The shuttling effects and associated mechanisms of different types of iron oxide nanoparticles for Cu(II) reduction by Geobacter sulfurreducens.

Iron oxide nanoparticles (IONPs), commonly occurring in soils, aquifers and subsurface sediments, may serve as important electron shuttles for the biotransformation of coexisting toxic metals. Here, we explored the impact of different IONPs (low-crystallinity goethite and ferrihydrite, high-crystallinity magnetite and hematite) on the reduction of Cu(II) by Geobacter sulfurreducens and the associated electron shuttle mechanisms. All four IONPs tested can function as electron shuttles to enhance long distance electron transfer from bacteria to Cu(II). Upon IONPs addition, the rate of Cu(II) reduction increased from 14.9 to 65.0-83.8 % in solution after 7 days of incubation. Formation of both Cu(I) and Cu(0) on the iron oxide nanoparticles was revealed by the X-ray absorption near-edge spectroscopy. The IONPs can be utilized as conduits by bacteria to directly transfer electrons and they can also reversibly accept and donate electrons as batteries through a charging-discharging cycle to transfer electron. The latter mechanism (geo-battery) played an important role in all four types of IONPs while the former one (geo-conductor) can only be found in the magnetite and hematite treatments due to the higher crystallinity. Our results shed new light on the biogeochemically mediated electron flux in microbe-IONPs-metal networks under anaerobic iron-reduction conditions.

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.

Multi-walled carbon nanotubes accelerate interspecies electron transfer between Geobacter cocultures.

Carbon nanotubes (CNTs) have been reported to promote symbiotic metabolism in bacteria by accelerating interspecies electron transfer. However, this phenomenon has not been investigated or proven in a cocultures system. In this study, multi-walled CNTs (MWCNTs) were added into Geobacter cocultures systems with the ability of direct interspecies electron transfer (DIET). Results showed that addition of MWCNTs accelerated the metabolic rate of the cocultures. Succinate production rate in a test with 1.0gL-1 MWCNTs was 1.12mM d-1, 1.67 times higher than without MWCNTs. However, the biotoxicity effect became evident with the addition of much higher levels of MWCNTs addition. This study supports the possibility that carbon nanotubes accelerate interspecies electron transfer and provides a theoretical basis for the MWCNTs application in the process of anaerobic wastewater treatment.

Response to inhibitory conditions of acetate-degrading methanogenic microbial community.

Investigating the effects of different kinds of inhibitors on the activity and structure of acetate-degrading microbial community involved in methane fermentation is critically important for developing countermeasures to make the fermentation process stable under different inhibitory conditions. In the present study, a mesophilic chemostat fed with acetate as the sole carbon source was constructed. Microbial community analysis based on high-throughput sequencing of 16S rRNA revealed that Methanothrix was the dominant methanogen and a variety of bacteria including acetate-oxidizing bacteria such as Tepidanaerobacter, Mesotoga, Geovibrio, and Geobacter were found. The activity and dynamic changes of the acetate-degrading microbial community under different inhibitory conditions were investigated. Addition of 600 mg L-1 ammonium and 150 mg L-1 sulfide reduced nearly half of the biogas production. The response of microbial community to sulfide inhibition was quicker than ammonium but the structure could recover in a short time. Addition of 8 mg L-1 chlortetracycline (CTC) and 160 mg L-1 enrofloxacin (EFX) exhibited a similar inhibitory effect on biogas production, with approximately 35% reduction. Compared to ammonium and sulfide, antibiotics showed stronger selective inhibition on some bacterial species. The genera related to acetate-oxidizing and sulfate-reducing bacteria showed stronger tolerance to CTC, which may be due to their low growth rates. Network analysis suggested that some genera which had close phylogenic relationship and similar functions showed constant positive correlation under different inhibitory conditions.

Tuning Geobacter sulfurreducens biofilm with conjugated polyelectrolyte for increased performance in bioelectrochemical system.

