmcA to α-Fe2O3 were in the sequence of carboxyl groups, amide II, and amide I. Further MD analysis reveals that both electrostatic interactions and hydrogen bonds played essential roles in the binding process, leading to the structural changes of OmcA to facilitate iron reduction. Moreover, the OmcA coating could store the photogenerated electrons from α-Fe2O3 like a capacitor and utilize the stored electrons for α-Fe2O3 reduction in dark and anoxic environments, further driving the biogeochemical cycle of iron. These investigations give the dynamic information on the OM protein/hematite interaction and provide fundamental insights into the biogeochemical cycle of iron by taking the photon-induced redox chemistry of iron oxide into consideration.

Ferric iron incorporation promotes brushite hydrolysis and enhances cadmium immobilization

Dissolution-precipitation processes on the surface of brushite (dicalcium phosphate dihydrate, DCPD) control the migration and transformation of potentially harmful elements (PHEs). The incorporation of impurities could affect the properties of DCPD and its interactions with PHEs. In this study, we synthesized Fe³⁺-bearing DCPD via coprecipitation and investigated the influence of Fe³⁺ incorporation on the crystal structure, hydrolysis process, and Cd removal performance. Fe-bearing DCPD had lattice expansion due to the coupled substitution of Fe³⁺ and NH₄⁺ for Ca²⁺. Therefore, the Cd removal performance of Fe-DCPD was enhanced, with a maximum Cd uptake capacity of 431.6 mg/g, which is 1.77 times that of Fe-free DCPD (244.4 mg/g). Furthermore, Fe-DCPD also exhibited a faster hydrolysis rate, which was up to 2.67 times that of Fe-free DCPD and accelerated Cd's transfer to the stable host mineral, hydroxylapatite. Cd was first caught by the DCPD surface in a weakly crystalline form and then incorporated into the hydroxylapatite structure during crystallization. Based on the X-ray photoelectron spectroscopy and thermogravimetric analysis results, we concluded that the decrease in interstitial water due to Fe incorporation was responsible for accelerating hydrolysis and enhancing Cd immobilization. In all, the incorporation of Fe³⁺ into DCPD could promote its transformation and improve its Cd uptake capacity. Our results suggest that Fe-DCPD could be a promising candidate for environmental remediation.

Forced Biomineralization: A Review

Biologically induced and controlled mineralization of metals promotes the development of protective structures to shield cells from thermal, chemical, and ultraviolet stresses. Metal biomineralization is widely considered to have been relevant for the survival of life in the environmental conditions of ancient terrestrial oceans. Similar behavior is seen among extremophilic biomineralizers today, which have evolved to inhabit a variety of industrial aqueous environments with elevated metal concentrations. As an example of extreme biomineralization, we introduce the category of "forced biomineralization", which we use to refer to the biologically mediated sequestration of dissolved metals and metalloids into minerals. We discuss forced mineralization as it is known to be carried out by a variety of organisms, including polyextremophiles in a range of psychrophilic, thermophilic, anaerobic, alkaliphilic, acidophilic, and halophilic conditions, as well as in environments with very high or toxic metal ion concentrations. While much additional work lies ahead to characterize the various pathways by which these biominerals form, forced biomineralization has been shown to provide insights for the progression of extreme biomimetics, allowing for promising new forays into creating the next generation of composites using organic-templating approaches under biologically extreme laboratory conditions relevant to a wide range of industrial conditions.

