Methanobacterium Capable of Direct Interspecies Electron Transfer

Fe3O4 nanoparticles promote methanogenesis in propionate acclimated system

Propionate accumulation is common in anaerobic digestion systems, while Fe3O4 nanoparticles (Fe3O4 NPs) show potential in steering direct interspecies electron transfer (DIET) to facilitate methanogenesis from propionate. However, the effect of Fe3O4 NPs in stable microbial communities remains unclear. This study demonstrated that 1.0 g/L Fe3O4 NPs enhanced propionate degradation and methane production by 220 % and 55 % of the propionate-acclimated microbial consortia post-shock loading, as evidenced by batch-test. High propionate concentrations suppressed Geobacter, yet Fe3O4 NPs enabled electron syntrophy between alternative DIET-participants (Arcobacter, Syntrophobacter) and methanogens (Methanothrix, Methanobacterium) serving as electron conduits. Network analysis further revealed that Fe3O4 NPs activated interactions among potential electroactive microbes, highlighting the potentially ubiquitous presence of electron syntrophy. Even in propionate-stressed microbial communities, such mutualism may be rapidly activated by Fe3O4 NPs, offering a practical solution for mitigating propionate accumulation and enhance digester performance.

Enhancement of the start-up and performance of an upflow anaerobic sludge blanket (UASB) reactor using electrochemically-enriched biofilm

A novel approach was developed to accelerate the start-up of a 20-L UASB reactor under mesophilic conditions. Two runs were conducted, where the first run (Run I) was inoculated with anaerobic sludge, and the second run (Run II) was inoculated with the same sludge supplemented with enriched electro-active biofilms collected from the working and counter electrodes of anodic and cathodic bio-electrochemical systems (BESs). Reactors' performance and microbial dynamics were monitored over 41 days. Methane production in Run II exceeded 200 mL-CH4/g-COD within 10 days, compared to 29 days in Run I. Run II achieved 80 % removal of soluble COD after 13 days as compared to 23 days in Run I. Sludge washout in Run II stabilized after 3 days, achieving 70 % VSS removal, whereas Run I required 17 days. Greater extracellular polymeric substance (EPS) values and higher protein-to-polysaccharide ratios in Run II may indicate accelerated granules formation mediated by EPS. 16S rRNA gene sequencing analysis results revealed shared genera between both runs but different relative abundances. Methanothrix dominated in Run I, while other archaeal genera, mainly Methanosarcina and Methanobacterium increased in abundance in the Run II. The Enterobacteriaceae family was prevalent in both reactors, with three genera, Citrobacter, Klebsiella, and Enterobacter distinctly dominating at different time points, suggesting potential links with the initial seed sludge or enriched biofilm consortia. The addition of electrochemically grown biofilm in Run II likely enhanced the microbial diversity, contributed to the rapid development of granular syntrophic communities, and improved reactor performance.

Enhanced methanogenesis of wastewater anaerobic digestion by nanoscale zero-valent iron: Mechanism on intracellular energy conservation and amino acid metabolism

Nanoscale zero-valent iron (nZVI)-mediated anaerobic digestion commonly focuses on electron transfer between syntrophic bacteria, neglecting intracellular energy conservation strategies and amino acid metabolism. In this study, F420H2 dehydrogenase abundance increased by 5.1 %, 27.0 %, and 31.5 % at 10 mM, mM， 30 mM, and 50 mM nZVI dosing, respectively, enabling an efficient transmembrane proton-coupled electron transfer mode. Electron bifurcation (EB) enzymes involved in methanogenesis responded differently to nZVI, with HdrA2B2C2 initially increasing at 10 mM and decreasing at 30 mM and 50 mM, while MvhADG-HdrABC was completely down-regulated. Metabolomics further demonstrated that nZVI reduced riboflavin and flavin mononucleotide content, which is detrimental to the EB. Instead, an alternative measure to maintain electron flow and energy conservation under high nZVI exposure is high expression of ndh and F-type or V/A-type ATPase genes. Additionally, enhancing C1-unit carrier expression through amino acid metabolism regulation emerged as a key strategy. This study provides new perspectives on nZVI-mediated anaerobic digestion.

Biogas production enhancement from anaerobic digestion with magnetic biochar: Insights into the functional microbes and DIET

The application of magnetic biochar in anaerobic digestion (AD) has gained increasing attention. However, the underlying mechanisms remain insufficiently understood. This study systematically investigated the effects of magnetic biochar on functional microbial communities involved in methanogenesis and elucidated its role in promoting direct interspecies electron transfer (DIET) within AD systems. The addition of 40 mg g-1 TSadded of magnetic biochar significantly enhanced methane production by 42.21 %, reaching 223.08 mL g-1 TS with highest organic matter degradation efficiency. Microbial community analysis showed that magnetic biochar significantly enriched microorganisms associated with hydrolysis, acidogenesis, and methanogenesis, as well as electroactive microorganisms' abundance such as Geobacter spp., Syntrophus spp., P. aestuarii, and M. harundinacea, providing direct evidence for the DIET process of AD with magnetic biochar. Furthermore, the abundance of key genes involved in the DIET, including pilA, Fpo, and the genes encoded outer-membrane c-type cytochromes, was respectively upregulated by 44.49 %, 22.04 %, and 37.6 % in the presence of magnetic biochar. These findings suggest that magnetic biochar enhances the production of conductive pili and cytochrome c, facilitating extracellular electron transfer between syntrophic microorganisms. This accelerated electron transfer promotes CO2 reduction to CH4, ultimately improving methane production efficiency in the AD system. Moreover, the enhancement of hydrogenotrophic methanogenesis was particularly pronounced with magnetic biochar, further contributing to the improved AD performance. This study provides novel mechanistic insights into biochar-mediated DIET, offering a theoretical basis for optimizing biochar applications in AD.

Enhancing sulfide mitigation via the synergistic dosing of calcium peroxide and ferrous ions in gravity sewers: Efficiency and mechanism

Chemical dosing constitutes an effective strategy for sulfide control in sewers; however, its efficacy requires further optimization and enhancement. In this study, a novel dosing strategy using the synergistic dosing of calcium peroxide (CaO2) and ferrous ions (Fe2+) for sulfide control was proposed, and its efficacy in controlling sulfides was evaluated using a long-term laboratory-scale reactor. The results showed that adding CaO2-Fe2+ improves the effect of sulfide control. When the ratio of the agent to the sewage (w/v) was 0.30 %, the RT50 of sulfide production rate was 8.34 days. The analysis of microbial communities in sewage biofilm revealed that the relative abundances of sulfate-reducing bacteria (SRB) and sulfide-oxidizing bacteria (SOB) demonstrated an overall downward tendency, suggesting that the potent oxidizing •OH generated by the synergism of CaO2 and Fe2+ could indiscriminately restrain the growth of microorganisms. Additionally, intracellular metabolic pathways, along with enzyme activities and the relative abundances of genes associated with sulfide metabolism, were significantly impaired. The cost of CaO2-Fe2+ synergistic dosing is 31.3 % of CaO2 and 63.4 % of Fe2+ alone addition. It can be reasonably proposed that the addition of CaO2-Fe2+ may provide an efficacious and cost-effective method for the mitigation of sulfide in sewer systems.

Effects of micro-magnetite on anaerobic co-digestion of waste activated sludge and slaughterhouse waste: Microbial community and metabolism analyses

Micro-magnetite has been widely applied to improve anaerobic digestion (AD) performance, while comprehensive investigation of microbial community succession, metabolic pathway and magnetite fate remains unclear. In the current study, the effects of micro-magnetite (Fe3O4) on anaerobic co-digestion (AcD) of waste activated sludge and slaughterhouse waste were investigated. Experimental results indicated that the cumulative methane production was significantly increased from 484.6 mL/g VS to 524.4 mL/g VS with 0.8 g/L Fe3O4 addition. Recycled magnetite remained the initial physicochemical properties, including morphology, particle size and crystal structure, as evidenced by various characterization methods. Microbial community analysis indicated that magnetite addition enriched syntrophic bacteria (Armatimonadota, Syntrophomonas and Petrimonas) and methanogens (Methanosarcina). Metagenomic sequencing analysis demonstrated that hydrolysis and acidogenesis metabolic pathways were reinforced by magnetite addition. Meanwhile, the magnetite stimulated the direct interspecies electron transfer via enriching syntrophic microbes (Syntrophomonas and Methanosarcina) and conductive pili functional genes (pilA, mshA and mshC), finally achieving higher cumulative methane yield. This study provided in-depth investigation of the methane production facilitated by micro-magnetite addition and the magnetite fate during the AcD process.

