Iron-reducing Bacteria Research Articles

The coupling between iron and carbon and iron reducing bacteria control carbon sequestration in paddy soils

The coupling between iron (Fe) and carbon (C) and the activity of Fe reducing bacteria (FeRB) can strongly affect C sequestration in paddy soils. Although the geochemical cycling of C and Fe has been reported at individual sites, a lack of information exists across soil types and regions. In this study, we investigated the coupling of Fe-C and the role of FeRB across five contrasting paddy soils, which account for 60 % of China’s rice cultivation area: Haplic Solonchaks (SCh), Calcaric Fluvisols in Henan (FLc1) and in Tianjin (FLc2), Cumulic Anthrosols (ATc), and Eutric Cambisols (CMe). ATc and CMe were in oxidizing environments and contained significantly higher Fe (III), free Fe oxide (Fed), amorphous Fe oxide (Feo), and organic matter complexed iron oxides (Fep) than SCh, FLc1, and FLc2 which were in reducing environments with higher relative abundance of FeRB. Nonmetric multidimensional scale analysis (NMDS) demonstrated the variability of FeRB in different soil types (r = 0.928, P = 0.001). The higher Fe-soil organic carbon (Fe-SOC) in CMe (2.27 g kg−1; 2.72 g kg−1) and ATc (1.32 g kg−1; 1.6 g kg−1) was likely due to the complexation of Fe (III) and its oxides within SOC and minimal dissimilatory Fe reduction. The existence of the “iron gate” in ATc and CMe also revealed the inhibitory effect of low quantities of Fe (II) on phenol oxidase (PO) enzyme activity and SOC mineralization. According to canonical correspondence analysis (CCA), our study indicates the importance of DOC aromaticity as characterized by UV light absorption at 254 nm (SUV254) (16.6 %), Fed (10.9 %), Feo (9.2 %) to FeRB. Moreover, the structural equation model (SEM) further revealed that SOC and DOC were regulated by PO activity, Fe, and FeRB (χ2 = 7.002, P = 0.536). Our study highlights the importance of soil characteristics on Fe-C coupling in the biogeochemical cycle through interactions among Fe morphology and microorganisms via different oxidation–reduction conditions.

Coupled reduction of structural Fe(III) in nontronite and oxidation of petroleum hydrocarbons

Fe-bearing clay minerals are frequently present in oil reservoirs as potential electron acceptor. Compounds in crude oil may mediate microbial reduction of structural Fe(III) in clay minerals through their roles as electron donor and shuttle. However, there is limited knowledge about these roles and resulting transformation of oil compounds. In this study, bio-reduction of structural Fe(III) in nontronite (NAu-2, Fe-rich smectite) by the dissimilatory iron-reducing bacterium (DIRB) Shewanella putrefaciens CN32 was investigated in the presence of either crude oil or specific fractions of oil. Lactate was added in some experiments as extra electron donor. The fractions of saturates (nC14-nC20 alkanes and nC8-nC14-alkyl cyclohexanes), aromatics (with two/three benzene rings), and polar compounds (carboxylic acids) of crude oil were adsorbed to NAu-2 surface. Without CN32, crude oil was able to slightly reduce structural Fe(III) in NAu-2. In the presence of CN32, reduction of Fe(III) in NAu-2 was coupled with oxidation of bulk oil and saturated/aromatic fractions. The reduction extent (21.5 ± 0.3%) and rate (0.04 ± 0.00 mM/h) by the saturated oil fraction (nC17-nC30 alkanes and nC10-nC23-alkyl cyclohexanes as electron donors) were slightly lower than those by bulk oil (22.8 ± 0.4% and 0.05 ± 0.00 mM/h, respectively), but those by the aromatic fraction with two benzene rings were much higher (30.8 ± 0.2% and 0.15 ± 0.00 mM/h, respectively). Fe(III) bio-reduction was coupled with oxidative transformation of oil compounds. Specifically, n-alkanes and alkyl cyclohexanes with shorter carbon chains and aromatic isomers with fewer methyl groups were more easily degraded (i.e., dibenzothiophene series) and/or desorbed (i.e., phenanthrene series) than n-alkanes and alkyl cyclohexanes with longer carbon chains and aromatic isomers with more methyl groups. These hydrocarbons were degraded to oxygen-containing compounds, mainly saturated fatty acids, monocyclic naphthenic acids, and phenols. In the presence of lactate as extra electron donor, the aromatic fraction significantly promoted the Fe(III) bio-reduction extent and rate, suggesting its role as electron shuttle. The oxidative transformation of oil compounds driven by bio-reduction of clay minerals has broad implications for the coupled biogeochemical cycles of carbon and iron in oil reservoirs and organic-rich sedimentary rocks.

