Iron-reducing Bacteria Research Articles

Increased arsenic mobilization in the rice rhizosphere is mediated by iron-reducing bacteria

Rice (Oryza sativa) tends to accumulate elevated levels of arsenic (As) in grain, threatening food safety and human health. The rice rhizosphere has a micro-environment that differs markedly from the bulk soil. Yet, little is known about how this micro-environment influences the mobility of As in the rhizosphere. Using rhizoboxes with two rice cultivars (cv. Shenyou 957 and Yangdao 6) differing in their radial oxygen loss (ROL), we investigated the in situ transformation of As in the rhizosphere associated with changes in microbial communities and As-related functional genes. Contrary to expectation, dissolved (porewater) As concentrations within the rhizosphere increased by 1.3–2.4 fold compared to the bulk soil during the seedling stage, with the magnitude of this difference gradually decreasing over time. The increased As mobilization in the rhizosphere was associated with increased soluble Fe. This increasing trend was associated with the increased abundance of both Fe-reducing bacteria (FeRB) and As-related functional genes within the rhizosphere. Furthermore, bacterial 16S rRNA gene sequencing data showed that the abundances of Geobacter and Clostridium were 3.1 times and 12.4 times higher in the rhizosphere, respectively. The importance of FeRB was also suggested by the fact that dissolved As concentrations were highly correlated with dissolved Fe concentrations (r2 = 0.83) and also with the relative abundance of genus Clostridium_sensu_stricto_10 (r2 = 0.85). This study highlights that although the rice rhizosphere favors a more aerobic condition compared to the bulk soil, As is more mobilized in the rhizosphere, and that Geobacter and some species of Clostridium play a critical role in controlling As mobilization in the rhizosphere.

Interplay between iron species transformation and hydroxyl radicals production in soils and sediments during anoxic-oxic cycles

Iron (Fe) redox transformation in soils and sediments plays an essential role in the biogeochemical cycling of associated nutrients and elements. It is documented recently that hydroxyl radicals (OH) can be produced upon oxygenation of Fe(II)-bearing minerals in soils and sediments. As OH induces serious oxidative stress to microbes, it is likely that OH produced upon Fe(II) oxygenation inactivates the coexisting dissimilatory iron-reducing bacteria and subsequently impacts Fe redox transformation. This hidden interplay between Fe species transformation and OH production in soils and sediments during anoxic-oxic cycles remains unexplored. In this study, a field sediment was incubated with 1 × 109 CFU/mL Shewanella oneidensis strain MR-1 and treated for six anoxic-oxic cycles (117-h anoxic and 6-h oxic periods). Fe species transformation, OH production and MR-1 activity variation were measured throughout the six cycles. Fe in the sediments prior to anoxic-oxic cycles mainly existed in Fe-bearing silicates (60–70%) and Fe (hydr)oxides (30–40%). For the anoxic periods, a portion of Fe(III) in sediments was either reduced to structural Fe(II) in (hydr)oxides and silicates or reductively dissolved to aqueous Fe2+. Total Fe(II) produced from Fe(III) bio-reduction increased from 2.46 to 8.24 mM in the first two anoxic periods, but decreased to 0.10 mM in the following four anoxic periods. For the oxic periods, Fe(II) was oxidized along with a production of OH, and the cumulative OH concentrations were around 31 µM in the first three periods but decreased to 6.3 µM in the subsequent three periods. MR-1 was inactivated by about 0.6 order of magnitude in the first two oxic periods, and the inactivation became insignificant for the later oxic periods. The six cycles of anoxic-oxic treatments promoted the transformation of Fe(III) (hydr)oxides into more amorphous phase. A strong interplay between Fe species transformation and OH production occurred in anoxic-oxic cycles. Fe(III) bio-reduction by MR-1 in anoxic periods determined the content and speciation of Fe(II), which governed OH production in the subsequent oxic periods. The generated OH affected MR-1 activity in oxic periods, which further controlled Fe(III) bio-reduction in anoxic periods. The hidden interplay identified herein improves our understanding of the cycling of Fe and associated elements under redox fluctuating environments.

