Redox potential control and applications in microaerobic and anaerobic fermentations

Application of exogenous electron mediator in fermentation to enhance the production of value-added products

Electron transfer is essential for the production efficiency of value-added products in anaerobic fermentation, such as butanol and ethanol as biofuels, and short-chain fatty acids (SCFAs) including butyric acid and acetic acid as platform chemicals. Electron mediators (EMs), also known as electron shuttles, can facilitate electron transfer to counter irreversible or slow redox reactions that limit fermentation. The addition of EMs has been shown to be an effective strategy to promote fermentation by various bacteria, particularly Clostridium species, for these valuable product syntheses. This paper reviews recent advancements in the application of exogenous electron mediators (EEMs) across various scenarios. Common EEM types, their characteristics, and mechanisms are summarized, and different application scenarios are discussed to elucidate the effect of EEMs. Key technical challenges and future directions for EEM application are also explored.

Waste iron shavings to advance anaerobic treatment of acidic poly (butylene adipate-co-terephthalate) wastewater in submerged anaerobic membrane reactor

The wastewater generated during the synthesis of biodegradable plastics, namely poly (butylene adipate-co-terephthalate) (PBAT), is greatly acidic and contains various toxic pollutants. Adding waste iron shavings (WIS) into the submerged anaerobic membrane bioreactor to construct the coupled reactor (WIS-Reactor) holds promise for improving the treatment efficiency of acidic PBAT wastewater. The results showed that the chemical oxygen demand (COD) and volatile fatty acids (VFAs) removal efficiencies of WIS-Reactor were increased by 2.36 and 9.92 times, respectively, compared with the control. Even under strongly acidic influent conditions (pH = 4.0), the methane conversion efficiency (227.07 mLCH4/gCODr) and COD removal rate (51.80 %) in WIS-Reactor were maintained consistently. The pH value in WIS-Reactor increased to around 6.0, the alkalinity increased by 1.5 times due to hydrogen evolution corrosion, and the sludge concentration increased by 19 % without a substantial increase in membrane fouling. Further analysis showed that iron ions released by WIS promoted the secretion of coenzyme F420, enhanced electron transfer between microorganisms, and accelerated CH4 production through enhancing the hydrogenotrophic methanogenesis pathway. Additionally, WIS promoted the enrichment of acidogenic bacteria (Corynebacterium) and electroactive microorganisms (Synergistaceae), and may accelerate the electron transfer efficiency between Syntrophomonas and Methanosaeta through direct interspecies electron transfer, thereby improving the anaerobic digestion of acidic PBAT wastewater.

Effects of impurities on the syngas fermentation: Mechanism and future perspectives

In the process of syngas bioconversion into high value-added chemicals, the presence and impact of impurities must be acknowledged. The present review aims to summarize the progress regarding the effects of various impurities on the syngas fermentation, with the focus on impurity formation in gasification, its inhibition on syngas conversion and influential mechanism. The production of impurities is influenced by various parameters in the gasification process, but substance characteristics is the most relevant factor on impurities composition and concentration. The inhibitory threshold of H2S, NH3 and CN- on syngas bioconversion was 108 ppm, 1520 ppm and 0.025 mM, respectively. In the response to impurities, functional microorganisms related to syngas bioconversion were normally inhibited. Furthermore, the inhibitory mechanisms in aspect of electron transfer and energy synthesis were revealed via the analysis of syngas and impurities metabolic pathway. To alleviate the impurity inhibition, the potential solutions are proposed.

Selective butyrate production from CO2 and methanol in microbial electrosynthesis - influence of pH

Methanol assisted microbial electrosynthesis (MES) enables butyrate production from carbon dioxide and methanol using external electricity. However, the effects of operational parameters on butyrate formation remain unclear. By running three flat plate MES reactors with fed-batch mode at three controlled pH values (5.5, 6 and 7), the present study investigated the influence of pH on methanol assisted MES by comparing the process performance, microbial community structure, and genetic potential. The highest butyrate selectivity (87 % on carbon basis) and the highest butyrate production rate of 0.3 g L-1 d-1 were obtained at pH 6. At pH 7, a comparable butyrate production rate was achieved, yet with a lower selectivity (70 %) accompanied with acetate production. Butyrate production rate was considerably hindered at pH 5.5, reaching 0.1 g L-1 d-1, while the selectivity reached was up to 81 %. Methanol and CO2 consumption increased with pH, along with more negative cathodic potential and more negative redox potential. Furthermore, pH affected the thermodynamical feasibility of involved reactions. The results of metagenomic analyses suggest that Eubacterium callanderi dominated the microbial communities at all pH values, which was responsible for methanol and CO2 assimilation via the Wood-Ljungdahl pathway and was likely the main butyrate producer via the reverse β-oxidation pathway.

Enhancing methane production in two-phase anaerobic digestion of perishable organic waste: Mini-review on acidogenic fermentation pathways and regulatory strategies

Two-phase anaerobic digestion is a highly effective approach for efficient reduction and resource recovery of perishable organic waste. Within this technological framework, organic wastes undergo multiple metabolic pathways during the acidogenic phase, which is classified into ethanol, butyrate, propionate, lactate, and mixed acid fermentation depending on the acidification end products. The nature of these acidification products critically influences the performance of the subsequent methanogenic phase. Strategic regulation of operational parameters during the acidogenic phase fosters the enrichment of specific microbial communities and establishment of dominant consortia, which enable the production of the targeted acidification end-products. This review provides a comprehensive analysis of the metabolic characteristics and regulatory strategies associated with various acidogenic fermentation types and methanogenic properties of different acidification products. The findings presented here are crucial for enhancing the stability and methanogenic efficiency of anaerobic digestion systems that process perishable organic waste.

Pretreatment of waste activated sludge by rotational generator of hydraulic shock

This study investigates hydrodynamic performance of a novel sludge pretreatment device based on periodic shock wave generation by a hydraulic hammer mechanism. A falling circular jet of thickened waste activated sludge was repeatedly impacted by a rotating blade, resulting in occurrence of hydraulic shock waves within the liquid region adjacent to the impact. The rotational generator of hydraulic shock (RGHS) treating 10 L of waste activated sludge was operated for 30 liquid passes and at two different rotational speeds producing blade impact velocities of 44 m/s and 70 m/s, respectively. At 70 m/s impact velocity and 30 passes, the device was able to achieve 41.3 % disintegration degree (DD), specific energy consumption (SEC) of 10.4 kWh/kg sCOD released and 9.0 % improvement of produced methane volume over unprocessed sample. Corresponding values for 44 m/s impact regime were DD = 18.7 %, SEC = 8.03 kWh/kg sCOD released and 33.1 % improvement in methane production. In both pretreatment regimes, sludge shear-dependent viscosity was reduced by about 60 %, while physicochemical analysis, FTIR spectra revealed substantial structural changes in WAS, namely median particle size reduction, degradation of proteins and polysaccharides, and microbial cell wall damage, what was notable also on SEM images. Compared to other rotary devices, the novel RGHS can achieve relatively high degree of sludge disintegration while consuming significantly less energy for sludge solubilization, and for methane production enhancement.

Metabolomics' Change Under β-Cypermethrin Stress and Detoxification Role of CYP5011A1 in Tetrahymena thermophila

BACKGROUND

β-cypermethrin (β-CYP) exhibits high toxicity to aquatic organisms and poses significant risks to aquatic ecosystems. Tetrahymena thermophila, a protozoa widely distributed in aquatic environments, can tolerate high concentrations of β-cypermethrin. However, the comprehensive detoxification mechanisms remain poorly understood in Tetrahymena.