"Candidatus Chlorobium masyuteum," a Novel Photoferrotrophic Green Sulfur Bacterium Enriched From a Ferruginous Meromictic Lake

Anoxygenic phototrophic bacteria can be important primary producers in some meromictic lakes. Green sulfur bacteria (GSB) have been detected in ferruginous lakes, with some evidence that they are photosynthesizing using Fe(II) as an electron donor (i.e., photoferrotrophy). However, some photoferrotrophic GSB can also utilize reduced sulfur compounds, complicating the interpretation of Fe-dependent photosynthetic primary productivity. An enrichment (BLA1) from meromictic ferruginous Brownie Lake, Minnesota, United States, contains an Fe(II)-oxidizing GSB and a metabolically flexible putative Fe(III)-reducing anaerobe. "Candidatus Chlorobium masyuteum" grows photoautotrophically with Fe(II) and possesses the putative Fe(II) oxidase-encoding cyc2 gene also known from oxygen-dependent Fe(II)-oxidizing bacteria. It lacks genes for oxidation of reduced sulfur compounds. Its genome encodes for hydrogenases and a reverse TCA cycle that may allow it to utilize H2 and acetate as electron donors, an inference supported by the abundance of this organism when the enrichment was supplied by these substrates and light. The anaerobe "Candidatus Pseudopelobacter ferreus" is in low abundance (∼1%) in BLA1 and is a putative Fe(III)-reducing bacterium from the Geobacterales ord. nov. While "Ca. C. masyuteum" is closely related to the photoferrotrophs C. ferroooxidans strain KoFox and C. phaeoferrooxidans strain KB01, it is unique at the genomic level. The main light-harvesting molecule was identified as bacteriochlorophyll c with accessory carotenoids of the chlorobactene series. BLA1 optimally oxidizes Fe(II) at a pH of 6.8, and the rate of Fe(II) oxidation was 0.63 ± 0.069 mmol day-1, comparable to other photoferrotrophic GSB cultures or enrichments. Investigation of BLA1 expands the genetic basis for phototrophic Fe(II) oxidation by GSB and highlights the role these organisms may play in Fe(II) oxidation and carbon cycling in ferruginous lakes.

Labile metal assessment in water by diffusive gradients in thin films in shipyards on the Brazilian subtropical coast

Shipyards impact on estuarine environments because of the use of antifouling paints and petroleum products, which release trace metals that may remain in their bioavailable or labile form. Regardless of its importance, the relation between continuous input of trace metals (hotspot area) and their availability in the water column has been scarcely studied. This study evaluated seasonal variations in the concentrations of labile fractions of metals in shipyards located in estuarine areas on the Brazilian subtropical coast. These fractions were determined by the Diffuse Gradients in Thin Films (DGT) technique. Maximum labile fraction concentrations of Cr (0.3 μg L^-1), Ni (2.2 μg L^-1) and V (2.0 μg L^-1) are directly related to (i) their specific source: antifouling paints (for Cr), metal and steel alloys (for Cr and Ni) and petroleum products (for V), besides (ii) periods of intensive traffic and vessel repair. Additionally, variations in labile fractions of Ni and V in the Patos Lagoon estuary were influenced by salinity, which is known to affect metal desorption from surface sediments in resuspension events. Even though Cr is affected by the same processes, it is available as Cr(III) and does not represent any ecological risk in the study areas. Although the areas under study are affected by variations in physical and chemical conditions, shipyards were effectively hotspots of trace metals in their labile fraction in various estuarine systems in southeastern and southern Brazil. Thus, they represent areas where Ecological Risk Assessment, mainly of V, should be carried out.

Insight to Microbial Fe(III) Reduction Mediated by Redox-Active Humic Acids with Varied Redox Potentials

