Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods

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.

Enhanced methane production from waste activated sludge by microbial electrolysis cell assisted anaerobic digestion: Fate and effect of humic substances

Humic substances as major components of waste activated sludge are refractory to degrade and have inhibition in traditional anaerobic digestion (AD). This study for the first time investigated the feasibility and mechanism of microbial electrolysis cell assisted anaerobic digestion (MEC-AD) to break the recalcitrance and inhibition of humic substances. The cumulative methane production of AD decreased from 134.7 to 117.6 mL/g-VS with the addition of humic acids and fulvic acids at 25.2-102.1 mg/g-VS. However, 0.6 V MEC-AD maintained stable methane production (155.5-158.2 mL/g-VS) under the effect of humic substances. 0.6 V MEC-AD formed electrical stimulation on microbial cells, provided anodic oxidation and cathodic reduction transformation pathways for humic substances (acting as carbon sources and electron shuttles), and aggregated functional microorganisms on electrodes, facilitating the degradation of humic substances and generation of methane. This study provides a theoretical basis for improving the energy recovery and system stability of sludge treatment.

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 lower 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. All these 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.

Mitigation of Triclocarban Inhibition in Microbial Electrolysis Cell-Assisted Anaerobic Digestion

Triclocarban (TCC), as a widely used antimicrobial agent, is accumulated in waste activated sludge at a high level and inhibits the subsequent anaerobic digestion of sludge. This study, for the first time, investigated the effectiveness of microbial electrolysis cell-assisted anaerobic digestion (MEC-AD) in mitigating the inhibition of TCC to methane production. Experimental results showed that 20 mg/L TCC inhibited sludge disintegration, hydrolysis, acidogenesis, and methanogenesis processes and finally reduced methane production from traditional sludge anaerobic digestion by 19.1%. Molecular docking revealed the potential inactivation of binding of TCC to key enzymes in these processes. However, MEC-AD with 0.6 and 0.8 V external voltages achieved much higher methane production and controlled the TCC inhibition to less than 5.8%. TCC in the MEC-AD systems was adsorbed by humic substances and degraded to dichlorocarbanilide, leading to a certain detoxification effect. Methanogenic activities were increased in MEC-AD systems, accompanied by complete VFA consumption. Moreover, the applied voltage promoted cell apoptosis and sludge disintegration to release biodegradable organics. Metagenomic analysis revealed that the applied voltage increased the resistance of electrode biofilms to TCC by enriching functional microorganisms (syntrophic VFA-oxidizing and electroactive bacteria and hydrogenotrophic methanogens), acidification and methanogenesis pathways, multidrug efflux pumps, and SOS response.

Efficient degradation of aqueous dichloromethane by an enhanced microbial electrolysis cell: Degradation kinetics, microbial community and metabolic mechanisms

Dichloromethane (DCM) has been listed as a toxic and harmful water pollutant, and its removal needs attention. Microbial electrolysis cells (MECs) are viewed as a promising alternative for pollutant removal, which can be strengthened from two aspects: microbial inoculation and acclimation. In this study, the MEC for DCM degradation was inoculated with the active sludge enhanced by Methylobacterium rhodesianum H13 (strain H13) and then acclimated in the form of a microbial fuel cell (MFC). Both the introduction of strain H13 and the initiation in MFC form significantly promoted DCM degradation. The degradation kinetics were fitted by the Haldane model, with Vmax, Kh, Ki and vmax values of 103.2 mg/L/hr, 97.8 mg/L, 268.3 mg/L and 44.7 mg/L/hr/cm2, respectively. The cyclic voltammogram implies that DCM redox reactions became easier with the setup of MEC, and the electrochemical impedance spectrogram shows that the acclimated and enriched microbes reduced the charge transfer resistance from the electrode to the electrolyte. In the biofilm, the dominant genera shifted from Geobacter to Hyphomicrobium in acclimation stages. Moreover, Methylobacterium played an increasingly important role. DCM metabolism mainly occurred through the hydrolytic glutathione S-transferase pathway, given that the gene dcmA was identified rather than the dhlA and P450/MO. The exogenous electrons facilitated the reduction of GSSG, directly or indirectly accelerating the GSH-catalyzed dehalogenation. This study provides support for the construction of an efficient and stable MEC for DCM removal in water environment.

Electrochemical promotion of organic waste fermentation: Research advances and prospects

The current methods of treating organic waste suffer from limited resource usage and low product value. Research and development of value-added products emerges as an unavoidable trend for future growth. Electro-fermentation (EF) is a technique employed to stimulate cell proliferation, expedite microbial metabolism, and enhance the production of value-added products by administering minute voltages or currents in the fermentation system. This method represents a novel research direction lying at the crossroads of electrochemistry and biology. This article documents the current progress of EF for a range of value-added products, including gaseous fuels, organic acids, and other organics. It also presents novel value-added products, such as 1,3-propanediol, 3-hydroxypropionic acid, succinic acid, acrylic acid, and lysine. The latest research trends suggest a focus on EF for cogeneration of value-added products, studying microbial community structure and electroactive bacteria, exploring electron transfer mechanisms in EF systems, developing effective methods for nutrient recovery of nitrogen and phosphorus, optimizing EF conditions, and utilizing biosensors and artificial neural networks in this area. In this paper, an analysis is conducted on the challenges that currently exist regarding the selection of conductive materials, optimization of electrode materials, and development of bioelectrochemical system (BES) coupling processes in EF systems. The aim is to provide a reference for the development of more efficient, advanced, and value-added EF technologies. Overall, this paper aims to provide references and ideas for the development of more efficient and advanced EF technology.

Co-enhancing effects of zero valent iron and magnetite on anaerobic methanogenesis of food waste at transition temperature (45 °C) and various organic loading rates

Deoiling of food waste (FW) after hydrothermal pretreatment occurs at high temperatures, and more energy is required for substrate cooling before the anaerobic digestion (AD) process. AD at the transition temperature (for example 45 °C) is good for energy saving and carbon emission reducing when treating deoiling FW. However, the metabolic activity of methanogens must increase at the transition temperatures. This study proposes the use of zero-valent iron (Fe0) and magnetite (Fe3O4) to boost CH4 yield from deoiling FW. The results showed a co-enhancing effect on CH4 yield upgradation when using Fe0 and Fe3O4 simultaneously, and the highest CH4 yield reached 536.23 mLCH4/gVS, which was 67.5 % higher than that of Fe0 alone (320.14 mLCH4/gVS). In addition, a high organic loading was favorable for increasing the CH4 yield from deoiling FW. Microbial diversity analysis suggested that the dominant methanogenic pathway at 45 °C was hydrogenotrophic methanogenesis. Herein, a potential metabolic pathway analysis revealed that the co-enhancing effects of Fe0 and Fe3O4 enhanced syntrophic methanogenesis and possibly boosted electron transfer efficiency.