Bioelectrochemical systems (BESs) are emerging as a platform technology with great application potentials such as wastewater remediation and power generation. Materials for electrode/microorganism modification are being examined in order to improve the current production in BESs. Herein, we report that the current production increased almost one fold in single-chamber BES reactors, by adding a conjugated polyelectrolyte (CPE-K) in the growth medium to co-form the anodic biofilm with Geobacter sulfurreducens cells. The CPE-K treated BESs had a maximum current density as high as 12.3 ±â€¯0.5 A/m2, with that of the controls being 6.2 ±â€¯0.7 A/m2. Improved current production was sustained even after CPE-K was no longer added to the medium. It was demonstrated that increased current resulted from improvement of certain biofilm properties. Analysis using electrochemical impedance spectroscopy (EIS) showed that CPE-K addition decreased the charge transfer resistance at the cell/electrode interface and the diffusion resistance through the biofilm. Protein quantification showed increased biomass growth on the electrode surface, and confocal scanning microscopy images revealed enhanced biofilm permeability. These results demonstrated for the first time that conjugated polyelectrolytes could be used for G. sulfurreducens biofilm augmentation to achieve high electricity production through tuning the anodic biofilm in BESs.

Inhibition of methanogens decreased sulfadiazine removal and increased antibiotic resistance gene development in microbial fuel cells.

The aim of this work was to study sulfadiazine (SDZ) biodegradation efficiency, antibiotic resistance genes (ARGs) development and shift of microbial communities under conditions of limited methanogens activity in Microbial fuel cells (MFCs). The results indicated that the removal performance of SDZ was decreased with the suppression of methanogens in both MFCs and open-circuit controls. The relative abundances of ARGs were even enhanced by the inhibition of methanogens. The biodegradation mechanism of SDZ was obtained, in which SDZ was initially divided into aniline and pyrimidin-2ylsulfamic acid, then converted into small molecules. Geobacter was found as the dominant microorganism, indicating its potential to degrade SDZ in the MFCs. These findings suggest there is a trade-off between electricity production and SDZ removal and ARG development by the mean of methanogen inhibition in MFCs.

Bacterial community shift and antibiotics resistant genes analysis in response to biodegradation of oxytetracycline in dual graphene modified bioelectrode microbial fuel cell.

This study explored the biodegradation mechanisms of oxytetracycline (OTC/O) and electrochemical characteristics from the perspective of bacterial community shift and OTC resistance genes in dual graphene modified bioelectrode microbial fuel cell (O-D-GM-BE MFC). In phylum level, Proteobacteria was accounted to 95.04% in O-GM-BA, Proteobacteria and Bacteroidetes were accounted to 59.13% and 20.52% in O-GM-BC, which were beneficial for extracellular electron transport (EET) process and OTC biodegradation. In genus level, the most dominant bacteria in O-GM-BA were Salmonella and Trabulsiella, accounting up to 83.04%, moreover, representative exoelectrogens (Geobacter) were enriched, which contributed to OTC biodegradation and electrochemical performances; abundant degrading bacteria (Moheibacter, Comamonas, Pseudomonas, Dechloromonas, Nitrospira, Methylomicrobium, Pseudorhodoferax, Thiobacillus, Mycobacterium) were enriched in O-GM-BC, which contributed to the maximum removal efficiency of OTC; coding resistance genes of efflux pump, ribosome protective protein and modifying or passivating were all found in O-GM-BE, and this explained the OTC removal mechanisms from gene level.

Magnetite production and transformation in the methanogenic consortia from coastal riverine sediments.