METHODS

Untargeted metabolomics was used to explore the detoxification mechanisms of T. thermophila under β-CYP stress.

RESULTS

Trehalose, maltose, glycerol, and D-myo-inositol were upregulated under β-CYP exposure in Tetrahymena. Furthermore, the expression level of CYP5011A1 was upregulated under β-CYP treatment. CYP5011A1 knockout mutants resulted in a decreasing proliferation rate of T. thermophila under β-CYP stress. The valine-leucine and isoleucine biosynthesis and glycine-serine and threonine metabolism were significantly affected, with significantly changed amino acids including serine, isoleucine, and valine.

CONCLUSIONS

These findings confirmed that T. thermophila develops β-CYP tolerance by carbohydrate metabolism reprogramming and Cyp5011A1 improves cellular adaptations by influencing amino acid metabolisms. Understanding these mechanisms can inform practices aimed at reducing the adverse effects of agricultural chemicals on microbial and environmental health.

Targeted volatile fatty acid production based on lactate platform in mixed culture fermentation: Insights into carbon conversion and microbial metabolic traits

In this study, the effects of fermentation pH and redox potential on the performance of the lactate platform were comprehensively evaluated. The results indicated that the type of acidogenic fermentation was influenced by redox potential, while pH was correlated with volatile fatty acid yield. The highest propionate yield was achieved under anaerobic conditions at a pH of9, with the dominant genus Serpentinicella producing propionate through the acrylate pathway. The highest acetate yield was produced under facultative conditions at a pH of 6. This production was primarily facilitated by the dominant genera unclassified_f__Enterobacteriaceae and Desulfovibrio, which exhibited significant upregulation of the expression of related genes. Furthermore, ecological processes were employed to establish the relationship between environmental factors and microbial communities. This study emphasized the process of converting lactate into volatile fatty acid, providing a theoretical basis for future strategies aimed at regulating targeted acid production.

Development and in-vitro assessment of novel oxygen-releasing feed additives to reduce enteric ruminant methane emissions

Ruminant livestock contribute significantly to global methane production and mitigation of which is of utmost importance. Feed additives represent a cost-effective means of achieving this. A potential target for such additives is rumen Oxidative Reduction Potential (ORP), a parameter which influences CH4 production rates, with methanogenesis occurring optimally at ORPs below -300 mV. Thus, a controlled elevation of rumen ORP represents a potentially benign means of methanogen suppression. This research involved assessing a range of oxygen-releasing compounds for their ability to increase rumen ORP and inhibit methanogenesis, using the in-vitro rumen simulation technique (RUSITEC). Seven potential CH4 inhibitors were tested in a 21-day trial monitoring biogas volume, CH4 content, ORP, digestibility, ammonia, and volatile fatty acids concentration. The additives evaluated included: liquid peroxide (H2O2) and urea hydrogen peroxide (UHP), as well as slower reacting species (calcium and magnesium peroxide), in addition to encapsulated liquid H2O2 for controlled, slow release. Consistent CH4 reductions of >50 % were observed from all additives. Reduced neutral detergent fibre (NDF) digestibility and a reduction in total volatile fatty acids (VFAs) was observed for some treatments, but MgO2 and encapsulated H2O2 reduced CH4 volume by 62 % and 58 %, respectively, and had no detrimental effects on digestibility (p > 0.05) or on VFA production. Ex-situ ORP measurements demonstrated significant increases in ORP upon addition of the additives, with MgO2 and encapsulated H2O2 inducing a more moderate effect suggesting a controlled additive release was achieved with the slow-release format of encapsulated liquid H2O2. Thus, potential slow-release forms deemed suitable to progress to bolus or pellet format in-vivo were identified and could enable a longer-lasting suppression of methanogens within the rumen, facilitating application in both intensive and pasture-based production systems.

Electro-Enhanced Gas Fermentation for Bioproduction of Volatile Fatty Acids and Alcohols

This study investigates sub-stoichiometric electron supply, also termed electro-fermentation, to influence product formation in gas fermentation. Two species, Clostridium carboxidivorans and Alkalibaculum bacchi, as well as a co-culture of A. bacchi and Clostridium kluyveri, were tested in batch cultures with and without an external cell potential of 800 mV. The supplied gas mixture was 50:40:10 N2:H2:CO2. The test unit was a single-chamber reactor with a cathode made from an electrically conducting composite of PP and black carbon. The observed current densities were generally very low, around 0.22 mA/m2. Despite that, a significant and reproducible change in product patterns and formation rates occurred. C. carboxidivorans increased the formation of acetate (+32%), butyrate (+300% relative to the control), and caproate (+600% relative to the control). In a similar manner, A. bacchi produced more acetate (+38%), butyrate (13 times more than the control), and caproate (only observed in the electrified setup). Additional trials using a modified gas phase composition, 80:20 H2:CO2, confirmed the finding that the application of an electric potential enhances chain elongation as well as alcohol formation. Moreover, an experiment with reversed electric polarity showed that a high cathode surface area is essential for inducing metabolic modifications. The results demonstrate that electro-fermentation holds significant potential for improving bioconversion processes aimed at producing green chemicals.

13C-Labelled Glucose Reveals Shifts in Fermentation Pathway During Cathodic Electro-Fermentation with Mixed Microbial Culture

Cathodic Electro-Fermentation (CEF) is an innovative approach to manage the spectrum of products deriving from anaerobic fermentation. Herein, mixed microbial culture fermentation using a ternary mixture containing labelled 13C glucose and non-labelled acetate and ethanol was studied to identify the role of polarization on the metabolic pathways of glucose fermentation. CEF at an applied potential of -700 mV (vs. SHE, Standard Hydrogen Electrode) enhanced the production yield of acetate, propionate, and butyrate (0.90±0.10, 0.22±0.03, and 0.34±0.05 mol/mol; respectively) compared to control tests performed at open circuit potential (OCP) (0.54±0.09, 0.15±0.04, and 0.20±0.001 mol/mol, respectively). Results indicate that CEF affected the 13C labelled fermented product levels and their fractional 13C enrichments, allowing to establish metabolic pathway models. This work demonstrates that, under cathodic polarization, the abundance of both fully 13C labelled propionate and butyrate isotopomers increased compared to control tests. The effect of CEF is mainly due to intermediates initially produced from the glucose metabolic transformation in the presence of non-labelled acetate and ethanol as external substrates. These findings represent a significant advancement in current knowledge of CEF, which offers a promising tool to control mixed cultures bioprocesses.

Long-term performance and mechanism of in-situ biogenetic sulfidated zero-valent iron for enhanced nitrate reduction

The biogenetic sulfidation of zero-valent iron (BS-ZVI) by sulfate-reducing bacteria (SRB) has been demonstrated to enhance the reactivity of ZVI. However, long-term performance of BS-ZVI and related mechanism were still unknown. Therefore, columns containing sponge iron and SRB are built to prepare BS-ZVI in-situ and study its long-term performance. Over 80 % of NO3‾ was reduced to NH4+ by in-situ BS-ZVI within 140 days, which was higher than the sole ZVI treatment (40 %-60 %). The bonding of ZVI and FeSx was in-situ firstly and finally loaded on ZVI. The reduction of Fe(III) by S(-II) and SRB contributed to the formation of FeSx, which improved the electrons transfer. Moreover, BS-ZVI enhanced the enzymes activity of SRB, thus accelerating the metabolic transformation of lactic acid to acetic acid. The accumulation of acetic acid enhanced the removal efficiency of NO3‾ through the dissolution of passivation layer. Overall, this study demonstrated a reactivity enhancement of ZVI through biogenetic sulfidation, which provided a new alternative method for the remediation of groundwater.