Joint response of chemistry and functional microbial community to oxygenation of the reductive confined aquifer

Oxygen can enter into reductive aquifer through natural and artificial processes. However, the joint response of groundwater chemistry and functional microbial communities to oxygenation is not well understood due to the gap between taxonomic and functional microbial composition. Here, two wells named CZK15 and CZK22 at the second confined aquifer in Central China were in situ aerated, and the chemical parameters of groundwater and microbial communities in bio-trapping sand sediment were analysed during aeration. The microbial metabolic functions related to C, N, S, Fe transformation were predicted by Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach and some key functional genes, such as phe, nah, narG, and soxB were verified by the real-time quantitative Polymerase Chain Reaction (qPCR) method. The biomass was promoted, microbial diversity fluctuated, and microbial composition changed remarkably with aeration mainly constrained by reduction-oxidation (redox) variation and SO42− concentration. Among functional microbes, aerobic chemoheterotrophs including aromatic compound degraders (also especially for relative abundance of phe and some nah gene) and methylotrophs are dramatically enriched interpreting dissolved oxygen (DO) consumption and total organic carbon (TOC) decomposing in sediment. Whilst fermenters and methanogen expectedly decreased during aeration. Denitrifying microbes and narG gene relative abundance increased corresponding to the NO3− increase after aeration, while microbes for N2 fixation, ammonification, and nitrification decreased relating to the source of NH4+. The sulfide oxidation causing increased SO42− was reflected by the blooming of sulfur-oxidizing microbes and soxB gene. Some sulfate reducers persisted in sediment after aeration due to sufficient SO42− as substrate. Fe(II) was mainly chemically oxidized as iron-oxidizing microbes were of low abundance and tended to decrease with aeration. The iron-reducing bacteria Geobacteraceae increased with aeration corresponding to the increased Fe(III) oxides formation. The findings of this study could have important implications in understanding the biogeochemical behaviours with cyclic redox conditions.

Integration of •SO4−-based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II

The sulfate radical (•SO4−)-based advanced oxidation processes (AOPs) for the degradation of refractory organic pollutants consume a large amount of persulfate activators and often generate toxic organic by-products. In this study, we proposed a novel iron-cycling process integrating •SO4−-based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II completely. The rusted waste iron particles (Fe0@FexOy), which contained FeII/FeIII oxides (FexOy) on the shell and zero-valent iron (Fe0) in the core, efficiently activated persulfate to produce •SO4− and hydroxyl radicals (•OH) to degrade over 95% of Orange II within 120 min. Both •SO4− and •OH destructed Orange II through a sequence of electron transfer, electrophilic addition and hydrogen abstraction reactions to generate several organic by-products (e.g., aromatic amines and phenol), which were more toxic than the untreated Orange II. The AOP-generated organic by-products were further mineralized and detoxified in a sulfidogenic bioreactor with sewage treatment together. In a 170-d trial, the organic carbon removal efficiency was up to 90%. The inhibition of the bioreactor effluents on the growth of Chlorella pyrenoidosa became negligible, due to the complete degradation and mineralization of toxic AOP-generated by-products by aromatic-degrading bacteria (e.g., Clostridium and Dechloromonas) and other bacteria. The sulfidogenic process also well recovered the used Fe0@FexOy particles through the reduction of surface FeIII back into FeII by hydrogen sulfide formed and iron-reducing bacteria (e.g., Sulfurospirillum and Paracoccus). The regenerated Fe0@FexOy particles had more reactive surface FeII sites and exhibited much better reactivity in activating persulfate in at least 20 reuse cycles. The findings demonstrate that the integrated process is a promising solution to the remediation of toxic and refractory organic pollutants because it reduces the chemical cost of persulfate activation and also completely detoxifies the toxic by-products.