Biomethane is produced by acetate cleavage, not direct interspecies electron transfer: genome-centric view and carbon isotope

Understanding the source of methane (CH4) is of great significance for improving the anaerobic fermentation efficiency in bioengineering, and for mitigating the emission potential of natural ecosystems. Microbes involved in the process named direct interspecies electron transfer coupling with CO2 reduction, i.e., electrons released from electroactive bacteria to reduce CO2 into CH4, have attracted considerable attention for wastewater treatment in the past decade. However, how the synergistic effect of microbiota contributes to this anaerobic carbon metabolism accompanied by CH4 production still remains poorly understood, especial for wastewater with antibiotic exposure. Results show that enhancing lower-abundant acetoclastic methanogens and acetogenic bacteria, rather than electroactive bacteria, contributed to CH4 production, based on a metagenome-assembled genomes network analysis. Natural and artificial isotope tracing of CH4 further confirmed that CH4 mainly originated from acetoclastic methanogenesis. These findings reveal the contribution of direct acetate cleavage (acetoclastic methanogenesis) and provide insightsfor further regulation of methanogenic strategies.

Biochar facilitates methanogens evolution by enhancing extracellular electron transfer to boost anaerobic digestion of swine manure under ammonia stress

This study explored the mechanisms by which biochar mitigates ammonia inhibition in anaerobic digestion (AD) of swine manure. Findings show 2-8 g/L exogenous ammonia dosages gradually inhibited AD, leading to decreases in the efficiencies of hydrolysis, acidogenesis and methanogenesis by 3.4-70.8%, 6.0-82.0%, and 4.9-93.8%, respectively. However, biochar addition mitigated this inhibition and facilitated methane production. Biochar enhanced microbial activities related to electron transport and extracellular electron transfer. Moreover, biochar primarily enriched Methanosarcina, which, consequently, upregulated the genes encoding formylmethanofuran dehydrogenase and methenyltetrahydromethanopterin cyclohydrolase for the CO2-reducing methanogenesis pathway by 26.9-40.8%. It is believed that biochar mediated direct interspecies electron transfer between syntrophic partners, thereby enhancing methane production under ammonia stress. Interestingly, biochar removal did not significantly impact the AD performance of the acclimated microbial community. This indicated the pivotal role of biochar in triggering methanogen evolution to mitigate ammonia stress rather than the indispensable function after the enrichment of ammonia-resistance methanogen.

Effect of thermal hydrolysis pretreatment on the occurrence of N-acyl homoserine lactone during anaerobic digestion of waste activated sludge

This study sought to investigate the relationship between N-acyl homoserine lactones (AHLs) and methanogenic microorganisms, focusing on endogenous AHLs in the anaerobic digestion (AD) process. By analyzing waste activated sludge (WAS) samples, we examine the changes in microbial communities and the AHLs-methanogens connection. The Mantel test and Spearman correlation analysis were conducted to gain novel insights into the AD process. Our findings demonstrate that thermal hydrolysis pretreatment (THP) modifies AHL concentrations during AD, thereby enhancing methanogenic bacteria activity and regulating social interactions among microorganisms. In the Eth group (AD of THP samples labeled Eth), Methanobacterium and Methanosarcina accounted for over 80% of the methanogenic bacteria, with correlation coefficients greater than 0.5 between these bacterial taxa and N-hexyl-l-homoserine lactone (C6-HSL) and N-enanthyl-l-homoserine lactone (C7-HSL).

Enhancing methane production from food waste with iron-carbon micro-electrolysis in a two-stage process

A two-stage process, consisting of a leach-bed reactor (LBR) and an up-flow anaerobic sludge blanket reactor (UASB), has been commonly adopted to improve food waste anaerobic digestion. However, its application is limited due to low hydrolysis and methanogenesis efficiencies. This study proposed a strategy of incorporating iron-carbon micro-electrolysis (ICME) into the UASB and recirculating its effluent to the LBR to improve the two-stage process efficiency. Results showed that the integration of the ICME with the UASB significantly increased the CH4 yield by 168.29%. The improvement of the food waste hydrolysis in the LBR mainly contributed to the enhanced CH4 yield (approximately 94.5%). The enrichment of hydrolytic-acidogenic bacterial activity, facilitated by the Fe2+ generated through ICME, might be the primary cause of the improved food waste hydrolysis. Moreover, ICME enriched the growth of hydrogenotrophic methanogens and stimulated the hydrogenotrophic methanogenesis pathway in the UASB, contributing partially to the enhanced CH4 yield.

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.

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.

Deciphering different effects of ZVI and NaOH on metabolic characteristics in the process of methanogenesis recovery from VFA suppression

Dosing zero valent iron (ZVI) or sodium hydroxide (NaOH) is the common method of addressing acidification in anaerobic digestion (AD) systems; however, few studies have discussed and compared their effects on microbial metabolism. In the present study, microbial syntrophy and metabolic pathways under ZVI and NaOH regulation are comparatively analyzed through microbial network analysis and metagenomic/metaproteomic analyses. CH₄ yield in the ZVI reactor was 414 mL/gVS, an increase of 23% when compared with that in the reactor with NaOH dosing (336 mL/gVS). The methanogenesis recovery period in the ZVI reactor (37 days) was shorter than that in the NaOH reactor (48 days). Co-occurrence networks indicated that ZVI promoted Methanoculleus and Methanosarcina to establish a complex syntrophic association with SAO bacteria (Syntrophaceticus and Aminobacterium) and syntrophic acetogens (Syntrophomonas), strengthening SAO-hydrogenotrophic methanogenesis (HM) and acetoclastic methanogenesis (AM) pathways simultaneously. Metagenomic analysis showed that the relative abundance of mcrA and fwdB in the ZVI reactor was higher 27% than that in the NaOH reactor. Furthermore, through metaproteomics analysis, much more enzymes related to glucose degradation, bioconversion of butyric acid and pyruvate, conversion of formate and acetate to CO₂, and production of CH₄ from acetate and CO₂ were significantly upregulated under ZVI regulation than under NaOH regulation (fold change relative to control [FC] > 1.5, p < 0.05). The results of the present study enhance our understanding of methanogenic mechanisms under the regulation of ZVI, providing a theoretical basis for its practical application in AD systems experiencing VFA suppression.