Minerals that contain ferric iron, such as amorphous Fe(III) oxides (A), can inhibit methanogenesis by competitively accepting electrons. In contrast, ferric iron reduced products, such as magnetite (M), can function as electrical conductors to stimulate methanogenesis, however, the processes and effects of magnetite production and transformation in the methanogenic consortia are not yet known. Here we compare the effects on methanogenesis of amorphous Fe (III) oxides (A) and magnetite (M) with ethanol as the electron donor. RNA-based terminal restriction fragment length polymorphism with a clone library was used to analyse both bacterial and archaeal communities. Iron (III)-reducing bacteria including Geobacteraceae and methanogens such as Methanosarcina were enriched in iron oxide-supplemented enrichment cultures for two generations with ethanol as the electron donor. The enrichment cultures with A and non-Fe (N) dominated by the active bacteria belong to Veillonellaceae, and archaea belong to Methanoregulaceae and Methanobacteriaceae, Methanosarcinaceae (Methanosarcina mazei), respectively. While the enrichment cultures with M, dominated by the archaea belong to Methanosarcinaceae (Methanosarcina barkeri). The results also showed that methanogenesis was accelerated in the transferred cultures with ethanol as the electron donor during magnetite production from A reduction. Powder X-ray diffraction analysis indicated that magnetite was generated from microbial reduction of A and M was transformed into siderite and vivianite with ethanol as the electron donor. Our data showed the processes and effects of magnetite production and transformation in the methanogenic consortia, suggesting that significantly different effects of iron minerals on microbial methanogenesis in the iron-rich coastal riverine environment were present.

Subminimal inhibitory concentration (sub-MIC) of antibiotic induces electroactive biofilm formation in bioelectrochemical systems.

Electroactive biofilms (EABs) generated from mixed inocula are attractive due to their unique direct extracellular electron transfer abilities and potential use in water pollution control. In this study, for the first time, we identified a chemical that can be used for EAB regulation (both inhibition and promotion). We confirmed that tobramycin, an antibiotic previously demonstrated to inhibit the activity of EABs, is an agonist of EAB formation at subminimal inhibitory concentrations (sub-MICs). Compared to the control, at tobramycin concentrations of 0.05 (1/80 MIC) and 0.1 mg/L (1/40 MIC), the time required to reach 3 A/m2 was shorter, and the limiting current densities increased by 17%. The enhanced EAB activity was primarily attributed to the 50% increase in biomass density from 289 ± 21 to 434 ± 12 µg protein/cm2 and the increased biofilm thickness from 28 ± 1 to 37 ± 0.5 µm. Geobacter species in the microbial communities were selectively increased from 76% to 82%, and their abundance was estimated to increase by 1.63-fold. The accelerated growth was further confirmed using the model strain G. sulfurreducens PCA. Transcriptomic analysis revealed that 0.05 mg/L of tobramycin led to a significant upregulation of genes related to cytochromes and the type IV pilus, suggesting a possible mechanism for the observed current enhancement. These findings extend our knowledge of the regulation of EAB formation by antibiotics and the selective enrichment of Geobacter from a mixed culture, with broader implications on the potential impact of trace antibiotics on the dissimilatory metal reduction process in water environment.

Genome Scale Mutational Analysis of Geobacter sulfurreducens Reveals Distinct Molecular Mechanisms for Respiration and Sensing of Poised Electrodes versus Fe(III) Oxides.