Redox-active humic acids (HA) are ubiquitous in terrestrial and aquatic systems and are involved in numerous electron transfer reactions affecting biogeochemical processes and fates of pollutants in soil environments. Redox-active contaminants are trapped in soil micropores (<2 nm) that have limited access to microbes and HA. Therefore, the contaminants whose molecular structure and properties are not damaged accumulate in the soil micropores and become potential pollution sources. Electron transfer capacities (ETC) of HA reflecting redox activities of low molecular weight fraction (LMWF, <2.5) HA can be detected by an electrochemical method, which is related to redox potentials (E_h) in soil and aquatic environments. Nevertheless, electron accepting capacities (EAC) and electron donating capacities (EDC) of these LMWF HA at different E_h are still unknown. EDC and EAC of different molecular weight HA at different E_h were analyzed using electrochemical methods. EAC of LMWF at -0.59 V was 12 times higher than that at -0.49 V, while EAC increased to 2.6 times when the Eh decreased from -0.59 V to -0.69 V. Afterward, LMWF can act as a shuttle to stimulate microbial Fe(III) reduction processes in microbial reduction experiments. Additionally, EAC by electrochemical analysis at a range of -0.49--0.59 V was comparable to total calculated ETC of different molecular weight fractions of HA by microbial reduction. Therefore, it is indicated that redox-active functional groups that can be reduced at E_h range of -0.49--0.59 are available to microbial reduction. This finding contributes to a novel perspective in the protection and remediation of the groundwater environment in the biogeochemistry process.

Geographical Distribution of Iron Redox Cycling Bacterial Community in Peatlands: Distinct Assemble Mechanism Across Environmental Gradient

Microbial-mediated iron (Fe) oxidation and reduction greatly contribute to the biogeochemistry and mineralogy of ecosystems. However, knowledge regarding the composition and distribution patterns of iron redox cycling bacteria in peatlands remains limited. Here, using high-throughput sequencing, we compared biogeographic patterns and assemblies of the iron redox cycling bacterial community between soil and water samples obtained from different types of peatland across four regions in Northeast China. A total of 48 phylotypes were identified as potential iron redox bacteria, which had greater than 97% similarity with Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). Among them, Rhodoferax, Clostridium, Geothrix, Sideroxydans, Geobacter, Desulfovibrio, and Leptothrix could be used as bioindicators in peatlands for characterizing different hydrological conditions and nutrient demands. Across all samples, bacterial communities associated with iron redox cycling were mainly affected by pH, dissolved organic carbon (DOC), and Fe²⁺. Distance-decay relationship (DDR) analysis indicated that iron redox cycling bacterial communities in soil, but not in water, were highly correlated with geographic distance. Additionally, null model analysis revealed that stochastic processes substituted deterministic processes from minerotrophic fens to ombrotrophic bogs in soils, whereas deterministic processes were dominant in water. Overall, these observations suggest that bacteria involved in iron redox cycling are widespread in diverse habitats and exhibit distinct patterns of distribution and community assembly mechanisms between soil and water in peatlands.

The Mechanism of Microbial-Ferromanganese Nodule Interaction and the Contribution of Biomineralization to the Formation of Oceanic Ferromanganese Nodules

Ferromanganese nodules are an important mineral resource in the seafloor; however, the genetic mechanism is still unknown. The biomineralization of microorganisms appears to promote ferromanganese nodule formation. To investigate the possible mechanism of microbial–ferromanganese nodule interaction, to test the possibility of marine microorganisms as deposition template for ferromanganese nodules minerals, the interactions between Jeotgalibacillus campisalis strain CW126-A03 and ferromanganese nodules were studied. The results showed that strain CW126-A03 increased ion concentrations of Fe, Mn, and other metal elements in solutions at first. Then, metal ions were accumulated on the cells’ surface and formed ultra-micro sized mineral particles, even crystalline minerals. Strain CW126-A03 appeared to release major elements in ferromanganese nodules, and the cell surface may be a nucleation site for mineral precipitation. This finding highlights the potentially important role of biologically induced mineralization (BIM) in ferromanganese nodule formation. This BIM hypothesis provides another perspective for understanding ferromanganese nodules’ genetic mechanism, indicating the potential of microorganisms in nodule formation.

Extracellular sulfite is protective against reactive oxygen species and antibiotic stress in Shewanella oneidensis MR-1

In this study, we investigated the extracellular reactive sulfur species produced by Shewanella oneidensis MR-1 during growth. The results showed that sulfite is the major extracellular sulfur metabolite released to the growth medium under both aerobic and anaerobic growth conditions. Exogenous sulfite at physiological concentrations protected S. oneidensis MR-1 from hydrogen peroxide toxicity and enhanced tolerance to the beta-lactam antibiotics cefazolin, meropenem, doripenem and ertapenem. These findings suggest that the release of extracellular sulfite is a bacterial defence mechanism that plays a role in the mitigation of environmental stress.