Comparative effect of conductive and dielectric materials on methanogenesis from highly concentrated volatile fatty acids

Various conductive materials and their dielectric counterparts were used to get deeper insights into contribution of direct interspecies electron transfer (DIET) in improving methanogenesis from highly concentrated volatile fatty acids (12.5 g/L). Potential CH₄ yield, maximum CH₄ production rate and lag phase were significantly (up to 1.4, 3.9 and 2.0 times, respectively) improved with addition of stainless-steel mesh (SM) and carbon felt (CF) compared to both control and dielectric counterparts (p < 0.05). k_app increased by 82% for SM and 63% for CF compared to control (p < 0.05). Short thick pili-like structures up to 150 nm in width were formed only in CF and SM biofilms, however, were more abundant for SM. Ureibacillus and Limnochordia specific for SM biofilms, and Coprothermobacter and Ca. Caldatribacterium for CF biofilms, were considered electrogenic. Promotion of DIET by conductive materials is governed by many factors, including specificity of electrogenic groups to material surface.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

Biochar Facilitated Direct Interspecies Electron Transfer in Anaerobic Digestion to Alleviate Antibiotics Inhibition and Enhance Methanogenesis: A Review

Efficient conversion of organic waste into low-carbon biofuels such as methane through anaerobic digestion (AD) is a promising technology to alleviate energy shortages. However, issues such as inefficient methane production and poor system stability remain for AD technology. Biochar-facilitated direct interspecies electron transfer (DIET) has recently been recognized as an important strategy to improve AD performance. Nonetheless, the underlying mechanisms of biochar-facilitated DIET are still largely unknown. For this reason, this review evaluated the role of biochar-facilitated DIET mechanism in enhancing AD performance. First, the evolution of DIET was introduced. Then, applications of biochar-facilitated DIET for alleviating antibiotic inhibition and enhancing methanogenesis were summarized. Next, the electrochemical mechanism of biochar-facilitated DIET including electrical conductivity, redox-active characteristics, and electron transfer system activity was discussed. It can be concluded that biochar increased the abundance of potential DIET microorganisms, facilitated microbial aggregation, and regulated DIET-associated gene expression as a microbial mechanism. Finally, we also discussed the challenges of biochar in practical application. This review elucidated the role of DIET facilitated by biochar in the AD system, which would advance our understanding of the DIET mechanism underpinning the interaction of biochar and anaerobic microorganisms. However, direct evidence for the occurrence of biochar-facilitated DIET still requires further investigation.

Polycyclic aromatic hydrocarbon removal from subsurface soil mediated by bacteria and archaea under methanogenic conditions: Performance and mechanisms

In situ anoxic bioremediation is an easy-to-use technology to remediate polycyclic aromatic hydrocarbon (PAH)-contaminated soil. Degradation of PAHs mediated by soil bacteria and archaea using CO₂ as the electron acceptor is an important process for eliminating PAHs under methanogenic conditions; however, knowledge of the performance and mechanisms involved is poorly unveiled. In this study, the effectiveness and efficiency of NaHCO₃ (CO₂) as an electron acceptor to stimulate the degradation of PAHs by bacteria and archaea in highly contaminated soil were investigated. The results showed that CO₂ addition (EC2000) promoted PAH degradation compared to soil without added CO₂ (EC0), with 4.18%, 9.01%-8.05%, and 6.19%-12.45% increases for 2-, 3- and 4-ring PAHs after 250 days of incubation, respectively. Soil bacterial abundances increased with increasing incubation time, especially for EC2000 (2.90 × 10⁸ g^-1 soil higher than EC0, p < 0.05). Different succession patterns of the soil bacterial and archaeal communities during PAH degradation were observed. According to the PCoA and ANOSIM results, the soil bacterial communities were greatly (ANOSIM: R = 0.7232, P = 0.001) impacted by electron acceptors, whereas significant differences in the archaeal communities were not observed (ANOSIM: R = 0.553, P = 0.001). Soil bacterial and archaeal co-occurrence network analyses showed that positive correlations outnumbered the negative correlations throughout the incubation period for both treatments (e.g., EC0 and EC2000), suggesting the prevalence of coexistence/cooperation within and between these two domains rather than competition. The higher complexity, connectance, edge, and node numbers in EC2000 revealed stronger linkage and a more stable co-occurrence network compared to EC0. The results of this study could improve the knowledge on the removal of PAHs and the responses of soil bacteria and archaea to CO₂ application, as well as a scientific basis for the in situ anoxic bioremediation of PAH-contaminated industrial sites.

Potassium channel blocker selectively enriched Geobacter from mixed-cultured electroactive biofilm: Insights from microbial community, functional prediction and gene expressions

This study investigated the effects of electrical signaling disruption induced by adding tetraethylammonium (TEA, a potassium channel blocker) on the formation of mixed-cultured electroactive biofilms, especially the relative abundance of Geobacter over time. Results showed that TEA addition decelerated the biofilm formation, but selectively enriched Geobacter over time (45.8% on Day 32, 67.7% on Day 60 and 78.1% on Day 90), thus resulting in higher final extracellular electron transfer (EET) efficiency. Redundancy analysis (RDA) confirmed that TEA and operation time were significant factors for the selective enrichment of Geobacter. Moreover, increase in cellular processes and signal processing by PICRUSt analysis indicated adaptive responses of electrogenic biofilms to electrical signaling disruption. Furthermore, qRT-PCR indicated the compensatory roles of key cytochromes and pilA in electrochemical communication, which induced Geobacter enrichment. This work provided a broader understanding of electroactive biofilm regulation and potential applications for electricity generation and biosensor in the future.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Effects of various materials used to promote the direct interspecies electron transfer on anaerobic digestion of low-concentration swine manure

The activation of direct interspecies electron transfer (DIET) by the supplementation of conductive materials is one of the effective and available methods to enhance anaerobic digestion (AD). Microorganisms that colonize the surface of these materials form biofilms, the study of which could provide new insights into the character of the DIET process and its effect on AD. The present study focused on AD performance, microbial community, as well as morphological and topological features of biofilms on various materials used to promote DIET during AD of low-concentration swine manure. The best AD characteristics were observed in stainless steel mesh (SM)/digested cow manure (CM) and polyester felt (PF)/digested sewage sludge (SS) combinations used as material/inoculum, respectively. Thus, potential methane yields in CM-SM and SS-PF were up to 26.4% and 26.2% higher compared to the corresponding controls. Microbial analysis of biofilms revealed the dominance of putatively syntrophic bacteria of the MBA03 group of the Limnochordia class in CM inoculated reactors, and syntrophic proteolytic bacteria of the genus Coprothermobacter and acetogenic Clostridium sensu stricto 1, known for their ability to carry out DIET, in SS inoculated reactors. Biofilms on non-conductive materials contained pili-like structures, which were observed only in SS inoculated reactors. Polyester felt tended to biofoul better than carbon felt, resulting in up to 2.8, 3.2 and 1.8 higher nucleic acid, extracellular polymeric substances, and total biomass content, respectively, depending on the inoculum. These results provide new insights into the different types of DIET that can occur in low-loaded AD systems with attached growth.