Geobacter sulfurreducens generates electrical current by coupling intracellular oxidation of organic acids to the reduction of proteins on the cell surface that are able to interface with electrodes. This ability is attributed to the bacterium's capacity to respire other extracellular electron acceptors that require contact, such as insoluble metal oxides. To directly investigate the genetic basis of electrode-based respiration, we constructed Geobacter sulfurreducens transposon-insertion sequencing (Tn-Seq) libraries for growth, with soluble fumarate or an electrode as the electron acceptor. Libraries with >33,000 unique insertions and an average of 9 insertions/kb allowed an assessment of each gene's fitness in a single experiment. Mutations in 1,214 different genomic features impaired growth with fumarate, and the significance of 270 genes unresolved by annotation due to the presence of one or more functional homologs was determined. Tn-Seq analysis of -0.1 V versus standard hydrogen electrode (SHE) electrode-grown cells identified mutations in a subset of genes encoding cytochromes, processing systems for proline-rich proteins, sensory networks, extracellular structures, polysaccharides, and metabolic enzymes that caused at least a 50% reduction in apparent growth rate. Scarless deletion mutants of select genes identified via Tn-Seq revealed a new putative porin-cytochrome conduit complex (extABCD) crucial for growth with electrodes, which was not required for Fe(III) oxide reduction. In addition, four mutants lacking components of a putative methyl-accepting chemotaxis-cyclic dinucleotide sensing network (esnABCD) were defective in electrode colonization but grew normally with Fe(III) oxides. These results suggest that G. sulfurreducens possesses distinct mechanisms for recognition, colonization, and reduction of electrodes compared to Fe(III) oxides.IMPORTANCE Since metal oxide electron acceptors are insoluble, one hypothesis is that cells sense and reduce metals using the same molecular mechanisms used to form biofilms on electrodes and produce electricity. However, by simultaneously comparing thousands of Geobacter sulfurreducens transposon mutants undergoing electrode-dependent respiration, we discovered new cytochromes and chemosensory proteins supporting growth with electrodes that are not required for metal respiration. This supports an emerging model where G. sulfurreducens recognizes surfaces and forms conductive biofilms using mechanisms distinct from those used for growth with metal oxides. These findings provide a possible explanation for studies that correlate electricity generation with syntrophic interspecies electron transfer by Geobacter and reveal many previously unrecognized targets for engineering this useful capability in other organisms.

Nano-graphene induced positive effects on methanogenesis in anaerobic digestion.

The effects of nano-graphene on methanogenesis in anaerobic digestion was investigated. Short-term results showed that graphene (30 and 120mg/L) had significantly positive effects on methane production rate, which increased by 17.0% and 51.4%. Further investigation indicated that acetate-consuming methanogenesis was enhanced. The failure of quinones to replicate graphene stimulation effects on methanogenesis suggested that graphene did not function as electron shuttles. After 55 day's operation at room temperature (from 20 to 10°C, the methane production rate with 30mg/L graphene was 14.3% higher than that of the control, while 120mg/L graphene showed a slight inhibition on methane yield. Illumina sequencing data showed that the archaeal community structure remained fairly constant as the incubated sludge with graphene at low temperature, in which Methanoregula, Methanosaeta and Methanospirillum were the dominant species. Besides, Geobacter enrichment was observed with graphene, suggesting that the direct interspecies electron transfer might be promoted.

Influence of Oxygen and Nitrate on Fe (Hydr)oxide Mineral Transformation and Soil Microbial Communities during Redox Cycling.

Oscillations between reducing and oxidizing conditions are observed at the interface of anaerobic/oxic and anaerobic/anoxic environments, and are often stimulated by an alternating flux of electron donors (e.g., organic carbon) and electron acceptors (e.g., O2 and NO3(-)). In iron (Fe) rich soils and sediments, these oscillations may stimulate the growth of both Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and their metabolism may induce cycling between Fe(II) and Fe(III), promoting the transformation of Fe (hydr)oxide minerals. Here, we examine the mineralogical evolution of lepidocrocite and ferrihydrite, and the adaptation of a natural microbial community to alternating Fe-reducing (anaerobic with addition of glucose) and Fe-oxidizing (with addition of nitrate or air) conditions. The growth of FeRB (e.g., Geobacter) is stimulated under anaerobic conditions in the presence of glucose. However, the abundance of these organisms depends on the availability of Fe(III) (hydr)oxides. Redox cycling with nitrate results in decreased Fe(II) oxidation thereby decreasing the availability of Fe(III) for FeRB. Additionally, magnetite is detected as the main product of both lepidocrocite and ferrihydrite reduction. In contrast, introduction of air results in increased Fe(II) oxidation, increasing the availability of Fe(III) and the abundance of Geobacter. In the lepidocrocite reactors, Fe(II) oxidation by dissolved O2 promotes the formation of ferrihydrite and lepidocrocite, whereas in the ferrihydrite reactors we observe a decrease in magnetite stoichiometry (e.g., oxidation). Understanding Fe (hydr)oxide transformation under environmentally relevant redox cycling conditions provides insight into nutrient availability and transport, contaminant mobility, and microbial metabolism in soils and sediments.