The characterization of arsenic biotransformation microbes in paddy soil after straw biochar and straw amendments

Straw biochar and straw application to paddy soil dramatically altered arsenic (As) biogeochemical cycling in soil-rice system, but it remains unknown how As biotransformation microbes (ABMs) contribute to these processes. In this study, rice pot experiments combining terminal restriction fragment length polymorphism (T-RFLP) analysis and clone library were performed to characterize ABMs. Through linear discriminant analysis (LDA) effect size (LEfSe) and correlation analysis, results revealed that arrA-harbouring iron-reducing bacteria (e.g., Geobacter and Shewanella) and arsC-harbouring Gammaproteobacteria (e.g., fermentative hydrogen-producing and lignin-degrading microorganisms) potentially mediated arsenate [As(V)] reduction under biochar and straw amendments, respectively. Methanogens and sulfate-reducing bacteria (SRB) carrying arsM gene might regulate methylated As concentration in soil-rice system. Network analysis demonstrated that the association among ABMs in rhizosphere was significantly stronger than that in bulk soil. Arsenite [As(III)] methylators carrying arsM gene exhibited much stronger co-occurrence pattern with arsC-harbouring As(V) reducers than with arrA-harbouring As(V) reducers. This study would broaden our insights for the dramatic variation of As biogeochemical cycling in soil-rice system after straw biochar and straw amendments through the activities of ABMs, which could contribute to the safe rice production and high rice yield in As-contaminated fields.

Effect of hydraulic retention time, ZVI concentration, and Fe2+ concentration on autotrophic denitrification efficiency with iron cycle bacterium strain CC76

ABSTRACT The immobilized reactor of iron-reducing bacteria and zero-valent iron (ZVI) integrated system was established. This study has shown that the effects of hydraulic retention times (9, 11, 13 h), ZVI concentrations (2, 4, 6, 8 mg/L), and Fe2+ concentrations (5, 10, 15 mg/L) on the denitrification characteristics of iron cycle bacterium strain CC76. The results show that the longer the HRT is, the stronger ability of bacteria to remove nitrate. When ZVI concentration was 4 mg/L and the Fe2+ concentration is 15 mg/L, the removal efficiency of nitrate was the highest, reaching the maximum value of 93.02% (1.07 mg/L/h). Since increasing ZVI concentration in a certain range can not only promote chemical reduction but also make use of strain CC76 as an electron donor. Also, the abundance of strain CC76 decreased with the increase of ZVI concentration, which indicated that adding a low concentration of ZVI could reduce the inhibitory effect on bacteria. Hypothesis analysis of principal components showed that a low concentration of ZVI is beneficial to increase nitrate removal rate. Community structure analysis indicated that strain CC76 and related bacteria were the most abundant bacteria in the reactor.

Understanding mercury methylation in the changing environment: Recent advances in assessing microbial methylators and mercury bioavailability

Methylmercury (MeHg) is a neurotoxin, mainly derived from microbial mercury methylation in natural aquatic environments, and poses threats to human health. Polar regions and paddy soils are potential hotspots of mercury methylation and represent environmental settings that are susceptible to natural and anthropogenic perturbations. The effects of changing environmental conditions on the methylating microorganisms and mercury speciation due to global climate change and farming practices aimed for sustainable agriculture were discussed for polar regions and paddy soils, respectively. To better understand and predict microbial mercury methylation in the changing environment, we synthesized current understanding of how to effectively identify active mercury methylators and assess the bioavailability of different mercury species for methylation. The application of biomarkers based on the hgcAB genes have demonstrated the occurrence of potential mercury methylators, such as sulfate-reducing bacteria, iron-reducing bacteria, methanogen and syntrophs, in a diverse variety of microbial habitats. Advanced techniques, such as enriched stable isotope tracers, whole-cell biosensor and diffusive gradient thin film (DGT) have shown great promises in quantitatively assessing mercury availability to microbial methylators. Improved understanding of the complex structure of microbial communities consisting mercury methylators and non-methylators, chemical speciation of inorganic mercury under geochemically relevant conditions, and the pathway of cellular mercury uptake will undoubtedly facilitate accurate assessment and prediction of in situ microbial mercury methylation.