Effect of tetracycline on bio-electrochemically assisted anaerobic methanogenic systems: Process performance, microbial community structure, and functional genes

Bio-electrochemically assisted anaerobic methanogenic systems (An-BES) are highly effective in wastewater treatment for methane production and degradation of toxic compounds. However, information on the treatment of antibiotic-bearing wastewater in An-BES is still very limited. This study therefore investigated the effect of tetracycline (TC) on the performance, microbial community, as well as functional and antibiotic resistance genes of An-BES. TC at 1 and 5 mg/L inhibited methane production by less than 4.8% compared to the TC-free control. At 10 mg/L TC, application of 0.5 and 1.0 V decreased methane production by 14 and 9.6%, respectively. Under the effect of 1-10 mg/L TC, application of 1.0 V resulted in a decrease of current from 42.3 to 2.8 mA. TC was mainly removed by adsorption; its removal extent increased by 19.5 and 32.9% with application of 0.5 and 1.0 V, respectively. At 1.0 V, current output was not recovered with the addition of granular activated carbon, which completely removed TC by adsorption. Metagenomic analysis showed that propionate oxidizing bacteria and methanogens were more abundant in electrode biofilms than in suspended culture. Antibiotic resistance genes (ARGs) were less abundant in biofilms than in suspended culture, regardless of whether voltage was applied or not. Application of 1.0 V resulted in the enrichment of Geobacter in the anode and Methanobacterium in the cathode. TC inhibited exoelectrogens, propionate oxidizing bacteria, and the methylmalonyl CoA pathway, leading to a decrease of current output, COD consumption, and methane production. These findings deepen our understanding of the inhibitory effect of TC in An-BES towards efficient bioenergy recovery from antibiotic-bearing wastewater, as well as the response of functional microorganisms to TC in such systems.

Understanding the mechanisms behind enhanced anaerobic digestion of corn straw by humic acids

Humic acids (HAs) are abundant on earth, yet their effects on anaerobic digestion (AD) of cellulosic substrate are not fully uncovered. The effects of HAs on AD of corn straw and the mechanisms behind were analyzed in this study. Results showed that the effects of HAs on methane yield were closely related to the total solids (TS) content. At relative high TS content of 5.0%, HAs benefited AD process by increasing 13.8% of methane yield, accelerating methane production rate by 43% and shortening lag phase time by 37.5%. Microbial community analysis indicated that HAs increased the relative abundance of syntrophic bacteria (Syntrophomonadaceae and Synergistaceae), facilitating the degradation of volatile fatty acids. HAs might act as electron shuttles to directly transfer electrons to hydrogenotrophic methanogens for CO₂ reduction to CH₄. This study provides a simple and efficient strategy to facilitate the AD of cellulosic substrate by HAs addition.

Effect of magnetite addition on transcriptional profiles of syntrophic Bacteria and Archaea during anaerobic digestion of propionate in wastewater sludge

Anaerobic digestion (AD) is an important technology for the effective conversion of waste and wastewater to methane. Here, syntrophic bacteria transfer molecular hydrogen (H₂ ), formate, or directly supply electrons (direct interspecies electron transfer, DIET) to the methanogens. Evidence is accumulating that the methanation of short-chain fatty acids can be enhanced by the addition of conductive material to the anaerobic digester, which has often been attributed to the stimulation of DIET. Since little is known about the transcriptional response of a complex AD microbial community to the addition of conductive material, we added magnetite to propionate-fed laboratory-scale reactors that were inoculated with wastewater sludge. Compared to the control reactors, the magnetite-amended reactors showed improved methanation of propionate. A genome-centric metatranscriptomics approach identified the active SCFA-oxidizing bacteria that affiliated with Firmicutes, Desulfobacterota and Cloacimonadota. The transcriptional profiles revealed that the syntrophic bacteria transferred acetate, H₂ and formate to acetoclastic and hydrogenotrophic methanogens, whereas transcription of potential determinants for DIET such as conductive pili and outer-membrane cytochromes did not significantly change with magnetite addition. Overall, changes in the transcriptional profiles of syntrophic Bacteria and Archaea in propionate-fed lab-scale reactors amended with magnetite refute a major role of DIET in the studied system.

Enhanced anaerobic digestion of dairy wastewater in a granular activated carbon amended sequential batch reactor

This study investigated the potential of granular activated carbon (GAC) supplementation to enhance anaerobic degradation of dairy wastewater. Two sequential batch reactors (SBRs; 0.8 L working volume), one control and another amended with GAC, were operated at 37°C and 1.5-1.6 m/h upflow velocity for a total of 120 days (four cycles of 30 days each). The methane production at the end of each cycle run increased by about 68%, 503%, 110%, and 125% in the GAC-amended SBR, compared with the Control SBR. Lipid degradation was faster in the presence of GAC. Conversely, the organic compounds, especially lipids, accumulated in the absence of the conductive material. In addition, a reduction in lag phase duration by 46%-100% was observed at all four cycles in the GAC-amended SBR. The peak methane yield rate was at least 2 folds higher with GAC addition in all cycles. RNA-based bacterial analysis revealed enrichment of Synergistes (0.8% to 29.2%) and Geobacter (0.4% to 11.3%) in the GAC-amended SBR. Methanolinea (85.8%) was the dominant archaea in the biofilm grown on GAC, followed by Methanosaeta (11.3%), at RNA level. Overall, this study revealed that GAC supplementation in anaerobic digesters treating dairy wastewater can promote stable and efficient methane production, accelerate lipid degradation and might promote the activity of electroactive microorganisms.

Light alters microbiota and electron transport: Evidence for enhanced mesophilic digestion of municipal sludge

Light as an environmental factor can affect the process of anaerobic digestion, but there is no systematic study in municipal wastewater sludge mesophilic digestion. In this study, the effects of light on the performance of the anaerobic digestion system and photo-anaerobic microbiota (PAM) were evaluated in lighted anaerobic batch digesters (LABRs). The methane yield from the reactor under the dark condition (LABR0) was 179.2 mL CH₄/g COD, which was lower than 305.4 mL CH₄/g COD and 223.0 mL CH₄/g COD (n = 3, p < 0.05) from reactors under the light intensity of 3600 lm (LABR1) and 7200 lm (LABR2), respectively. The dominant genera in the bacterial and archaeal communities were Bacillus and Methanosarcina under light conditions, Enterococcus and Methanobacterium under dark conditions. And these two bacteria acted as electroactive bacterial genera, indicating that light changes the combination of direct interspecies electron transfer (DIET) microbial partners and activates the DIET pathway for methane production. The electron conduction pathways analysis further suggests that biological DIET (bDIET) between microbial biomass, rather than DIET via conductive material (cDIET) between microbes and conductive materials, is promoted and behaves as the dominant factor enhancing methane production under light conditions. The morphology of microorganisms and the amount and properties of EPS corroborate these views. Our findings are guided to anaerobic digester constructions under the outdoor environment with light exposure.