Enhanced methane production through synergistic integration of quorum sensing Signals and microbial electrolysis in anaerobic digestion of low-quality biomass

This study explored the synergistic effects of exogenous acyl homoserine lactones (AHLs) and microbial electrolysis (ME) on enhancing anaerobic digestion (AD) of low-quality biomass. The combined AHLs + ME strategy achieved a 79.50 % increase in cumulative methane production compared to the control, outperforming individual treatments. This enhancement was attributed to accelerated substrate degradation, selective enrichment of hydrogenotrophic methanogens (Methanobrevibacter, Methanobacterium, Methanofollis), and strengthened quorum sensing, electron transfer, and methane synthesis pathways. Metagenomic analysis revealed abundance upregulation of key genes involved in AHL synthesis (bjaI, rpaI, braI, rhiI) and sensing (solR, cepR, tofR), direct interspecies electron transfer (pilA, mtrC), and hydrogenotrophic methanogenesis (ftr, mvhD, vhuD, vhcD). This study offers a novel and sustainable strategy to optimize methane production from recalcitrant biomass, advancing AD-based waste-to-energy systems.

Rethinking the biochar impact on the anaerobic digestion of food waste in bench-scale digester: Spatial distribution and biogas production

The improvement of biogas production in anaerobic digestion (AD) by biochar introduction has been demonstrated. However, the distribution of biochar in the digester and its effect on AD have been seldom explored. In this study, the distribution of biochar and their impact on AD were investigated in a 30 L semi-continuously operated bench-scale anaerobic digester. The results demonstrated that the biochar significantly increased biogas yields by 23.38 % under an organic loading rate (OLR) of 3.0 g VS/L·d. The stability of the AD under an OLR of 4.0 g VS/L·d was also improved by biochar introduction. The increased stirring speed of the digester enhanced the spatial distribution uniformity of biochar and increased biogas production by 5.89 %. Reducing the particle size of biochar improved its spatial distribution uniformity but did not significantly increase biogas production, likely due to excessive microbial accumulation on the biochar, which have caused substrate competition. Biochar aided AD by boosting microbial genera of Syntrophomonas, Bacteroidota, Cloacimonadot, and Methanosaeta, accelerating volatile fatty acids consumption, and improving microorganisms' spatial ecological niches. The economic analysis showed that applying residue-based biochar for biogas production presented superior benefits and greater development potential than residue incineration in the food waste AD process. Overall, this study presented a novel and comprehensive understanding of the biochar distribution and impact on food waste AD in digesters.

Activated carbon and anthraquinone-2,6-disulfonate as electron shuttles for enhancing carbon and nitrogen removal from simultaneous methanogenesis, Feammox and denitrification system

Anthraquinone-2,6-disulfonate (AQDS) and activated carbon (AC) were employed as exogenous electron shuttles (ESs) for enhancing the performance of an integrated simultaneous methanogenesis, Feammox, and denitrification (SMFD) system treating fish sludge. The addition of AQDS and AC led to an increased total nitrogen removal efficiency by 30.2 % and 66.5 %, an increased total chemical oxygen demand removal efficiency by 9.5 % and 24.5 %, and an improved methane yield by 5.2 % and 12.6 %, respectively. Regarding nitrogen removal, AQDS mainly facilitated NH4+-N oxidation into NO3--N via Feammox, while AC facilitated both Feammox and denitrification. Regarding carbon removal, both ESs promoted the hydrolysis-acidification process via stimulating dissimilatory iron reduction and established direct interspecies electron transfer (DIET) between methanogens and syntrophic bacteria. Microbial analysis confirmed the enrichment of iron-reducing bacteria, denitrifiers, DIET-related methanogens and syntrophic partners in the presence of ESs. The study provides an ESs-assisted strategy for enhancing simultaneous nitrogen and carbon removal from high-strength wastewater.

Microbial activity of the inoculum determines the impact of activated carbon, magnetite and zeolite on methane production

The conversion of organic matter to methane through anaerobic digestion (AD) process can be enhanced by different materials. However, literature reports show inconsistent results on the effect of materials in different AD systems. In this study, we evaluated the influence of the inoculum's activity on methane production (MP) efficiency in the presence of different materials (activated carbon (AC), magnetite (Mag), and zeolite (Zeo)). The inocula included pure cultures of methanogens, syntrophic cocultures, and complex microbial communities, and the kinetic parameters assessed were the lag phase duration and methane production rates (MPR). The results showed that the microbial activity of the inocula is an important factor determining materials' effect on MP kinetics. AC, Mag, and Zeo significantly enhanced the MP profiles of less active microbial communities or low-active microorganisms by decreasing lag phases duration up to 85 %, consequently increasing MPR up to 15 times. Contrarily, these materials did not affect highly active microbial communities or pure cultures, as MP profiles tend to be similar with and without materials. These results indicate that from an applied point of view, the addition of materials to anaerobic bioreactors should be considered only when the methanogenic activity of the sludge is low or compromised.

Unlocking anaerobic digestion potential via extracellular electron transfer by exogenous materials: Current status and perspectives

The efficiency of energy transfer among microorganisms presents a substantial hurdle for the widespread implementation of anaerobic digestion techniques. Nonetheless, recent studies have demonstrated that enhancing the extracellular electron transfer (EET) can markedly enhance this efficiency. This review highlights recent advancements in EET for anaerobic digestion and examines the contribution of external additives to fostering enhanced efficiency within this context. Diverse mechanisms through which additives are employed to improve EET in anaerobic environments are delineated. Furthermore, specific strategies for effectively regulating EET are proposed, aiming to augment methane production from anaerobic digestion. This review thus offers a perspective on future research directions aimed at optimizing waste resources, enhancing methane production efficiency, and improving process predictability in anaerobic digestion.

Enhancing CO2-reduction methanogenesis in microbial electrosynthesis: Role of oxygen-containing groups on carbon-based cathodes

Microbial electrosynthesis is a promising technology that recovers energy from wastewater while converting CO2 into CH4. Constructing a biocathode with both strong H2-mediated and direct electron transfer capacities is crucial for efficient startup and long-term stable CH4 production. This study found that introducing carboxyl groups onto the cathode effectively enhanced both electron transfer pathways, improving the reduction rate and coulombic efficiency of CH4 production and increasing the CH4 yield by 2-3 times. Carboxyl groups decreased the overpotential for H2 evolution and increased current density, thereby enhancing H2-mediated electron transfer. Additionally, carboxyl groups increased the relative abundance of Methanosaeta by 3%-10%, doubled the protein content in extracellular polymeric substances, and boosted the expression of cytochrome c-related genes, thereby enhancing direct electron transfer capacity. These findings present a novel and efficient approach for constructing a stable, high-performance biocathode, contributing to energy recovery and CO2 fixation.

Investigating role of corn stover biochar supplementation on continuous pilot scale anaerobic digestion: Performance and microbial community dynamics

Biochar supplementation could facilitate microbial colonization and increase biogas and methane yield in anaerobic digestion (AD). This study investigated the impact of corn stover biochar (CSB) augmentation on the continuous pilot scale AD of rice straw. The CSB supplementation exhibited the daily average specific biogas and methane yield of 368.6 L/kg volatile solids (VS) and 230 L/kg VS, which were 35% and 37% higher than the control. Principle component analysis indicated that the VS reduction positively correlated with the daily average specific biogas and methane yield. Metagenomic analysis revealed that the CSB supplementation facilitated microbial colonization with the enrichment of unclassified genera from families Planococcaceae, Clostridiaceae, and Ruminococcaceae, and genus Clostridium and methanogenic archaea (Methanosarcina and Methanobacterium). The co-occurrence network revealed a notable shift in microbial interactions following the supplementation of CSB as an additive in AD. These results offer insights for enhancing the performance of the AD using CSB.

