Extracellular Electron Acceptors Research Articles

MmcA is an electron conduit that facilitates both intracellular and extracellular electron transport in Methanosarcina acetivorans

Methanogens are a diverse group of Archaea that obligately couple energy conservation to the production of methane. Some methanogens encode alternate pathways for energy conservation, like anaerobic respiration, but the biochemical details of this process are unknown. We show that a multiheme c-type cytochrome called MmcA from Methanosarcina acetivorans is important for intracellular electron transport during methanogenesis and can also reduce extracellular electron acceptors like soluble Fe3+ and anthraquinone-2,6-disulfonate. Consistent with these observations, MmcA displays reversible redox features ranging from −100 to −450 mV versus SHE. Additionally, mutants lacking mmcA have significantly slower Fe3+ reduction rates. The mmcA locus is prevalent in members of the Order Methanosarcinales and is a part of a distinct clade of multiheme cytochromes that are closely related to octaheme tetrathionate reductases. Taken together, MmcA might act as an electron conduit that can potentially support a variety of energy conservation strategies that extend beyond methanogenesis.

Nonelectroactive clostridium obtains extracellular electron transfer-capability after forming chimera with Geobacter.

Extracellular electron transfer (EET) of microorganisms is a major driver of the microbial growth and metabolism, including reactions involved in the cycling of C, N, and Fe in anaerobic environments such as soils and sediments. Understanding the mechanisms of EET, as well as knowing which organisms are EET-capable (or can become so) is fundamental to electromicrobiology and geomicrobiology. In general, Gram-positive bacteria very seldomly perform EET due to their thick non-conductive cell wall. Here, we report that a Gram-positive Clostridium intestinale (C.i) attained EET-capability for ethanol metabolism only after forming chimera with electroactive Geobacter sulfurreducens (G.s). Mechanism analyses demonstrated that the EET was possible after the cell fusion of the two species was achieved. Under these conditions, the ethanol metabolism pathway of C.i was integrated by the EET pathway of G.s, by which achieved the oxidation of ethanol for the subsequent reduction of extracellular electron acceptors in the coculture. Our study displays a new approach to perform EET for Gram-positive bacteria via recruiting the EET pathway of an electroactive bacterium, which suggests a previously unanticipated prevalence of EET in the microbial world. These findings also provide new perspectives to understand the energetic coupling between bacterial species and the ecology of interspecies mutualisms.

Multipathway of Nitrogen Metabolism Revealed by Genome-Centered Metatranscriptomics from Pyrite-Guided Mixotrophic Partial Denitrification/Anammox Installations.

Further reducing the organic requirements is essential for the sustainable development of partial denitrification/anammox technology. Here, an innovative mixotrophic partial denitrification/anammox (MPD/A) installation fed with pyrite and few organics was realized, and the average nitrogen and phosphorus removal rates were as high as 96.24 ± 0.11% and 79.23 ± 2.06%, respectively, with a C/N ratio of 0.5. To understand the nature by which MPD/A achieves efficient nitrogen removal and organic conservation, the electron transfer-dependent nitrogen escape and energy metabolism were first elucidated using multiomics analysis. Apart from heterotrophic denitrification and anammox, the results revealed some unexpected metabolic couplings of MPD/A systems, in particular, putative nitrate-dependent organic and pyrite oxidation among nominally heterotrophic Denitratisoma (PRO3) strains, which accelerated nitrate gasification with a low-carbon supply. Additionally, Candidatus Brocadia (AMX) employed extracellular solid-state electron acceptors as terminal electron sinks for high-rate ammonium removal. AMX transported ammonium electrons to extracellular γFeO(OH) (generated from pyrite oxidation) through the transient storage of menaquinoline pools, cytoplasmic migration via multiheme cytochrome(s), and OmpA protein/nanowires-mediated electron hopping on cell surfaces. Further investigation observed that extracellular electron flux resulted in the transfer of more energy from the increased oxidation of the electron donor to the ATP, supporting nitrite-independent ammonium removal.

Lactiplantibacillus plantarum uses ecologically relevant, exogenous quinones for extracellular electron transfer.