Orange leads to black: evaluating the efficacy of co-culturing iron-oxidizing and sulfate-reducing bacteria to discern ecological relationships

Two global cycles, iron and sulfur, are critically interconnected in estuarine environments by microbiological actors. To this point, the methods of laboratory study of this interaction have been limited. Here we propose a methodology for co-culturing from numerous coastal environments, from the same source inocula, iron-oxidizing and sulfate-reducing bacteria. The use of same source inocula is largely beneficial to understand real-world interactions that are likely occurring in situ. Through the use of this methodology, the ecological interactions between these groups can be studied in a more controlled environment. Here, we characterize the oxygen and hydrogen sulfide concentrations using microelectrode depth profiling in the co-cultures of iron-oxidizing bacteria and sulfate-reducing bacteria. These results suggest that while oxygen drives the relationship between these organisms and sulfate-reducers are reliant on iron-oxidizers in this culture to create an anoxic environment, there is likely another environmental driver that also influences the interaction as the two remain spatially distinct, as trends in FeS precipitation changed within the anoxic zone relative to the presence of Fe(III) oxyhydroxides. Understanding the relationship between iron-oxidizing and sulfate-reducing bacteria will ultimately have implications for understanding microbial cycling in estuarine environments as well as in processes such as controlling microbially influenced corrosion.

Anaerobic Ammonium Removal Pathway Driven by the Fe(II)/Fe(III) Cycle through Intermittent Aeration

Feammox, that is, Fe(III) reduction coupled to anaerobic ammonium oxidation, has been reported to play an important role in the nitrogen cycle in natural environments. However, the application of Feammox in wastewater treatment is limited because continuous Fe(III) supplementation is required for achieving continuous nitrogen removal, which is not feasible in practice. In this study, air was aerated intermittently into the Feammox system containing iron and high-content ammonium for oxidizing Fe(II) generated from Feammox to Fe(III), then, the produced Fe(III) participated in the next round of Feammox, leading to continuous nitrogen removal through the Fe(II)/Fe(III) cycle. The results showed that after each 10 min of aeration (150 mL/min), every 6-7 days, dissolved oxygen (DO) increased from 0 to about 0.4 mg/L, accompanied by a decrease in Fe(II) and an increase in Fe(III). One day after the aeration, DO was undetectable, and then, Fe(II) content increased and Fe(III) content decreased. On day 90, NH₄⁺-N content in the aerated reactor was only 10.2 mg/L, while it remained at around 288.3 mg/L in the aeration-free group. X-ray diffraction showed that the generated Fe(III) through air aeration was Fe(OH)₃. Microbial analysis showed that anammox and nitrification/denitrification could be excluded in the system. This NH₄⁺ removal process, driven by the Fe(II)/Fe(III) cycle with O₂ as the terminal electron acceptor, might be used as an in situ remediation method for treating high-content NH₄⁺.

Biogeochemical dynamics and microbial community development under sulfate- and iron-reducing conditions based on electron shuttle amendment

