Characterization of Fe(III)-reducing enrichment cultures and isolation of Fe(III)-reducing bacteria from the Savannah River site, South Carolina

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

Uranium dioxide (UO₂) and metaschoepite (UO₃•nH₂O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and μ-focus X-ray spectroscopy. In the UO₂ lysimeter, oxidative dissolution of UO₂ and subsequent migration of U was observed over 1-2 cm in the direction of waterflow and against it. Sequential extractions of the UO₂ sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

Volatile fatty acids changed the microbial community during feammox in coastal saline-alkaline paddy soil

In order to indicate the effect of volatile fatty acids (VFAs) on the characteristics of feammox and dissimilatory iron reducing bacteria (DIRB) in paddy soils, different VFAs were selected with paddy soils for anaerobic cultivation. Five treatments were set up, respectively, only adding N and both adding N and C (formate + NH₄⁺ (Fo-N), acetate + NH₄⁺ (Ac-N), propionate + NH₄⁺ (Pr-N), and butyrate + NH₄⁺ (Bu-N)) treatments. The concentration of Fe(II), Fe(III), NH₄⁺, and VFAs was assessed within 45 d, and the bacterial community was determined after cultivation. The oxidation rates of NH₄⁺ were the highest in N treatment, while it was the lowest in Fo-N treatment. Under the four C treatments, the consumption of NH₄⁺ and Fe(III) was the fastest in Pr-N treatment, which was consumed by 31.2% and 76.3%, respectively. Different VFAs selected for distinct DIRB. Compared with N treatment, Ac-N and Bu-N treatment increased the relative abundance of DIRB, such as Geobacter and Clostridia, which increased the consumption of VFAs during incubation. Overall, VFAs, especially formate, could promote Fe(III) reduction and compete with the feammox process for the electron acceptors to decrease the feammox reaction, and prohibited soil NH₄⁺ loss. Therefore, VFAs, which was released from organic fertilizer, could reduce NH₄⁺ loss in feammox process of saline-alkaline paddy soils.

Review on corrosion and corrosion scale formation upon unlined cast iron pipes in drinking water distribution systems

The qualified finished water from water treatment plants (WTPs) may become discolored and deteriorated during transportation in drinking water distribution systems (DWDSs), which affected tap water quality seriously. This water stability problem often occurs due to pipe corrosion and the destabilization of corrosion scales. This paper provides a comprehensive review of pipe corrosion in DWDSs, including corrosion process, corrosion scale formation, influencing factors and monitoring technologies utilized in DWDSs. In terms of corrosion process, corrosion occurrence, development mechanisms, currently applied assays, and indices used to determine the corrosion possibility are summarized, as well as the chemical and bacterial influences. In terms of scale formation, explanations for the nature of corrosion and scale formation mechanisms are discussed and its typical multilayered structure is illustrated. Furthermore, the influences of water quality and microbial activity on scale transformation are comprehensively discussed. Corrosion-related bacteria at the genus level and their associated corrosion mechanism are also summarized. This review helps deepen the current understanding of pipe corrosion and scale formation in DWDSs, providing guidance for water supply utilities to ensure effective measures to maintain water quality stability and guarantee drinking water safety.

Direct and Indirect Reduction of Cr(VI) by Fermentative Fe(III)-Reducing Cellulomonas sp. Strain Cellu-2a

Hexavalent chromium (Cr(VI)) is recognized to be carcinogenic and toxic and registered as a contaminant in many drinking water regulations. It occurs naturally and is also produced by industrial processes. The reduction of Cr(VI) to Cr(III) has been a central topic for chromium remediation since Cr(III) is less toxic and less mobile. In this study, fermentative Fe(III)-reducing bacterial strains (Cellu-2a, Cellu-5a, and Cellu-5b) were isolated from a groundwater sample and were phylogenetically related to species of Cellulomonas by 16S rRNA gene analysis. One selected strain, Cellu-2a showed its capacity of reduction of both soluble iron (ferric citrate) and solid iron (hydrous ferric oxide, HFO), as well as aqueous Cr(VI). The strain Cellu-2a was able to reduce 15 μM Cr(VI) directly with glucose or sucrose as a sole carbon source under the anaerobic condition and indirectly with one of the substrates and HFO in the same incubations. The heterogeneous reduction of Cr(VI) by the surface-associated reduced iron from HFO by Cellu-2a likely assisted the Cr(VI) reduction. Fermentative features such as large-scale cell growth may impose advantages on the application of bacterial Cr(VI) reduction over anaerobic respiratory reduction.