While quinones are essential for respiratory microorganisms, their importance for microbes that rely on fermentation metabolism is not understood. This gap in knowledge hinders our understanding of anaerobic microbial habitats, such in mammalian digestive tracts and fermented foods. We show that Lactiplantibacillus plantarum, a model fermentative lactic acid bacteria species abundant in human, animal, and insect microbiomes and fermented foods, uses multiple exogenous, environmental quinones as electron shuttles for a hybrid metabolism involving EET. Interestingly, quinones both stimulate this metabolism as well as cause oxidative stress when extracellular electron acceptors are absent. We also found that quinone-producing, lactic acid bacteria species commonly enriched together with L. plantarum in food fermentations accelerate L. plantarum growth and medium acidification through a mainly quinone- and EET-dependent mechanism. Thus, our work provides evidence of quinone cross-feeding as a key ecological feature of anaerobic microbial habitats.

Cellular reductive stress: Is plasma membrane electron transport an evolutionarily-conserved safety valve?

Cellular respiration is highly regulated, changes dynamically in response to the microenvironment of individual cells and during differentiation and differs between cell and tissue types. Too little cell respiration can cause an accumulation of reductants, leading to reductive stress, while inefficient respiration, that causes a build-up of reactive oxygen species (ROS), can result in oxidative stress. Most of the discussion of this central redox dichotomy has centred around oxidative stress because the damaging effects of cellular oxidants on DNA, lipids and proteins are well-established, and have been shown to contribute to health issues including, mitochondrial and cardiovascular diseases, tumorigenesis, and to the effects of ageing. Much less attention has been paid to cellular reductive stress. Nevertheless, excessive levels of key cellular reductants including NADH, NADPH and glutathione, as well as an imbalance in protein thiols, and insufficient levels of ROS to maintain cell signalling pathways, can be harmful to cells and result in poor health outcomes. Recently, cellular mechanisms that sense and regulate cellular reductive stress associated with low ROS levels have been identified. In addition, plasma membrane electron transport has been shown to be a key player in cellular redox homeostasis involving NAD(P)H/NAD(P)+ ratios. It is now well-established that the plasma membrane contains coenzyme Q-mediated electron transport pathways capable of oxidizing intracellular NAD(P)H and reducing extracellular electron acceptors such as molecular oxygen. A better understanding of the origins, cellular and subcellular compartmentalization and regulation of cellular reductants could lead to the development of new anticancer strategies.

Sustainable disposal of Fenton sludge and enhanced organics degradation based on dissimilatory iron reduction in the hydrolytic acidification process

Fenton sludge generated in the flocculation stage of the Fenton oxidation process contains significant amounts of ferric iron and organic pollutants, which require proper treatment. Previous studies have demonstrated that adding Fenton sludge to an anaerobic digester can decompose some of the organic pollutants in the Fenton sludge to lower its environmental risk, but iron gradually accumulates in the reactor, which weakens the sustainability of the method. In this study, Fenton sludge was introduced into a hydrolytic acidification reactor with a weak acid environment to relieve the iron accumulation as well as improve the degradation of organic matter. The results showed that the added Fenton sludge acted as an extracellular electron acceptor to induce dissimilatory iron reduction, which increased chemical oxygen demand (COD) removal and acidification efficiency by 16.1% and 19.8%, respectively, compared to the group without Fenton sludge. Along with the operation, more than 90% of the Fe(III) in Fenton sludge was reduced to Fe(II), and part of them was released to the effluent. Moreover, the Fe(II) in the effluent could be used as flocculants and Fenton reagents to further decrease the effluent COD by 29.8% and 44.5%, respectively. It provided a sustainable strategy to reuse Fenton sludge to enhance organic degradation based on the iron cycle.

Study on the mechanism of enhanced anaerobic ammonia oxidation performance by extracellular electron acceptor biochar