Long-term effects of soluble and insoluble ferric irons on anaerobic oxidation of methane in paddy soil

Iron-dependent anaerobic oxidation of methane (Fe-AOM) is an important process to reduce methane emissions into the atmosphere. It is well known that iron bioavailability largely influences microbial iron reduction, but the long-term effects of different ferric irons on soil Fe-AOM remain unknown. In this work, paddy soil in the ferruginous zone was collected and inoculated with insoluble ferrihydrite and soluble EDTA-Fe(III) for 420 days. Stable isotope experiments, activity inhibition tests, and molecular biological techniques were performed to reveal the activity, microbial community, and possible mechanism of paddy soil Fe-AOM. The results showed that ferrihydrite was a better electron acceptor for long-term Fe-AOM cultivation. Although EDTA-Fe(III) is highly bioavailable and could stimulate Fe-AOM activity for a short time, it restricted the activity increase in the long term. The abundances of archaea, iron-reducing bacteria (IRB), and gene mcrA largely increased after cultivation, indicating the important roles of mcrA-carrying archaea and IRB. Remarkably, archaeal communities were similar, but bacteria were totally different with different ferric irons. The results of the microbial community and activity inhibition suggested that Fe-AOM was performed likely by the cooperation between archaea (Methanomassiliicoccaceae or pGrfC26) and IRB in the cultures.

Methylmercury formation in biofilms of Geobacter sulfurreducens.

Mercury (Hg) is a major environmental pollutant that accumulates in biota predominantly in the form of methylmercury (MeHg). Surface-associated microbial communities (biofilms) represent an important source of MeHg in natural aquatic systems. In this work, we report MeHg formation in biofilms of the iron-reducing bacterium Geobacter sulfurreducens. Biofilms were prepared in media with varied nutrient load for 3, 5, or 7 days, and their structural properties were characterized using confocal laser scanning microscopy, cryo-scanning electron microscopy and Fourier-transform infrared spectroscopy. Biofilms cultivated for 3 days with vitamins in the medium had the highest surface coverage, and they also contained abundant extracellular matrix. Using 3 and 7-days-old biofilms, we demonstrate that G. sulfurreducens biofilms prepared in media with various nutrient load produce MeHg, of which a significant portion is released to the surrounding medium. The Hg methylation rate constant determined in 6-h assays in a low-nutrient assay medium with 3-days-old biofilms was 3.9 ± 2.0 ∙ 10-14 L ∙ cell-1 ∙ h-1, which is three to five times lower than the rates found in assays with planktonic cultures of G. sulfurreducens in this and previous studies. The fraction of MeHg of total Hg within the biofilms was, however, remarkably high (close to 50%), and medium/biofilm partitioning of inorganic Hg (Hg(II)) indicated low accumulation of Hg(II) in biofilms. These findings suggest a high Hg(II) methylation capacity of G. sulfurreducens biofilms and that Hg(II) transfer to the biofilm is the rate-limiting step for MeHg formation in this systems.

Metabolic turnover of cysteine-related thiol compounds at environmentally relevant concentrations by Geobacter sulfurreducens.

Low-molecular-mass (LMM) thiol compounds are known to be important for many biological processes in various organisms but LMM thiols are understudied in anaerobic bacteria. In this work, we examined the production and turnover of nanomolar concentrations of LMM thiols with a chemical structure related to cysteine by the model iron-reducing bacterium Geobacter sulfurreducens. Our results show that G. sulfurreducens tightly controls the production, excretion and intracellular concentration of thiols depending on cellular growth state and external conditions. The production and cellular export of endogenous cysteine was coupled to the extracellular supply of Fe(II), suggesting that cysteine excretion may play a role in cellular trafficking to iron proteins. Addition of excess exogenous cysteine resulted in a rapid and extensive conversion of cysteine to penicillamine by the cells. Experiments with added isotopically labeled cysteine confirmed that penicillamine was formed by a dimethylation of the C-3 atom of cysteine and not via indirect metabolic responses to cysteine exposure. This is the first report of de novo metabolic synthesis of this compound. Penicillamine formation increased with external exposure to cysteine but the compound did not accumulate intracellularly, which may suggest that it is part of G. sulfurreducens' metabolic strategy to maintain cysteine homeostasis. Our findings highlight and expand on processes mediating homeostasis of cysteine-like LMM thiols in strict anaerobic bacteria. The formation of penicillamine is particularly noteworthy and this compound warrants more attention in microbial metabolism studies.