Iron reduction and sulfate reduction are two of the major biogeochemical processes that occur in anoxic sediments. Microbes that catalyze these reactions are therefore some of the most abundant organisms in the subsurface, and some of the most important. Due to the variety of mechanisms that microbes employ to derive energy from these reactions, including the use of soluble electron shuttles, the dynamics between iron- and sulfate-reducing populations under changing biogeochemical conditions still elude complete characterization. Here, we amended experimental bioreactors comprised of freshwater aquifer sediment with ferric iron, sulfate, acetate, and the model electron shuttle AQDS (9,10-anthraquinone-2,6-disulfonate) and monitored both the changing redox conditions as well as changes in the microbial community over time. The addition of the electron shuttle AQDS did increase the initial rate of Fe^III reduction; however, it had little effect on the composition of the microbial community. Our results show that in both AQDS- and AQDS+ systems there was an initial dominance of organisms classified as Geobacter (a genus of dissimilatory Fe^III-reducing bacteria), after which sequences classified as Desulfosporosinus (a genus of dissimilatory sulfate-reducing bacteria) came to dominate both experimental systems. Furthermore, most of the ferric iron reduction occurred under this later, ostensibly “sulfate-reducing” phase of the experiment. This calls into question the usefulness of classifying subsurface sediments by the dominant microbial process alone because of their interrelated biogeochemical consequences. To better inform models of microbially-catalyzed subsurface processes, such interactions must be more thoroughly understood under a broad range of conditions.

Disentangling Microbial Syntrophic Mechanisms for Hexavalent Chromium Reduction in Autotrophic Biosystems

Hexavalent chromium [Cr(VI)] is one of the common heavy-metal contaminants in groundwater, and the availability of electron donors is considered to be a key parameter for Cr(VI) biotransformation. During the autotrophic remediation process, however, much remains to be illuminated about how complex syntrophic microbial communities couple Cr(VI) reduction with other elemental cycles. Two series of Cr(VI)-reducing groundwater bioreactors were independently amended by elemental sulfur and iron and inoculated with the same inoculum. After 160 days of incubation, both bioreactors showed similar archaea-dominating microbiota compositions, whereas a higher Cr(VI)-reducing rate and more methane production were detected in the Fe⁰-driven one. Metabolic reconstruction of 23 retrieved genomes revealed complex symbiotic relationships driving distinct elemental cycles coupled with Cr(VI) reduction in bioreactors. In both bioreactors, these Cr(VI) reducers were assumed to live in syntrophy with oxidizers of sulfur, iron, hydrogen, and volatile fatty acids and methane produced by carbon fixers and multitrophic methanogens, respectively. The significant difference in methane production was mainly due to the fact that the yielded sulfate greatly retarded acetoclastic methanogenesis in the S-bioreactor. These findings provide insights into mutualistic symbioses of carbon, sulfur, iron, and chromium metabolisms in groundwater systems and have implications for bioremediation of Cr(VI)-contaminated groundwater.

Microorganisms from deep-sea hydrothermal vents

With a rich variety of chemical energy sources and steep physical and chemical gradients, hydrothermal vent systems offer a range of habitats to support microbial life. Cultivation-dependent and independent studies have led to an emerging view that diverse microorganisms in deep-sea hydrothermal vents live their chemolithoautotrophic, heterotrophic, or mixotrophic life with versatile metabolic strategies. Biogeochemical processes are mediated by microorganisms, and notably, processes involving or coupling the carbon, sulfur, hydrogen, nitrogen, and metal cycles in these unique ecosystems. Here, we review the taxonomic and physiological diversity of microbial prokaryotic life from cosmopolitan to endemic taxa and emphasize their significant roles in the biogeochemical processes in deep-sea hydrothermal vents. According to the physiology of the targeted taxa and their needs inferred from meta-omics data, the media for selective cultivation can be designed with a wide range of physicochemical conditions such as temperature, pH, hydrostatic pressure, electron donors and acceptors, carbon sources, nitrogen sources, and growth factors. The application of novel cultivation techniques with real-time monitoring of microbial diversity and metabolic substrates and products are also recommended.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-020-00086-4.