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

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

Nickel ferrite/MXene-coated carbon felt anodes for enhanced microbial fuel cell performance

In recent years, the modification of electrode materials for enhancing the power generation of microbial fuel cells (MFCs) has attracted considerable attention. In this study, a conventional carbon felt (CF) electrode was modified by NiFe₂O₄ (NiFe₂O₄@CF), MXene (MXene@CF), and NiFe₂O₄-MXene (NiFe₂O₄-MXene@CF) using facile dip-and-dry and hydrothermal methods. In these modified CF electrodes, the electrochemical performance considerably improved, while the highest power density (1385 mW/m²), which was 5.6, 2.8, and 1.4 times higher than those of CF, NiFe₂O₄@CF, and MXene@CF anodes, respectively, was achieved using NiFe₂O₄-MXene@CF. Furthermore, electrochemical impedance spectroscopy and cyclic voltammetry results confirmed the superior bioelectrochemical activity of a NiFe₂O₄-MXene@CF anode in a MFC. The improved performance could be attributed to the low charge transfer resistance, high conductivity and number of catalytically active sites of the NiFe₂O₄-MXene@CF anode. Microbial community analysis demonstrated the relative abundance of electroactive bacteria on a NiFe₂O₄-MXene@CF anodic biofilm rather than CF, MXene@CF, and NiFe₂O₄@CF anodes. Therefore, these results suggest that combining the favorable properties of composite materials such as NiFe₂O₄-MXene@CF anodes can open up new directions for fabricating novel electrodes for renewable energy-related applications.

Shifts in short-chain fatty acid profile, Fe(III) reduction and bacterial community with biochar amendment in rice paddy soil

Biochar, a valuable product from the pyrolysis of agricultural and forestry residues, has been widely applied as soil amendment. However, the effect of different types of biochar on soil microorganisms and associated biochemical processes in paddy soil remains ambiguous. In this study, we investigated the impact of biochars derived from different feedstocks (rice straw, orange peel and bamboo powder) on the dynamics of short-chain fatty acids (SCFAs), iron concentration and bacterial community in paddy soil within 90 days of anaerobic incubation. Results showed that biochar amendment overall inhibited the accumulation of SCFAs while accelerating the Fe(III) reduction process in paddy soil. In addition, 16S rRNA gene sequencing results demonstrated that the α-diversity of the bacterial community significantly decreased in response to biochar amendments at day 1 but was relatively unaffected at the end of incubation, and incubation time was the major driver for the succession of the bacterial community. Furthermore, significant correlations between parameters (e.g. SCFAs and iron concentration) and bacterial taxa (e.g. Clostridia, Syntrophus, Syntrophobacter and Desulfatiglans) were observed. Overall, our findings demonstrated amendment with different types of biochar altered SCFA profile, Fe(III) reduction and bacterial biodiversity in rice paddy soil.

Characterization and application of an anaerobic, iron and sulfate reducing bacterial culture in enhanced bioremediation of acid mine drainage impacted soil

Development of an appropriate bioremediation strategy for acid mine drainage (AMD) impacted environment is imperative for sustainable mining but remained critically challenged due to the paucity of knowledge on desired microbiological factors and their nutrient requirements. The present study was conducted to utilize the potential of an anaerobic, acid-tolerant, Fe³⁺ and SO₄^2- reducing microbial consortium for in situ remediation of highly acidic (pH 3.21), SO₄^2- rich (6285 mg/L) mine drainage impacted soil (AIS). A microbial consortium enriched from AMD system and composed of Clostridiales and Bacillales members was characterized and tested for in situ application through microcosms. A combination of bioaugmentation (enriched consortium) and biostimulation (cellulose) allowed 97% reduction in dissolved sulfate and rise in pH up to 7.5. 16S rRNA gene-based amplicon sequencing confirmed that although the bioaugmented community could survive in AIS, availability of carbon source was necessary for superior iron- and sulfate- reduction. Quantitative PCR of dsrB gene confirmed the role of carbon source in boosting the SO₄^2- reduction activities of sulfate reducers. This study demonstrated that native AIS harbored limited catabolic activities required for the remediation but addition of catabolically active microbial populations along with necessary carbon and energy source facilitate the bioremediation of AIS.