Structural basis for metallic-like conductivity in microbial nanowires.

UNLABELLED: Direct measurement of multiple physical properties of Geobacter sulfurreducens pili have demonstrated that they possess metallic-like conductivity, but several studies have suggested that metallic-like conductivity is unlikely based on the structures of the G. sulfurreducens pilus predicted from homology models. In order to further evaluate this discrepancy, pili were examined with synchrotron X-ray microdiffraction and rocking-curve X-ray diffraction. Both techniques revealed a periodic 3.2-Å spacing in conductive, wild-type G. sulfurreducens pili that was missing in the nonconductive pili of strain Aro5, which lack key aromatic acids required for conductivity. The intensity of the 3.2-Å peak increased 100-fold when the pH was shifted from 10.5 to 2, corresponding with a previously reported 100-fold increase in pilus conductivity with this pH change. These results suggest a clear structure-function correlation for metallic-like conductivity that can be attributed to overlapping π-orbitals of aromatic amino acids. A homology model of the G. sulfurreducens pilus was constructed with a Pseudomonas aeruginosa pilus model as a template as an alternative to previous models, which were based on a Neisseria gonorrhoeae pilus structure. This alternative model predicted that aromatic amino acids in G. sulfurreducens pili are packed within 3 to 4 Å, consistent with the experimental results. Thus, the predictions of homology modeling are highly sensitive to assumptions inherent in the model construction. The experimental results reported here further support the concept that the pili of G. sulfurreducens represent a novel class of electronically functional proteins in which aromatic amino acids promote long-distance electron transport. IMPORTANCE: The mechanism for long-range electron transport along the conductive pili of Geobacter sulfurreducens is of interest because these "microbial nanowires" are important in biogeochemical cycling as well as applications in bioenergy and bioelectronics. Although proteins are typically insulators, G. sulfurreducens pilus proteins possess metallic-like conductivity. The studies reported here provide important structural insights into the mechanism of the metallic-like conductivity of G. sulfurreducens pili. This information is expected to be useful in the design of novel bioelectronic materials.

Coupled mercury-cell sorption, reduction, and oxidation on methylmercury production by Geobacter sulfurreducens PCA.

G. sulfurreducens PCA cells have been shown to reduce, sorb, and methylate Hg(II) species, but it is unclear whether this organism can oxidize and methylate dissolved elemental Hg(0) as shown for Desulfovibrio desulfuricans ND132. Using Hg(II) and Hg(0) separately as Hg sources in washed cell assays in phosphate buffered saline (pH 7.4), we report how cell-mediated Hg reduction and oxidation compete or synergize with sorption, thus affecting the production of toxic methylmercury by PCA cells. Methylation is found to be positively correlated to Hg sorption (r = 0.73) but negatively correlated to Hg reduction (r = -0.62). These reactions depend on the Hg and cell concentrations or the ratio of Hg to cellular thiols (-SH). Oxidation and methylation of Hg(0) are favored at relatively low Hg to cell-SH molar ratios (e.g., <1). Increasing Hg to cell ratios from 0.25 × 10(-19) to 25 × 10(-19) moles-Hg/cell (equivalent to Hg/cell-SH of 0.71 to 71) shifts the major reaction from oxidation to reduction. In the absence of five outer membrane c-type cytochromes, mutant ΔomcBESTZ also shows decreases in Hg reduction and increases in methylation. However, the presence of competing thiol-binding ions such as Zn(2+) leads to increased Hg reduction and decreased methylation. These results suggest that the coupled cell-Hg sorption and redox transformations are important in controlling the rates of Hg uptake and methylation by G. sulfurreducens PCA in anoxic environments.

Proteome of Geobacter sulfurreducens in the presence of U(VI).

