Enrichment of Clostridia enhances Geobacter population and electron harvesting in a complex electroactive biofilm

Geobacter sulfurreducens strain 60473, a potent bioaugmentation agent for improving the performances of bioelectrochemical systems

Bioaugmentation with electrochemically active bacteria (EAB) has been suggested useful for improving the performance of bioelectrochemical systems (BESs) for sustainable energy generation, while its success is dependent on EAB introduced into the systems. Here we report on the isolation of a novel EAB, Geobacter sulfurreducens strain 60473, from microbes that colonized on an anode of a sediment microbial fuel cell. This strain is highly adhesive to graphite electrodes, forms dense biofilms on electrode surfaces, and generates high current densities in BESs. When microbial electrolysis cells (MECs) inoculated with paddy-field soil and fed starch as the major organic substrate were augmented with strain 60473, Geobacter bacteria predominantly colonized on anodes, and MEC performances, including current generation, hydrogen production and organics removal, were substantially improved compared to non-bioaugmented controls. Results suggest that bioaugmentation with electrode-adhesive EAB, such as strain 60473, is a promising approach for improving the performance of BESs, including MECs treating fermentable organics and biomass wastes.

From "resistance genes expression" to "horizontal migration" as well as over secretion of Extracellular Polymeric Substances: Sludge microorganism's response to the increasing of long-term disinfectant stress

Glutaraldehyde-Didecyldimethylammonium bromides (GDs) has been frequently and widely employed in livestock and poultry breeding farms to avoid epidemics such as African swine fever, but its long-term effect on the active sludge microorganisms of the receiving wastewater treatment plant was keep unclear. Four simulation systems were built here to explore the performance of aerobic activated sludge with the long-term exposure of GDs and its mechanism by analyzing water qualities, resistance genes, extracellular polymeric substances and microbial community structure. The results showed that the removal rates of CODCr and ammonia nitrogen decreased with the exposure concentration of GDs increasing. It is worth noting that long-term exposure to GDs can induce the horizontal transfer and coordinated expression of a large number of resistance genes, such as qacE, sul1, tetx, and int1, in drug-resistant microorganisms. Additionally, it promotes the secretion of more extracellular proteins, including arginine, forming a "barrier-like" protection. Therefore, long-term exposure to disinfectants can alter the treatment capacity of activated sludge receiving systems, and the abundance of resistance genes generated through horizontal transfer and coordinated expression by drug-resistant microorganisms does pose a significant threat to ecosystems and health. It is recommended to develop effective pretreatment methods to eliminate disinfectants.

Towards a new understanding of bioelectrochemical systems from the perspective of microecosystems: A critical review

Bioelectrochemical system (BES) holds promise for sustainable energy generation and wastewater treatment. The microbial communities, as the core of BES, play a crucial role in its performance, thus needing to be systematically studied. However, researches considering microbial communities in BES from an ecological perspective are limited. This review provided a comprehensive summary of the BES with special emphasis on microecological principles, commencing with the dynamic formation and succession of the microbial communities. It also clarified the intricate interspecies relationships and quorum-sensing mechanisms regulated by dominant species. Furthermore, this review addressed the crucial themes in BES-related researches on ecological processes, including growth patterns, ecological structures, and defense strategies against external disturbances. By offering this novel perspective, it would contribute to enhancing the understanding of BES-centered technologies and facilitating future research in this field.

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.

Boosting thermophilic anaerobic digestion with conductive materials: Current outlook and future prospects

Thermophilic anaerobic digestion (TAD) can provide superior process kinetics, higher methane yields, and more pathogen destruction than mesophilic anaerobic digestion (MAD). However, the broader application of TAD is still very limited, mainly due to process instabilities such as the accumulation of volatile fatty acids and ammonia inhibition in the digesters. An emerging technique to overcome the process disturbances in TAD and enhance the methane production rate is to add conductive materials (CMs) to the digester. Recent studies have revealed that CMs can promote direct interspecies electron transfer (DIET) among the microbial community, increasing the TAD performance. CMs exhibited a high potential for alleviating the accumulation of volatile fatty acids and inhibition caused by high ammonia levels. However, the types, properties, sources, and dosage of CMs can influence the process outcomes significantly, along with other process parameters such as the organic loading rates and the type of feedstocks. Therefore, it is imperative to critically review the recent research to understand the impacts of using different CMs in TAD. This review paper discusses the types and properties of CMs applied in TAD and the mechanisms of how they influence methanogenesis, digester start-up time, process disturbances, microbial community, and biogas desulfurization. The engineering challenges for industrial-scale applications and environmental risks were also discussed. Finally, critical research gaps have been identified to provide a framework for future research.