Enhancement of tribromophenol removal in a sequencing batch reactor via submicron magnetite.

Conductive magnetite (Fe3O4) has been applied into some anaerobic bioprocesses to accelerate direct interspecies electron transfer (DIET), however, Fe3O4 is usually dissolved by iron-reducing bacteria under anaerobic conditions, resulting in the loss of magnetite. Therefore, submicron magnetite particles were added to the sequencing batch reactor (SBR) to build a Fe3O4/SBR system, which could alleviate magnetite dissolution and simultaneously remove tribromophenol (TBP) effectively. The average removal efficiencies of chemical oxygen demand (COD) and TBP in Fe3O4/SBR system were 81% and 91%, respectively, which were 51% and 18% higher than those of the control group without Fe3O4 (SBR system). The enhanced TBP biodegradation was likely related to potential DIET, which was supported by the scanning electron microscopy (SEM) analysis, the increase of dehydrogenase and heme c (fivefold and 1.7-fold), and the enrichment of iron-redoxing bacteria (Geobacter and Thiobacillus). Furthermore, magnetite mainly remained intact in structure as indicated by X-ray diffraction (XRD), which might be ascribed to in situ iron redox cycle and magnetite biosynthesis via Magnetospirillum. Notably, the content of hydrogen peroxide (H2O2) and hydroxyl radical (⋅OH) in Fe3O4/SBR system was 4-5 times higher than that of SBR system. These findings could provide insights into the development of cost-effective strategy for the removal of refractory organic pollutants.

Insight into ferrihydrite effects on methanogenesis in UASB reactors treating high sulfate wastewater: reactor performance and microbial community

Ferrihydrite supplemented to establish DIET between iron-reducing bacteria and methanogens with Fe oxides in anaerobic digestion.

Appropriate Location and Deployment Method for Successful Iron Fertilization

“High nutrient, low chlorophyll (HNLC)” regions were created by locking iron into sedimentary iron sulfides with hydrogen sulfide available from volcanic eruptions in surrounding oceans. Appropriate locations and deployment methods for the iron fertilization were far from volcanoes, earthquakes and boundaries of tectonic plates to reduce the chance of iron-locking by volcanic sulfur compounds. The appropriate locations for the large-scale iron fertilization are proposed as Shag Rocks in South Georgia and the Bransfield Strait in Drake Passage in the Southern Ocean due to their high momentum flux causing efficient iron deployment. The iron (Fe) replete compounds, consisting of natural clay, volcanic ash, agar, N2-fixing mucilaginous cyanobacteria, carbon black, biodegradable plastic foamed polylactic acid, fine wood chip, and iron-reducing marine bacterium, are deployed in the ocean to stay within a surface depth of 100 m for phytoplankton digestion. The deployment method of Fe-replete composite with a duration of at least several years for the successful iron fertilization, is configured to be on the streamline of the Antarctic Circumpolar Current (ACC). This will result in high momentum flux for its efficient dispersion on the ocean surface where diatom, copepods, krill and humpback whale stay together (~100 m). Humpback whales are proposed as a biomarker for the successful iron fertilization in large-scale since humpback whales feed on krill, which in turn feed on cockpods and diatoms. The successful large-scale iron fertilization may be indicated by the return of the humpback whales if they could not be found for a long period before the iron fertilization. On-line monitoring for the successful iron fertilization focuses on the simultaneous changes of the following two groups; the increase concentration group (chlorophyll, O2, Dissolved Oxygen (DO), Di Methyl Sulfide (DMS)) and the decrease concentration group (nitrate, phosphate, silicate, CO2, Dissolved CO2 (DCO2)). The monitoring of chlorophyll-a, nitrate phosphate, and silicate concentrations after deploying the Fe-replete complex is carried out throughout the day and night for the accurate measurement of algal blooms.