ABSTRACT Anaerobic ammonia oxidation process has the advantages of energy and cost reduction, therefore, it has been considered as one of the main alternatives to conventional biological denitrification process in recent years. Biochar has been applied in the anammox process for nitrogen removal efficiency. But, due to its extracellular electron transfer capacity and abundance of redox-specific functional groups, which served as extracellular electron acceptor to anaerobically oxidize NH4 + is still controversy. In this study, the anaerobic ammonia oxidation was investigated when biochar was used as electron acceptor in the wastewater. According to the optimal process variables determined in the batch tests, when the influent NH4 +-N concentration in the anaerobic ammonia oxidation reaction was 30–50 mg/L and the biochar dosing was at 10 g/L, it showed some promotion in the long-term experiments. The anaerobic ammonia oxidation process with biochar as the electron acceptor reached more than 60% NH4 +-N removal efficiency in the system, and the ΔNO3 --N/ΔNH4 +-N ratio reached 0.19 which tended to the theoretical value. After 20 days, the voltage in the system keeps fluctuating about 4 mV, indicated that the functional bacteria using biochar as the electron acceptor gradually dominated the system. In addition, the abundance of norank_f__norank_o__SBR1031 in the Chloroflexi phylum has increased significantly at 29.92%, while the abundance of the major genus Candidatus_Kuenenia in AnAOB has decreased slightly at 11.47%.

The microbial oxidation of pharmaceuticals in an anaerobic aqueous environment: Effect of dissolved organic matter fractions from different sources

Previous studies have demonstrated the importance of dissolved organic matter (DOM) on the biodegradation of trace organic contaminants occurred in the hyporheic zone. However, the role of diverse DOM fractions with distinct physicochemical properties on the biodegradation of pharmaceuticals under reducing conditions is scarcely known. To address this knowledge gap, DOMs derived from road-deposited sediment, soil, and active sludge (namely allochthonous DOM) and algae (namely autochthonous DOM) were collected and isolated into different fractions. Thereafter, the effect of DOM fractions on the anaerobic microbial oxidation of two typical pharmaceuticals, i.e., ritonavir (RTV) and tetracycline (TC) was explored by using simulated anaerobic microcosms. Mechanistic insights into how DOM fractions from different sources influence pharmaceutical biodegradation processes were provided by optical and electrochemical analyses. Results showed that humic acid and fulvic acid fractions from allochthonous DOM could enhance the biodegradation of TC (12.2 % per mgC/L) and RTV (14.5 % per mgC/L), while no significant impact was observed for that of hydrophilic fractions. However, autochthonous DOM promoted the biodegradation of TC (4.17 % per mgC/L) and inhibited that of RTV. Mechanistic analysis showed that the higher of humification and aromatization level of DOM components, the stronger their promotive effect on the biodegradation of TC and RTV. Further, the promotive mechanism could be attributed to the response of quinone moieties in DOM as extracellular electron acceptors that yields more energy to support microbial metabolism. These results provide a more comprehensive understanding of diverse DOM fractions mediating microbial anaerobic oxidation of trace organic pollutants, and extend our insights into contamination control and remediation technologies.

Geobacter sulfurreducens electroactive biofilms on Fe2O3/FTO support-electrodes for developing a sodium acetate electrochemical biosensor

Geobacter sulfurreducens is an essential microorganism in biogeochemical cycles. It is used biotechnologically in bioremediation and electricity production. The importance of G. sulfurreducens lies in its capacity to contact extracellular electron acceptors, including several electrode materials. Taking advantage of the versatility of G. sulfurreducens to grow on different surfaces, it is possible to design and synthesize new electrodes that can expand and improve the applications of these bacteria. In this work, we performed multidisciplinary research studies to demonstrate an innovative application of this well-known microbial system. The interaction between fluorine-doped tin oxide (FTO) and ordinary glass with G. sulfurreducens was enhanced by modifying the surface with Fe2O3 films. As confocal laser microscopy analyses revealed, the enhancement promoted biofilm development, whose parameters were significantly better than the bare support-electrodes. Furthermore, we provided evidence that G. sulfurreducens interacted with and dissolved the Fe2O3 film during incubation. Then, when we added sodium acetate to the system, the microbial metabolism was activated and decreased the electrochemical response of this compound. We benefited from that behavior to develop a voltammetric-based robust biosensor that could measure sodium acetate at high concentrations with an acceptable linear interval, from 10 to 110 mM, with LOD = 5.9 ± mM and LOQ = 18.1 ± 1.5 mM.