Anaerobic Bioreduction of Jarosites and Biofilm Formation by a Natural Microbial Consortium

Jarosite occurs naturally in acid sulphate soils and is a common feature of streams impacted by acid mine drainage (AMD). Biological reduction of iron-sulphate minerals, such as jarosite, has the potential to contribute to the natural attenuation of acid mine drainage sites. The reduction of different jarosites (including minerals containing precious and toxic metals) by a natural bacterial/microbial consortium was examined in this study. Jarosites was used as a sole terminal electron acceptor via the reductive dissolution of Fe(III) minerals. The production of Fe(II) and the presence of sulphate-reducing bacteria in the consortium lead to the precipitation of metal sulphides immobilizing toxic heavy metals. Microbial attachment and biofilm formation of minerals have a great impact on the production and transformation of minerals and can influence the mobility of metals. After the adaptation to different jarosites, a unique specie was found: Desulfosporosinus orientis. Desulfosporosinus species are sulphate-reducing bacteria and can be found in sulphate-rich heavy metal-polluted environments, such as acid mine/rock drainage sites, being responsible for the sulphides formation. D. orientis is an obligate anaerobic microorganism and is able to reduce Fe(III) D. orientis is an obligate anaerobic microorganism and is able to reduce Fe(III). Confocal laser scanning microscopy and fluorescent lectin-binding analyses (FLBA) were used to study the arrangement and composition of the exopolysaccharides/glycoconjugates in biofilms indicating the presence of mannose, glucose, and N-acetylglucosamine residues. This study provides insights to understand the processes leading to the mobility or retention of metals in mine waste and industrial landfill environments.

Low-Abundance Members of the Firmicutes Facilitate Bioremediation of Soil Impacted by Highly Acidic Mine Drainage From the Malanjkhand Copper Project, India

Sulfate- and iron-reducing heterotrophic bacteria represented minor proportion of the indigenous microbial community of highly acidic, oligotrophic acid mine drainage (AMD), but they can be successfully stimulated for in situ bioremediation of an AMD impacted soil (AIS). These anaerobic microorganisms although played central role in sulfate- and metal-removal, they remained inactive in the AIS due to the paucity of organic carbon and extreme acidity of the local environment. The present study investigated the scope for increasing the abundance and activity of inhabitant sulfate- and iron-reducing bacterial populations of an AIS from Malanjkhand Copper Project. An AIS of pH 3.5, high soluble SO4 2- (7838 mg/l) and Fe (179 mg/l) content was amended with nutrients (cysteine and lactate). Thorough geochemical analysis, 16S rRNA gene amplicon sequencing and qPCR highlighted the intrinsic metabolic abilities of native bacteria in AMD bioremediation. Following 180 days incubation, the nutrient amended AIS showed marked increase in pH (to 6.6) and reduction in soluble -SO4 2- (95%), -Fe (50%) and other heavy metals. Concomitant to physicochemical changes a vivid shift in microbial community composition was observed. Members of the Firmicutes present as a minor group (1.5% of total community) in AIS emerged as the single most abundant taxon (∼56%) following nutrient amendments. Organisms affiliated to Clostridiaceae, Peptococcaceae, Veillonellaceae, Christensenellaceae, Lachnospiraceae, Bacillaceae, etc. known for their fermentative, iron and sulfate reducing abilities were prevailed in the amended samples. qPCR data corroborated with this change and further revealed an increase in abundance of dissimilatory sulfite reductase gene (dsrB) and specific bacterial taxa. Involvement of these enhanced populations in reductive processes was validated by further enrichments and growth in sulfate- and iron-reducing media. Amplicon sequencing of these enrichments confirmed growth of Firmicutes members and proved their sulfate- and iron-reduction abilities. This study provided a better insight on ecological perspective of Firmicutes members within the AMD impacted sites, particularly their involvement in sulfate- and iron-reduction processes, in situ pH management and bioremediation.