Synchronous reinforcement azo dyes decolorization and anaerobic granular sludge stability by Fe, N co-modified biochar: Enhancement based on extracellular electron transfer

Anaerobic digestion (AD) treatment of azo dyes wastewater often suffers from low decolorization efficiency and poor stability of anaerobic granular sludge (AnGS). In this study, iron and nitrogen co-modified biochar (FNC) was synthesized based on the secondary calcination method, and the feasibility of this material for enhanced AD treatment of azo dye wastewater and its mechanism were investigated. FNC not only formed richer conducting functional groups, but also generated Fe2+/Fe3+ redox pairs. The decolorization efficiency of Congo red and AD properties (e.g., methane production) were enhanced by FNC. After adding FNC, the content of extracellular polymeric substances (EPS) and the ratio of proteins remained stable under the impact of Congo red, which greatly protected the internal microbial community. This was mainly contributed to the excellent electrochemical properties of FNC, which strengthened the microbial extracellular electron transfer and realized the coupled mechanism of action: On the one hand, an electron transfer bridge between decolorizing bacteria and dyes was constructed to achieve rapid decolorization of azo dyes and mitigate the impact on methanogenic bacteria; On the other hand, the stability of AnGS was enhanced based on enhanced extracellular polymeric substances secretion, microbial community and direct interspecies electron transfer (DIET) process. This study provides a new idea for enhanced AD treatment of azo dyes wastewater.

Ammonia-stressed anaerobic digestion: Sensitivity dynamics of key syntrophic interactions and methanogenic pathways-A review

The problematic anaerobic digestion (AD) of protein-rich substrates owing to their high ammonia content continues to hinder optimum methanation despite their high potential for offsetting greenhouse gas (GHG) emissions. This review focuses on the analyses of the sensitivity dynamics of key AD processes as well as the microbial interactions and exchanges that occur with them. Aside from the apparent increased risk associated with thermophilic ammonia-rich substrate AD, the marginally higher energy generation compared to mesophilic systems is not commensurate to the energy requirement. Moreover, while comparable FAN thresholds have been confirmed, TAN thresholds are susceptible to physical chemistry and so vary greatly. Profiling of the metabolic capability of front-end AD microbiome revealed Bacteroidetes, Firmicutes, and Synergistetes as some of the ammonia-resilient bacteria groups while Proteobacteria and Actinobacteria were the most fragile taxa. Besides the predominance of incomplete propionate oxidizing bacteria under ammonia stress conditions, syntrophic propionate oxidation (SPO) is usually shifted from the methylmalonyl CoA to the dismutation pathway. Furthermore, besides their different recoverability potentials, distinct methanogenic groups are differentially impacted by different ammonia species. Prevailing literature evidence suggests that conductive material assisted bioaugmentation with SAO-HM consortia, and in-situ H2 supplementation are the most effective for expediting electron transfer and relieving ammonia stress. These valuable insights should inform the design of targeted ammonia inhibition mitigation strategies.

Long-term and multiscale assessment of methanogenesis enhancement mechanisms in magnetite nanoparticle-mediated anaerobic digestion reactor

Magnetite nanoparticles (Fe3O4-NPs) have been demonstrated to be involved in direct interspecies electron transfer between syntrophic bacteria, yet a comprehensive assessment of the ability of Fe3O4-NPs to cope with process instability and volatile fatty acids (VFAs) accumulation in scaled-up anaerobic reactors is still lacking. Here, we investigated the start-up characteristics of an expanded granular sludge bed (EGSB) with Fe3O4-NPs as an adjuvant at high organic loading rate (OLR). The results showed that the methane production rate of R1 (with Fe3O4-NPs) was approximately 1.65 folds of R0 (control), and effluent COD removal efficiency was maintained at approximately 98.32% upon 20 kg COD/(m3·d) OLR. The components of volatile fatty acids are acetate and propionate, and the rapid scavenging of propionate accumulation was the difference between R1 and the control. The INT-ETS activity of R1 was consistently higher than that of R0 and R2, and the electron transfer efficiencies increased by 68.78% and 131.44%, respectively. Meanwhile, the CV curve analysis showed that the current of R1 was 40% higher than R3 (temporary addition of Fe3O4-NPs), indicating that multiple electron transfer modes might coexist. High-throughput analysis further revealed that it was difficult to reverse the progressive deterioration of system performance with increasing OLR by simply reconfiguring bacterial community structure and abundance, demonstrating that the Fe3O4-NPs-mediated DIET pathway is a prerequisite for establishing multiple electron transfer systems. This study provides a long-term and multi-scale assessment of the gaining effect of Fe3O4-NPs in anaerobic digestion scale-up devices, and provides technical support for their practical engineering applications.

Insights into the biocorrosion of Q235A steel influenced by the electron transfer process between iron and methanogenic microbiota

Anaerobic microbiologically influenced corrosion (MIC) of Fe (0) metals causes great harm to the environment and economy, which depends on the key electron transfer process between anaerobic microorganisms and Fe (0) metals. However, the key electron transfer process in microbiota dominating MIC remains unclear, especially for methanogenic microbiota wildly distributed in the environment. Herein, three different methanogenic microbiota (Methanothrix, Methanospirillum, and Methanobacterium) were acclimated to systematically investigate electron transfer pathways on corroding Q235A steel coupons. Results indicated that microbiota dominated by Methanothrix, Methanospirillum, or Methanobacterium accelerated the steel corrosion mainly through direct electron transfer (DET) pathway, H2 mediated electron transfer (HMET) pathway, and combined DET and HMET pathways, respectively. Compared with Methanospirillum dominant microbiota, Methanothrix or Methanobacterium dominant microbiota caused more methane production, higher weight loss, corrosion pits with larger areas, higher corrosion depth, and smaller corrosion pits density. Such results reflected that the DET process between microbiota and Fe (0) metals decided the biocorrosion degree and behavior of Fe (0) metals. This study insightfully elucidates the mechanisms of methanogenic microbiota on corroding steels, in turn providing new insights for anti-corrosion motives.

Non-antibiotic disinfectant synchronously interferes methane production and antibiotic resistance genes propagation during sludge anaerobic digestion: Activation of microbial adaptation and reconfiguration of bacteria-archaea synergies

Waste activated sludge (WAS) presents both resource recovery potential and pollution risks, making its efficient treatment challenging. Anaerobic digestion is broadly recognized as a green and sustainable approach to WAS treatment, whose efficiency is easily impacted by the exogeneous pollutants in WAS. However, the impact of polyhexamethylene guanidine (PHMG), as a widely-used non-antibiotic disinfectant, on WAS digestion under semi-continuous flow conditions remains unclear. In this study, CH4 production decreased from 16.1 mL/g volatile suspended solids (VSS) in the control to 13.2 mL/g VSS and 0.3 mL/g VSS under low and high PHMG exposure, respectively, while PHMG increased the number of antibiotic resistance gene (ARG) copies per bacterium by 4.6-12.7 %. Molecular docking analysis revealed that PHMG could spontaneously bind to and disintegrate WAS (binding energy:2.35 and -9.62 kcal/mol), increasing the likelihood of microbial exposure to PHMG. This led to an increase in bacterial abundance and a reduction in archaeal populations, resulting in bacterial dominance in ecological niches. The network topology index in PHMG-treated reactors was consistently lower than in the control, with a higher proportion of negatively correlated links, indicating a more antagonistic relationship between bacteria and archaea. Consequently, PHMG significantly interfered with key genes involved in CH4 biosynthesis (e.g., mch and mtd). Interestingly, methanogenic activity and archaeal chemotaxis (e.g., rfk and cheA) partially recovered under low PHMG exposure due to archaeal adaptation through quorum sensing and two-component systems. However, this adaptation process also contributed to the propagation of ARGs through horizontal gene transfer, facilitated by the enhancement of mobile genetic elements and ARGs hosts. These findings confirm the ecological risks of PHMG and highlight the need for effective WAS disposal strategies.

Advances in microbial community, mechanisms and stimulation effects of direct interspecies electron transfer in anaerobic digestion

Anaerobic digestion (AD) has been proven to be an effective green technology for producing biomethane while reducing environmental pollution. The interspecies electron transfer (IET) processes in AD are critical for acetogenesis and methanogenesis, and these IET processes are carried out via mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET). The latter has recently become a topic of significant interest, considering its potential to allow diffusion-free electron transfer during the AD process steps. To date, different multi-heme c-type cytochromes, electrically conductive pili (e-pili), and other relevant accessories during DIET between microorganisms of different natures have been reported. Additionally, several studies have been carried out on metagenomics and metatranscriptomics for better detection of DIET, the role of DIET's stimulation in alleviating stressed conditions, such as high organic loading rates (OLR) and low pH, and the stimulation mechanisms of DIET in mixed cultures and co-cultures by various conductive materials. Keeping in view this significant research progress, this study provides in-depth insights into the DIET-active microbial community, DIET mechanisms of different species, utilization of various approaches for stimulating DIET, characterization approaches for effectively detecting DIET, and potential future research directions. This study can help accelerate the field's research progress, enable a better understanding of DIET in complex microbial communities, and allow its utilization to alleviate various inhibitions in complex AD processes.