Interactive effects of salinity, redox, and colloids on greenhouse gas production and carbon mobility in coastal wetland soils

Coastal wetlands, including freshwater systems near large lakes, rapidly bury carbon, but less is known about how they transport carbon either to marine and lake environments or to the atmosphere as greenhouse gases (GHGs) such as carbon dioxide and methane. This study examines how GHG production and organic matter (OM) mobility in coastal wetland soils vary with the availability of oxygen and other terminal electron acceptors. We also evaluated how OM and redox-sensitive species varied across different size fractions: particulates (0.45-1μm), fine colloids (0.1-0.45μm), and nano particulates plus truly soluble (<0.1μm; NP+S) during 21-day aerobic and anaerobic slurry incubations. Soils were collected from the center of a freshwater coastal wetland (FW-C) in Lake Erie, the upland-wetland edge of the same wetland (FW-E), and the center of a saline coastal wetland (SW-C) in the Pacific Northwest, USA. Anaerobic methane production for FW-E soils were 47 and 27,537 times greater than FW-C and SW-C soils, respectively. High Fe2+ and dissolved sulfate concentrations in FW-C and SW-C soils suggest that iron and/or sulfate reduction inhibited methanogenesis. Aerobic CO2 production was highest for both freshwater soils, which had a higher proportion of OM in the NP+S fraction (64±28% and 70±10% for FW-C and FW-E, respectively) and organic C:N ratios reflective of microbial detritus (5.3±5.3 and 5.3±7.0 for FW-E and FW-C, respectively) compared to SW-C, which had a higher fraction of particulate (58±9%) and fine colloidal (19±7%) OM and organic C:N ratios reflective of vegetation detritus (11.4 ± 1.7). The variability in GHG production and shifts in OM size fractionation and composition observed across freshwater and saline soils collected within individual and across different sites reinforce the high spatial variability in the processes controlling OM stability, mobility, and bioavailability in coastal wetland soils.

Electron transfer in biological systems

Examples of how metalloproteins feature in electron transfer processes in biological systems are reviewed. Attention is focused on the electron transport chains of cellular respiration and photosynthesis, and on metalloproteins that directly couple electron transfer to a chemical reaction. Brief mention is also made of extracellular electron transport. While covering highlights of the recent and the current literature, this review is aimed primarily at introducing the senior undergraduate and the novice postgraduate student to this important aspect of bioinorganic chemistry.

Recent developments on sustainable biobutanol production: a novel integrative review

Renewable and sustainable biofuel production, such as biobutanol, is becoming increasingly popular as a substitute for non-renewable and depleted petrol fuel. Many researchers have studied how to produce butanol cheaply by considering appropriate feedstock materials and bioprocess technologies. The production of biobutanol through acetone-butanol-ethanol (ABE) is highly sought after around the world because of its sustainable supply and lack of competition with food. The purpose of this study is to present the current biobutanol production research and to analyse the biobutanol research conducted during 2006 to 2023. The keyword used in this study is "Biobutanol," and the relevant data was extracted from the Web of Science database (WoS). According to the results, institutions and scholars from the People's Republic of China, the USA, and India have the highest number of cited papers across a broad spectrum of topics including acetone-butanol-ethanol (ABE) fermentation, biobutanol, various pretreatment techniques, and pervaporation. The success of biobutanol fermentation from biomass depends on the ability of the fermentation operation to match the microbial behaviour along with the appropriate bioprocessing strategies to improve the entire process to be suitable for industrial scale. Based on the review data, we will look at the biobutanol technologies and appropriate strategies that have been developed to improve biobutanol production from renewable biomass.

Methanol as a co-substrate with CO2 enhances butyrate production in microbial electrosynthesis

Methanol is a promising feedstock for the bio-based economy as it can be derived from organic waste streams or produced electrochemically from CO2. Acetate production from CO2 in microbial electrosynthesis (MES) has been widely studied, while more valuable compounds such as butyrate are currently attracting attention. In this study, methanol was used as a co-substrate with CO2 to enhance butyrate production in MES. Feeding with CO2 and methanol resulted in the highest butyrate production rates and titres of 0.36 ± 0.01 g L-1 d-1 and 8.6 ± 0.2 g L-1, respectively, outperforming reactors with only CO2 feeding (0.20 ± 0.03 g L-1 d-1 and 5.2 ± 0.1 g L-1, respectively). Methanol acted as electron donor and as carbon source, both of which contributed ca. 50% of the carbon in the products. Eubacterium was the dominant genus with 52.6 ± 2.5% relative abundance. Thus, we demonstrate attractive route for the use of the C1 substrates, CO2 and methanol, to produce mainly butyrate. KEY POINTS: • Butyrate was the main product from methanol and CO2 in MES • Methanol acted as both carbon and electron source in MES • Eubacterium dominating microbial culture was enriched in MES.

Insight into the mechanism of chlorinated nitroaromatic compounds anaerobic reduction with mackinawite (FeS) nanoparticles

Anaerobic biotechnology for wastewaters treatment can nowadays be considered as state of the art methods. Nonetheless, this technology exhibits certain inherent limitations when employed for industrial wastewater treatment, encompassing elevated substrate consumption, diminished electron transfer efficiency, and compromised system stability. To address the above issues, increasing interest is being given to the potential of using conductive non-biological materials, e,g., iron sulfide (FeS), as a readily accessible electron donor and electron shuttle in the biological decontamination process. In this study, Mackinawite nanoparticles (FeS NPs) were studied for their ability to serve as electron donors for p-chloronitrobenzene (p-CNB) anaerobic reduction within a coupled system. This coupled system achieved an impressive p-CNB removal efficiency of 78.3 ± 2.9% at a FeS NPs dosage of 1 mg/L, surpassing the efficiencies of 62.1 ± 1.5% of abiotic and 30.6 ± 1.6% of biotic control systems, respectively. Notably, the coupled system exhibited exclusive formation of aniline (AN), indicating the partial dechlorination of p-CNB. The improvements observed in the coupled system were attributed to the increased activity in the electron transport system (ETS), which enhanced the sludge conductivity and nitroaromatic reductases activity. The analysis of equivalent electron donors confirmed that the S2- ions dominated the anaerobic reduction of p-CNB in the coupled system. However, the anaerobic reduction of p-CNB would be adversely inhibited when the FeS NPs dosage exceeded 5 g/L. In a continuous operation, the p-CNB concentration and HRT were optimized as 125 mg/L and 40 h, respectively, resulting in an outstanding p-CNB removal efficiency exceeding 94.0% after 160 days. During the anaerobic reduction process, as contributed by the predominant bacterium of Thiobacillus with a 6.6% relative abundance, a mass of p-chloroaniline (p-CAN) and AN were generated. Additionally, Desulfomonile was emerged with abundances ranging from 0.3 to 0.7%, which was also beneficial for the reduction of p-CNB to AN. The long-term stable performance of the coupled system highlighted that anaerobic technology mediated by FeS NPs has a promising potential for the treatment of wastewater containing chlorinated nitroaromatic compounds, especially without the aid of organic co-substrates.