A seasonal study of a passive abandoned coalmine drainage remediation system reveals three distinct zones of contaminant levels and microbial communities

A passive remediation system that treats coalmine drainage was sampled to determine the impact seasonal changes had on water quality and microbial diversity. Every quarter for 1 year, water-soil slurries were collected at the influent of the 5 settling ponds and the wetlands, and the effluent of the system. The concentration of 12 metals and sulfate, as well as sequences from the V4 region of the bacterial 16S rrn genes were determined. The water quality analysis revealed high levels of iron and sulfate, and measurable levels of Al, Ba, Cu, Pb, Mn, Sr, and Zn. Iron increased 25-fold in the summer and spikes in metal concentrations were observed during several seasons in pond 3 and the wetlands. These spikes cannot be explained by abiotic chemical reactions in the neutral pH found in the pond. Based on contaminant levels and microbial community composition, our results indicate that there were 3 unique environments in the system (ponds 1 and 2; pond 3; pond 4 through the end) and that changes in contaminant levels and bacterial composition in these environments correlated with seasonal variation. Iron and sulfate are the most prevalent contaminants in the system. An examination of sequences from known iron- and sulfur-cycling bacteria demonstrated that there were more iron-reducing (IRB) bacterial sequences than iron-oxidizing (IOB) (137,912 IRB vs. 98,138 IOB), the two groups of bacteria were found mainly in the fall and winter samples, and were prevalent in different ponds. There were more sulfur/sulfide-oxidizing (SOB) bacterial sequences than sulfur/sulfate-reducing (SRB) bacterial sequences (72,978 SOB vs 30,504 SRB), they were found mainly in the fall and winter samples, and the sequences were mixed in ponds 4, 5 and the wetlands effluent. Iron is remediated in this system but sulfate is not.

Interactive effect of trivalent iron on activated sludge digestion and biofilm structure in attached growth reactor of waste tire rubber

Waste tire rubber (WTR) has been introduced as an alternative, novel media for biofilm development in several experimental systems including attached growth bioreactors. In this context, four laboratory-scale static batch bioreactors containing WTR as a support material for biofilm development were run under anoxic condition for 90 days using waste activated sludge as an inoculum under the influence of different concentrations (2.5, 6.5, 8.5 mg/l) of trivalent ferric iron (Fe³⁺). The data revealed that activated sludge with a Fe³⁺ concentration of 8.5 mg/l supported the maximum bacterial biomass [4.73E + 10 CFU/ml cm²]; besides, it removed 38% more Chemical oxygen demand compared to Fe³⁺ free condition from the reactor. Biochemical testing and 16S rDNA phylogenetic analysis of WTR-derived biofilm communities further suggested the role of varying concentrations of Fe³⁺ on the density and diversity of members of Enterobacteria(ceae), ammonium (AOB) and nitrite oxidizing bacteria. Furthermore, Fluorescent in situ hybridization with phylogenetic oligonucleotide probes and confocal laser scanning microscopy of WTR biofilms indicated a significant increase in density of eubacteria (3.00E + 01 to.05E + 02 cells/cm²) and beta proteobacteria (8.10E + 01 to 1.42E + 02 cells/cm²), respectively, with an increase in Fe³⁺ concentration in the reactors, whereas, the cell density of gamma proteobacteria in biofilms decreased.

Response of humic-reducing microorganisms to the redox properties of humic substance during composting

Humic substance (HS) could be utilized by humus-reducing microorganisms (HRMs) as the terminal acceptors. Meanwhile, the reduction of HS can support the microbial growth. This process would greatly affect the redox conversion of inorganic and organic pollutants. However, whether the redox properties of HS lined with HRMs community during composting still remain unclear. This study aimed to assess the relationships between the redox capability of HS [i.e. humic acids (HA) and fulvic acids (FA)] and HRMs during composting. The results showed that the changing patterns of electron accepting capacity and electron donating capacity of HS were diverse during seven composting. Electron transfer capacities (ETC) of HA was significantly correlated with the functional groups (i.e. alkyl C, O-alkyl C, aryl C, carboxylic C, aromatic C), aromaticity and molecular weight of HA. Aromatic C, phenols, aryl C, carboxylic C, aromaticity and molecular weight of HS were the main structuralfeatures associated with the ETC of FA. Ten key genera of HRMs were found significantly determine these redox-active functional groups of HS during composting, thus influencing the ETC of HS in composts. In addition, a regulating method was suggested to enhance the ETC of HS during composting based on the relationships between the key HRMs and redox-active functional groups as well as environmental variables.