Microbial interactions within beneficial consortia promote soil health

By ecologically interacting with various biotic and abiotic agents acting in soil ecosystems, highly diverse soil microorganisms establish complex and stable assemblages and survive in a community context in natural settings. Besides facilitating soil microbiome to maintain great levels of population homeostasis, such microbial interactions drive soil microbes to function as the major engine of terrestrial biogeochemical cycling. It is verified that the regulative effect of microbe-microbe interplay plays an instrumental role in microbial-mediated promotion of soil health, including bioremediation of soil pollutants and biocontrol of soil-borne phytopathogens, which is considered an environmentally friendly strategy for ensuring the healthy condition of soils. Specifically, in microbial consortia, it has been proven that microorganism-microorganism interactions are involved in enhancing the soil health-promoting effectiveness (i.e., efficacies of pollution reduction and disease inhibition) of the beneficial microbes, here defined as soil health-promoting agents. These microbial interactions can positively regulate the soil health-enhancing effect by supporting those soil health-promoting agents utilized in combination, as multi-strain soil health-promoting agents, to overcome three main obstacles: inadequate soil colonization, insufficient soil contaminant eradication and inefficient soil-borne pathogen suppression, all of which can restrict their probiotic functionality. Yet the mechanisms underlying such beneficial interaction-related adjustments and how to efficiently assemble soil health-enhancing consortia with the guidance of microbe-microbe communications remain incompletely understood. In this review, we focus on bacterial and fungal soil health-promoting agents to summarize current research progress on the utilization of multi-strain soil health-promoting agents in the control of soil pollution and soil-borne plant diseases. We discuss potential microbial interaction-relevant mechanisms deployed by the probiotic microorganisms to upgrade their functions in managing soil health. We emphasize the interplay-related factors that should be taken into account when building soil health-promoting consortia, and propose a workflow for assembling them by employing a reductionist synthetic community approach.

Biodegradation of 1,4-dioxane in a continuous-flow bioelectrochemical reactor by biofilm of Pseudonocardia dioxanivorans CB1190 and microbial community on conductive carriers

Bioelectrochemical degradation is an environmentally friendly, cost-effective and controllable way of providing electron acceptor to the microorganisms. A two-chamber continuous-flow bioelectrochemical reactor (BER) was developed in this study. The objective was to investigate the potential for enhancing the bioelectrochemical degradation of 1,4-dioxane (DX) by Pseudonocardia dioxanivorans CB1190 (CB1190) and microbial community biofilm on conductive and non-conductive carriers in low potentials (1.0-1.2 V) and currents (<2 mA). In the case of CB1190, biodegradation experiments at 1.0 V did not result in any observable change in DX removal efficiency (32.63 ± 2.48%) compared to the 0.0 V (31.69 ± 2.33%). However, the removal efficiency was much higher at 1.2 V (59.08 ± 0.86%). The higher removal at 1.2 V was attributed to an increase in dissolved oxygen (DO) concentration from 3.77 ± 0.33 mg/L at 0.0 V to 5.40 ± 0.11 mg/L at 1.2 V, which resulted from water electrolysis. In the case of microbial community, on the other hand, DX removal efficiency increased at 1.0 V (30.98 ± 1.10%) compared to 0.0 V (23.40 ± 1.02%) that can be attributed to a simultaneous increase in microbial activity from 2389 ± 118.5 ngATP/mgVSS at 0.0 V to 2942 ± 109 ngATP/mgVSS at 1.0 V. Analysis of the changes in microbial composition indicated enrichment of Alistipes and Lutispora at 1.0 V due to the ability of these genera to directly transfer electrons with conductive surface. On the other hand, no change was observed in the microbial community in the case of non-conductive carriers. Results of this study showed that electro-assisted biodegradation of DX at low potentials is possible through two different mechanisms (oxygen production and direct electron transfer with electrode) which makes this technique flexible and cost-effective. The novelty of this work lies in exploring the use of electrical assistance to enhance the biodegradation of DX in the presence of CB1190 and the microbial community. This study more specifically investigated lower potential than required water electrolysis potential, allowing microorganisms to be stimulated through mechanisms unrelated to oxygen generation.