Intracytoplasmic membranes develop in Geobacter sulfurreducens under thermodynamically limiting conditions

Geobacter sulfurreducens is an electroactive bacterium capable of reducing metal oxides in the environment and electrodes in engineered systems1,2. Geobacter sp. are the keystone organisms in electrogenic biofilms, as their respiration consumes fermentation products produced by other organisms and reduces a terminal electron acceptor e.g. iron oxide or an electrode. To respire extracellular electron acceptors with a wide range of redox potentials, G. sulfurreducens has a complex network of respiratory proteins, many of which are membrane-bound3–5. We have identified intracytoplasmic membrane (ICM) structures in G. sulfurreducens. This ICM is an invagination of the inner membrane that has folded and organized by an unknown mechanism, often but not always located near the tip of a cell. Using confocal microscopy, we can identify that at least half of the cells contain an ICM when grown on low potential anode surfaces, whereas cells grown at higher potential anode surfaces or using fumarate as electron acceptor had significantly lower ICM frequency. 3D models developed from cryo-electron tomograms show the ICM to be a continuous extension of the inner membrane in contact with the cytoplasmic and periplasmic space. The differential abundance of ICM in cells grown under different thermodynamic conditions supports the hypothesis that it is an adaptation to limited energy availability, as an increase in membrane-bound respiratory proteins could increase electron flux. Thus, the ICM provides extra inner-membrane surface to increase the abundance of these proteins. G. sulfurreducens is the first Thermodesulfobacterium or metal-oxide reducer found to produce ICMs.

Direct extracellular electron transfer to an indium tin doped oxide electrode via heme redox reactions in Desulfovibrio ferrophilus IS5

Soluble redox mediators have been hypothesized to play a major role in cellular electron transportation to extracellular insoluble electron acceptors in sulfate-reducing bacteria (SRB) through a diffusion-based electron shuttle mechanism. Herein, in vivo electrochemistry shows that outer-membrane cytochromes (OMCs) directly mediated extracellular electron transfer (EET) in Desulfovibrio ferrophilus IS5. Anodic current production by IS5, coupled with lactate oxidation, was detected on indium-tin-doped oxide electrodes poised at +400 mV (vs. standard hydrogen electrode), which was the sole electron acceptor. Subsequent electrolyte replacement experiments with a fresh one showed that cells attached to the electrode contributed to 73% of current production by direct EET, while the electron shuttle mechanism may have contributed to the rest. Differential pulse voltammetry detected two redox species at E = −180 and 20 mV, with the former being most likely assignable to cell-surface cytochromes because nitric oxide decreased the peak current at −180 mV, as previously demonstrated with OMCs in iron-reducing bacteria. The EET onset potential at −200 mV further supports the notion that OMCs mediated EET in IS5, and that EET is thermodynamically favorable for reducing Fe(III) oxide. These results suggest that the direct EET mechanism regulates the reduction rate of insoluble substrates in SRB.

OXIDATIVE PROCESSES IN GRANULOSA CELLS OF COW OVARIAN FOLLICLES AT FOLLICULAR GROWTH

Article presents the research results on the respiratory activity and redox capacity of granulose layer cells of cow ovarian follicles under the physiological state of follicular growth. After the slaughter of the cows, the ovaries in physiological state of follicular growth were selected and the size of the follicles was evaluated. Cells of the granulosa layer were aspirated from small (< 4 mm), medium (4–7 mm) and large (> 7 mm) follicles and determined: intensity of oxygen absorption – polarographically, redox capacity – potentiometrically. K3[Fe(CN)6] was used as an extracellular electron acceptor. The share of oxygen consumed by aerobic glycolysis was determined by using sodium fluoride inhibitor, the NAD-dependent link of the mitochondrial electron transport chain - amytal, and the terminal (cytochrome oxidase) – sodium azide. The share of free-radical oxidation of unsaturated fatty acids in the realization of oxygen was detected by Na2EDTA.
 Granulosa cells from the ovary of follicular growth are characterized by respiratory activity and regenerative capacity, the intensity of which depends on the size of the follicles from which they are extracted. The highest values of the investigated indicators are characteristic of cells extracted from large follicles, lower – from small follicles, and the lowest values are from medium follicles. The high intensity of oxidative processes in granulosa from small follicles ensures the growth of oocytes, and from large follicles – maturation of the germ cell in the dominant follicle. At the same time, the increased intensity of oxygen consumption by the cells of small and dominant follicles is manifested against the background of limitations in electron acceptors in the extracellular environment.
 The use of inhibitors of ATP resynthesis of both glycolysis and individual links of the mitochondrial respiratory chain testify the activity of the indicated metabolic pathways in granulosa and the possibility of using alternative substrates to ensure the existence of cells. The main way of utilization of oxygen in the granulosa layer cells of small and large follicles is aerobic glycolysis, and from the medium there are alternative ways of supplying and using substrates. The largest flow of reducing equivalents into the extracellular space from the granulosa of a medium-sized follicle due to inhibition of glycolysis indicates that the source of the supply of reducing equivalents into the follicle cavity is glycolysis and this characterizes the dependence of the existence of cells on the indicated metabolic pathway.