Geobacter species often play an important role in the in situ bioremediation of uranium-contaminated groundwater, but little is known about how these microbes avoid uranium toxicity. To evaluate this further, the proteome of Geobacter sulfurreducens exposed to 100 µM U(VI) acetate was compared to control cells not exposed to U(VI). Of the 1363 proteins detected from these cultures, 203 proteins had higher abundance during exposure to U(VI) compared with the control cells and 148 proteins had lower abundance. U(VI)-exposed cultures expressed lower levels of proteins involved in growth, protein and amino acid biosynthesis, as well as key central metabolism enzymes as a result of the deleterious effect of U(VI) on the growth of G. sulfurreducens. In contrast, proteins involved in detoxification, such as several efflux pumps belonging to the RND (resistance-nodulation-cell division) family, and membrane protection, and other proteins, such as chaperones and proteins involved in secretion systems, were found in higher abundance in cells exposed to U(VI). Exposing G. sulfurreducens to U(VI) resulted in a higher abundance of many proteins associated with the oxidative stress response, such as superoxide dismutase and superoxide reductase. A strain in which the gene for superoxide dismutase was deleted grew more slowly than the WT strain in the presence of U(VI), but not in its absence. The results suggested that there is no specific mechanism for uranium detoxification. Rather, multiple general stress responses are induced, which presumably enable Geobacter species to tolerate high uranium concentrations.

Enhanced uranium immobilization and reduction by Geobacter sulfurreducens biofilms.

Biofilms formed by dissimilatory metal reducers are of interest to develop permeable biobarriers for the immobilization of soluble contaminants such as uranium. Here we show that biofilms of the model uranium-reducing bacterium Geobacter sulfurreducens immobilized substantially more U(VI) than planktonic cells and did so for longer periods of time, reductively precipitating it to a mononuclear U(IV) phase involving carbon ligands. The biofilms also tolerated high and otherwise toxic concentrations (up to 5 mM) of uranium, consistent with a respiratory strategy that also protected the cells from uranium toxicity. The enhanced ability of the biofilms to immobilize uranium correlated only partially with the biofilm biomass and thickness and depended greatly on the area of the biofilm exposed to the soluble contaminant. In contrast, uranium reduction depended on the expression of Geobacter conductive pili and, to a lesser extent, on the presence of the c cytochrome OmcZ in the biofilm matrix. The results support a model in which the electroactive biofilm matrix immobilizes and reduces the uranium in the top stratum. This mechanism prevents the permeation and mineralization of uranium in the cell envelope, thereby preserving essential cellular functions and enhancing the catalytic capacity of Geobacter cells to reduce uranium. Hence, the biofilms provide cells with a physically and chemically protected environment for the sustained immobilization and reduction of uranium that is of interest for the development of improved strategies for the in situ bioremediation of environments impacted by uranium contamination.

Effect of divalent metals on Hg(II) uptake and methylation by bacteria.

The production of methylmercury by some bacteria is a key first step in the accumulation and biomagnification of this toxic substance in aquatic food webs, a major human health concern. By direct measurement of cellular Hg(II) uptake in model iron and sulfate reducing bacteria, we have observed that specific trace metals, such as Zn(II) and Cd(II), inhibit uptake and methylation in these organisms, whereas other metals, such as Ni(II), Co(II), or Fe(II), do not. The inhibition of Hg(II) methylation by Zn(II) was competitive in nature and related to the concentration of inorganically complexed Zn(II) (Zn'). The inhibition of Hg(II) methylation was alleviated by decreasing the free Zn' concentration through complexation with nitrilotriacetic acid without altering the speciation of Hg(II). The inhibitory effect by Zn(II) was observed when either Hg-cysteine complexes or neutral HgCl2 dominated the speciation of Hg(II), demonstrating that both charged and neutral species are transported into the cytosol by an active rather than passive process. We propose that Hg(II) uptake is the result of its accidental uptake by metal transporter(s), possibly one effecting the transport of Zn(II).