Evolution and interaction of microbial communities in mangrove microbial fuel cells and first description of Shewanella fodinae as electroactive bacterium

Understanding exoelectrogenic bacteria mechanisms and their interactions in complex biofilm is critical for the development of microbial fuel cells (MFCs). In this article, assumptions concerning the benefits of the complex sediment microbial community for electricity production were explored with both the complex microbial community and isolates identified as Shewanella. Analysis of the microbial community revealed a strong influence of the sediment community on anodes and electrolytes compared to that of only water. Moreover, while Pelobacteraceae-related genera were dominant in our MFCs instead of Desulfuromonas and Geobacter as usually reported, the electroactive Shewanella algae and Shewanella fodinae were isolated and cultivated from the anodic biofilm. S. fodinae, described for the first time as an electroactive bacterium to the best of our knowledge, led to a maximal current density of 3.6 A/m² set as 0.3 V/SCE in a three-electrode set-up fed with lactate. S. algae, in a complex medium containing several available substrates, showed several preferential oxidative behaviors including a diauxic behavior. In pure culture and under our conditions, S. fodinae and S. algae were not able to use acetate as a sole electron donor. However, their presence in our acetate-fed MFCs and the adaptive behavior of S. algae hint a syntrophic interaction between the bacteria to optimize the use of the substrate in a complex environment.

Isolation and Characterisation of Electrogenic Bacteria from Mud Samples

To develop efficient microbial fuel cell systems for green energy production using different waste products, establishing characterised bacterial consortia is necessary. In this study, bacteria with electrogenic potentials were isolated from mud samples and examined to determine biofilm-formation capacities and macromolecule degradation. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identifications have revealed that isolates represented 18 known and 4 unknown genuses. They all had the capacities to reduce the Reactive Black 5 stain in the agar medium, and 48 of them were positive in the wolfram nanorod reduction assay. The isolates formed biofilm to different extents on the surfaces of both adhesive and non-adhesive 96-well polystyrene plates and glass. Scanning electron microscopy images revealed the different adhesion potentials of isolates to the surface of carbon tissue fibres. Eight of them (15%) were able to form massive amounts of biofilm in three days at 23 °C. A total of 70% of the isolates produced proteases, while lipase and amylase production was lower, at 38% and 27% respectively. All of the macromolecule-degrading enzymes were produced by 11 isolates, and two isolates of them had the capacity to form a strong biofilm on the carbon tissue one of the most used anodic materials in MFC systems. This study discusses the potential of the isolates for future MFC development applications.

Bacterial community structure of electrogenic biofilm developed on modified graphite anode in microbial fuel cell

Formation of electrogenic microbial biofilm on the electrode is critical for harvesting electrical power from wastewater in microbial biofuel cells (MFCs). Although the knowledge of bacterial community structures in the biofilm is vital for the rational design of MFC electrodes, an in-depth study on the subject is still awaiting. Herein, we attempt to address this issue by creating electrogenic biofilm on modified graphite anodes assembled in an air-cathode MFC. The modification was performed with reduced graphene oxide (rGO), polyaniline (PANI), and carbon nanotube (CNTs) separately. To accelerate the growth of the biofilm, soybean-potato composite (plant) powder was blended with these conductive materials during the fabrication of the anodes. The MFC fabricated with PANI-based anode delivered the current density of 324.2 mA cm-2, followed by CNTs (248.75 mA cm-2), rGO (193 mA cm-2), and blank (without coating) (151 mA cm-2) graphite electrodes. Likewise, the PANI-based anode supported a robust biofilm growth containing maximum bacterial cell densities with diverse shapes and sizes of the cells and broad metabolic functionality. The alpha diversity of the biofilm developed over the anode coated with PANI was the loftiest operational taxonomic unit (2058 OUT) and Shannon index (7.56), as disclosed from the high-throughput 16S rRNA sequence analysis. Further, within these taxonomic units, exoelectrogenic phyla comprising Proteobacteria, Firmicutes, and Bacteroidetes were maximum with their corresponding level (%) 45.5, 36.2, and 9.8. The relative abundance of Gammaproteobacteria, Clostridia, and Bacilli at the class level, while Pseudomonas, Clostridium, Enterococcus, and Bifidobacterium at the genus level were comparatively higher in the PANI-based anode.