Simultaneous removal of NOx− and NH4+ driven by Fe(II)/Fe(III) cycle under the continuously-feeding mode

Fe(III) (hydr)oxides have been reported to serve as the extracellular electron acceptor of iron-reducing bacteria to oxidize NH4+, and the resulted Fe(II) may be oxidized by NOx− to Fe(III) that continuously participates to the next round of NH4+ removal, but whether such the Fe(II)/Fe(III) cycle can simultaneously remove NOx− and NH4+ under continuously feeding modes remains unknown. In this study, a NH4+-containing wastewater with and without NO2− or NO3− was fed into anaerobic reactors continuously to investigate the nitrogen removal with the Fe(II)/Fe(III) cycle. The results showed that, under a hydraulic retention time (HRT) of 48 h, the NH4+-N removal efficiencies in the NO3−-fed and NO2−-fed reactors reached about 50.3 % and 42.7 % respectively, accompanied with the complete removal of NO3− and NO2−. The experiment of intermittent addition of 14NOx− into the serum bottle containing 15NH4+ showed that the Fe(II) content decreased rapidly after NOx− addition, and then gradually increased, indicating the occurrence of iron cycle. The isotope experiment also demonstrated that 29N2 and 30N2 were generated after adding NOx−, and the Anammox microorganisms were undetected in all groups. The above results indicated that NOx− and NH4+ could be simultaneously removed under the continuously-feeding mode, in which Fe(II)/Fe(III) cycle was the essential reason for the nitrogen removal.

Rewiring the respiratory pathway of Lactococcus lactis to enhance extracellular electron transfer.

Lactococcus lactis, a lactic acid bacterium with a typical fermentative metabolism, can also use oxygen as an extracellular electron acceptor. Here we demonstrate, for the first time, that L. lactis blocked in NAD+ regeneration can use the alternative electron acceptor ferricyanide to support growth. By electrochemical analysis and characterization of strains carrying mutations in the respiratory chain, we pinpoint the essential role of the NADH dehydrogenase and 2-amino-3-carboxy-1,4-naphtoquinone in extracellular electron transfer (EET) and uncover the underlying pathway systematically. Ferricyanide respiration has unexpected effects on L. lactis, e.g., we find that morphology is altered from the normal coccoid to a more rod shaped appearance, and that acid resistance is increased. Using adaptive laboratory evolution (ALE), we successfully enhance the capacity for EET. Whole-genome sequencing reveals the underlying reason for the observed enhanced EET capacity to be a late-stage blocking of menaquinone biosynthesis. The perspectives of the study are numerous, especially within food fermentation and microbiome engineering, where EET can help relieve oxidative stress, promote growth of oxygen sensitive microorganisms and play critical roles in shaping microbial communities.

Multiple roles of released c-type cytochromes in tuning electron transport and physiological status of Geobacter sulfurreducens.

Dissimilatory metal-reducing bacteria (DMRB) can transfer electrons to extracellular insoluble electron acceptors and play important roles in geochemical cycling, biocorrosion, environmental remediation, and bioenergy generation. c-type cytochromes (c-Cyts) are synthesized by DMRB and usually transported to the cell surface to form modularized electron transport conduits through protein assembly, while some of them are released as extracellularly free-moving electron carriers in growth to promote electron transport. However, the type of these released c-Cyts, the timing of their release, and the functions they perform have not been unrevealed yet. In this work, after characterizing the types of c-Cyts released by Geobacter sulfurreducens under a variety of cultivation conditions, we found that these c-Cyts accumulated up to micromolar concentrations in the surrounding medium and conserved their chemical activities. Further studies demonstrated that the presence of c-Cyts accelerated the process of microbial extracellular electron transfer and mediated long-distance electron transfer. In particular, the presence of c-Cyts promoted the microbial respiration and affected the physiological state of the microbial community. In addition, c-Cyts were observed to be adsorbed on the surface of insoluble electron acceptors and modify electron acceptors. These results reveal the overlooked multiple roles of the released c-Cyts in acting as public goods, delivering electrons, modifying electron acceptors, and even regulating bacterial community structure in natural and artificial environments.