Anode potential regulates gas composition and microbiome in anaerobic electrochemical digestion

Anaerobic electrochemical digestion (AED) is an effective system for recovering biogas from organic wastes. However, the effects of different anode potentials on anaerobic activated sludge remain unclear. This study confirmed that biofilms exhibited the best electroactivity at -0.2 V (vs. Ag/AgCl) compared to -0.4 V and 0 V. Gas was further regulated, with the highest hydrogen content (47 ± 7 %) observed at -0.2 V. The 0 V system produced the largest amount of methane (70 ± 8 %) and exhibited the greatest presence of hydrogen-utilizing microorganisms. The gas yield at -0.4 V was the lowest, with no hydrogen detected. Excess bioelectrohydrogen at -0.2 V and 0 V caused the co-enrichment of Methanobacterium and Acetoanaerobium, establishing a thermodynamically feasible current-acetate-hydrogen electron cycle to improve electrogenesis. These results provide insights into the regulatory strategies of MEC technology during anaerobic digestion, which play a decisive role in determining the composition of biogas.

Correlation between intracellular electron transfer and gene expression for electrically conductive pili in electroactive bacteria during anaerobic digestion with ethanol

Ethanol feeding has been widely documented as an economical and effective strategy for establishing direct interspecies electron transfer (DIET) during anaerobic digestion. However, the mechanisms involved are still unclear, especially on correlation between intracellular electron transfer in electroactive bacteria and their gene expression for electrically conductive pili (e-pili), the most essential electrical connection component for DIET. Upon cooling from room temperature, the conductivity of digester aggregates with ethanol exponentially increased by an order of magnitude (from 45.5 to 125.4 μS/cm), whereas which with its metabolites (acetaldehyde [from 40.5 to 54.4 μS/cm] or acetate [from 32.1 to 50.4 μS/cm]) did not increase significantly. In addition, the digester aggregates only with ethanol were observed with a strong dependence of conductivity on pH. Metagenomic and metatranscriptomic analysis showed that Desulfovibrio desulfuricans was the most dominant and metabolically active bacterium that contained and highly expressed the genes for e-pili. Abundance of genes encoding the total type IV pilus assembly proteins (6.72E-04 vs 1.24E-03, P < 0.05), PilA that determined the conductive properties (2.22E-04 vs 2.44E-04, P > 0.05), and PilB that proceeded the polymerization of pilin (1.56E-04 vs 3.52E-03, P < 0.05) with ethanol was lower than that with acetaldehyde. However, transcript abundance of these genes with ethanol was generally higher than that with acetaldehyde. In comparison to acetaldehyde, ethanol increased the transcript abundance of genes encoding the key enzymes involved in NADH/NAD+ transformation on complex I and ATP synthesis on complex V in intracellular electron transport chain. The improvement of intracellular electron transfer in D. desulfuricans suggested that electrons were intracellularly energized with high energy to activate e-pili during DIET.

Metabolism mechanisms of biogenic methane production by synergistic biodegradation of lignite and guar gum

Coalbed methane (CBM) presents a promising energy source for addressing global energy shortages. Nonetheless, challenges such as low gas production from individual wells and difficulties in breaking gels at low temperatures during extraction hinder its efficient utilization. Addressing this, we explored native microorganisms within coal seams to degrade guar gum, thereby enhancing CBM production. However, the underlying mechanisms of biogenic methane production by synergistic biodegradation of lignite and guar gum remain unclear. Research results showed that the combined effect of lignite and guar gum enhanced the production, yield rate and concentration of biomethane. When the added guar gum content was 0.8 % (w/w), methane production of lignite and guar gum reached its maximum at 561.9 mL, which was 11.8 times that of single lignite (47.3 mL). Additionally, guar gum addition provided aromatic and tryptophan proteins and promoted the effective utilization of CC/CH and OCO groups on the coal surface. Moreover, the cooperation of lignite and guar gum accelerated the transformation of volatile fatty acids into methane and mitigated volatile fatty acid inhibition. Dominant bacteria such as Sphaerochaeta, Macellibacteroides and Petrimonas improved the efficiency of hydrolysis and acidification. Electroactive microorganisms such as Sphaerochaeta and Methanobacterium have been selectively enriched, enabling the establishment of direct interspecies electron transfer pathways. This study offers valuable insights for increasing the production of biogenic CBM and advancing the engineering application of microbial degradation of guar gum fracturing fluid. Future research will focus on exploring the methanogenic capabilities of lignite and guar gum in in-situ environments, as well as elucidating the specific metabolic pathways involved in their co-degradation.

Transitioning weathered oil fields towards new energy: A review on utilizing hydrogenotrophic methanogens for petroleum hydrocarbons remediation

The weathering process can cause the volatilization of light components in crude oil, leading to the accumulation of total petroleum hydrocarbons (TPH) in weathered oil field soils. These TPH compounds are relatively resistant to biodegradation, posing a significant environmental hazard by contributing to soil degradation. TPH represents a complex mixture of petroleum-based hydrocarbons classified as persistent organic pollutants in soil and groundwater. The release of TPH pollutants into the environment poses serious threats to ecosystems and human health. Currently, various methods are available for TPH-contaminated soil remediation, with bioremediation technology recognized as an environmentally friendly and cost-effective approach. While converting TPH to CO2 is a common remediation method, the complex structures and diverse types of petroleum hydrocarbons (PHs) involved can result in excessive CO2 generation, potentially exacerbating the greenhouse effect. Alternatively, transforming TPH into energy forms like methane through bioremediation, followed by collection and reuse, can reduce greenhouse gas emissions and energy consumption. This process relies on the synergistic interaction between Methanogens archaea and syntrophic bacteria, forming a consortium known as the oil-degrading bacterial consortium. Methanogens produce methane through anaerobic digestion (AD), with hydrogenotrophic methanogens (HTMs) utilizing H2 as an electron donor, playing a crucial role in biomethane production. Candidatus Methanoliparia (Ca. Methanoliparia) was found in the petroleum archaeal community of weathered Oil field in northeast China. Ca. Methanoliparia has demonstrated its independent ability to decompose and produce new energy (biomethane) without symbiosis, contribute to transitioning weathered oil fields towards new energy. Therefore, this review focuses on the principles, mechanisms, and developmental pathways of HTMs during new energy production in the degradation of PHs. It also discusses strategies to enhance TPH degradation and recovery methods.

Natural defence mechanisms of electrochemically active biofilms: From the perspective of microbial adaptation, survival strategies and antibiotic resistance

Electrochemically active biofilms (EABs) play an ever-growingly critical role in the biological treatment of wastewater due to its low carbon footprint and sustainability. However, how the multispecies biofilms adapt, survive and become tolerant under acute and chronic toxicity such as antibiotic stress still remains well un-recognized. Here, the stress responses of EABs to tetracycline concentrations (CTC) and different operation schemes were comprehensively investigated. Results show that EABs can quickly adapt (start-up time is barely affected) to low CTC (≤ 5 μM) exposure while the adaptation time of EABs increases and the bioelectrocatalytic activity decreases at CTC ≥ 10 μM. EABs exhibit a good resilience and high anti-shocking capacity under chronic and acute TC stress, respectively. But chronic effects negatively affect the metabolic activity and extracellular electron transfer, and simultaneously change the spatial morphology and microbial community structure of EABs. Particularly, the typical exoelectrogens Geobacter anodireducens can be selectively enriched under chronic TC stress with relative abundance increasing from 45.11% to 85.96%, showing stronger TC tolerance than methanogens. This may be attributed to the effective survival strategies of EABs in response to TC stress, including antibiotic efflux regulated by tet(C) at the molecular level and the secretion of more extracellular proteins in the macro scale, as the C=O bond in amide I of aromatic amino acids plays a critical role in alleviating the damage of TC to cells. Overall, this study highlights the versatile defences of EABs in terms of microbial adaptation, survival strategies, and antibiotic resistance, and deepens the understanding of microbial communities' evolution of EABs in response to acute and chronic TC stress.