Nitrogen and carbon removal from anaerobic digester effluents with low carbon to nitrogen ratios under feammox conditions

Removal of nitrogen and carbon from anaerobic digester (AD) effluents is challenging for currently available technologies. Herein, effective treatment for real AD effluents was achieved via the feammox process by using a Multistage Feammox Bioreactor (MSFB). The reactor achieved the best performance with AD effluent of a low carbon to nitrogen (C/N) ratio of 2.5. A 6-day retention time reached removal efficiencies for NH4+ and COD at 99 % and 97 %, respectively, with a thorough conversion of NH4+ to N2. Accordingly, the MSFB achieved removal rates for N and C of 14 and 34 mg L−1 d−1, respectively. The C/N ratio of 2.5 is regarded to be the critical point above which the feammox is shifted to conventional iron reduction with organic carbon. Iron-reducing bacteria of the γ- Proteobacteria (Pseudomonas and Acinetobacter), and δ- Proteobacteria (Geobacter) were dominant in the MSFB and were supposed to drive the feammox process.

Genetic and metabolic engineering of Methanococcus spp

Methanococcus is a genus of well-characterized mesophilic and hydrogenotrophic methanogens with a robust genetic system. Commonly found in tidal marshes, they compete with sulfate- and iron-reducing bacteria for hydrogen gas (H2) and formate, their two major energy sources. They are characterized by rapid growth in mineral medium and requirements for marine levels of salts for optimal growth. Genetic tools developed over the last twenty years include improvements in shuttle vectors for production of recombinant proteins and CRISPR systems for mutagenesis as well as a variety of marker proteins for studies of gene expression. These tools have enabled functional characterization of the genome, including regulation of the transcriptome and proteome, mapping of transcription start and termination sites, and identification of essential genes. Metabolic and gene regulatory network models have been created. This foundation in the basic biology of Methanococcus provides an opportunity for development of biotechnological applications of these hydrogenotrophic methanogens.

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

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

Enhanced removal of organic, nutrients, and PFCs in the iron-carbon micro-electrolysis constructed wetlands: Mechanism and iron cycle

The poor removal capacity of the wastewater treatment plants (WWTPs) on perfluorinated compounds (PFCs) results in the desirability of advanced treatment. Therefore, this study mainly discusses the enhanced removal mechanism of iron-carbon constructed wetlands on chemical oxygen demand (COD), phosphorus, nitrogen, perfluorooctanoic acid (PFOA), and perfluorobutanesulfonic acid (PFBS). The results showed that the removal efficiencies of most pollutants increased with the extension of hydraulic retention time (HRT) (1d, 2d, 3d). During the 3-day HRT, the average removal efficiencies in F1 (iron-carbon wetland, 100 μg/L PFCs) for COD, total phosphorus (TP), total nitrogen (TN), PFOA, and PFBS were 61.88 %, 67.92 %, 58.26 %, 78.12 %, and 80.60 %, respectively. Compared with C1 (common gravel wetland, 100 μg/L PFCs), the corresponding removal efficiencies increased by 4.68 %, 24.53 %, 10.07 %, 16.52 %, and 13.16 %, respectively. However, the removal effect of F2 (iron-carbon wetland, 200 μg/L PFCs) became worse due to the high concentration of PFCs. Further study found that the efficient removal of phosphorus in F1 was the precipitation and coagulation of Fe(III) and Fe(II). Meanwhile, the removal of nitrogen mainly relied on autotrophic and heterotrophic denitrification. Simultaneously, the combination with Feammox (the process of anaerobic ammonium oxidation coupled to iron reduction) and other pathways promoted nitrogen removal and iron cycling in F1. Further mass balance calculation of PFOA and PFBS in iron-carbon wetland found that among all matrixes (coarse sand, gravel, and iron-carbon), the contribution rate of iron-carbon to the removal PFCs was the highest. In addition, the enrichment of microorganisms related to Fe(II)-dependent autotrophic denitrification and the cultivation of iron-reducing bacteria reconfirmed the iron cycle.