Phage lysate can regulate the humification process of composting

Phages play a crucial role in orchestrating top-down control within microbial communities, influencing the dynamics of the composting process. Despite this, the impact of phage-induced thermophilic bacterial lysis on humification remains ambiguous. This study investigates the effects of phage lysate, derived explicitly from Geobacillus subterraneus, on simulated composting, employing ultrahigh-resolution mass spectrometry and 16S rRNA sequencing techniques. The results show the significant role of phage lysate in expediting humus formation over 40 days. Notably, the rapid transformation of protein-like precursors released from phage-induced lysis of the host bacterium resulted in a 14.8 % increase in the proportion of lignins/CRAM-like molecules. Furthermore, the phage lysate orchestrated a succession in bacterial communities, leading to the enrichment of core microbes, exemplified by the prevalence of Geobacillus. Through network analysis, it was revealed that these enriched microbes exhibit a capacity to convert protein and lignin into essential building blocks such as amino acids and phenols. Subsequently, these components were polymerized into humus, aligning with the phenol-protein theory. These findings enhance our understanding of the intricate microbial interactions during composting and provide a scientific foundation for developing engineering-ready composting humification regulation technologies.

Regulation of metabolism and proton motive force generation during mixed carbon fermentation by an Escherichia coli strain lacking the FOF1-ATPase

Proton FOF1-ATPase is the key enzyme in E. coli under fermentative conditions. In this study the role of E. coli proton ATPase in the μ and formation of metabolic pathways during the fermentation of mixture of glucose, glycerol and formate using the DK8 (lacking FOF1) mutant strain was investigated. It was shown that the contribution of FOF1-ATPase in the specific growth rate was ∼45 %. Formate was not taken up in the DK8 strain during the initial hours of the growth. The utilization rates of glucose and glycerol were unchanged in DK8, however, the production of succinate, lactate and ethanol was decreased causing a reduction of the redox state up to -450 mV. Moreover, the contribution of FOF1-ATPase in the interplay between H+ and H2 cycles was described depending on the bacterial growth phase and main utilizing substrate. Besides, the H2 production rate in the DK8 strain was decreased by ∼60 % at 20 h and was absent at 72 h. Δp was decreased from -157 ± 4.8 mV to -140 ± 4.2 mV at 20 h and from -195 ± 5.9 mV to -148 ± 4.4 mV at 72 h, compared to WT. Taken together it can be concluded that during fermentation of mixed carbon sources metabolic cross talk between FOF1-ATPase-TrkA-Hyd-Fdh-H is taking place for maintaining the cell energy balance via regulation proton motive force.

Applying machine learning to anaerobic fermentation of waste sludge using two targeted modeling strategies

Anaerobic fermentation is an effective method to harvest volatile fatty acids (VFAs) from waste activated sludge (WAS). Accurately predicting and optimizing VFAs production is crucial for anaerobic fermentation engineering. In this study, we developed machine learning models using two innovative strategies to precisely predict the daily yield of VFAs in a laboratory anaerobic fermenter. Strategy-1 focuses on model interpretability to comprehend the influence of variables of interest on VFAs production, while Strategy-2 takes into account the cost of variable acquisition, making it more suitable for practical applications in prediction and optimization. The results showed that Support Vector Regression emerged as the most effective model in this study, with testing R2 values of 0.949 and 0.939 for the two strategies, respectively. We conducted feature importance analysis to identify the critical factors that influence VFAs production. Detailed explanations were provided using partial dependence plots and Shepley Additive Explanations analyses. To optimize VFAs production, we integrated the developed model with optimization algorithms, resulting in a maximum yield of 2997.282 mg/L. This value was 45.2 % higher than the average VFAs level in the operated fermenter. Our study offers valuable insights for predicting and optimizing VFAs production in sludge anaerobic fermentation, and it facilitates engineering practice in VFAs harvesting from WAS.

Enhancing short-chain fatty acids production via acidogenic fermentation of municipal sewage sludge: Effect of sludge characteristics and peroxydisulfate pre-oxidation

This study first employed a combined pretreatment of low-dose peroxy-disulfate (PDS) and initial pH 10 to promote short-chain fatty acids (SCFAs) production via acidogenic fermentation using different types of sewage sludge as substrates. The experimental results showed that the yield of maximal SCFAs and acetate proportion after the combined pretreatment were 1513.82 ± 28.25 mg chemical oxygen demand (COD)/L and 53.64%, and promoted by 1.28 and 1.56 times higher, respectively, compared to the sole initial pH 10 pretreatment. Furthermore, in terms of the disintegration degree of sewage sludge, it increased by more than 18% with the combined pretreatment compared to the pretreatment of sole initial pH 10. Waste-activated sludge (WAS) from A2/O and Bardenpho processes were more biodegradable, explained by the 1.47- and 1.35-times higher disintegration rate than those from oxidation ditch and they favored acetate dominant fermentation. Correlation analysis revealed a strong correlation (p ≤ 0.01) between SCFAs production and soluble COD, total proteins, proteins in soluble-extracellular polymeric substances (SEPS), total polysaccharides, and polysaccharides in SEPS. Mechanism explorations showed that preoxidation with PDS enhanced the solubilization and biodegradability of complex substrates, and altered the microbial community structure during the fermentation process. Firmicutes and Tetrasphaera were proven to play a key role in improving SCFA production, especially in promoting acetate production by converting additional SCFAs into acetate. Additionally, the addition of PDS greatly promoted sulfur and iron-related metabolic activities. Finally, the combined pretreatment was estimated to be a cost-effective solution for reutilizing and treating Fe-sludge.

Sustainable Production of Biofuels and Biochemicals via Electro-Fermentation Technology

The energy crisis and climate change are two of the most concerning issues for human beings nowadays. For that reason, the scientific community is focused on the search for alternative biofuels to conventional fossil fuels as well as the development of sustainable processes to develop a circular economy. Bioelectrochemical processes have been demonstrated to be useful for producing bioenergy and value-added products from several types of waste. Electro-fermentation has gained great attention in the last few years due to its potential contribution to biofuel and biochemical production, e.g., hydrogen, methane, biopolymers, etc. Conventional fermentation processes pose several limitations in terms of their practical and economic feasibility. The introduction of two electrodes in a bioreactor allows the regulation of redox instabilities that occur in conventional fermentation, boosting the overall process towards a high biomass yield and enhanced product formation. In this regard, key parameters such as the type of culture, the nature of the electrodes as well as the operating conditions are crucial in order to maximize the production of biofuels and biochemicals via electro-fermentation technology. This article comprises a critical overview of the benefits and limitations of this emerging bio-electrochemical technology and its contribution to the circular economy.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Metagenomics insights into the effect of co-landfill of incineration fly ash and refuse for bacterial community succession and metabolism pathway of VFAs production

With the development of incineration technologies, incineration has become the most common treatment method of municipal solid waste in China. However, stabilized fly ash may enter landfills during the transition from landfill to incineration, which caused uncertain impact on landfill waste stabilization. Two simulated co-landfill columns were constructed based on different co-landfill methods (layer co-landfill and mixed co-landfill) to investigate the effect of stabilized fly ash co-landfilled municipal solid waste for bacterial community succession and change in metabolic pathways during hydrolysis-acidogenesis stage. The mixed co-landfill method resulted in higher degree of organic matter degradation, and the concentrations of volatile fatty acids (VFA) and chemical oxygen demand (COD) in leachate were higher. The dominant phyla were Firmicutes in the layered co-landfill column and Bacteroidetes in mixed co-landfill column. The dominant genera for the total bacterial composition and VFA production were different, Pseudomonas and Propionibacterium, Proteiniphilum and unclassified Bacteroides were the dominant genera responsible for VFA generation in the layered and mixed co-landfill columns. The genes for butyrate production were enriched in the layered co-landfill column, whereas those related to acetate production were enriched in mixed co-landfill column. However, the layered co-landfill inhibited the microbial metabolic activity at the end of the co-landfill process.