Microbial iron reduction enhances in-situ control of biogenic hydrogen sulfide by FeOOH granules in sediments of polluted urban waters

This paper discusses the abiotic and biotic processes in the in-situ control of biogenic hydrogen sulfide generated from microbial sulfate reduction by ferric (FeIII) (hydr)oxides (FeOOH) granules in the sediments of polluted urban waters. Granular ferric hydroxide (GFH, β-FeOOH) and granular ferric oxide (GFO, α-FeOOH) dosed in the organic- and sulfate-rich sediments had 180% and 19% higher sulfide removal capacities than those used for the purely abiotic removal of dissolved sulfide, respectively. The enhancement was attributable to the involvement of the biotic pathways, besides the abiotic pathways (mainly sulfide oxidation). The FeOOH granules stimulated the microbial reduction of surface FeIII by iron-reducing bacteria (e.g., Desulfovibrio and Carnobacterium), and increased the microbial sulfate reduction by 24%–30% under an organic-rich condition, likely due to the enhanced organic fermentation. The microbial iron reduction significantly enhanced the removal of the formed biogenic hydrogen sulfide through increasing sulfide precipitation because it remarkably promoted the release of Fe2+ ions from the granule surface, likely due to the involvement of siderophores as ligands. This biotic pathway led to the formation of amorphous FeS(s) as a major sulfur product (56%–81%), instead of elemental sulfur. The enhancement in the sulfide control performance was much more pronounced when the poorly ordered GFH was used, because of the faster Fe2+ release, compared to the highly ordered GFO. The abiotic and biotic mechanisms elucidated in this study provide insights into the iron-sulfur chemistry in the sediments of various polluted waters (e.g., storm drains, urban rivers, and estuary), where the manually-dosed and naturally-occurring FeIII (hydr)oxides control biogenic hydrogen sulfide.

Pathway for the Production of Hydroxyl Radicals during the Microbially Mediated Redox Transformation of Iron (Oxyhydr)oxides.

The reduction of ferric iron (Fe(III)) to ferrous iron (Fe(II)) by dissimilatory iron-reducing bacteria is widespread in anaerobic environments. The oxidation of Fe(II) in aerobic environments has been found to produce hydroxyl radicals (•OH); however, the role of iron-reducing bacteria in the process has not been well understood. Here, Shewanella oneidensis MR-1-mediated redox transformation of four typical iron (oxyhydr)oxides and the production of reactive oxygen species were investigated. The results showed that the production of •OH was mainly determined by the insoluble Fe(II) formed during microbially mediated reduction and also mediated by the mineralogical phase. Moreover, this study for the first time observed the exogenetic iron-independent production of •OH by S. oneidensis MR-1, and the integrated pathway of •OH generation during the iron redox process was revealed. Superoxide (O2•-) was indicated as a key intermediate species that was produced by both abiotic and biotic pathways, and •OH was generated by both the exogenetic iron-dependent Fenton-like reaction and exogenetic iron-independent pathways. S. oneidensis MR-1 played a pivotal role in both the reduction of Fe(III) and the production of O2•-. These findings contribute substantially to our understanding of the generation mechanism of reactive oxygen species at oxidation-reduction boundaries in the environment.