Successions and diversity of humic-reducing microorganisms and their association with physical-chemical parameters during composting

Humic-reducing microorganisms (HRMs) could utilize humic substances (HS) as terminal electron mediator to promote the biodegradation of recalcitrant pollutants. However, the dynamics of HRMs during composting has not been explored. Here, high throughput sequencing technology was applied to investigate the patterns of HRMs during three composting systems. A total of 30 main genera of HRMs were identified in three composts, with Proteobacteria being the largest phylum. HRMs were detected with increased diversity and abundance and distinct patterns during composting, which were significantly associated with dissolved organic carbon, dissolved organic nitrogen and germination index. Regulating key physical-chemical parameters is a process control of HRMs community composition, thus promoting the redox capability of the compost. The redox capability of HRMs were strengthened during composting, suggesting that HRMs of the compost may play an important role on pollutant degradation of the compost or when they are applied to the contaminated soils.

Microbial functional genes enriched in the Xiangjiang River sediments with heavy metal contamination

Background

Xiangjiang River (Hunan, China) has been contaminated with heavy metal for several decades by surrounding factories. However, little is known about the influence of a gradient of heavy metal contamination on the diversity, structure of microbial functional gene in sediment. To deeply understand the impact of heavy metal contamination on microbial community, a comprehensive functional gene array (GeoChip 5.0) has been used to study the functional genes structure, composition, diversity and metabolic potential of microbial community from three heavy metal polluted sites of Xiangjiang River.

Results

A total of 25595 functional genes involved in different biogeochemical processes have been detected in three sites, and different diversities and structures of microbial functional genes were observed. The analysis of gene overlapping, unique genes, and various diversity indices indicated a significant correlation between the level of heavy metal contamination and the functional diversity. Plentiful resistant genes related to various metal were detected, such as copper, arsenic, chromium and mercury. The results indicated a significantly higher abundance of genes involved in metal resistance including sulfate reduction genes (dsr) in studied site with most serious heavy metal contamination, such as cueo, mer, metc, merb, tehb and terc gene. With regard to the relationship between the environmental variables and microbial functional structure, S, Cu, Cd, Hg and Cr were the dominating factor shaping the microbial distribution pattern in three sites.

Conclusions

This study suggests that high level of heavy metal contamination resulted in higher functional diversity and the abundance of metal resistant genes. These variation therefore significantly contribute to the resistance, resilience and stability of the microbial community subjected to the gradient of heavy metals contaminant in Xiangjiang River.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0800-x) contains supplementary material, which is available to authorized users.

Identification of the microbes mediating Fe reduction in a deep saline aquifer and their influence during managed aquifer recharge

In this study, indigenous microbes enabling Fe reduction under saline groundwater conditions were identified, and their potential contribution to Fe release from aquifer sediments during managed aquifer recharge (MAR) was evaluated. Sediment and groundwater samples were collected from a MAR feasibility test site in Korea, where adjacent river water will be injected into the confined aquifer. The residual groundwater had a high salinity over 26.0 psu, as well as strong reducing conditions (dissolved oxygen, DO<2.0mg/L; oxidation-reduction potential, ORP<-100 mV) with high Fe(2+) concentrations. The indigenous microbes that mediate the reduction of Fe-minerals in this deep saline aquifer were found to be Citrobacter sp. However, column experiments to simulate field operation scenarios indicated that additional Fe release would be limited during MAR, as the dominant microbial community in the sediment would shift from Citrobacter sp. to Pseudomonas sp. and Limnohabitans sp. as river water injection alters the pore water chemistry.