Performance of volatile fatty acids production from food waste at the presence of alkyl ethoxy polyglycosides and sodium dodecyl sulfate

In the current context of technological and industrial development, strategies for sustainable development and resource utilization have become increasingly important. FW anaerobic fermentation (Fermentation of Wastes) is a process that utilizes organic waste for biotransformation and is widely used for the production of volatile fatty acids (VFAs). Volatile fatty acids (VFAs) are a kind of high value-added product generated from anaerobic fermentation process, and has extensive applications in chemical synthesis and electricity generation. This study investigated the performance of VFAs production from food waste at the presence of alkyl ethoxy polyglycosides (AEG) and sodium dodecyl sulfate (SDS). The highest yield of VFAs was obtained at 0.1 g AEG/g TS (14.53 g COD/L), which increased by 25.80% than the Blank. But inhibited phenomenon was observed at other reactors with relatively low yield and delayed fermentation time. The inhibition of lactate's production and bioconversion delayed the fermentation time, and SDS has changed the acidogenic fermentation type from lactate-butyrate fermentation to acetate fermentation. In addition, more organic matter dissolved in the fermentation liquor with the addition of AEG and SDS, but the hydrolysis and acidification of polysaccharide were inhibited to some extent. Microbial community analysis showed that the abundance of key bacteria Clostridium has significantly decreased from 82.71% (Blank) to 33.54% (AEG) and 23.72% (SDS), leading to low VFAs production performance.

Perspectives for Using CO2 as a Feedstock for Biomanufacturing of Fuels and Chemicals

Microbial cell factories offer an eco-friendly alternative for transforming raw materials into commercially valuable products because of their reduced carbon impact compared to conventional industrial procedures. These systems often depend on lignocellulosic feedstocks, mainly pentose and hexose sugars. One major hurdle when utilizing these sugars, especially glucose, is balancing carbon allocation to satisfy energy, cofactor, and other essential component needs for cellular proliferation while maintaining a robust yield. Nearly half or more of this carbon is inevitably lost as CO2 during the biosynthesis of regular metabolic necessities. This loss lowers the production yield and compromises the benefit of reducing greenhouse gas emissions-a fundamental advantage of biomanufacturing. This review paper posits the perspectives of using CO2 from the atmosphere, industrial wastes, or the exhausted gases generated in microbial fermentation as a feedstock for biomanufacturing. Achieving the carbon-neutral or -negative goals is addressed under two main strategies. The one-step strategy uses novel metabolic pathway design and engineering approaches to directly fix the CO2 toward the synthesis of the desired products. Due to the limitation of the yield and efficiency in one-step fixation, the two-step strategy aims to integrate firstly the electrochemical conversion of the exhausted CO2 into C1/C2 products such as formate, methanol, acetate, and ethanol, and a second fermentation process to utilize the CO2-derived C1/C2 chemicals or co-utilize C5/C6 sugars and C1/C2 chemicals for product formation. The potential and challenges of using CO2 as a feedstock for future biomanufacturing of fuels and chemicals are also discussed.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Supplementing branched-chain volatile fatty acids in dual-flow cultures varying in dietary forage and corn oil concentrations. I: Digestibility, microbial protein, and prokaryotic community structure

Branched-chain amino acids are deaminated by amylolytic bacteria to branched-chain volatile fatty acids (BCVFA), which are growth factors for cellulolytic bacteria. Our objective was to determine the dietary conditions that would increase the uptake of BCVFA by rumen bacteria. We hypothesized that increased forage would increase cellulolytic bacterial abundance and incorporation of BCVFA into their structure. Supplemental polyunsaturated fatty acids, supplied via corn oil (CO), should inhibit cellulolytic bacteria growth, but we hypothesized that additional BCVFA would alleviate that inhibition. Further, supplemental BCVFA should increase neutral detergent fiber degradation and efficiency of bacterial protein synthesis more with the high forage and low polyunsaturated fatty acid dietary combination. The study was an incomplete block design with 8 dual-flow continuous cultures used in 4 periods with 8 treatments (n = 4 per treatment) arranged as a 2 × 2 × 2 factorial. The factors were: high forage (HF) or low forage (LF; 67 or 33%), without or with supplemental CO (3% dry matter), and without or with 2.15 mmol/d (which included 5 mg/d of 13C each of BCVFA isovalerate, isobutyrate, and 2-methylbutyrate). The isonitrogenous diets consisted of 33:67 alfalfa:orchardgrass pellet, and was replaced with a concentrate pellet that mainly consisted of ground corn, soybean meal, and soybean hulls for the LF diet. The main effect of supplementing BCVFA increased neutral detergent fiber (NDF) degradability by 7.6%, and CO increased NDF degradability only in LF diets. Supplemental BCVFA increased bacterial N by 1.5 g/kg organic matter truly degraded (6.6%) and 0.05 g/g truly degraded N (6.5%). The relative sequence abundance decreased with LF for Fibrobacter succinogenes, Ruminococcus flavefaciens, and genus Butyrivibrio compared with HF. Recovery of the total 13C dose in bacterial pellets decreased from 144 µg/ mg with HF to 98.9 µg/ mg with LF. Although isotope recovery in bacteria was greater with HF, BCVFA supplementation increased NDF degradability and efficiency of microbial protein synthesis under all dietary conditions. Therefore, supplemental BCVFA has potential to improve feed efficiency in dairy cows even with dietary conditions that might otherwise inhibit cellulolytic bacteria.

Production of poly (3-hydroxybutyrate) and extracellular polymeric substances from glycerol by the acidophile Acidiphilium cryptum

Acidiphilium cryptum is an acidophilic, heterotrophic, and metallotolerant bacteria able to use dissolved oxygen or Fe(III) as an electron sink. The ability of this extremophile to accumulate poly(3-hydroxybutyrate) (PHB) and secrete extracellular polymeric substances (EPS) has also been reported. Hence, the aim of this work is to characterize the production of PHB and EPS by the wild strain DSM2389 using glycerol in shaken flasks and bioreactor. Results showed that maximum PHB accumulation (37-42% w/w) was obtained using glycerol concentrations of 9 and 15 g L-1, where maximum dry cell weight titers reached 3.6 and 3.9 g L-1, respectively. The culture in the bioreactor showed that PHB accumulation takes place under oxygen limitation, while the redox potential of the culture medium could be used for online monitoring of the PHB production. Recovered EPS was analyzed by Fourier-transform infrared spectroscopy and subjected to gas chromatography-mass spectrometry after cleavage and derivatization steps. These analyses showed the presence of sugars which were identified as mannose, rhamnose and glucose, in a proportion near to 3.2:2.3:1, respectively. Since glycerol had not been used in previous works, these findings suggest the potential of A. cryptum to produce biopolymers from this compound at a large scale with a low risk of microbial contamination due to the low pH of the fermentation process.