Microbiological insights into anaerobic digestion for biogas, hydrogen or volatile fatty acids (VFAs): a review

In the past decades, considerable attention has been directed toward anaerobic digestion (AD), which is an effective biological process for converting diverse organic wastes into biogas, volatile fatty acids (VFAs), biohydrogen, etc. The microbial bioprocessing takes part during AD is of substantial significance, and one of the crucial approaches for the deep and adequate understanding and manipulating it toward different products is process microbiology. Due to highly complexity of AD microbiome, it is critically important to study the involved microorganisms in AD. In recent years, in addition to traditional methods, novel molecular techniques and meta-omics approaches have been developed which provide accurate details about microbial communities involved AD. Better understanding of process microbiomes could guide us in identifying and controlling various factors in both improving the AD process and diverting metabolic pathway toward production of selective bio-products. This review covers various platforms of AD process that results in different final products from microbiological point of view. The review also highlights distinctive interactions occurring among microbial communities. Furthermore, assessment of these communities existing in the anaerobic digesters is discussed to provide more insights into their structure, dynamics, and metabolic pathways. Moreover, the important factors affecting microbial communities in each platform of AD are highlighted. Finally, the review provides some recent applications of AD for the production of novel bio-products and deals with challenges and future perspectives of AD.

Electrotrophy: Other microbial species, iron, and electrodes as electron donors for microbial respirations

Electrotrophy, the growth of microbes on extracellular electron donors, drives important biogeochemical cycles and has practical applications. Studies of Fe(II)-based electrotrophy have provided foundational cytochrome-based mechanistic models for electron transport into cells. Direct electron uptake from other microbial species, Fe(0), or cathodes is of intense interest due to its potential roles in the production and anaerobic oxidation of methane, corrosion, and bioelectrochemical technologies. Other cells or Fe(0) can serve as the sole electron donor supporting the growth of several Geobacter and methanogen strains that are unable to use H₂ as an electron donor, providing strong evidence for electrotrophy. Additional evidence for electrotrophy in Geobacter strains and Methanosarcina acetivorans is a requirement for outer-surface c-type cytochromes. However, in most instances claims for electrotrophy in anaerobes are based on indirect inference and the possibility that H₂ is actually the electron donor supporting growth has not been rigorously excluded.

Promotion of methane production by magnetite via increasing acetogenesis revealed by metagenome-assembled genomes

Metal oxides are wildly studied to enhance anaerobic digestion and the methanogenic process, which is generally interpreted by increased direct interspecies electron transfer (DIET). Yet microbial mechanisms involved are under debate. Herein, methane production dynamics were analyzed, and acceleration on biogas accumulation was presented. Complementing previous findings, Fe₃O₄ nanoparticles stimulated bacterial fermentation rather than methanogenesis or syntropy between electro-microorganism and methanogen. More importantly, metagenome-assembled genomes proved that Fe₃O₄ nanoparticles increased acetogenesis by Parabacteroides chartae, which provided abundant substrates for acetoclastic methanogenesis. Interestingly, the weakly conductive V₃O₇·H₂O nanowires increased potential hydrogen-producing bacteria, Brevundimonas, and electro-microorganisms, Clostridium and Rhodoferax, which is convenient for conducting DIET. Collectively, conductivity may not be a critical factor in mediating DIET and distinct strategies of metal oxides on methane production propose more possibilities, such as fermentation process.

Overview on agricultural potentials of biogas slurry (BGS): applications, challenges, and solutions

The residual slurry obtained from the anaerobic digestion (AD) of biogas feed substrates such as livestock dung is known as BGS. BGS is a rich source of nutrients and bioactive compounds having an important role in establishing diverse microbial communities, accelerating nutrient use efficiency, and promoting overall soil and plant health management. However, challenges such as lower C/N transformation rates, ammonia volatilization, high pH, and bulkiness limit their extensive applications. Here we review the strategies of BGS valorization through microbial and organomineral amendments. Such cohesive approaches can serve dual purposes viz. green organic inputs for sustainable agriculture practices and value addition of biomass waste. The literature survey has been conducted to identify the knowledge gaps and critically analyze the latest technological interventions to upgrade the BGS for potential applications in agriculture fields. The major points are as follows: (1) Bio/nanotechnology-inspired approaches could serve as a constructive platform for integrating BGS with other organic materials to exploit microbial diversity dynamics through multi-substrate interactions. (2) Advancements in next-generation sequencing (NGS) pave an ideal pathway to study the complex microflora and translate the potential information into bioprospecting of BGS to ameliorate existing bio-fertilizer formulations. (3) Nanoparticles (NPs) have the potential to establish a link between syntrophic bacteria and methanogens through direct interspecies electron transfer and thereby contribute towards improved efficiency of AD. (4) Developments in techniques of nutrient recovery from the BGS facilities' negative GHGs emissions and energy-efficient models for nitrogen removal. (5) Possibilities of formulating low-cost substrates for mass-multiplication of beneficial microbes, bioprospecting of such microbes to produce bioactive compounds of anti-phytopathogenic activities, and developing BGS-inspired biofertilizer formulations integrating NPs, microbial inoculants, and deoiled seed cakes have been examined.

Effect of magnetite on anaerobic digestion treating saline wastewater: Methane production, biomass aggregation and microbial community dynamics

In this study, the effect of magnetite amendment on anaerobic digestion was investigated at three increasing salinity levels (0.5%, 1% and 2% NaCl). The amendment of magnetite enhanced the methane yield by 36.3%, 33.3% and 16.5% at low salinity (0.5% NaCl) and high salinity (1% and 2% NaCl), respectively. Meanwhile, a larger proportion of granules was obtained in the magnetite amended reactor (48.05% vs 33.16% at the end of operation). Microbial analysis suggested magnetite could induce more methanogenesis partnerships between hydrogenotrophic methanogens and syntrophic bacteria. Methanosaeta and Methanocorpusculum were the alternating dominant methanogens at low salinity and high salinity. While Streptococcus and Mesotoga were two prevalent bacteria that showed totally different transition tendency in two reactors. Additionally, the supplement of magnetite could relieve the suppression of methanogenesis-related gene expression caused by salinity, thus facilitated the higher methane production.

Anaerobic bacterial responses to carbonaceous materials and implications for contaminant transformation: Cellular, metabolic, and community level findings

Carbonaceous materials (CM) enhance the abundance and activity of bacteria capable of persistent organic (micro)pollutant (POP) degradation. This review synthesizes anaerobic bacterial responses to minimally modified CM in non-fuel cell bioremediation applications at three stages: attachment, metabolism, and biofilm genetic composition. Established relationships between biological behavior and CM surface properties are identified, but temporal relationships are not well understood, making it difficult to connect substratum properties and "pioneer" bacteria with mature microorganism-CM systems. Stark differences in laboratory methodology at each temporal stage results in observational, but not causative, linkages as system complexity increases. This review is the first to critically examine relationships between material and cellular properties with respect to time. The work highlights critical knowledge gaps that must be addressed to accurately predict microorganism-CM behavior and to tailor CM properties for optimized microbial activity, critical frontiers in establishing this approach as an effective bioremediation strategy.