Fe(OH)3 induced the Anammox system to perform extracellular electron transfer for enhancement of NH4+ removal

It is widely found that the ratio of nitrite consumption to ammonia consumption (ΔNO2--N/ΔNH4+-N) in Anammox systems is lower than its stoichiometry of 1.32, but the reason is unclear. In this study, ΔNO2--N/ΔNH4+-N of an Anammox system decreased to 0.80 with the addition of Fe(OH)3, which was significantly lower than a control system without the addition of Fe(OH)3. The lower ratio or more NH4+ removal was likely related to the reduction of Fe(OH)3 that served as extracellular electron acceptor to anaerobically oxidize NH4+, namely Feammox. Chemical or biological oxidation of the produced Fe2+ formed the Fe(III)/Fe(II) cycle to likely cause the continuous Feammox in the Anammox system. Isotopic analysis verified that the Anammox sludge collected from the Fe(OH)3-added reactor produced more 30N2 by Feammox. Raman analysis showed that the outer membrane cytochrome c of Fe(OH)3-added Anammox sludge could participate in the extracellular electron transfer during Feammox. Metagenomic analysis further confirmed that the gene encoding redox enzymes of cytochrome c of Candidatus Brocadia were enriched with adding Fe(OH)3. The results suggested that the Fe(OH)3 induced Anammox bacteria to perform extracellular electron transfer, allowing the Anammox system to effectively remove NH4+ via the Feammox pathway.

Structure of Geobacter cytochrome OmcZ identifies mechanism of nanowire assembly and conductivity.

OmcZ nanowires produced by Geobacter species have high electron conductivity (>30 S cm-1). Of 111 cytochromes present in G. sulfurreducens, OmcZ is the only known nanowire-forming cytochrome essential for the formation of high-current-density biofilms that require long-distance (>10 µm) extracellular electron transport. However, the mechanisms underlying OmcZ nanowire assembly and high conductivity are unknown. Here we report a 3.5-Å-resolution cryogenic electron microscopy structure for OmcZ nanowires. Our structure reveals linear and closely stacked haems that may account for conductivity. Surface-exposed haems and charge interactions explain how OmcZ nanowires bind to diverse extracellular electron acceptors and how organization of nanowire network re-arranges in different biochemical environments. In vitro studies explain how G. sulfurreducens employ a serine protease to control the assembly of OmcZ monomers into nanowires. We find that both OmcZ and serine protease are widespread in environmentally important bacteria and archaea, thus establishing a prevalence of nanowire biogenesis across diverse species and environments.

Genetic Manipulation of Desulfovibrio ferrophilus and Evaluation of Fe(III) Oxide Reduction Mechanisms.

The sulfate-reducing microbe Desulfovibrio ferrophilus is of interest due to its relatively rare ability to also grow with Fe(III) oxide as an electron acceptor and its rapid corrosion of metallic iron. Previous studies have suggested multiple agents for D. ferrophilus extracellular electron exchange including a soluble electron shuttle, electrically conductive pili, and outer surface multiheme c-type cytochromes. However, the previous lack of a strategy for genetic manipulation of D. ferrophilus limited mechanistic investigations. We developed an electroporation-mediated transformation method that enabled replacement of D. ferrophilus genes of interest with an antibiotic resistance gene via double-crossover homologous recombination. Genes were identified that are essential for flagellum-based motility and the expression of the two types of D. ferrophilus pili. Disrupting flagellum-based motility or expression of either of the two pili did not inhibit Fe(III) oxide reduction, nor did deleting genes for multiheme c-type cytochromes predicted to be associated with the outer membrane. Although redundancies in cytochrome or pilus function might explain some of these phenotypes, overall, the results are consistent with D. ferrophilus primarily reducing Fe(III) oxide via an electron shuttle. The finding that D. ferrophilus is genetically tractable not only will aid in elucidating further details of its mechanisms for Fe(III) oxide reduction but also provides a new experimental approach for developing a better understanding of some of its other unique features, such as the ability to corrode metallic iron at high rates and accept electrons from negatively poised electrodes. IMPORTANCE Desulfovibrio ferrophilus is an important pure culture model for Fe(III) oxide reduction and the corrosion of iron-containing metals in anaerobic marine environments. This study demonstrates that D. ferrophilus is genetically tractable, an important advance for elucidating the mechanisms by which it interacts with extracellular electron acceptors and donors. The results demonstrate that there is not one specific outer surface multiheme D. ferrophilus c-type cytochrome that is essential for Fe(III) oxide reduction. This finding, coupled with the lack of apparent porin-cytochrome conduits encoded in the D. ferrophilus genome and the finding that deleting genes for pilus and flagellum expression did not inhibit Fe(III) oxide reduction, suggests that D. ferrophilus has adopted strategies for extracellular electron exchange that are different from those of intensively studied electroactive microbes like Shewanella and Geobacter species. Thus, the ability to genetically manipulate D. ferrophilus is likely to lead to new mechanistic concepts in electromicrobiology.