Microbial electrolysis cell simultaneously enhancing methanization and reducing hydrogen sulfide production in anaerobic digestion of sewage sludge

The effects of microbial electrolysis cells (MECs) at three applied voltages (0.8, 1.3, and 1.6 V) on simultaneously enhancing methanization and reducing hydrogen sulfide (H2S) production in the anaerobic digestion (AD) of sewage sludge were studied. The results showed that the MECs at 1.3 V and 1.6 V simultaneously enhanced the methane production by 57.02 and 12.70% and organic matter removal by 38.77 and 11.13%, and reduced H2S production by 94.8 and 98.2%, respectively. MECs at 1.3 V and 1.6 V created a micro-aerobic conditions for the digesters with oxidation-reduction potential as -178∼-232 mv, which enhanced methanization and reduced H2S production. Sulfur reduction, H2S and elemental sulfur oxidation occurred simultaneously in the ADs at 1.3 V and 1.6 V. The relative abundances of sulfur-oxidizing bacteria increased from 0.11% to 0.42% and those of sulfur-reducing bacteria decreased from 1.24% to 0.33% when the applied voltage of MEC increased from 0 V to 1.6 V. Hydrogen produced by electrolysis enhanced the abundance of Methanobacterium and changed the methanogenesis pathway.

Intermittent Oxygen Supply Facilitates Codegradation of Trichloroethene and Toluene by Anaerobic Consortia

Biodegradation is commonly employed for remediating trichloroethene- or toluene-contaminated sites. However, remediation methods using either anaerobic or aerobic degradation are inefficient for dual pollutants. We developed an anaerobic sequencing batch reactor system with intermittent oxygen supply for the codegradation of trichloroethylene and toluene. Our results showed that oxygen inhibited anaerobic dechlorination of trichloroethene, but dechlorination rates remained comparable to that at dissolved oxygen levels of 0.2 mg/L. Intermittent oxygenation engendered reactor redox fluctuations (-146 to -475 mV) and facilitated rapid codegradation of targeting dual pollutants, with trichloroethene degradation constituting only 27.5% of the noninhibited dechlorination. Amplicon sequencing analysis revealed the predominance of Dehalogenimonas (16.0% ± 3.5%) over Dehalococcoides (0.3% ± 0.2%), with ten times higher transcriptomic activity in Dehalogenimonas. Shotgun metagenomics revealed numerous genes related to reductive dehalogenases and oxidative stress resistance in Dehalogenimonas and Dehalococcoides, as well as the enrichment of diversified facultative populations with functional genes related to trichloroethylene cometabolism and aerobic and anaerobic toluene degradation. These findings suggested that the codegradation of trichloroethylene and toluene may involve multiple biodegradation mechanisms. Overall results of this study demonstrate the effectiveness of intermittent micro-oxygenation in aiding trichloroethene-toluene degradation, suggesting the potential for the bioremediation of sites with similar organic pollutants.

Production of acetone, butanol, and ethanol by electro-fermentation with Clostridium saccharoperbutylacetonicum N1-4

This manuscript describes the effect of altering the extracellular redox potential during the production of acetone, butanol, and ethanol on a dual chamber H-type microbial fuel cell by fermenting glucose with Clostridium saccharoperbutylacetonicum N1-4. Extracellular redox potential modification was achieved by either supplementing the microbial broth with the redox agent NADH or by poising the cathode potential at -600 mV vs. Ag/AgCl. The addition of NADH was found to foment the production of acetone via fermentation of glucose. The addition of 200 mM of NADH to the catholyte rendered the highest production of acetone (2.4 g L^-1), thus outperforming the production of acetone by conventional fermentation means (control treatment) by a factor of 2.2. The experimental evidence gathered here, indicates that cathodic electro-fermentation of glucose favors the production of butanol. When poising the cathode potential at -600 mV vs Ag/AgCl (electro-fermentation), the largest production of butanol was achieved (5.8 g L^-1), outperforming the control treatment by a factor of 1.5. The production of ABE solvents and the electrochemical measurements demonstrate the electroactive properties of C. saccharoperbutylacetonicum N1-4 and illustrates the usefulness of bio-electrochemical systems to improve conventional fermentative processes.

The intraruminal redox potential is stabilised by opposing influences during fermentation

An optimal fermentation process in the forestomach is pivotal for the wellbeing and performance of ruminants. Complex carbohydrates are broken down into short-chain fatty acids (SCFA) which form the major energy source for the animal. A strong interrelationship of this process with intraruminal pH and redox potential (Eh) exists. These parameters can be measured with intraruminal sensors, but the interpretation of the measurements, especially of Eh, and their meaning for intraruminal homeostasis is not completely clear. In this study, factors influencing intraruminal Eh were elucidated. We hypothesised that intraruminal Eh is influenced by the fermentation process as such, but not by its end products SCFA. We measured Eh and pH in ruminal fluid from fasting cannulated sheep after the addition of 0.06 m Na-acetate, -propionate, -butyrate or glucose in vitro. Furthermore, we assessed the interrelation of pH and Eh. Basal Eh and pH values were -120 ± 41 mV and 7.0 ± 0.3, respectively, in native ruminal fluid in vitro. While the addition of SCFA did not induce any changes, glucose addition caused a significant decrease in both pH and Eh compared to the values before the addition (paired Student's t-test, p < 0.05). We attribute the decrease in Eh to an increased production of H₂ in the process of generating SCFA, predominantly acetate. By titrating both native and particle-free ruminal fluid to more acidic and basic pH values (4.5-8.5), we found a non-linear inverse correlation of pH and Eh, counteracting the H₂ -driven decrease of Eh during fermentation. Thus, the intraruminal Eh is influenced by pH and H₂ output during SCFA formation. The opposed character of these factors stabilises the intraruminal homeostasis which might help maintain symbiotic microbiota in the rumen. Understanding, monitoring, and supporting this system will be an essential part of modern cattle production.

Advanced Monitoring and Control of Redox Potential in Wine Fermentation across Scales

Combined with real-time monitoring of density and temperature, the control of the redox potential provides a new approach to influencing cell metabolism during growth, cell viability and non-growing yeast activity in wine fermentations. Prior research indicates that the problem of sluggish and incomplete fermentation can be alleviated by maintaining a constant redox potential during the ethanol fermentation. A secondary trait of hydrogen sulfide formation from elemental sulfur also seems to be associated with the development of low redox potentials during fermentation and this might be prevented by the deliberate control of redox potentials in a certain range. While the control of the redox potential during wine fermentations has been demonstrated previously at the research scale (100 L), the ability to control it in larger volumes typically seen in commercial conditions remained unanswered. Wine fermentations from the same load of Cabernet Sauvignon grapes from the 2021 harvest were conducted at three volumes: 100 L and 1500 L in a research winery and 10,000 L in a commercial winery. Using only pulses of air delivery, the redox potential was successfully controlled to −40 mV referenced to a silver/silver chloride electrode throughout the fermentations, at all scales. This appears to be the first published result of a controlled fermentation trial that includes the commercial scale and demonstrates the scalability of control of redox potential in wine fermentations.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Recent achievements in platform chemical production from food waste

Food waste conversion/valorization to produce bio-based chemicals plays a key role toward achieving carbon neutrality by 2050. Food waste valorization to renewable chemicals is thus an attractive and eco-friendly approach to handling food waste. The production of platform chemicals from food waste is crucial for making highly value-added renewable chemicals. However, earlier reviews dealing with food waste valorization to produce value-added chemicals have emphasized the enhancement of methane, hydrogen, and ethanol production. Along these lines, the existing methods of food waste to produce platform chemicals (e.g., volatile fatty acids, glucose, hydroxymethylfurfural, levulinic acid, lactic acid, and succinic acid) through physical, chemical, and enzymatic pretreatments, hydrolysis, fermentation, and hydrothermal conversion are extensively reviewed. Finally, the challenges faced under these methods are discussed, along with suggestions for future research on platform chemical production from food waste.