Biogeochemical Factors of Cs, Sr, U, Pu Immobilization in Bottom Sediments of the Upa River, Located in the Zone of Chernobyl Accident.

Laboratory modeling of Cs, Sr, U, Pu immobilization by phytoplankton of the river Upa, affected after the Chernobyl accident, has been carried out. Certain conditions are selected for strong fixation of radionuclides in bottom sediments due to biogeochemical processes. The process of radionuclide removal from the water phase via precipitation was based on their accumulation by phytoplankton, stimulated by nitrogen and phosphorus sources. After eight days of stimulation, planktonic phototrophic biomass, dominated by cyanobacteria of the genus Planktothrix, appears in the water sample. The effectiveness of U, Pu and Sr purification via their transfer to bottom sediment was observed within one month. The addition of ammonium sulfate and phosphate (Ammophos) led to the activation of sulfate- and iron-reducing bacteria of the genera Desulfobacterota, Desulfotomaculum, Desulfosporomusa, Desulfosporosinus, Thermodesulfobium, Thiomonas, Thiobacillus, Sulfuritallea, Pseudomonas, which form sulphide ferrous precipitates such as pyrite, wurtzite, hydrotroillite, etc., in anaerobic bottom sediments. The biogenic mineral composition of the sediments obtained under laboratory conditions was verified via thermodynamic modeling.

The intensity of biofilm formation by heterotrophic bacteria isolated from soil ferrosphere

The intensity of biofilm formation by heterotrophic bacteria possessed ammonifying ability (Bacillus simplex ChNPU F1, Streptomyces canus NUChC F2, Streptomyces gardneri ChNPU F3), ammonifying and iron-reducing ability (Fictibacillus sp. ChNPU ZVB1) previously isolated from soil ferrosphere was studied. Methods used: indirect measurement of the biomass of the bacterial biofilm using the adsorption/desorption of crystal violet, the aggregation test (to determine aggregation properties), the salt aggregation test (to determine hydrophobicity). The correlation analysis between the intensity of biofilm formation and aggregation of strains showed a significant positive correlation. The studied strains of microorganisms did not show high adhesive properties, they were moderately-adhesive (B. simplex, S. canus and S. gardneri) and weakly-adhesive (Fictibacillus sp.). It is supposed that the role these bacteria in the microbial damage of materials is determined preferably by bioelectrochemical reactions (iron-reducing bacteria) and the production of corrosive and/or antimicrobial metabolites (ammonifying and iron-reducing bacteria), but not by the biofilms formation. The prospect of further research is to analyze the antagonistic properties and biofilm formation of heterotrophic bacteria under co-cultivation conditions, in particular, with sulfate-reducing bacteria.

Promising antimicrobial protein from <em>Klebsiella</em>

Increasing antibody resistance necessitates the use of new antimicrobial products. Antimicrobial peptides (AMP`s) And Proteins are a promising alternative to antibiotics currently being used due to their high degree of specificity & wide range of action Here we report a novel antimicrobial protein from iron reducing bacteria isolated from the coffee estate soils of Coorg district, Karnataka, India. Iron reducing bacteria are known to be antagonistic to others due to the presence of Pseuderophores. It makes them a good source of antimicrobial active principles. Fifteen isolates were obtained by selectively screening for iron reduction. These isolates were tested for antimicrobial activity against Gram positive [Staphylococcus aureus (MTCC3160), Bacillus cereus (MTCC6620)] & Gram negative [Escherichia coli (MTCC1650), Salmonella typhi (MTCC3224) and Pseudomonas aeruginosa (MTCC2453)] bacteria. Of the 15 isolates, one isolate, CBI-8, which showed good antimicrobial activity, was selected for further studies. The isolate was identified by MALDI-TOF & 16S rDNA sequencing as Klebsiella variicola/quasipneumonae (strain IS93) (genebank - MN814029). To induce and augment the production of antimicrobial molecules, CBI-8 was grown in the presence of Salmonella typhi and the spent medium was subjected to ammonium sulphate precipitation. The fractions which showed promising activity were further subjected to preparatory HPLC. The distinct bands obtained in the SDS-PAGE corresponding to the HPLC peak that showed antimicrobial activity from the induced sample was analyzed by Mass Spectrometry. Antimicrobial Peptide Database (APD3) calculator, Collection of Anti-Microbial Peptides (CAMPR3) software used to predict AMPs in the proteins identified from MS data.