Co-occurrence of Methanosarcina mazei and Geobacteraceae in an iron (III)-reducing enrichment culture

Methanosaeta harundinacea and Methanosarcina barkeri, known as classic acetoclastic methanogens, are capable of directly accepting electrons from Geobacter metallireducens for the reduction of carbon dioxide to methane, having been revealed as direct interspecies electron transfer (DIET) in the laboratory co-cultures. However, whether their co-occurrences are ubiquitous in the iron (III)-reducing environments and the other species of acetoclastic methanogens such as Methanosarcina mazei are capable of DIET are still unknown. Instead of initiating the co-cultures with pure cultures, two-step cultivation was employed to selectively enrich iron (III)-reducing microorganisms in a coastal gold mining river, Jiehe River, with rich iron content in the sediments. First, iron (III) reducers including Geobacteraceae were successfully enriched by 3-months successive culture on amorphous Fe(III) oxides as electron acceptor and acetate as electron donor. High-throughput Illumina sequencing, terminal restriction fragment length polymorphism (T-RFLP) and clone library analysis based on 16S rRNA genes revealed that the enrichment cultures actively contained the bacteria belong to Geobacteraceae and Bacilli, exclusively dominated by the archaea belong to Methanosarcinaceae. Second, the enrichment cultures including methanogens and Geobacteraceae were transferred with ethanol as alternative electron donor. Remarkably, aggregates were successively formed in the enrichments after three transfers. The results revealed by RNA-based analysis demonstrate that the co-occurrence of Methanosarcina mazei and Geobacteraceae in an iron (III)-reducing enrichment culture. Furthermore, the aggregates, as close physical contact, formed in the enrichment culture, indicate that DIET could be a possible option for interspecies electron transfer in the aggregates.

Formation and release behavior of iron corrosion products under the influence of bacterial communities in a simulated water distribution system

To understand the formation and release behavior of iron corrosion products in a drinking water distribution system, annular reactors (ARs) were used to investigate the development processes of corrosion products and biofilm community as well as the concomitant iron release behavior. Results showed that the formation and transformation of corrosion products and bacterial community are closely related to each other. The presence of sulfate-reducing bacteria (SRB, e.g. Desulfovibrio and Desulfotomaculum), sulfur-oxidizing bacteria (SOB, e.g. Sulfuricella), and iron-oxidizing bacteria (IOB, e.g. Acidovorax, Gallionella, Leptothrix, and Sphaerotilus) in biofilms could speed up iron corrosion; however, iron-reducing bacteria (IRB, e.g. Bacillus, Clostridium, and Pseudomonas) could inhibit iron corrosion and iron release. Corrosion scales on iron coupons could develop into a two-layered structure (top layer and inner layer) with time. The relatively stable constituents such as goethite (α-FeOOH) and magnetite (Fe3O4) mainly existed in the top layers, while green rust (Fe6(OH)12CO3) mainly existed in the inner layers. The IOB (especially Acidovorax) contributed to the formation of α-FeOOH, while IRB and the anaerobic conditions could facilitate the formation of Fe3O4. Compared with the AR test without biofilms, the iron corrosion rate with biofilms was relatively higher (p < 0.05) during the whole experimental period, but the iron release with biofilms was obviously lower both at the initial stage and after 3 months. Biofilm and corrosion scale samples formed under different water supply conditions in an actual drinking water distribution system verified the relationships between the bacterial community and corrosion products.

Bacterial community of biofilms developed under different water supply conditions in a distribution system

In order to understand the bacterial community characteristics of biofilms developed under different finished water supply histories in drinking water distribution systems (DWDS), biofilm samples on different type of iron corrosion scales in a real DWDS were collected and systematically investigated using 454 pyrosequencing of 16S rRNA gene. The richness and diversity estimators showed that biofilms formed in DWDS transporting finished groundwater (GW) had the lowest level of bacterial diversity. From phylum to genus level, the dominant bacterial groups found in the biofilms under finished surface water (SW) and GW conditions were distinct. Proteobacteria was the dominant group in all biofilm samples (in the range of 40%-97%), but was relatively higher in biofilms with GW. The relative abundance of Firmicutes in biofilms with SW (28%-35%) was significantly higher (p<0.01) than that in biofilms with GW (0.5%-2.88%). Statistical analysis (Spearman's rank) revealed that alkalinity and chemical oxygen demand (CODMn) positively correlated with the relative abundance of Proteobacteria and Firmicutes, respectively. The abundance of sequences affiliated to iron-reducing bacteria (mainly Bacillus) and iron-oxidizing bacteria (mainly Acidovorax) were relatively higher in biofilms with SW, which might contribute to the formation of much thicker or tubercle-formed corrosion scales under SW supply condition. Several potential opportunistic pathogens, such as Burkholderia fungorum, Mycobacterium neoaurum, Mycobacterium frederiksbergense were detected in the biofilms.