Simultaneous immobilization of the cadmium, lead and arsenic in paddy soils amended with titanium gypsum

In situ immobilization of heavy metals in contaminated soils using industrial by-products is an attractive remediation technique. In this work, titanium gypsum (TG) was applied at two levels (TG-L: 0.15% and TG-H: 0.30%) to simultaneously reduce the uptake of cadmium (Cd), lead (Pb) and arsenic (As) in rice grown in heavy metal contaminated paddy soils. The results showed that the addition of TG significantly decreased the pH and dissolved organic carbon (DOC) in the bulk soil. TG addition significantly improved the rice plants growth and reduced the bioavailability of Cd, Pb and As. Particularly, bioavailable Cd, Pb and As decreased by 35.2%, 38.1% and 38.0% in TG-H treatment during the tillering stage, respectively. Moreover, TG application significantly reduced the accumulation of Cd, Pb and As in brown rice. Real-time PCR analysis demonstrated that the relative abundance of sulfate-reducing bacteria increased with the TG application, but not for the iron-reducing bacteria. In addition, 16S rRNA sequencing analysis revealed that the relative abundances of heavy metal-resistant bacteria such as Bacillus, Sulfuritalea, Clostridium, Sulfuricella, Geobacter, Nocardioides and Sulfuricurvum at the genus level significantly increased with the TG addition. In conclusion, the present study implied that TG is a potential and effective amendment to immobilize metal(loid)s in soil and thereby reduce the exposure risk of metal(loid)s associated with rice consumption.

Impacts of biochar and silicate fertilizer on arsenic accumulation in rice (Oryza sativa L.).

Human exposure to arsenic (As) through rice consumption is a global food safety issue, especially in Southeast Asia. To investigate the impacts of biochar amendment (rice husk and smooth cordgrass-derived biochar) and/or silicate fertilizer on As mobility/phytoavailability in soil and on As accumulation in rice, pot and microcosm experiments were conducted. The results showed that both single application of low doses of biochar (0.5%, w/w) and coapplication of biochar with silicate fertilizer decreased As levels in grain (brown rice) by 14-16%, but not in straw and roots. The biodilution of As in grain resulting from increased grain biomass (by 6-21%) could be mainly a response to the decline in grain As levels with biochar and/or silicate fertilizer amendment. However, both applications exerted limited effects to decrease the overall As uptake by rice grain and straw, potentially due to the small changes in As mobility/phytoavailability in amended soil relative to the control, although plant-available silicon (Si) from amendment could potentially inhibit As uptake. Furthermore, microcosm-based anaerobic incubation experiments demonstrated that As levels in soil solution increased (up to 11-14-fold) with increasing doses of biochar amendment (up to 5%, w/w), possibly due to biochar enhancing the reductive dissolution of iron (oxyhydr) oxides via an increase in the total number of iron-reducing bacteria (up to 1.6-3.2-fold). Our findings suggested that a low application rate of biochar may not be a very effective approach for mitigating As accumulation in rice, while a high application rate could enhance the health risk of As in As-contaminated flooded soil.

Inhibition effect of polyvinyl chloride on ferrihydrite reduction and electrochemical activities of Geobacter metallireducens.

Geobacter metallireducens GS15, a model of dissimilatory iron-reducing bacteria, is the key regulator in biogeochemical iron cycling. How the emerging contaminant microplastics involved in the iron cycling are driven by microbes on the microscale remains unknown. Hence, the influences of two typical microplastics, polybutylene terephthalate-hexane acid (PBAT) and polyvinyl chloride (PVC), were explored on the activity of G. metallireducens GS15 with ferrihydrite or ferric citrate as the respective electron acceptors. The results showed that the iron (II) contents in PBAT- and PVC-treatment groups were 16.79 and 6.81 mM, respectively, at the end of the experiment. Compared with the PBAT-treatment group, scanning electron microscopy and energy dispersive spectrometery revealed that merely a small amount of iron-containing products covered the surface of PVC. Moreover, PBAT and PVC could both retard the electroactivity of G. metallireducens GS15 at the beginning of microbial fuel cell operation. On the basis of the results above, microplastic PVC might exhibit potential inhibition of the iron cycling process driven by G. metallireducens GS15 with ferrihydrite as the terminal electron acceptor. This study extended our understanding of the influence of the microplastics PBAT and PVC on microbially mediated biogeochemical iron cycling. The findings might have an important implication on the biogeochemical elements cycling in the ecosystem with the involvement of emerging contaminant microplastics.