Enrichment of microbial consortia for MEOR in crude oil phase of reservoir-produced liquid and their response to environmental disturbance

Developing microbial consortiums is necessary for microbial enhanced oil recovery (MEOR) in heavy crude oil production. The aqueous phase of produced fluid has long been considered an ideal source of microorganisms for MEOR. However, it is recently found that rich microorganisms (including hydrocarbon-degrading bacteria) are present in the crude oil phase, which is completely different from the aqueous phase of produced fluid. So, in this study, the microbial consortia from the crude oil phase of produced fluids derived from four wells were enriched, respectively. The microbial community structure during passage was dynamically tracked, and the response of enriched consortia to successive disturbance of environmental factors was investigated. The results showed the crude oil phase had high microbial diversity, and the original microbial community structure from four wells was significantly different. After ten generations of consecutive enrichment, different genera were observed in the four enriched microbial consortia, namely, Geobacillus, Bacillus, Brevibacillus, Chelativorans, Ureibacillus, and Ornithinicoccus. In addition, two enriched consortia (eG1614 and eP30) exhibited robustness to temperature and oxygen perturbations. These results further suggested that the crude oil phase of produced fluids can serve as a potential microbial source for MEOR.

Roles of quorum-sensing molecules in methane production from anaerobic digestion aided by biochar

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted.

Unnatural Direct Interspecies Electron Transfer Enabled by Living Cell-Cell Click Chemistry

Direct interspecies electron transfer (DIET) is essential for maintaining the function and stability of anaerobic microbial consortia. However, only limited natural DIET modes have been identified and DIET engineering remains highly challenging. In this study, an unnatural DIET between Shewanella oneidensis MR-1 (SO, electron donating partner) and Rhodopseudomonas palustris (RP, electron accepting partner) was artificially established by a facile living cell-cell click chemistry strategy. By introducing alkyne- or azide-modified monosaccharides onto the cell outer surface of the target species, precise covalent connections between different species in high proximity were realized through a fast click chemistry reaction. Remarkably, upon covalent connection, outer cell surface C-type cytochromes mediated DIET between SO and RP was achieved and identified, although this was never realized naturally. Moreover, this connection directly shifted the natural H2 mediated interspecies electron transfer (MIET) to DIET between SO and RP, which delivered superior interspecies electron exchange efficiency. Therefore, this work demonstrated a naturally unachievable DIET and an unprecedented MIET shift to DIET accomplished by cell-cell distance engineering, offering an efficient and versatile solution for DIET engineering, which extends our understanding of DIET and opens up new avenues for DIET exploration and applications.

Mechanisms of microbial co-aggregation in mixed anaerobic cultures

Co-aggregation of anaerobic microorganisms into suspended microbial biofilms (aggregates) serves ecological and biotechnological functions. Tightly packed aggregates of metabolically interdependent bacteria and archaea play key roles in cycling of carbon and nitrogen. Additionally, in biotechnological applications, such as wastewater treatment, microbial aggregates provide a complete metabolic network to convert complex organic material. Currently, experimental data explaining the mechanisms behind microbial co-aggregation in anoxic environments is scarce and scattered across the literature. To what extent does this process resemble co-aggregation in aerobic environments? Does the limited availability of terminal electron acceptors drive mutualistic microbial relationships, contrary to the commensal relationships observed in oxygen-rich environments? And do co-aggregating bacteria and archaea, which depend on each other to harvest the bare minimum Gibbs energy from energy-poor substrates, use similar cellular mechanisms as those used by pathogenic bacteria that form biofilms? Here, we provide an overview of the current understanding of why and how mixed anaerobic microbial communities co-aggregate and discuss potential future scientific advancements that could improve the study of anaerobic suspended aggregates. KEY POINTS: • Metabolic dependency promotes aggregation of anaerobic bacteria and archaea • Flagella, pili, and adhesins play a role in the formation of anaerobic aggregates • Cyclic di-GMP/AMP signaling may trigger the polysaccharides production in anaerobes.

Electrical stress and acid orange 7 synergistically clear the blockage of electron flow in the methanogenesis of low-strength wastewater

Energy recovery from low-strength wastewater through anaerobic methanogenesis is constrained by limited substrate availability. The development of efficient methanogenic communities is critical but challenging. Here we develop a strategy to acclimate methanogenic communities using conductive carrier (CC), electrical stress (ES), and Acid Orange 7 (AO7) in a modified biofilter. The synergistic integration of CC, ES, and AO7 precipitated a remarkable 72-fold surge in methane production rate compared to the baseline. This increase was attributed to an altered methanogenic community function, independent of the continuous presence of AO7 and ES. AO7 acted as an external electron acceptor, accelerating acetogenesis from fermentation intermediates, restructuring the bacterial community, and enriching electroactive bacteria (EAB). Meanwhile, CC and ES orchestrated the assembly of the archaeal community and promoted electrotrophic methanogens, enhancing acetotrophic methanogenesis electron flow via a mechanism distinct from direct electrochemical interactions. The collective application of CC, ES, and AO7 effectively mitigated electron flow impediments in low-strength wastewater methanogenesis, achieving an additional 34% electron recovery from the substrate. This study proposes a new method of amending anaerobic digestion systems with conductive materials to advance wastewater treatment, sustainability, and energy self-sufficiency.

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Riboflavin modified carbon cloth enhances anaerobic digestion treating food waste in a pilot-scale system

Previous laboratory-scale studies have consistently shown that carbon-based conductive materials can notably improve the anaerobic digestion of food waste, typically employing reactors with regular capacity of 1-20 L. Furthermore, incorporating riboflavin-loaded conductive materials can further address the imbalance between fermentation and methanogenesis in anaerobic systems. However, there have been few reports on pilot-scale investigation. In this study, a 10 m2 of riboflavin modified carbon cloth was incorporated into a pilot-scale (2 m3) food waste anaerobic reactor to improve its treatment efficiency. The study found that the addition of riboflavin-loaded carbon cloth can increase the maximum organic loading rate (OLR) by 40% of the pilot-scale reactor, compared to the system using carbon cloth without riboflavin loading, while ensuring efficient operation of the reaction system, effectively alleviating system acidification, sustaining methanogen activity, and increasing daily methane production by 25%. Analysis of the microbial community structure revealed that riboflavin-loaded carbon cloth enriched the methanogenic archaea in the genera of Methanothrix and Methanobacterium, which are capable of extracellular direct interspecies electron transfer (DIET). And metabolic pathway analysis identified the methane production pathway, highly enriched on the reduction of acetic acid and CO2 at riboflavin-loaded carbon cloth sample. The expression levels of genes related to methane production via DIET pathway were also significantly upregulated. These results can provide important guidance for the practical application of food waste anaerobic digestion engineering.

Material and microbial perspectives on understanding the role of biochar in mitigating ammonia inhibition during anaerobic digestion

With the increasing adoption of carbon-based strategies to enhance methanogenic processes, there is a growing concern regarding the correlation between biochar properties and its stimulating effects on anaerobic digestion (AD) under ammonia inhibition. This study delves into the relevant characteristics and potential mechanisms of biochar in the context of AD system under ammonia inhibition. The introduction of optimized biochar, distinguished by rich CO bond, abundant defect density, and high electronic capacity, resulted in a significant reduction in the lag period of anaerobic digestion system under 5.0 g/L ammonia stress, approximately by around 63 % compared to the control one. Biochar helps regulate the community structure, promotes the accumulation of acetate-consuming bacteria, in the AD system under ammonia inhibition. More examinations show that biochar promotes direct interspecies electron transfer in AD system under ammonia inhibition, as evidenced by diminished levels of bound electroactive extracellular polymeric substances, increased abundance of electroactive bacteria, and notably, the up-regulation of direct interspecies electron transfer associated genes, including the conductive pili and Cytochrome C genes, as revealed by meta-transcriptomic analysis. Additionally, gene expression related to proteins associated with ammonium detoxification were found to be up-regulated in systems supplemented with biochar. These findings provide essential evidence and insights for the selection and potential engineering of effective biochar to enhance AD performance under ammonia inhibition.