Machine learning estimation of biodegradable organic matter concentrations in municipal wastewater

This study investigates whether a novel estimation method based on machine learning can feasibly predict the readily biodegradable chemical oxygen demand (RB-COD) and slowly biodegradable COD (SB-COD) in municipal wastewater from the oxidation-reduction potential (ORP) data of anoxic batch experiments. Anoxic batch experiments were conducted with highly mixed liquor volatile suspended solids under different RB-COD and SB-COD conditions. As the RB-COD increased, the ORP breakpoint appeared earlier, and fermentation occurred in the interior of the activated sludge, even under anoxic conditions. Therefore, the ORP decline rates before and after the breakpoint were significantly correlated with the RB-COD and SB-COD, respectively (p < 0.05). The two biodegradable CODs were estimated separately using six machine learning models: an artificial neural network (ANN), support vector regression (SVR), an ANN-based AdaBoost, a SVR-based AdaBoost, decision tree, and random forest. Against the ORP dataset, the RB-COD and SB-COD estimation correlation coefficients of SVR-based AdaBoost were 0.96 and 0.88, respectively. To identify which ORP data are useful for estimations, the ORP decline rates before and after the breakpoint were separately input as datasets to the estimation methods. All six machine learning models successfully estimated the two biodegradable CODs simultaneously with accuracies of ≥0.80 from only ORP time-series data. Sensitivity analysis using the Shapley additive explanation method demonstrated that the ORP decline rates before and after the breakpoint obviously contributed to the estimation of RB-COD and SB-COD, respectively, indicating that acquiring the ORP data with various decline rates before and after the breakpoint improved the estimations of RB-COD and SB-COD, respectively. This novel estimation method for RB-COD and SB-COD can assist the rapid control of biological wastewater treatment when the biodegradable organic matter concentration dynamically changes in influent wastewater.

A redox-based strategy to enhance propionic and butyric acid production during anaerobic fermentation

This study investigated the selective production of volatile fatty acids (VFAs) during anaerobic mixed-culture fermentation. The experiment used chicken manure (CM) as a potential substrate to produce high added-value propionic acid and butyric acid under an alkaline environment. The conversion of CM into selective VFAs depends highly on operational conditions such as pH and redox balance. Therefore, the current experiment is designed to employ amino acid addition and develop a redox balance control method to control the final VFA profile. This study showed that 0.2-5.0 % valine and threonine addition successfully enhanced propionic acid and butyric acid production during alkaline fermentation and hence decreased the proportion of acetic acid from 83 % to approximately 47 %. The oxidation-reduction potential (ORP) and redox cofactor ratio (NADH/NAD⁺) were measured to support the selective VFA production mechanism. The results obtained in this study bring extra value to the valorization of CM within the circular economy concept for selective value-added VFA production.

Micro-aeration: an attractive strategy to facilitate anaerobic digestion

Micro-aeration can facilitate anaerobic digestion (AD) by regulating microbial communities and promoting the growth of facultative taxa, thereby increasing methane yield and stabilizing the AD process. Additionally, micro-aeration contributes to hydrogen sulfide stripping by oxidization to produce molecular sulfur or sulfuric acid. Although micro-aeration can positively affect AD, it must be strictly regulated to maintain an overall anaerobic environment that permits anaerobic microorganisms to thrive. Even so, obligate anaerobes, especially methanogens, could suffer from oxidative stress during micro-aeration. This review describes the applications of micro-aeration in AD and examines the cutting-edge advances in how methanogens survive under oxygen stress. Moreover, barriers and corresponding solutions are proposed to move micro-aeration technology closer to application at scale.

Conversion of Syngas from Entrained Flow Gasification of Biogenic Residues with Clostridium carboxidivorans and Clostridium autoethanogenum

Synthesis gas fermentation is a microbial process, which uses anaerobic bacteria to convert CO-rich gases to organic acids and alcohols and thus presents a promising technology for the sustainable production of fuels and platform chemicals from renewable sources. Clostridium carboxidivorans and Clostridium autoethanogenum are two acetogenic bacteria, which have shown their high potential for these processes by their high tolerance toward CO and in the production of industrially relevant products such as ethanol, 1-butanol, 1-hexanol, and 2,3-butanediol. A promising approach is the coupling of gasification of biogenic residues with a syngas fermentation process. This study investigated batch processes with C. carboxidivorans and C. autoethanogenum in fully controlled stirred-tank bioreactors and continuous gassing with biogenic syngas produced by an autothermal entrained flow gasifier on a pilot scale >1200 °C. They were then compared to the results of artificial gas mixtures of pure gases. Because the biogenic syngas contained 2459 ppm O2 from the bottling process after gasification of torrefied wood and subsequent syngas cleaning for reducing CH4, NH3, H2S, NOX, and HCN concentrations, the oxygen in the syngas was reduced to 259 ppm O2 with a Pd catalyst before entering the bioreactor. The batch process performance of C. carboxidivorans in a stirred-tank bioreactor with continuous gassing of purified biogenic syngas was identical to an artificial syngas mixture of the pure gases CO, CO2, H2, and N2 within the estimation error. The alcohol production by C. autoethanogenum was even improved with the purified biogenic syngas compared to reference batch processes with the corresponding artificial syngas mixture. Both acetogens have proven their potential for successful fermentation processes with biogenic syngas, but full carbon conversion to ethanol is challenging with the investigated biogenic syngas.

Distribution, horizontal transfer and influencing factors of antibiotic resistance genes and antimicrobial mechanism of compost tea

Compost tea was alternatives of chemical pesticide for green agriculture, but there were no reports about antibiotic resistance genes (ARGs) in compost tea. This study investigated the effect of livestock manures, sewage sludge, their composting products and liquid fermentation on ARGs, mobile genetic elements (MGEs), metal resistance genes (MRGs) and antimicrobial properties of various compost tea. The results showed aerobic liquid fermentation reduced ARGs by 65.93 % and 45.20 % in the compost tea of chicken manure and sludge, enriched ARGs by 8.57 % and 37.41 % in the compost tea of pig manure and bovine manure, and increased MGEs and MRGs by 1.25 × 10^-5-5.53 × 10^-3 and 2.03 × 10^-5-2.03 × 10^-3 in the four compost tea. The correlation coefficient of tetracycline and sulfonamide resistance genes between compost product and compost tea were 0.98 and 0.91. aadA2-02, sul2 and tetX abundant in the compost tea were positively correlated with MGEs and MRGs. Furthermore, liquid fermentation enriched the potential host of tetracycline and vancomycin resistance genes. Tetracycline resistance genes occupied 62.7 % of total ARGs in the compost tea. Alcaligenes and Bacillus enriched by 0.78-39.31 % in the four compost tea, which metabolites had high antimicrobial activity. The potential host of ARGs accounted for 42.1 % bacteria abundance in the four compost tea.

Control of redox potential in a novel continuous bioelectrochemical system led to remarkable metabolic and energetic responses of Clostridium pasteurianum grown on glycerol

BACKGROUND

Electro-fermentation (EF) is an emerging tool for bioprocess intensification. Benefits are especially expected for bioprocesses in which the cells are enabled to exchange electrons with electrode surfaces directly. It has also been demonstrated that the use of electrical energy in BES can increase bioprocess performance by indirect secondary effects. In this case, the electricity is used to alter process parameters and indirectly activate desired pathways. In many bioprocesses, oxidation-reduction potential (ORP) is a crucial process parameter. While C. pasteurianum fermentation of glycerol has been shown to be significantly influenced electrochemically, the underlying mechanisms are not clear. To this end, we developed a system for the electrochemical control of ORP in continuous culture to quantitatively study the effects of ORP alteration on C. pasteurianum by metabolic flux analysis (MFA), targeted metabolomics, sensitivity and regulation analysis.