Hematite facet-mediated microbial dissimilatory iron reduction and production of reactive oxygen species during aerobic oxidation

Microbial dissimilatory iron reduction and aerobic oxidation affect the biogeochemical cycles of many elements. Although the processes have been widely studied, the underlying mechanisms, and especially how the surface structures of iron oxides affect these redox processes, are poorly understood. Therefore, {001} facet-dominated hematite nanoplates (HNP) and {100} facet-dominated hematite nanorods (HNR) were used to explore the effects of surface structure on the microbial dissimilatory iron reduction and aerobic oxidation processes. During the reduction stage, the production of total Fe(II) normalized by specific surface area (SSA) was higher for HNP than HNR due to steric effects and the ligand-bound conformation of the connection between iron on different exposed facets and electron donors from microorganisms. However, during the aerobic oxidation stage, both the SSA- and Fe(II)-normalized reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and hydroxyl radical (•OH), were higher for HNR than HNP. Theoretical calculation results showed that the {100} facets exhibited a lower activation energy barrier for oxygen reduction reaction than {001} facets, supporting the experimental observation that {100} facet-dominated HNR had a higher ROS production efficiency than {001} facet-dominated HNP. These results indicated that surface characteristics not only mediated the microbial reduction of Fe(III) but also affected the aerobic oxidation of microbially reduced Fe(II). Accessibility of electron donors to surface iron atom determined the reduction of Fe(III), and activation energy barrier for oxygen reduction by surface Fe(II) dominated the ROS production during the redox processes. This study advances our understanding of the mechanisms through which ROS are produced by iron (oxyhydr)oxides during microbial dissimilatory iron reduction and aerobic oxidation processes.

Nanosized non-proteinaceous complexes III and IV mimicking electron transfer of mitochondrial respiratory chain

Synthetic biology pursues the understanding of biological processes and their possible mimicry with artificial bioinspired materials. A number of materials have already been used to mimic the active site of simple redox proteins, including nanosized iron oxides due to their redox properties. However, the mimicry of membrane redox protein complexes is still a challenge. Herein, magnetic iron oxide nanoparticles (NPs), incorporated as non-proteinaceous complexes III and IV in a mitochondrial model membrane, catalyze electron transfer (ET) similarly to the natural complexes towards cytochrome c. The associated molecular mechanism is experimentally proven in solution and in a Langmuir-Blodgett film. A direct and entropy-driven ET, with rate constant of 2.63 ± 0.05Lmol^-1 at 25 °C, occurs between the iron sites of the NPs and the cytochrome c heme group, not affecting the protein secondary and tertiary structures. This process requires an activation energy of 40.2 ± 1.5 kJ mol^-1 resulting in an overall Gibbs free energy of -55.3 kJ mol^-1. Furthermore, the protein-NP system is governed by electrostatic and non-polar forces that contribute to an associative mechanism in the transition state. Finally, the incorporated NPs in a model membrane were able to catalyze ET, such as the natural complexes in respiratory chain. This work presents an experimental approach demonstrating that inorganic nanostructured systems may behave as embedded proteins in the eukaryotic cells membrane, opening the way for more sophisticated and robust mimicry of membrane protein complexes.

Effect of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge system

The effects of voltage intensity on the nutrient removal performance and microbial community in the iron electrolysis-integrated aerobic granular sludge (AGS) system were investigated over a period of 15 weeks. Results revealed that the application outcomes of iron electrolysis for AGS systems relied on voltage intensity. When a constant voltage of 1.5 V was applied, the sludge granulation was most obviously accelerated with a specific growth rate of the sludge diameter of 0.078 day^-1, and the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) increased by 14.1% and 20.2%, respectively, compared to the control reactor (without the iron electrolysis-integration). Moreover, the AGS developed at different voltages included different microbial communities, whose shifts were driven by the Fe content and the average diameter of AGS. Both heterotrophic nitrifiers and mixotrophic denitrifiers were significantly enriched in the AGS developed at 1.5 V, which effectively enhanced TN removal. Together with the response of the functional genes involved in Fe, N, and P metabolism, the electrolytic iron-driven nutrient degradation pathway was further elaborated. Overall, this study clarified the optimum voltage condition when iron electrolysis was integrated into the AGS system, and revealed the enhancement mechanism of this coupling technology on nutrient removal during the treatment of low-strength municipal wastewater.