Biogas upgrading performance and underlying mechanism in microbial electrolysis cell and anaerobic digestion integrated system

The productivity and efficiency of two-chamber microbial electrolysis cell and anaerobic digestion integrated system (MEC-AD) were promoted by a complex of anaerobic granular sludge and iron oxides (Fe-AnGS) as inoculum. Results showed that MEC-AD with Fe-AnGS achieved biogas upgrading with a 23%-29% increase in the energy recovery rate of external circuit current and a 26%-31% decrease in volatile fatty acids. The energy recovery rate of MEC-AD remained at 52%-57%, indicating a stable operation performance. The selectively enriched methanogens and electroactive bacteria resulted in dominant hydrogenotrophic and acetoclastic methanogenesis in the cathode and anode chambers. Mechanistic analysis revealed that MEC-AD with Fe-AnGS led to specifically upregulated enzymes related to energy metabolism and electron transfer. Fe-AnGS as inoculum could improve the long-term operation performance of MEC-AD. Consequently, this study provides an efficient strategy for biogas upgrading in MEC-AD.

Atomic Pyridinic Nitrogen as Highly Active Metal-Free Coordination Sites at the Biotic-Abiotic Interface for Bio-Electrochemical CO2 Reduction

Bio-electrochemical conversion of anthropogenic CO2 into value-added products using cost-effective metal-free catalysts represents a promising strategy for sustainable fuel production. Herein, N-doped carbon nanosheets synthesized via pyrolysis of the zeolitic-imidazolate framework (ZIF) are developed for constructing efficient biohybrids to facilitate CO2-to-CH4 conversion. The microbial enrichment and bio-interfacial charge transfer are significantly affected by the proportion of the co-existed graphitic-N, pyridinic-N, and pyrrolic-N in the defective carbon nanosheets. It is unfolded that pyridinic-N and pyrrolic-N with the doped N atoms near the edge can significantly enhance the adsorption of their adjacent C atoms toward O, leading to improved microbe enrichment. Especially, pyridinic-N which can provide one p electron to the aromatic π system, greatly enhances the electron-donating capability of the carbon nanosheets to the microorganisms. Correspondingly, due to its largest amount of pyridinic-N doping, the N-doped carbon nanosheets derived from ZIF pyrolysis at 900 °C (denoted 900-NC) achieve the highest methane production of ≈215.7 mmol m-2 day-1 with a high selectivity (Faradaic efficiency = ≈94.2%) at -0.9 V versus Ag/AgCl. This work demonstrates the effectiveness of N-doped carbon catalysts for bio-electrochemical CO2 fixation and contributes to the understanding of N functionalities toward microbiome response and biotic-abiotic charge transfer in various bio-electrochemical systems.

Proposed mechanisms of electron uptake in metal-corroding methanogens and their potential for CO2 bioconversion applications

Some methanogens are electrotrophic bio-corroding microbes that can acquire electrons from solid surfaces including metals. In the laboratory, pure cultures of methanogenic cells oxidize iron-based materials including carbon steel, stainless steel, and Fe0. For buried or immersed pipelines or other metallic structures, methanogens are often major components of corroding biofilms with complex interspecies relationships. Models explaining how these microbes acquire electrons from solid donors are multifaceted and include electron transfer via redox mediators such as H2 or by direct contact through membrane proteins. Understanding the electron uptake (EU) routes employed by corroding methanogens is essential to develop efficient strategies for corrosion prevention. It is also beneficial for the development of bioenergy applications relying on methanogenic EU from solid donors such as bioelectromethanogenesis, hybrid photosynthesis, and the acceleration of anaerobic digestion with electroconductive particles. Many methanogenic species carrying out biocorrosion are the same ones forming the extensive abiotic-biological interfaces at the core of these bio-applications. This review will discuss the interactions between corrosive methanogens and metals and how the EU capability of these microbes can be harnessed for different sustainable biotechnologies.

Alternating polarity as a novel strategy for building synthetic microbial communities capable of robust Electro-Methanogenesis

Electro-methanogenic microbial communities can produce biogas with high efficiency and have attracted extensive research interest. In this study an alternating polarity strategy was developed to build electro-methanogenic communities. In two-chamber bioelectrochemical systems amended with activated carbon, the electrode potential was alternated between +0.8 V and -0.4 V vs. standard hydrogen electrode every three days. Cumulative biogas production under alternating polarity increased from 45 L/L/kg-activated carbon after start-up to 125 L/L/kg after the 4th enrichment, significantly higher than that under intermittent cathode (-0.4 V/open circuit), continuous cathode (-0.4 V), and open circuit. The communities assembled under alternating polarity were electroactive and structurally different from those assembled under other conditions. One Methanobacterium population and two Geobacter populations were consistently abundant and active in the communities. Their 16S rRNA was up-regulated by electrode potentials. Bayesian networks inferred close associations between these populations. Overall, electro-methanogenic communities have been successfully assembled with alternating polarity.

Effect of zero-valent iron (ZVI) and biogas slurry reflux on methane production by anaerobic digestion of waste activated sludge

This study aimed to improve anaerobic digestion (AD) efficiency through the addition of zero-valent iron (ZVI) and biogas slurry. This paper demonstrated that methane production was most effectively promoted at a biogas slurry reflux ratio of 60%. The introduction of ZVI into anaerobic systems does not enhance its bioavailability. However, both biogas slurry reflux and the combination of ZVI with biogas slurry reflux increase the relative abundance of microorganisms involved in the direct interspecific electron transfer (DIET) process. Among them, the dominant microorganisms Methanosaeta, Methanobacterium, Methanobrevibacter, and Methanolinea accounted for over 60% of the total methanogenic archaea. The Tax4Fun function prediction results indicate that biogas slurry reflux and the combination of ZVI with biogas slurry reflux can increase the content of key enzymes in the acetotrophic and hydrotrophic methanogenesis pathways, thereby strengthening these pathways. The corrosion of ZVI promotes hydrogen production, and the biogas slurry reflux provided additional alkaline and anaerobic microorganisms for the anaerobic system. Their synergistic effect promoted the growth of hydrotrophic methanogens and improved the activities of various enzymes in the hydrolysis and acidification phases, enhanced the system's buffer capacity, and prevented secondary environmental pollution. PRACTITIONER POINTS: Optimal methane production was achieved at a biogas slurry reflux ratio of 60%. Biogas slurry reflux in anaerobic digestion substantially reduced discharge. ZVI addition in combination with biogas slurry reflux facilitates the DIET process.

Dual role of birnessite on the modulation of acid production and reinforcement of interspecific electron transfer in anaerobic digestion

Achieving efficient anaerobic digestion of highly loaded substrates is one of the most challenging problems in the field of waste resourcing. Here, the addition of birnessite (2.0 g/L) to kitchen wastewater increased the acetate and final methane yields by 40.53 and 99.18 %, respectively, while reducing the yields of propionate and butyrate by 38.17 and 48.86 %, respectively. There were two main pathways for birnessite to enhance anaerobic digestion, one of which is to act as an electron acceptor, by inducing an alteration in the ratio of reduced-state coenzyme I in the microorganism, allowing the acid production process to proceed towards deeper oxidation. Another pathway enhances the interspecific electron transfer between bacteria and archaea and improves methane yield by optimizing the metabolic relationship. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional predictions suggest that the extracellular electron transport pathway of the microorganism is enhanced with the addition of birnessite and that its intracellular metabolic pathway is biased towards the nicotinamide adenine dinucleotide (NADH) generation pathway. This work demonstrated that anaerobic digestion facilitation by metallic minerals was not monolithic; that is, different properties of the minerals were employed to intensify the different stages of anaerobic digestion and obtain an amplification cascade.