Proteomic Analysis of a Syntrophic Coculture of Syntrophobacter fumaroxidans MPOBT and Geobacter sulfurreducens PCAT

We established a syntrophic coculture of Syntrophobacter fumaroxidans MPOBT (SF) and Geobacter sulfurreducens PCAT (GS) growing on propionate and Fe(III). Neither of the bacteria was capable of growth on propionate and Fe(III) in pure culture. Propionate degradation by SF provides acetate, hydrogen, and/or formate that can be used as electron donors by GS with Fe(III) citrate as electron acceptor. Proteomic analyses of the SF-GS coculture revealed propionate conversion via the methylmalonyl-CoA (MMC) pathway by SF. The possibility of interspecies electron transfer (IET) via direct (DIET) and/or hydrogen/formate transfer (HFIT) was investigated by comparing the differential abundance of associated proteins in SF-GS coculture against (i) SF coculture with Methanospirillum hungatei (SF-MH), which relies on HFIT, (ii) GS pure culture growing on acetate, formate, hydrogen as propionate products, and Fe(III). We noted some evidence for DIET in the SF-GS coculture, i.e., GS in the coculture showed significantly lower abundance of uptake hydrogenase (43-fold) and formate dehydrogenase (45-fold) and significantly higher abundance of proteins related to acetate metabolism (i.e., GltA; 62-fold) compared to GS pure culture. Moreover, SF in the SF-GS coculture showed significantly lower abundance of IET-related formate dehydrogenases, Fdh3 (51-fold) and Fdh5 (29-fold), and the rate of propionate conversion in SF-GS was 8-fold lower than in the SF-MH coculture. In contrast, compared to GS pure culture, we found lower abundance of pilus-associated cytochrome OmcS (2-fold) and piliA (5-fold) in the SF-GS coculture that is suggested to be necessary for DIET. Furthermore, neither visible aggregates formed in the SF-GS coculture, nor the pili-E of SF (suggested as e-pili) were detected. These findings suggest that the IET mechanism is complex in the SF-GS coculture and can be mediated by several mechanisms rather than one discrete pathway. Our study can be further useful in understanding syntrophic propionate degradation in bioelectrochemical and anaerobic digestion systems.

Algae biochar enhanced methanogenesis by enriching specific methanogens at low inoculation ratio during sludge anaerobic digestion

Carbon materials are promising in improving the performance of anaerobic digestion, however, interactive mechanisms between the carbon-based enhancement and operating parameters remained unclear. Using anaerobic digested sludge as inoculum, the effects of Taihu blue algae biochar (ABC) on methanogenesis at different inoculation ratios were investigated during sludge anaerobic digestion. Results showed that ABC enhanced methane productions at the lower inoculation ratios (4% and 1%, v/v), but not at the higher ratio (10%, v/v). Mechanism analysis demonstrated methanogenic improvements at the lower inoculation ratios were not owing to initial organic loading rate increments. Otherwise, ABC addition at the lower inoculation ratios were more favorable for the enrichment of Methanosarcina than the higher ratio, which might be benefit for methanogenesis through directed interspecies electron transfer. Thus, for the improvement of sludge anaerobic digestion, the microbial enrichments at different inoculation ratios would be more important than the merely biochar addition.

The role of conductive nanoparticles in anaerobic digestion: Mechanism, current status and future perspectives

In the current scenario, alternative energy sources are the need of the hour. Organic wastes having a larger fraction of biodegradable constituents present a sustainable bioenergy source. It has been reported that the calorific value of biogas generated by anaerobic digestion (AD) is 21-25 MJ/m³ with the treatment which makes it an excellent replacement of natural gas and fossil fuels and can reduce more than 80% greenhouse gas emission to the surroundings. However, there are some limitations associated with the AD process for instance ammonia build-up at the first stage reduces the rate of hydrolysis of biomass, whereas, in the last stage it interferes with methane formation. Owing to special physicochemical properties such as high activity, high reactive surface area, and high specificity, tailor-made conductive nanoparticles can improve the performance of the AD process. In the AD process, H₂ is used as an electron carrier, referred as mediated interspecies electron transfer (MIET). Due to the diffusion limitation of these electron carriers, the MIET efficiency is relatively low that limits the methanogenesis. Direct interspecies electron transfer (DIET), which enables direct cell-to-cell electron transport between bacteria and methanogen, has been considered an alternative efficient approach to MIET that creates metabolically favorable conditions and results in faster conversion of organic acids and alcohols into methane. This paper discusses in detail the application of conductive nanoparticles to enhance the AD process efficiency. Interaction between microbes in anaerobic conditions for electron transfer with the help of CNPs is discussed. Application of a variety of conductive nanomaterials as an additive is discussed with their potential biogas production and treatment enhancement in the anaerobic digestion process.

Desulfovibrio feeding Methanobacterium with electrons in conductive methanogenic aggregates from coastal zones

Geobacter, as a typical electroactive microorganism, is the "engine" of interspecies electron transfer (IET) between microorganisms. However, it does not have a dominant position in all natural environments. It is not known what performs a similar function as Geobacter in coastal zones. Metagenomic and metatranscriptomic analysis revealed that Desulfovibrio and Methanobacterium species were the most abundant in electrochemically active aggregates. Metatranscriptomic analysis showed that Desulfovibrio species highly expressed genes for ethanol metabolism and extracellular electron transfer involving cytochromes, pili and flagella. Methanobacterium species in the aggregates also expressed genes for enzymes involved in reducing carbon dioxide to methane. Pure cultures demonstrated that the isolated Desulfovibrio sp. strain JY contributed to aggregate conductivity and directly transferred electrons to Methanothrix harundinacea, which is unable to use H₂ or formate. Most importantly, further coculture studies indicated that Methanobacterium strain YSL might directly accept electrons from the Desulfovibrio strain JY for the reduction of carbon dioxide to methane in the aggregate. This finding suggested that the possibility of DIET by Desulfovibrio similar to Geobacter species in conductive methanogenic aggregates can not be excluded.

Zeolitic imidazolate framework-derived porous carbon enhances methanogenesis by facilitating interspecies electron transfer: Understanding fluorimetric and electrochemical responses of multi-layered extracellular polymeric substances

Modulating microbial electron transfer during anaerobic digestion can significantly improve syntrophic interactions for enhanced biogas production. As a carbonaceous conductive material, zeolite imidazolate framework-67 (ZIF-67)-derived porous carbon (PC) was hypothesized to act as a microbial electron transfer highway and assessed with respect to understanding the fluorimetric and electrochemical responses of multilayered extracellular polymeric substances (EPS). The highest biomethane yield (614.0 mL/g) from ethanol was achieved in the presence of 100 mg/L PC prepared at a carbonization temperature of 800 °C (PC-800), which was 28.2% higher than that without PC addition. Electrochemical analysis revealed that both the redox peak currents and conductivity of the methanogenic sludge increased, while the free charge transfer resistance decreased with PC-800 addition. The conductive PC-800 potentially functioned as an abiotic electron conduit to promote direct interspecies electron transfer, thereby resulting in decreased expression of functional genes associated with electrically conductive pili (e-pili) and hemeproteins. Additionally, PC-800 stimulated the secretion of redox-active humic substances (HSs), and excitation emission matrix spectra analysis indicated that the largest increase in percent fluorescence response of HSs occurred in the tightly bound EPS (TB-EPS) with addition of PC-800. This was attributed to the strong complexation ability of PC-800 particles to hydroxyl/carboxylic/phenolic moieties of HSs contained in the TB-EPS. Microbial analysis revealed that syntrophic/exoelectrogenic bacteria such as Pelotomaculum and Syntrophomonas, as well as hydrogenotrophic/electrotrophic methanogens such as Methanoculleus and Methanobacterium, were enriched in methanogenic sludge with adding PC-800. This study provided comprehensive insights for understanding the interactions among ZIF-derived PC, methanogenic microorganisms and their multilayered EPS.