Enriched Iron(III)-Reducing Bacterial Communities are Shaped by Carbon Substrate and Iron Oxide Mineralogy

Iron (Fe) oxides exist in a spectrum of structures in the environment, with ferrihydrite widely considered the most bioavailable phase. Yet, ferrihydrite is unstable and rapidly transforms to more crystalline Fe(III) oxides (e.g., goethite, hematite), which are poorly reduced by model dissimilatory Fe(III)-reducing microorganisms. This begs the question, what processes and microbial groups are responsible for reduction of crystalline Fe(III) oxides within sedimentary environments? Further, how do changes in Fe mineralogy shape oxide-hosted microbial populations? To address these questions, we conducted a large-scale cultivation effort using various Fe(III) oxides (ferrihydrite, goethite, hematite) and carbon substrates (glucose, lactate, acetate) along a dilution gradient to enrich for microbial populations capable of reducing Fe oxides spanning a wide range of crystallinities and reduction potentials. While carbon source was the most important variable shaping community composition within Fe(III)-reducing enrichments, both Fe oxide type and sediment dilution also had a substantial influence. For instance, with acetate as the carbon source, only ferrihydrite enrichments displayed a significant amount of Fe(III) reduction and the well-known dissimilatory metal reducer Geobacter sp. was the dominant organism enriched. In contrast, when glucose and lactate were provided, all three Fe oxides were reduced and reduction coincided with the presence of fermentative (e.g., Enterobacter spp.) and sulfate-reducing bacteria (e.g., Desulfovibrio spp.). Thus, changes in Fe oxide structure and resource availability may shift Fe(III)-reducing communities between dominantly metal-respiring to fermenting and/or sulfate-reducing organisms which are capable of reducing more recalcitrant Fe phases. These findings highlight the need for further targeted investigations into the composition and activity of speciation-directed metal-reducing populations within natural environments.

Time-course correlation of biofilm properties and electrochemical performance in single-chamber microbial fuel cells

The relationship between anode microbial characteristics and electrochemical parameters in microbial fuel cells (MFCs) was analyzed by time-course sampling of parallel single-bottle MFCs operated under identical conditions. While voltage stabilized within 4days, anode biofilms continued growing during the six-week operation. Viable cell density increased asymptotically, but membrane-compromised cells accumulated steadily from only 9% of total cells on day 3 to 52% at 6weeks. Electrochemical performance followed the viable cell trend, with a positive correlation for power density and an inverse correlation for anode charge transfer resistance. The biofilm architecture shifted from rod-shaped, dispersed cells to more filamentous structures, with the continuous detection of Geobacter sulfurreducens-like 16S rRNA fragments throughout operation and the emergence of a community member related to a known phenazine-producing Pseudomonas species. A drop in cathode open circuit potential between weeks two and three suggested that uncontrolled biofilm growth on the cathode deleteriously affects system performance.