Comprehending the impact of berberine on anaerobic digestion of waste activated sludge

Berberine is a natural isoquinoline alkaloid performing wide-spectrum antimicrobial and antiviral effects like antibiotics. Its production generates berberine containing wastewater, and berberine adsorbed on waste activated sludge (WAS) will unavoidably enter the anaerobic digestion (AD) system while its impact on the AD process is unknown. Our research found that berberine of 20 mg/L (BBR20) slightly enhanced the methane yield (4.2 ± 0.6%) under mesophilic condition (35.0 ± 1.0 °C). However, 100 and 500 mg/L (BBR100 and BBR500) depressed methane production by 17.3 ± 4.3% and 83.2 ± 0.4%; meanwhile more soluble chemical oxygen demand (SCOD) including volatile fatty acid (VFA), protein, and polysaccharide were left in the fermentation broth, which led to an increase in sludge reduction. 88.3 ± 0.09%-99.1 ± 0.04% of berberine was distributed in the sludge phase and could be efficiently removed even under a high berberine level of 500 mg/L during the AD process. Exposure to different berberine concentrations promoted sludge dissolution and triggered more sludge extracellular polymeric substances (EPS) being dissolved. Lower berberine concentration (20 mg/L) enhanced acidification and methanogenesis steps, resulting in a final methane generation increase. While hydrolysis, acidification and methanogenesis processes were all inhibited by 100 and 500 mg/L berberine. Microbial analysis revealed that the main acid-producing bacteria genera were changed as Bacteroidetes vadinHA17 dominated in control, BBR20 and BBR100 groups, was replaced by Petrimonas in BBR500. Additionally, Methanosaeta, as a strict acetoclastic methanogen, was suppressed under exposure to 100 and 500 mg/L berberine. Accordingly, the declined abundance of archaea genera consuming acetic acid caused more VFA accumulation and less methane production in BBR100 and BBR500 groups.

Self-assembled electrochemically active biofilms doped with carbon nanotubes: Electron exchange efficiency and cytotoxicity evaluation

Thick electrochemically active biofilms (EABs) will lead to insufficient extracellular electron transfer (EET) rate because of the limitation of both substrate diffusion and electron exchange. Herein, carbon nanotubes (CNTs)-doped EABs are developed through self-assembly. The highly conductive biofilms (internal resistance of ∼211 Ω) are efficiently enriched at CNTs dosage of 1 g L-1, with the stable power output of 0.568 W m-2 over three months. The embedded CNTs can act as electron tunnel to accelerate the EET rate in thick biofilm. Self-charging/discharging experiments and Nernst-Monod model stimulation demonstrate a higher net charge storage capacity (0.15 C m-2) and more negative half-saturation potential (-0.401 V) for the hybrid biofilms than that of the control (0.09 C m-2, and -0.378 V). Enzyme activity tests and the observation of confocal laser scanning microscopy by live/dead staining show a nearly negligible cytotoxicity of CNTs, and non-targeted metabonomics analysis reveals fourteen differential metabolites that do not play key roles in microbial central metabolic pathways according to KEGG compound database. The abundance of typical exoelectrogens Geobacter sp. is 2-fold of the control, resulting in a better bioelectrocatalytic activity. These finding provide a possible approach to prolong electron exchange and power output by developing a hybrid EABs doped with conductive material.

Co-NC@Co-NP hierarchical nanoforest steering charge exchange efficiency at biotic-abiotic interface for microbial electrochemical carbon reduction

Converting anthropogenic carbon dioxide (CO2) to value-added products using bio-electrochemical conversions represents a promising strategy for producing sustainable fuel. However, the reaction kinetics are hindered by insufficient attachment of microorganisms and limited charge extraction at the bioinorganic interface. A hierarchical nanoforest with doped cobalt‑nitrogen-doped carbon covering cobalt nanoparticle (Co-NC@Co-NP) was integrated with a CO2-to-CH4 conversion microbiome for methane production to address these shortcomings. In-situ nanoforests were developed on the nanosheet by chemical vapor deposition with Co nanoparticles catalyzed. The bio-nanowire-like carbon nanotubes enhanced the electrostatic force for microbe enrichment via the tip effect, providing a maximum of 3.6-fold electron-receiving microbes to utilize reducing equivalents. The Co-NC@Co-NP enhanced the direct electron transfer between microbes and electrodes, reducing the adoption of energy barriers for heme-like proteins. Thus, the optimized electron transfer pathway improved selectivity by a factor of 2.0 compared to the pristine nanosheet biohybrid. Furthermore, the adjusted microbial community structure provided sufficient methanogenesis genes to match the strong electron flow, achieving maximal methane production rates (311.1 mmol/m2/day at -0.9 V vs. Ag/AgCl), 8.62 times higher than those of the counterpart nanosheet biohybrid (36.06 mmol/m2/day). This work demonstrates a comprehensive assessment of biotic-abiotic energy transfer, which may serve as a guiding principle for designing efficient bio-electrochemical systems.

A novel approach to enhance methane production during anaerobic digestion of waste activated sludge by combined addition of trypsin, nano-zero-valent iron and activated carbon

A novel approach with a combination of trypsin, nano-zero-valent iron (NZVI) and activated carbon (AC) was conducted to promote the methane production of waste activated sludge (WAS) during the anaerobic digestion (AD) processes. Results showed that the combined addition of trypsin-NZVI-AC exhibited the synergistic effect during different AD stages. Trypsin mainly facilitated the hydrolysis process and the acetic acid conversion, while NZVI-AC enhanced the substrate metabolism and the electronic transfer to subsequently produce methane. A dose of 1000 mg/L trypsin was optimal to maximize this synergistic effect. Metagenomic analysis showed that trypsin-NZVI-AC addition effectively improved the relative abundance of acetyl-CoA carboxylase, and then strengthened both acetoclastic methanogenesis (M00357) and hydrogenotrophic methanogenesis (M00567). Hydrogenotrophic methanogens such as Methanobacterium, Methanoculleus, and Methanosarcina were greatly enriched with trypsin-NZVI-AC compared with trypsin or NZVI-AC addition. Moreover, electroactive bacteria G. sulfurreducens and G. metallireducens were also enriched by this method to conduct direct interspecies electron transfer among methanogens, leading to the better improvement of methane production. These findings supply a promising way to optimize the enzyme pretreatment technology and elevate the methanogenic efficiency of WAS.

A review of application of combined biochar and iron-based materials in anaerobic digestion for enhancing biogas productivity: Mechanisms, approaches and performance

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application.

Direct interspecies electron transfer enables anaerobic oxidation of sulfide to elemental sulfur coupled with CO2-reducing methanogenesis

Electric syntrophy between fatty acid oxidizers and methanogens through direct interspecies electron transfer (DIET) is essential for balancing acidogenesis and methanogenesis in anaerobic digestion. Promoting DIET using electrically conductive additives proved effective in enhancing methanogenesis; however, its possibility to affect other microbial redox reactions in methanogenic systems has been little studied. This study provides the first confirmation of the electro-syntrophic coupling of sulfide oxidation to S0 with CO2-reducing methanogenesis in sulfur-rich methanogenic cultures supplemented with conductive magnetite (100-700-nm particle size). The H2S content in biogas, initially exceeding 5000 ppmv, decreased to below 1 ppmv along with an accumulation of extracellular S0 (60-70 mg/L; initially <1 mg/L) at a magnetite dose of 20 mM Fe, while there were no significant changes in methane yield. A comprehensive polyphasic approach demonstrated that the S0 formation occurs through electro-syntrophic oxidation of sulfide coupled with CO2-reducing methanogenesis, involving Methanothrix as the dominant methanogen.

Methane production by Methanothrix thermoacetophila via direct interspecies electron transfer with Geobacter metallireducens

Methanothrix is widely distributed in natural and artificial anoxic environments and plays a major role in global methane emissions. It is one of only two genera that can form methane from acetate dismutation and through participation in direct interspecies electron transfer (DIET) with exoelectrogens. Although Methanothrix is a significant member of many methanogenic communities, little is known about its physiology. In this study, transcriptomics helped to identify potential routes of electron transfer during DIET between Geobacter metallireducens and Methanothrix thermoacetophila. Additions of magnetite to cultures significantly enhanced growth by acetoclastic methanogenesis and by DIET, while granular activated carbon (GAC) amendments impaired growth. Transcriptomics suggested that the OmaF-OmbF-OmcF porin complex and the octaheme outer membrane c-type cytochrome encoded by Gmet_0930, were important for electron transport across the outer membrane of G. metallireducens during DIET with Mx. thermoacetophila. Clear differences in the metabolism of Mx. thermoacetophila when grown via DIET or acetate dismutation were not apparent. However, genes coding for proteins involved in carbon fixation, the sheath fiber protein MspA, and a surface-associated quinoprotein, SqpA, were highly expressed in all conditions. Expression of gas vesicle genes was significantly lower in DIET- than acetate-grown cells, possibly to facilitate better contact between membrane-associated redox proteins during DIET. These studies reveal potential electron transfer mechanisms utilized by both Geobacter and Methanothrix during DIET and provide important insights into the physiology of Methanothrix in anoxic environments. IMPORTANCE Methanothrix is a significant methane producer in a variety of methanogenic environments including soils and sediments as well as anaerobic digesters. Its abundance in these anoxic environments has mostly been attributed to its high affinity for acetate and its ability to grow by acetoclastic methanogenesis. However, Methanothrix species can also generate methane by directly accepting electrons from exoelectrogenic bacteria through direct interspecies electron transfer (DIET). Methane production through DIET is likely to further increase their contribution to methane production in natural and artificial environments. Therefore, acquiring a better understanding of DIET with Methanothrix will help shed light on ways to (i) minimize microbial methane production in natural terrestrial environments and (ii) maximize biogas formation by anaerobic digesters treating waste.