Degradation of phenol in the bio-cathode of a microbial desalination cell with power generation and salt removal

In this study, the performance of a three-chamber microbial desalination cell (MDC) was assessed to simultaneously remove salt (35 g.L^-1) from water and degrade phenol as a hazardous compound. Two parallel MDCs with the same configurations were run using glucose as the chemical oxygen demand (COD) at an initial concentration of 1.5 g.L^-1 as the anolyte. MDC#1 operated with 10 mM phosphate buffer solution (PBS), while MDC#2 operated with bio-cathode as the catholyte for the degradation of 100 mg.L^-1 of phenol. The use of MDC#1 resulted in a power density, desalination efficiency, and COD removal of 366.2 mW.m^-2, 50.3 ± 4.0 %, and 79.3 ± 2.2 %, respectively. All performance parameters were improved in MDC#2 with bio-cathode so that power density, desalination efficiency, and COD removal reached 660.1 mW.m^-2, 72.1 ± 3.0 %, and 92.6 ± 2.4 %, respectively. Also, more than 96 % of phenol was degraded using bio-cathode within 7 h of operation. Bio-cathode could enhance the performance of the MDC reactor through catalyzing the final reactions of electron acceptors compared to MDC#1 with a chemical cathode. In general, the results indicated that heterotrophic microorganisms, able to grow alongside autotrophic bacteria, could effectively extend the applications of MDC reactors to degrade hazardous compounds in cathode chambers.

Performance of bioelectrode based on different carbon materials in bioelectrochemical anaerobic digestion for methanation of maize straw

The performance of the bioelectrochemical anaerobic digestion (BEAD) reactor was investigated with different carbon material-modified electrodes for the methanation of maize straw. The carbon material-modified electrodes used titanium (Ti) mesh modified with carbon nanotube (CNT), carbon black (CB), and activated carbon (AC). The maximum cumulative methane production obtained in the Ti-CNT reactor was (616.4 ± 9.3) mL/g VS, while the maximum methane production rate in the Ti-AC reactor was (61.9 ± 1.0) mL/g VS.d.The electroactive bacteria were well enriched by the different electrodes, and the enriched electroactive bacteria further facilitate the direct interspecies electron transfer (DIET) for methane production. Additionally, we found the phylum Firmicutes showed a linear relationship to methanogenic performance, as well as the Genus Proteiniborus. The Ti-CNT electrode shows better performance by the electrochemical analysis. These findings provide critical knowledge for the large-scale use of the BEAD process and the treatment of maize straw.

High current density with spatial distribution of Geobacter in anodic biofilm of the microbial electrolysis desalination and chemical-production cell with enlarged volumetric anode

The aim of this study was to establish the relationship between spatial distribution of Geobacter and electric intensity in the microbial electrolysis desalination and chemical-production cell (MEDCC) and to investigate the effect of enlarged volumetric anode on the performance of MEDCC. The MEDCC was constructed with nine carbon brush anodes (length × diameter = 11 cm × 3 cm) as enlarged volumetric anode, and operated by feeding with 1 g/L acetate as substrate and 35 g/L NaCl as artificial seawater under the applied voltages of 1.2-4.5 V. Spatial distribution of Geobacter in the anodic biofilm was determined according to the bacterial community analysis on 27 biofilm samples from the top, middle and bottom layers of anodes (i.e., with distance of 4.5, 10, and 15.5 cm to the cathode, respectively). Results showed that the enlarged volumetric anode significantly improved the performance of MEDCC. The maximum desalination rate and current density reached 338.5 ± 21.8 mg/L∙h and 55.7 ± 3.7 A/m² in the MEDCC, respectively. The electric intensity values decreased with the distance from the anode to the cathode and formed an uneven distribution in the anode chamber. The samples in the top layer of anodes had the highest average 16S rRNA gene copy number of Geobacter of 1.55 × 10⁷ copies/μL, which was 18 times higher than that in the bottom layer of anodes. A linear relation was established between the spatial distribution of Geobacter and electric intensity (R² = 0.994-0.999). The electric intensity gradient created the uneven spatial distribution of Geobacter in the biofilms of volumetric anode. Results from this study could be useful to enrich Geobacter in the anodic biofilm thus to improve the performance of MEDCC.