Phenolic root exudates enhance Avicennia marina tolerance to cadmium under the mediation of functional bacteria in mangrove sediments.

This study was carried out to demonstrate the mechanism of phenolic root exudates affecting microbial-mediated cadmium (Cd) speciation transformation thus enhancing the Avicennia marina tolerance to Cd. A rhizo-box experiment was conducted including eight treatments with four Cd levels (0, 1, 2, and 4mgCdkg-1) and two phenol levels (0, 15mgkg-1). The results showed that the addition of phenols increased the pH, reduced the number of iron-reducing bacteria (IRB) and sulfur-oxidizing bacteria (SOB) in the rhizosphere sediments, meanwhile promoted the transformation of Cd to low activity speciation. Furthermore, the sulfate accumulation and synthesis of flavonoid phenols in plants were also enhanced. The results indicated that phenolic root exudates inhibit functional bacteria-mediated Fe and S cycles and promote the immobilization of Cd in the sediments. In conclusion, the mitigation of Cd phytotoxicity induced by phenolic root exudates enhanced the Cd tolerance of A. marina.

Microbial reduction of schwertmannite by co-cultured iron- and sulfate-reducing bacteria

Schwertmannite (Sch) is an iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment. The transformation mechanism of Sch mediated by pure cultured iron-reducing bacteria (FeRB) or sulfate-reducing bacteria (SRB) has been studied. However, FeRB and SRB widely coexist in the environment, the mechanism of Sch transformation by the consortia of FeRB and SRB is still unclear. This study investigated the Sch reduction by co-cultured Shewanella oneidensis (FeRB) and Desulfosporosinus meridiei (SRB). The results showed that co-culture of FeRB and SRB could accelerate the reductive dissolution of Sch, but not synergistically, and there were two distinct phases in the reduction of Sch mediated by FeRB and SRB: an initial phase in which FeRB predominated and Fe3+ in Sch was reduced, accompanied with the release of SO42−, and the detected secondary minerals were mainly vivianite; the second phase in which SRB predominated and mediated the reduction of SO42−, producing minerals including mackinawite and siderite in addition to vivianite. Compared to pure culture, the abundance of FeRB and SRB in the consortia decreased, and more minerals aggregated inside and outside the cell; correspondingly, the transcription levels of genes (cymA, omcA, and mtrCBA) related to Fe3+ reduction in co-culture was down-regulated, while the transcription levels of SO42−-reducing genes (sat, aprAB, dsr(C)) was generally up-regulated. These phenomena suggested that secondary minerals produced in co-culture limited but did not inhibit bacterial growth, and the presence of SRB was detrimental to dissimilatory Fe3+ reduction, while existed FeRB was in favor of dissimilatory SO42− reduction. SRB mediated SO42− reduction by up-regulating the expression of SO42− reduction-related genes when its abundance was limited, which may be a strategy to cope with external coercion. These findings allow for a better understanding of the process and mechanism of microbial mediated reduction of Sch in the environment.

Microbial community characteristics of cadmium speciation transformation in soil after iron-based materials application

Although numerous studies on the stabilization of heavy metals in contaminated soil with iron-based materials have been conducted, the role of microorganisms in iron species and Cd speciation transformation has not been fully elucidated. Here, 2-line ferrihydrite (MGE) and goethite (GE) were prepared and used to stabilize Cd in contaminated soil. Based on the microbial community characteristics in the early, middle, and later stages, the role of microorganisms in Cd speciation transformation was explored. The exchangeable Cd decreased by 10.73 %–51.31 %, while the iron‑manganese oxide-bound speciation, and the residual speciation increased after MGE/GE application. The relative abundance of Proteobacteria, and Sphingomonas were obviously increased, and Trichoderma and Mortierella were decreased in the MGE group. The outcome in the GE group was the opposite. The complexity of the bacterial and fungal community network was increased in the MGE and GE groups. The lignin in MGE could stimulate the growth of iron-reducing bacteria (FeRB) and promote the reduction of iron minerals to Fe2+. The Fe2+ in the soil was oxidized and hydrolysed, which might lead to an increase in the soil pH. During the process of iron species transformation and iron redox mediated by FeRB, Cd was transformed to more stable speciation.