Changes in Phosphorus and Sediment Oxygen Demand in Coastal Sediments Promoted by Functionalized Oyster Shell Powder as an Oxygen Release Compound

In this study, we performed a sediment elution experiment to evaluate water quality in terms of phosphorus, as influenced by the dissolved oxygen consumed by sediments. Three separate model column treatments, namely, raw, calcined, and sonicated oyster shell powders, were used in this experiment. Essential phosphorus fractions were examined to verify their roles in nutrient release from sediment based on correlation analyses. When treated with calcined or sonicated oyster shell powder, the sediment-water interface became “less anaerobic,” thereby producing conditions conducive to partial oxidation and activities of aerobic bacteria. Sediment Oxygen Demand (SOD) was found to be closely correlated with the growth of algae, which confirmed an intermittent input of organic biomass at the sediment surface. SOD was positively correlated with exchangeable and loosely adsorbed phosphorus and organic phosphorus, owing to the accumulation of unbound algal biomass-derived phosphates in sediment, whereas it was negatively correlated with ferric iron-bound phosphorus or calcium fluorapatite-bound phosphorus, which were present in the form of insoluble complexes, thereby facilitating the free migration of sulfate-reducing bacteria or limiting the release from complexes, depending on applied local conditions. PCR-denaturing gradient gel electrophoresis revealed that iron-reducing bacteria were the dominant species in control and non-calcined oyster shell columns, whereas certain sulfur-oxidizing bacteria were identified in the column treated with calcined oyster powder.

The effects of biochar as the electron shuttle on the ferrihydrite reduction and related arsenic (As) fate

The effects of electron shuttles (biochar/anthraquinone-2,6-disulphonate (AQDS)) on the process of the Shewanella oneidensis MR-1-induced As(V)-adsorbed ferrihydrite reduction were studied. The results showed that biochar could stimulate Fe(Ⅱ) and As release during the ferrihydrite bioreduction. After the addition of biochar, more dissolved organic matter (DOM) can be consumed as an electron donor to promote the metabolism of microorganisms by the fluorescence excitation-emission matrix spectra analysis. After microbial treatment, cyclic voltammetry (CV) showed that a unique cathodic peak and a distinct anodic peak appeared, which may represent the reduction of Fe(OH)3 to Fe(OH)2 and the complexed oxidation of Fe2+ to Fe3+. No characteristic peak was associated with arsenate reduction or arsenite oxidation. The mineralogical characterization of the final products indicated that AQDS can promote solid-state conversion from ferrihydrite to vivianite (Fe3(PO4)2·8H2O). However, the addition of biochar inhibited solid-state conversion of ferrihydrite. It was shown that after 6 d, the secondary mineral vivianite production in the bacteria alone and AQDS treatments was 8.12% and 15.6% respectively by mössbauer spectroscopy analysis. Moreover, the XPS indicated that As(V) has no species transformation. It provided new data for understanding the iron-reducing bacteria induced mineralization process and related biogeochemical cycles of Fe and As.

Mercury methylation-related microbes and genes in the sediments of the Pearl River Estuary and the South China Sea

Methylmercury (MeHg) is a toxicant that mainly originates from in situ microbial methylation of inorganic mercury (Hg) in the environment and poses a severe health risk to the public. However, the characteristics of the Hg-methylating microbial community and its relationship with MeHg production in various environments remain to be understood. In the present study, Hg-methylating microbial communities and genes (hgcAB cluster) in the sediments of the Pearl River (PR), Pearl River Estuary (PRE) and South China Sea (SCS) were investigated at a large spatial scale using high-throughput sequencing-based approaches. The results showed that sulfur-reducing bacteria (SRB) and iron-reducing bacteria (IRB) were consistently the dominant microbial strains responsible for the methylation of inorganic Hg in all three regions investigated. The abundance and diversity of Hg-methylating communities and genes were both found to be higher in the PR sediments compared to that in the PRE and SCS sediments, and in good agreement with the spatial distribution of MeHg. Furthermore, a significant correlation was observed between the MeHg concentration and the abundance of both hgcA and hgcB genes in the sediments of the PR, PRE and SCS regions. Overall, the present study suggested that there was the presence of a close link between MeHg and Hg-methylating communities or genes in the ambient aquatic environment, which could be used to reflect the potential of in situ MeHg production.