Population analysis of mesophilic microbial fuel cells fed with carbon monoxide.

Electricity generation in a microbial fuel cell (MFC) fed with carbon monoxide (CO) has been recently demonstrated; however, the microbial ecology of this system has not yet been described. In this work the diversity of the microbial community present at the anode of CO-fed MFCs was studied by performing denaturing gradient gel electrophoresis (DGGE) and high-throughput sequencing (HTS) analyses. HTS indicated a significant increase of the archaeal genus Methanobacterium and of the bacterial order Clostridiales, notably including Clostridium species, while in both MFCs DGGE identified members of the bacterial genera Geobacter, Desulfovibrio, and Clostridium, and of the archaeal genera Methanobacterium, Methanofollis, and Methanosaeta. In particular, the presence of Geobacter sulfurreducens was identified. Tolerance of G. sulfurreducens to CO was confirmed by growing G. sulfurreducens with acetate under a 100 % CO atmosphere. This observation, along with the identification of acetogens, supports the hypothesis of the two-step process in which CO is converted to acetate by the carboxidotrophic Bacteria and acetate is then oxidized by CO-tolerant electricigenic bacteria to produce electricity.

Fate of Cd during microbial Fe(III) mineral reduction by a novel and Cd-tolerant Geobacter species.

Fe(III) (oxyhydr)oxides affect the mobility of contaminants in the environment by providing reactive surfaces for sorption. This includes the toxic metal cadmium (Cd), which prevails in agricultural soils and is taken up by crops. Fe(III)-reducing bacteria can mobilize such contaminants by Fe(III) mineral dissolution or immobilize them by sorption to or coprecipitation with secondary Fe minerals. To date, not much is known about the fate of Fe(III) mineral-associated Cd during microbial Fe(III) reduction. Here, we describe the isolation of a new Geobacter sp. strain Cd1 from a Cd-contaminated field site, where the strain accounts for 10(4) cells g(-1) dry soil. Strain Cd1 reduces the poorly crystalline Fe(III) oxyhydroxide ferrihydrite in the presence of at least up to 112 mg Cd L(-1). During initial microbial reduction of Cd-loaded ferrihydrite, sorbed Cd was mobilized. However, during continuous microbial Fe(III) reduction, Cd was immobilized by sorption to and/or coprecipitation within newly formed secondary minerals that contained Ca, Fe, and carbonate, implying the formation of an otavite-siderite-calcite (CdCO3-FeCO3-CaCO3) mixed mineral phase. Our data shows that microbially mediated turnover of Fe minerals affects the mobility of Cd in soils, potentially altering the dynamics of Cd uptake into food or phyto-remediating plants.

The impact of ionic mercury on antioxidant defenses in two mercury-sensitive anaerobic bacteria.

While the toxicological effects of mercury (Hg) are well studied in mammals, little is known about the mechanisms of toxicity to bacterial cells lacking an Hg resistance (mer) operon. We determined that Shewanella oneidensis MR-1 is more sensitive to ionic mercury [Hg(II)] under aerobic conditions than in fumarate reducing conditions, with minimum inhibitory concentrations of 0.25 and 2 µM respectively. This increased sensitivity in aerobic conditions is not due to increased import, as more Hg is associated with cellular material in fumarate reducing conditions than in aerobic conditions. In fumarate reducing conditions, glutathione may provide protection, as glutathione levels decrease in a dose-dependent manner, but this does not occur in aerobic conditions. Hg(II) does not change the redox state of thioredoxin in MR1 in either fumarate reducing conditions or aerobic conditions, although thioredoxin is oxidized in Geobacter sulfurreducens PCA in response to Hg(II) treatment. However, treatment with 0.5 µM Hg(II) increases lipid peroxidation in aerobic conditions but not in fumarate reducing conditions in MR-1. We conclude that the enhanced sensitivity of MR-1 to Hg(II) in aerobic conditions is not due to differences in intracellular responses, but due to damage at the cell envelope.