Effect of Pipe Materials on Bacterial Community, Redox Reaction, and Functional Genes

In the present study, the effect of pipe materials on water quality as well as the microbial community was researched with static devices as well as dynamic ones. Five kinds of pipe materials (SP: steel plastic composite pipe, SS: stainless steel pipe, DI: ductile iron pipe, CI: cast iron pipe, GS: galvanized steel pipe) were chosen, and the soaking experiment was carried out with bench-scale devices. To further investigate the performance of pipe materials over a long term, a pilot-scale simulated drinking water distribution system was constructed, and the water quality parameters were monitored for six months. The pipe materials were ranked as SP, DI, and CI by the order of increasing turbidity, CODMn, and NH3-N. Furthermore, the biofilm samples were analyzed via pyrosequencing and COG functional categories. The DI biofilm possessed the highest bacterial diversity with a Shannon index of 3.56, followed by SP (3.14) and CI (0.77). The presence of nitrate-reducing bacteria (NRB), iron-oxidizing bacteria (IOB), iron-reducing bacteria (IRB), and sulfate-reducing bacteria (SRB)was identified, and NRB composed the largest share in all pipe materials (13.0%–17.2%), with other redox bacteria making up a minor proportion (0.02%–1.52%). NRB and IRB inhibited the corrosion process while IOB and SRB enhanced it. Most dominant genera present in samples were derived firstly from soil or active sludge, indicating a turbidity problem due to soil contamination in the distribution network.

Heat-assisted potassium ferrate pretreatment enhancing short-chain fatty acids production from waste activated sludge: Performance and mechanisms

This work presented an innovative strategy for short-chain fatty acids (SCFAs) production from anaerobic waste activated sludge (WAS) fermentation via heat assisted potassium ferrate (PF) pretreatment. Results showed that under optimal pretreatment condition (55 °C + 0.1 g PF/g TSS), SCFAs production achieved 4068.4 mg COD/L with acetic acid reaching 1766.1 mg COD/L, significantly higher than those of the control (77.39 mg COD/L, only acetic acid contained) as well as individual heat (2930.8 and 1203.5 mg COD/L) and individual 0.1 g PF/g TSS (1970.1 and 982.7 mg COD/L) pretreatment groups. Mechanism analysis revealed that heat-assisted PF pretreatment aggravated WAS disintegration by facilitating the disruption of EPS and cells, hence accelerating organics release from sludge flocs to the supernatant. Besides, the biodegradability of released substances was also improved thereby providing quantities of directly available substrates for hydrolysis and acidification process. Analysis of key enzymes involved in SCFAs accumulation showed that heat-assisted PF pretreatment promoted the relative bioactivities of protease, butyrate kinase (BK) and acetate kinase (AK) by 51.6%, 39.2% and 155.6% compared to the control respectively, while suppressing the bioactivity of coenzyme F420. Microbial community analysis demonstrated the enrichment of Proteobacteria and Firmicutes, which were related to SCFAs accumulation. Meanwhile, in genus level, hydrolytic bacteria (e.g., Acinetobacter sp.), acidification bacteria (e.g., Petrimonas sp.) and iron-reducing bacteria (e.g., Romboutsia sp.) were enriched while SCFAs consuming bacteria (e.g., Candidatus_Competibacter sp.) were inhibited.

Remediation of arsenic-containing ferrihydrite in soil using iron- and sulfate-reducing bacteria: Implications for microbially-assisted clean technology