Geobacter: the microbe electric's physiology, ecology, and practical applications

Geobacter species specialize in making electrical contacts with extracellular electron acceptors and other organisms. This permits Geobacter species to fill important niches in a diversity of anaerobic environments. Geobacter species appear to be the primary agents for coupling the oxidation of organic compounds to the reduction of insoluble Fe(III) and Mn(IV) oxides in many soils and sediments, a process of global biogeochemical significance. Some Geobacter species can anaerobically oxidize aromatic hydrocarbons and play an important role in aromatic hydrocarbon removal from contaminated aquifers. The ability of Geobacter species to reductively precipitate uranium and related contaminants has led to the development of bioremediation strategies for contaminated environments. Geobacter species produce higher current densities than any other known organism in microbial fuel cells and are common colonizers of electrodes harvesting electricity from organic wastes and aquatic sediments. Direct interspecies electron exchange between Geobacter species and syntrophic partners appears to be an important process in anaerobic wastewater digesters. Functional and comparative genomic studies have begun to reveal important aspects of Geobacter physiology and regulation, but much remains unexplored. Quantifying key gene transcripts and proteins of subsurface Geobacter communities has proven to be a powerful approach to diagnose the in situ physiological status of Geobacter species during groundwater bioremediation. The growth and activity of Geobacter species in the subsurface and their biogeochemical impact under different environmental conditions can be predicted with a systems biology approach in which genome-scale metabolic models are coupled with appropriate physical/chemical models. The proficiency of Geobacter species in transferring electrons to insoluble minerals, electrodes, and possibly other microorganisms can be attributed to their unique "microbial nanowires," pili that conduct electrons along their length with metallic-like conductivity. Surprisingly, the abundant c-type cytochromes of Geobacter species do not contribute to this long-range electron transport, but cytochromes are important for making the terminal electrical connections with Fe(III) oxides and electrodes and also function as capacitors, storing charge to permit continued respiration when extracellular electron acceptors are temporarily unavailable. The high conductivity of Geobacter pili and biofilms and the ability of biofilms to function as supercapacitors are novel properties that might contribute to the field of bioelectronics. The study of Geobacter species has revealed a remarkable number of microbial physiological properties that had not previously been described in any microorganism. Further investigation of these environmentally relevant and physiologically unique organisms is warranted.

Bio-reduction of Fe(III) ores using three pure strains of Aeromonas hydrophila, Serratia fonticola and Clostridium celerecrescens and a natural consortium

The present work describes a research approach to the anaerobic bioleaching of Fe(III) ores. Three strains (Serratia fonticola, Aeromonas hydrophila and Clostridium celerecrescens) isolated from an acidic abandoned mine were selected to test their ability to reduce dissimilatory Fe(III). Total iron bio-reduction was achieved after 48 h using either the consortium or the Aeromonas cultures. In the latter case, there was no evidence of precipitates and Fe(II) remained in solution at neutral pH through complex formation with citrate. None of the other cultures tested (mixed culture and the two isolates) exhibited this behaviour. Biotechnologically, this is a very promising result since it obviates the problem associated with undesirable precipitation of iron compounds in Fe(III)-reducing bacterial cultures. The performance of the Aeromonas culture was improved progressively by adaptation to moderately acidic pH values (up to 4.5) and to three different Fe(III)-oxyhydroxides as the sole source of iron: ferrihydrite, hematite and jarosite, commonly found as weathering compounds at mine sites. Dissimilatory Fe(III)-reducers for iron extraction from ores is therefore especially attractive in that acidification of the surrounding area can be minimized.

Fe(III)-enhanced anaerobic transformation of 2,4-dichlorophenoxyacetic acid by an iron-reducing bacterium Comamonas koreensis CY01

This work studied the ability of Comamonas koreensis CY01 to reduce Fe(III) (hydr)oxides by coupling the oxidation of electron donors and the enhanced biodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by the presence of Fe(III) (hydr)oxides. The experimental results suggested that strain CY01 can utilize ferrihydrite, goethite, lepidocrocite or hematite as the terminal electron acceptor and citrate, glycerol, glucose or sucrose as the electron donor. Strain CY01 could transform 2,4-D to 4-chlorophenol through reductive side-chain removal and dechlorination. Under the anaerobic conditions, Fe(III) reduction and 2,4-D biodegradation by strain CY01 occurred simultaneously. The presence of Fe(III) (hydr)oxides would significantly enhance 2,4-D biodegradation, probably due to the fact that the reactive mineral-bound Fe(II) species generated from Fe(III) reduction can abiotically reduce 2,4-D. This is the first report of a strain of C. koreensis capable of reducing Fe(III) (hydr)oxides and 2,4-D, which extends the diversity of iron-reducing bacteria associated with dechlorination.