Disentangling the syntrophic electron transfer mechanisms of Candidatus geobacter eutrophica through electrochemical stimulation and machine learning

Interspecies hydrogen transfer (IHT) and direct interspecies electron transfer (DIET) are two syntrophy models for methanogenesis. Their relative importance in methanogenic environments is still unclear. Our recent discovery of a novel species Candidatus Geobacter eutrophica with the genetic potential of IHT and DIET may serve as a model species to address this knowledge gap. To experimentally demonstrate its DIET ability, we performed electrochemical enrichment of Ca. G. eutrophica-dominating communities under 0 and 0.4 V vs. Ag/AgCl based on the presumption that DIET and extracellular electron transfer (EET) share similar metabolic pathways. After three batches of enrichment, Geobacter OTU650, which was phylogenetically close to Ca. G. eutrophica, was outcompeted in the control but remained abundant and active under electrochemical stimulation, indicating Ca. G. eutrophica's EET ability. The high-quality draft genome further showed high phylogenomic similarity with Ca. G. eutrophica, and the genes encoding outer membrane cytochromes and enzymes for hydrogen metabolism were actively expressed. A Bayesian network was trained with the genes encoding enzymes for alcohol metabolism, hydrogen metabolism, EET, and methanogenesis from dominant fermentative bacteria, Geobacter, and Methanobacterium. Methane production could not be accurately predicted when the genes for IHT were in silico knocked out, inferring its more important role in methanogenesis. The genomics-enabled machine learning modeling approach can provide predictive insights into the importance of IHT and DIET.

In Situ Spectroelectrochemical Characterization Reveals Cytochrome-Mediated Electric Syntrophy in Geobacter Coculture

Direct interspecies electron transfer (DIET) between microbial species prevails in some key microbial consortia. However, the electron transfer mechanism(s) in these consortia is controversial due to lack of efficient characterization methods. Here, we provide an in situ anaerobic spectroelectrochemical coculture cell (in situ ASCC) to induce the formation of DIET coculture biofilm on the interdigitated microelectrode arrays and characterize the electron transfer directly. Two typical Geobacter DIET cocultures, Geobacter metallireducens and wild-type Geobacter sulfurreducens (G.m&G.s) and G. metallireducens and a G. sulfurreducens strain deficient in citrate synthase (G.m&G.s-ΔgltA), were selected. In situ Raman and electrochemical Fourier transform infrared (FTIR) spectroscopy indicated that cytochromes are abundant in the electric syntrophic coculture. Cyclic voltammetry and potential step experiment revealed a diffusion-controlled electron transfer process and the electrochemical gating measurements further demonstrated a cytochrome-mediated electron transfer in the DIET coculture. Furthermore, the G.m&G.s-ΔgltA coculture displayed a higher redox conductivity than the G.m&G.s coculture, consistent with the existence of an intimate and efficient electrical connection between these two species. Our findings provide the first report of a redox-gradient-driven electron transport facilitated by c-type cytochromes in DIET coculture, supporting the model that DIET is mediated by cytochromes and suggest a platform to explore the other DIET consortia.

Direct interspecies electron transfer mechanism in enhanced methanogenesis: A mini-review

The role of direct interspecies electron transfer (DIET) on enhancement of methanogenesis has been studied. This mini-review updated the current researches on the potential role of DIET on enhanced performance for anaerobic digestion of organic substrates with effective strategies implemented. Since most experimental observations correlated with the DIET mechanism are yet to be consolidated, this article categorized and discussed the current experimental observations supporting DIET mechanism for methanogenesis, mainly based on those with supplement of carbon materials, from which the prospects and challenges for further studies to confirm the role of DIET in anaerobic digestion processes were highlighted.

The application status, development and future trend of nano-iron materials in anaerobic digestion system

Growing environment problem and emphasis of environmental protection motivate intense research efforts in exploring technology to improve treatment efficiency on refractory organic pollutants. Hence, finding a method to make up for the deficiency of anaerobic digestion (AD) is very attractive and challenging tasks. The recent spark in the interest for the usage of some nanomaterials as an additive to strengthen AD system. The adoption of iron compounds can influence the performance and stability in AD system. However, different iron species and compounds can influence AD system in significantly different ways, both positive and negative. Therefore, strengthening mechanism, treatment efficiency, microbial community changes in Nanoscale Zero Valent Iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) added AD systems were summarized by this review. The strengthening effects of nZVI and Fe₃O₄ NPs in different pollutants treatment system were analyzed. Previous study on the effects of nZVI and Fe₃O₄ NPs addition on AD have reported the concentration of nZVI and Fe₃O₄ NPs, and the types and biodegradability of pollutants might be the key factors that determine the direction and extent of effect in AD system. This review provides a summary on the nZVI and Fe₃O₄ NPs added AD system to establish experiment systems and conduct follow-up experiments in future study.

Electrochemical evidence for in situ microbial activity at the Deep Mine Microbial Observatory (DeMMO), South Dakota, USA

The subsurface is Earth's largest reservoir of biomass. Micro-organisms are the dominant lifeforms in this habitat, but the nature of their in situ activities remains largely unresolved. At the Deep Mine Microbial Observatory (DeMMO) located in the Sanford Underground Research Facility (SURF) in Lead, South Dakota (USA), we performed in situ electrochemical incubations designed to assess the potential for deep groundwater microbial communities to utilize extracellular electron transfer to support microbial respiration. DeMMO 4 was chosen for its stable geochemistry and microbial community. Graphite and indium tin oxide electrodes poised at -200 mV versus SHE were incubated along with open circuit controls and various minerals in a parallel flow reactor that split access to fluids across different treatments. From the patterns of net current over time (fluctuating between anodic and cathodic currents over the course of a few days to weeks) and the catalytic features measured using periodic cyclic voltammetry, evidence of both oxidative and reductive microbe-electrode interactions was observed. The predominant catalytic activity ranged from -210 to -120 mV. The observed temporal variability in electrochemical activity was unexpected given the documented stability in major geochemical parameters. This suggests that the accessed fluids are more heterogeneous in electrochemically active microbial populations than previously predicted from the stable community composition. As previously reported, the fracture fluid and surface-attached microbial communities at SURF differed significantly. However, only minimal differences in community composition were observed between poised potential electrodes, open circuit electrodes, and mineral incubations. These data support that in this environment the ability to attach to surfaces is a stronger driver of microbial community structure than the type or reactivity of the surface. We demonstrate that insight into specific activities can be gained from electrochemical methods, specifically chronoamperometry coupled with routine cyclic voltammetry, which provide a sensitive approach to evaluate microbial activities in situ.