Detrimental impact of the Geobacter metallireducens type VI secretion system on direct interspecies electron transfer

Direct interspecies electron transfer (DIET) is important in anaerobic communities of environmental and practical significance. Other than the need for close physical contact for electrical connections, the interactions of DIET partners are poorly understood. Type VI secretion systems (T6SSs) typically kill competitive microbes. Surprisingly, Geobacter metallireducens highly expressed T6SS genes when DIET-based co-cultures were initiated with Geobacter sulfurreducens. T6SS gene expression was lower when the electron shuttle anthraquinone-2,6-disulfonate was added to alleviate the need for interspecies contact. Disruption of hcp, the G. metallireducens gene for the main T6SS needle-tube protein subunit, and the most highly upregulated gene in DIET-grown cells eliminated the long lag periods required for the initiation of DIET. The mutation did not aid DIET in the presence of granular-activated carbon (GAC), consistent with the fact that DIET partners do not make physical contact when electrically connected through conductive materials. The hcp-deficient mutant also established DIET quicker with Methanosarcina barkeri. However, the mutant also reduced Fe(III) oxide faster than the wild-type strain, a phenotype not expected from the loss of the T6SS. Quantitative PCR revealed greater gene transcript abundance for key components of extracellular electron transfer in the hcp-deficient mutant versus the wild-type strain, potentially accounting for the faster Fe(III) oxide reduction and impact on DIET. The results highlight that interspecies interactions beyond electrical connections may influence DIET effectiveness. The unexpected increase in the expression of genes for extracellular electron transport components when hcp was deleted emphasizes the complexities in evaluating the electromicrobiology of highly adaptable Geobacter species. IMPORTANCE Direct interspecies electron transfer is an alternative to the much more intensively studied process of interspecies H2 transfer as a mechanism for microbes to share electrons during the cooperative metabolism of energy sources. DIET is an important process in anaerobic soils and sediments generating methane, a significant greenhouse gas. Facilitating DIET can accelerate and stabilize the conversion of organic wastes to methane biofuel in anaerobic digesters. Therefore, a better understanding of the factors controlling how fast DIET partnerships are established is expected to lead to new strategies for promoting this bioenergy process. The finding that when co-cultured with G. sulfurreducens, G. metallireducens initially expressed a type VI secretion system, a behavior not conducive to interspecies cooperation, illustrates the complexity of establishing syntrophic relationships.

Insights into the roles of humic acids in facilitating the anaerobic digestion process

Humic acids (HAs) are important byproducts of anaerobic digestion (AD), which have complex structures and dynamic electrochemical activities. However, the effects of HAs on AD process were usually misestimated due to the neglect of the in situ generated HAs and the interaction between HAs and metal ions. This study explored the effects of HAs on AD performance using corn straw as typical "clean" substrate (rare in metals content) via commercial HAs (C-HAs) addition and in-situ-generated HAs (In-HAs) removal. Results showed that C-HAs (1 g/L) addition promoted the maximum methane production rate (Rm) by 20.6%, while In-HAs removal decreased the Rm by 42.7%. Meanwhile, C-HAs showed little effect on the acidification of corn straw but increased the Rm during the methanation of ethanol by 41.6%. Both the C-HAs and In-HAs were rich in surface oxygen-containing functional groups, which enabled them to act as electron shuttles and facilitate the syntrophic methanogenesis. HAs also acted in regulation of syntrophic microorganisms. For instance, C-HAs addition enriched the relative abundances of Cloacimonadia, Spirochaetia, Synergistia and Methanosarcina, while the removal of In-HAs reduced the relative abundances of Spirochaetia and Synergistia. In conclusion, HAs addition to the AD process could be a feasible approach to improve methane production by enhancing direct interspecies electron transfer during AD of lignocellulosic biomass.

Microbial response behavior to powdered activated carbon in high-solids anaerobic digestion of kitchen waste: Metabolism and functional prediction analysis

Anaerobic digestion (AD) can not only treat organic waste, but also recycle energy. However, high-solids AD of kitchen waste usually failed due to excessive acidification. In this study, the effect of activated carbon (AC) on kitchen waste AD performance was investigated under high-solids conditions (total solids contents = 15%). The results showed that efficiencies of acidogenesis and methanogenesis were promoted in presence of moderate concentration (50 g/L > AC >5 g/L), but high concentration (AC >70 g/L) weakened AD performance. Moreover, AC addition enhanced the methane production rate from 66.0 mL/g VS to 231.50 mL/g VS, i.e., up to 250.7%. High-throughput sequencing results demonstrated that the abundance of electroactive DMER64 increased from less than 1%-29.7% (20 g/L AC). As AC gradually increased，aceticlastic methanogenesis changed to hydrogenotrophic pathway. Predicted functional analysis indicated that AC can enhance abundances of energy and inorganic ion metabolism, resulting in high methane production.

Chitosan-Fe3O4 composites enhance anaerobic digestion of liquor wastewater under acidic stress

Acid stress in the anaerobic digestion process of liquor wastewater leads to low anaerobic treatment efficiency. Herein, chitosan-Fe₃O₄ was prepared, and its effects on anaerobic digestion processes under acid stress were studied. Results showed that chitosan-Fe₃O₄ increased the methanogenesis rate of anaerobic digestion of acidic liquor wastewater by 1.5-2.3 times and accelerated the restoration of acidified anaerobic systems. The analysis of sludge characteristics showed that chitosan-Fe₃O₄ promoted the secretion of proteins and humic substances in extracellular polymeric substances and increased the electron transfer activity of the system by 71.4%. Microbial community analysis indicated that chitosan-Fe₃O₄ enriched the abundance of Peptoclostridium, and Methanosaeta participated in direct interspecies electron transfer. Chitosan-Fe₃O₄ could promote the direct interspecies electron transfer pathway to maintain stable methanogenesis. These methods and results regarding the use of chitosan-Fe₃O₄ could be referred to for improving the efficiency of anaerobic digestion of high concentration organic wastewater under acid inhibition.

The greenhouse effect of antibiotics: The influence pathways of antibiotics on methane release from freshwater sediment

The impact of antibiotics on methane (CH₄) release from sediment involves both CH₄ production and consumption processes. However, most relevant studies lack a discussion of the pathways by which antibiotics affect CH₄ release and do not highlight the role played by the sediment chemical environment in this influence mechanism. Here, we collected field surface sediments and grouped them with various antibiotic combination concentration gradients (50, 100, 500, 1000 ng g^-1) under a 35-day indoor anaerobic constant temperature incubation. We found that the positive effect of antibiotics on sediment CH₄ release potential appeared later than the positive effect on sediment CH₄ release flux. Still, the positive effect of high-concentration antibiotics (500, 1000 ng g^-1) occurred with a lag in both processes. Also, the positive effect of high-concentration antibiotics was significantly higher than low-concentration antibiotics (50, 100 ng g^-1) in the later incubation period (p < 0.05). We performed a multi-collinearity assessment of sediment biochemical indicators, followed by a generalized linear model with negative binomial regression (GLM-NB) to obtain essential variables. In particular, we conducted the interaction analysis on CH₄ release potential and flux regression for the influence pathways construction. The partial least-squares path modeling (PLS-PM) demonstrated that the positive effect of antibiotics on CH₄ release (Total effect = 0.2579) was primarily attributed to their effect on the sediment chemical environment (Direct effect = 0.5107). These findings greatly expand our understanding of the antibiotic greenhouse effect in freshwater sediment. Further studies should more carefully consider the effects of antibiotics on the sediment chemical environment, and continuously improve the mechanistic studies of antibiotics on sediment CH₄ release.