Fluid-like electrodes and Purple Phototrophic Bacteria: bridging the gap in wastewater biorefineries

Wastewater biorefineries aim to generate value-added products in an economically viable process while removing pollutants. In this scenario, Purple phototrophic bacteria (PPB), the most versatile microorganisms on earth, are highly effective for sustainable wastewater treatment and nutrient recovery as cell protein. One of the most innovative approaches for applying PPB in the wastewater sector is their capacity for interchanging electrons with electroconductive materials. In contrast with classical biofilm-based techniques, we have demonstrated that a fluid-like electrode can accept electrons from planktonically grown PPB. We anticipate that such findings will impact in wastewater electrobioremediating capacity of PPB. Moreover, controlling the electrochemical nature of the extracellular electron acceptor (fluid-like electrode) allows for fine-tuning the metabolism of a planktonic PPB-dominated community to enhance their biodegradation rate (2-fold) while growing on brewery wastewater. For this purpose, a twin set of microbial electrochemical fluidized bed reactors (ME-FBR) were operated in identical conditions, except for illumination conditions (dark vs. infrared), to promote the development of PPB. Illumina sequencing revealed that both infrared radiation and polarization led to changes in the microbial population while producing an electrical current of 7 A·m-3. Indeed, the Geobacter genus was the electroactive bacteria outcompeting under dark conditions. In contrast, electroactive PPB genera like the Rhodopseudomonas and Rhodobacter outcompeted others under infrared illumination and electrostimulation. In this work, we have demonstrated how microbial selection can contribute to the sustainability of an electrobioremediation wastewater treatment by avoiding emissions of greenhouse gases such as methane. In addition, fluid-like bed bioreactors have shown their usefulness in recovering nutrients as PPB biomass, favoring planktonic growth and thus facilitating the recovery of a valuable product: the biomass of PPB.

Co-fitness analysis identifies a diversity of signal proteins involved in the utilization of specific c-type cytochromes

Purposec-Type cytochromes are essential for extracellular electron transfer (EET) in electroactive microorganisms. The expression of appropriate c-type cytochromes is an important feature of these microorganisms in response to different extracellular electron acceptors. However, how these diverse c-type cytochromes are tightly regulated is still poorly understood.MethodsIn this study, we identified the high co-fitness genes that potentially work with different c-type cytochromes by using genome-wide co-fitness analysis. We also constructed and studied the co-fitness networks that composed of c-type cytochromes and the top 20 high co-fitness genes of them.ResultsWe found that high co-fitness genes of c-type cytochromes were enriched in signal transduction processes in Shewanella oneidensis MR-1 cells. We then checked the top 20 co-fitness proteins for each of the 41 c-type cytochromes and identified the corresponding signal proteins for different c-type cytochromes. In particular, through the analysis of the high co-fitness signal protein for CymA, we further confirmed the cooperation between signal proteins and c-type cytochromes and identified a novel signal protein that is putatively involved in the regulation of CymA. In addition, we showed that these signal proteins form two signal transduction modules.ConclusionTaken together, these findings provide novel insights into the coordinated utilization of different c-type cytochromes under diverse conditions.