Role of cost-effective organic carbon substrates in bioremediation of acid mine drainage-impacted soil of Malanjkhand Copper Project, India: a biostimulant for autochthonous microbial populations.

Development of an efficient bioremediation strategy for the mitigation of low pH (3.21), high dissolved SO42- (6285 mg/L), and Fe (7292 mg/kg)-rich acid mine drainage-impacted soil (AIS) was investigated through amendment of readily available organic carbon substrates (rice husk, compost, leaf litter, and grass clippings). An organic carbon mixture (OCM) formulated by mixing the test substrates was used to biostimulate microbial processes (SO42-/Fe3+reduction) necessary for efficient attenuation of the hazards imposed by AIS. OCM amendment in calcium carbonate-treated AIS enhanced reductive processes and removed dissolved SO42- and Fe3+ considerably raising the pH close to neutrality. 16S rRNA gene amplicon sequencing performed with total DNA and RNA elucidated the microbial population dynamics of treated AIS. Metabolically active populations comprised of fermentative (Clostridium sensu stricto 1 and Fonticella), iron-reducing (Acidocella, Anaeromyxobacter, and Clostridium sensu stricto 1), and sulfate-reducing (Desulfovibrio, Desulfotomaculum, Desulfosporosinus, and Desulfobacteraceae) bacteria. Microbial guilds obtained highlighted the synergistic role of cellulolytic, fermentative, and SO42-/Fe3+-reducing bacteria in attenuation of hazardous contaminants. Quantitative PCR analysis well supported the role of OCM in stimulating the indigenous bacterial populations, including those harboring the dissimilatory sulfite reductase (dsrB) gene and involved actively in SO42- reduction. The study demonstrated the suitability of locally available organic substrates as a low-cost and efficient biostimulation agentfor in situ bioremediation of acid mine drainage (AMD)-impacted soil system.

Arsenic mobilization in a high arsenic groundwater revealed by metagenomic and Geochip analyses

Microbial metabolisms of arsenic, iron, sulfur, nitrogen and organic matter play important roles in arsenic mobilization in aquifer. In this study, microbial community composition and functional potentials in a high arsenic groundwater were investigated using integrated techniques of RNA- and DNA-based 16S rRNA gene sequencing, metagenomic sequencing and functional gene arrays. 16S rRNA gene sequencing showed the sample was dominated by members of Proteobacteria (62.3–75.2%), such as genera of Simplicispira (5.7–6.7%), Pseudomonas (3.3–5.7%), Ferribacterium (1.6–4.4%), Solimonas (1.8–3.2%), Geobacter (0.8–2.2%) and Sediminibacterium (0.6–2.4%). Functional potential analyses indicated that organics degradation, assimilatory sulfate reduction, As-resistant pathway, iron reduction, ammonification, nitrogen fixation, denitrification and dissimilatory nitrate reduction to ammonia were prevalent. The composition and function of microbial community and reconstructed genome bins suggest that high level of arsenite in the groundwater may be attributed to arsenate release from iron oxides reductive dissolution by the iron-reducing bacteria, and subsequent arsenate reduction by ammonia-producing bacteria featuring ars operon. This study highlights the relationship between biogeochemical cycling of arsenic and nitrogen in groundwater, which potentially occur in other aquifers with high levels of ammonia and arsenic.