In situ biogas upgrading and enhancement of anaerobic digestion of cheese whey by addition of scrap or powder zero-valent iron (ZVI)

Cheese whey is an easily biodegradable substrate with high organic matter that can be anaerobically digested to biogas; however, the process is often inhibited by excess acidification due to the presence of undissociated volatile fatty acids and requires considerable concentration of alkaline buffer. The current study investigates a new approach for biogas upgrading, and increase of total CH₄ in conjunction with buffering acidification by using zero-valent iron (powder and scrap metals at concentrations 25, 50, and 100 g/L) in anaerobic granular sludge and cheese whey under mesophilic batch conditions. During the first 2 cycles (total 34 days), a high performance was found in anaerobic bottles with 25 g/L powder zero valent iron (PZVI) and 50 g/L scrap zero valent iron (SZVI) since they had a higher total CH₄ production compared to anaerobic bottles free of ZVI, as well as 97% CH₄ composition in produced biogas compared to 74% CH₄ for anaerobic bottles free of ZVI. Under these conditions, no additional NaOH was added to anaerobic bottles with 25 g/L PZVI and 50 g/L SZVI to increase the pH and at the end of 2nd cycle the concentration of VFAs was substantially lower compared to the anaerobic bottles free of ZVI. However, no positive effects of ZVI in terms of alkaline buffer were found at the 3rd and 4th cycle probably due to ZVI inactivation outer surface layer. Based on the experimental findings (anaerobic bottles: (a) 25 g/L PZVI, (b) 50 g/L SZVI and (c) free of ZVI) an economic comparison for anaerobic digestion of cheese whey by large scale was contacted and pointed out that the best scenario was the anaerobic digestion by addition of 50 g/L SZVI, followed by anaerobic digestion free of ZVI and last was the anaerobic digestion by addition of 25 g/L PZVI. This study highlights a new proof of concept for in-situ biogas upgrading and alleviation of acidification by addition of 50 g/L SZVI or 25 g/L PZVI during anaerobic digestion of cheese whey.

Microbes trading electricity in consortia of environmental and biotechnological significance

Favorable interspecies associations prevail in natural microbial assemblages. Some of these favorable associations are co-metabolic dependent partnerships in which extracellular electrons are exchanged between species. For such electron exchange to occur, the cells must exhibit electroactive interfaces and get involved in direct cell-to-cell contact (Direct Interspecies Electron Transfer/DIET) or use available conductive mineral grains from their environment (Conductive-particle-mediated Interspecies Electron Transfer/CIET). This review will highlight recent discoveries and knowledge gaps regarding DIET and CIET interspecies associations in artificial co-cultures and consortia from natural and man-made environments and emphasize approaches to validate DIET and CIET. Additionally, we acknowledge the initiation of a movement towards applying electric syntrophies in biotechnology, bioremediation and geoengineering for natural attenuation of toxic compounds. Next, we have highlighted the urgent research needs that must be met to develop such technologies.

Modeling of interspecies electron transfer in anaerobic microbial communities

Interspecies electron transfer (IET) is a key phenomenon in anaerobic ecosystems, which is traditionally modeled as hydrogen transfer. Recently discovered alternative mediated IET (MIET) or direct IET (DIET) offer exciting alternative mechanisms of microbial partnerships that could lead to new strategies for the improvement of biotechnologies. Here, we analyze mathematical modeling of DIET and MIET in anaerobic ecosystems. Bioenergetics approaches already enable the evaluation of different energy sharing scenarios between microorganisms and give interesting clues on redox mediators and on possible ways of driving microbial communities relying on IET. The modeling of DIET kinetics however is currently only in its infancy. Recent concepts introduced for the modeling of electroactive biofilms should be further exploited. Recent modeling examples confirms the potential of DIET to increase the IET rates compared to H₂-MIET, but also point out the need for additional characterizations of biological components supporting IET to improve predictions.

A high-rate anaerobic biofilm reactor for biomethane recovery from source-separated blackwater at ambient temperature

Anaerobic bioreactors for source-separated blackwater are mostly operated at low organic loading rates (OLRs) due to low biodegradability and the potential of ammonia inhibition. In this study, an anaerobic biofilm reactor having conductive carbon fibers as the media was investigated for the high-rate treatment of blackwater collected from vacuum toilets. The bioreactor was operated at different OLRs ranged from 0.77 to 3.01 g COD/L-d in four stages for a total operating period of ~ 250 days. With the increase of OLRs, the specific methane production rate increased from 105.3 to 304.6 ml/L-d with high methane content in biogas (75.5%-83%). The maximum methane yield was achieved at hydraulic retention time (HRT) of 15 days. Highest organics and suspended solids removal (80%-83%) were achieved at 20-days HRT, while increased OLRs resulted in diminished removal efficiencies. The state variables, including pH, total ammonia nitrogen, short-chain volatile fatty acids, and soluble chemical oxygen demand, indicated the system had a great capability to withstand the high OLRs. Microbial community analysis revealed that the high performance might be attributed to direct interspecies electron transfer (DIET) facilitated by potentially electroactive bacteria (e.g., Syntrophomonas, Clostridium) and electrotrophic archaea (e.g., Methanosaeta and Methanosarcina species) enriched on the carbon fibers. PRACTITIONER POINTS: An anaerobic biofilm reactor was investigated for biomethane recovery from source-separated blackwater. Conductive carbon fibers were utilized as the media to stimulate enrichment of potentially electroactive methanogenic communities. The bioreactor was operated at ambient temperature for over 250 days. High methane production rate and high-quality biogas were achieved at OLRs ranged from 0.77 to 3.01 g COD/L-d. Microbial community analysis suggested direct interspecies electron transfer (DIET) between specific electroactive bacteria and electrotrophic archaea.

Insight into sludge anaerobic digestion with granular activated carbon addition: Methanogenic acceleration and methane reduction relief

In this study, the multiple effects of granular activated carbon (GAC) on sludge anaerobic digestion at ambient (16-24 °C), mesophilic (35 °C) and thermophilic (55 °C) temperature were investigated. After GAC addition, although the methane yields of raw sludge were reduced by 6.5%-36.9%, the lag phases of methanogenesis were shortened by 19.3%-30.6% and the reductions of methane yields were declined to only 5.9%-8.1% simultaneously for pretreated sludge. The inhibitory substances like phenols that generated by thermal pretreatment were reduced after GAC addition, which were demonstrated to be responsible for the methanogenic acceleration. Meanwhile, the methane reduction due to the non-selective adsorption by GAC could be mitigated by pretreatment and elevated temperature. Thus, a strategy coupling thermal pretreatment with detoxification by GAC was proposed to improve the methane production rate and avoid the negative effects during sludge anaerobic digestion with GAC addition.