The iron-reducing bacterium (IRB) Shewanella oneidensis MR-1 and a sulfate-reducing bacterium (SRB) Desulfovibrio vlugaris miyazaki, were used in varying sequences for batch and column arsenic extraction experiments to assess their use in alleviating arsenic pollution from arsenic-containing ferrihydrite in soil. Both batch (four conditions) and column experiments show variations in biological sequence affect the arsenic release and species. The results of arsenic release from batch conditions are: IRB-SRB (2.28 mg/g) > SRB-IRB (1.72 mg/g) > SRB+IRB (0.55 mg/g) > Control group (pure water, 0.04 mg/g). Further cycle operations from batch experiments showed that IRB-SRB operation always obtained the best performance of releasing arsenic. The column experiments achieved greater arsenic release: SRB+IRB (10.87 mg/g) > IRB-SRB (7.54 mg/g) > SRB-IRB (6.48 mg/g) relative to arsenic in the pristine samples (36.6 mg/g). Flow methods could obtain more releasing performance than batch experiment. The hydraulic conductivity in the column experiments was best in the SRB-IRB operation (average water difference: 0.39 psi for SRB-IRB, 0.86 psi for IRB-SRB and 1.07 psi for SRB+IRB). After the biological operation, chemical treatment (0.5 M, (NH4)2HPO4) only desorbed 11.14–39.81% of the total arsenic quantity of each operation. Reductive dissolution is the main driving reason for arsenic release, though the formation of FeS was found to hinder arsenic release. Analysis of the solids showed that the SRB+IRB operation results in 58.07% arsenic released. These results suggest that microbially-assisted reduction using iron- and sulfate-reducing bacteria may be an effective technology for remediating arsenic-bearing soil e.g. paddy soil using systems such as an in-situ injection and pump.

Geobacter sulfurreducens' Unique Metabolism Results in Cells with a High Iron and Lipid Content.

Geobacter sulfurreducens is a ubiquitous iron-reducing bacterium in soils, and in engineered systems, it can respire an electrode to produce measurable electric current. Its unique metabolism, heavily dependent on an extensive network of cytochromes, requires a unique cell composition. In this work, we used metallomics, cell fraction and elemental analyses, and transcriptomics to study and analyze the cell composition of G. sulfurreducens. Elemental composition studies (C, H, O, N, and ash content) showed high C:O and H:O ratios of approximately 1.7:1 and 0.25:1, indicative of more reduced cell composition that is consistent with high lipid content. Our study shows that G. sulfurreducens cells have a large amount of iron (2 ± 0.2 μg/g dry weight) and lipids (32 ± 0.5% dry weight/dry weight) and that this composition does not change whether the cells are grown with a soluble or an insoluble electron acceptor. The high iron concentration, higher than similar microorganisms, is attributed to the production of cytochromes that are abundant in transcriptomic analyses in both solid and soluble electron acceptor growth. The unique cell composition of G. sulfurreducens must be considered when growing this microorganism for lab studies and commercial applications. IMPORTANCE Geobacter sulfurreducens is an electroactive microorganism. In nature, it grows on metallic minerals by transferring electrons to them, effectively "breathing" metals. In a manmade system, it respires an electrode to produce an electric current. It has become a model organism for the study of electroactive organisms. There are potential biotechnological applications of an organism that can bridge the gap between biology and electrical signal and, as a ubiquitous iron reducer in soils around the world, G. sulfurreducens has an impact on the global iron cycle. We measured the concentrations of metals, macromolecules, and basic elements in G. sulfurreducens to define this organism's composition. We also used gene expression data to discuss which proteins those metals could be associated with. We found that G. sulfurreducens has a large amount of lipid and iron compared to other bacteria-these observations are important for future microbiologists and biotechnologists working with the organism.

Continuously feeding fenton sludge into anaerobic digesters: Iron species change and operating stability

Fenton sludge generated from the Fenton process contains a large number of ferric species and organic pollutants, which need to be properly treated before discharge. In this study, Fenton sludge as an Fe(III) source for dissimilatory iron reduction (DIR) was continuously added with increasing dosage into an anaerobic digester to enhance the treatment. Results showed continuously feeding Fenton sludge to the anaerobic digester did not deteriorate the performance and increased methane production and COD removal rate by 2.2 folds and 14.0%, respectively. The Fe content of sludge in the digester increased from 40.25 mg/g (dry weight) to 131.53 mg/g after continuously feeding for 77days, and then declined to 109.17 mg/g when the feeding was stopped. Mass balance analysis showed that 20.5 to 48.4% of Fe in the Fenton sludge was released to the effluent. After experiment, the ratio of reducible Fe species to the total Fe was 75.1%, which maintained the high activity in DIR. Microbial community analysis showed that iron-reducing bacteria were enriched with the addition of Fenton sludge and the sludge in the digester had a higher conductivity and capacitance to strengthen the electron transfer of DIR. All results suggested that feeding Fenton sludge into anaerobic digesters was a feasible method to dispose of Fenton sludge as well as to enhance the performance of anaerobic digestion.