RESULTS

In the ORP range of -462 mV to -250 mV, the developed algorithm enabled a stable anodic electrochemical control of ORP at desired set-points and a fixed dilution rate of 0.1 h-1. An overall increase of 57% in the molar yield for 1,3-propanediol was observed by an ORP increase from -462 to -250 mV. MFA suggests that C. pasteurianum possesses and uses cellular energy generation mechanisms in addition to substrate-level phosphorylation. The sensitivity analysis showed that ORP exerted its strongest impact on the reaction of pyruvate-ferredoxin-oxidoreductase. The regulation analysis revealed that this influence is mainly of a direct nature. Hence, the observed metabolic shifts are primarily caused by direct inhibition of the enzyme upon electrochemical production of oxygen. A similar effect was observed for the enzyme pyruvate-formate-lyase at elevated ORP levels.

CONCLUSIONS

The results show that electrochemical ORP alteration is a suitable tool to steer the metabolism of C. pasteurianum and increase product yield for 1,3-propanediol in continuous culture. The approach might also be useful for application with further anaerobic or anoxic bioprocesses. However, to maximize the technique's efficiency, it is essential to understand the chemistry behind the ORP change and how the microbial system responds to it by transmitted or direct effects.

Oxidoreduction potential controlling for increasing the fermentability of enzymatically hydrolyzed steam-exploded corn stover for butanol production

Background

Lignocellulosic biomass is recognized as an effective potential substrate for biobutanol production. Though many pretreatment and detoxification methods have been set up, the fermentability of detoxicated lignocellulosic substrate is still far lower than that of starchy feedstocks. On the other hand, the number of recent efforts on rational metabolic engineering approaches to increase butanol production in Clostridium strains is also quite limited, demonstrating the physiological complexity of solventogenic clostridia. In fact, the strain performance is greatly impacted by process control. developing efficient process control strategies could be a feasible solution to this problem.

Results

In this study, oxidoreduction potential (ORP) controlling was applied to increase the fermentability of enzymatically hydrolyzed steam-exploded corn stover (SECS) for butanol production. When ORP of detoxicated SECS was controlled at − 350 mV, the period of fermentation was shortened by 6 h with an increase of 27.5% in the total solvent (to 18.1 g/L) and 34.2% in butanol (to 10.2 g/L) respectively. Silico modeling revealed that the fluxes of NADPH, NADH and ATP strongly differed between the different scenarios. Quantitative analysis showed that intracellular concentrations of ATP, NADPH/NADP⁺, and NADH/NAD⁺ were increased by 25.1%, 81.8%, and 62.5%. ORP controlling also resulted in a 2.1-fold increase in butyraldehyde dehydrogenase, a 1.2-fold increase in butanol dehydrogenase and 29% increase in the cell integrity.

Conclusion

ORP control strategy effectively changed the intracellular metabolic spectrum and significantly improved Clostridium cell growth and butanol production. The working mechanism can be summarized into three aspects: First, Glycolysis and TCA circulation pathways were strengthened through key nodes such as pyruvate carboxylase [EC: 6.4.1.1], which provided sufficient NADH and NADPH for the cell. Second, sufficient ATP was provided to avoid “acid crash”. Third, the key enzymes activities regulating butanol biosynthesis and cell membrane integrity were improved.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01824-2.

Consolidation of hydrogenotrophic methanogenesis by sulfidated nanoscale zero-valent iron in the anaerobic digestion of food waste upon ammonia stress

The feasibility of adding sulfidated nanoscale zero-valent iron (S-nZVI) into anaerobic systems to improve anaerobic digestion of food waste (FW) under ammonia stress was evaluated in this study. The addition of S-nZVI improved the methane production compared to nanoscale zero-valent iron (nZVI), indicating that sulfidation significantly reinforced the enhancement effect of nZVI in consolidating the hydrogenotrophic methanogenesis. The promoted methanogenic performance was associated with chemical reaction and variances of microbial community induced by S-nZVI. With the characteristics of generation of Fe²⁺ and slow-release of H₂, S-nZVI made the anaerobic system respond positively in facilitating extracellular polymeric substances secretion and optimizing the microbial community structure. Moreover, microbial community analysis showed that S-nZVI addition enriched the species related to biohydrogen production (e.g., Prevotella) and ammonia-tolerant hydrogenotrophic methanogenesis (e.g., Methanoculleus), possibly enhancing the hydrogenotrophic methanogenesis pathway to accelerate methane production. Therefore, adding S-nZVI into the anaerobic systems might propose a feasible engineering strategy to improve the methanogenic performance of the anaerobic digestion of FW upon ammonia stress.

Low-temperature anaerobic digestion of chicken manure at high organic and nitrogen loads - strategies for controlling short chain fatty acids

Objective of this work was to investigate the technical feasibility of low-temperature, closed-loop two-stage (liquid-solid) anaerobic digesters to treat chicken-manure (TS:68%; NH₃:8 g/L) as a sole-feedstock. Effect of pH, temperature, treatment-duration, organic loading rate (OLR) and inoculum-recirculation ratio on short chain fatty acids (SCFA) production was studied. Digesters were operated at 20 ± 1 °C for 282-d over 4 batch-runs (∼70-d/batch) at an OLR of 8.78-4.3 gVS/L/d. Results showed that specific methane yield above 0.6 LCH₄/gVS was feasible with a methane concentration > 60%. SCFA speciation of the entire system was monitored through the liquid-digester. Among SCFA indicators, the ratios of propionic-to-acetic acids, (butyric + valeric)-to-acetic acids, and total SCFA-to-alkalinity were observed within the limit, i.e., below 1.4, 0.3 and 0.8, respectively, indicating high-digester stability. This strategy allowed early detection, diagnosis of process failures in high-solids digester in fed-batch mode, and re-evaluation of operating protocol to enrich performance with economic-benefits.

Electro-fermentation: Sustainable bioproductions steered by electricity

The market of biobased products obtainable via fermentation processes is steadily increasing over the past few years, driven by the need to create a decarbonized economy. To date, industrial fermentation (IF) employs either pure or mixed microbial cultures (MMC) whereby the type of the microbial catalysts and the used feedstock affect metabolic pathways and, in turn, the type of product(s) generated. In many cases, especially when dealing with MMC, the economic viability of IF is hindered by factors such as the low attained product titer and selectivity, which ultimately challenge the downstream recovery and purification steps. In this context, electro-fermentation (EF) represents an innovative approach, based on the use of a polarized electrode interface to trigger changes in the rate, yield, titer or product distribution deriving from traditional fermentation processes. In principle, the electrode in EF can act as an electron acceptor (i.e., anodic electro-fermentation, AEF) or donor (i.e., cathodic electro-fermentation, CEF), or simply as a mean to control the oxidation-reduction potential of the fermentation broth. However, the molecular and biochemical basis underlying the EF process are still largely unknown. This review paper provides a comprehensive overview of recent literature studies including both AEF and CEF examples with either pure or mixed microbial cultures. A critical analysis of biochemical, microbiological, and engineering aspects which presently hamper the transition of the EF technology from the laboratory to the market is also presented.

The Measurement, Application and Effect of Oxygen in Microbial Fermentations: Focusing on Methane and Carboxylate Production

Oxygen is considered detrimental to anaerobic fermentation processes by many practitioners. However, deliberate oxygen sparging has been used successfully for decades to remove H2S in anaerobic digestion (AD) systems. Moreover, microaeration techniques during AD have shown that small doses of oxygen may enhance process performance and promote the in situ degradation of recalcitrant compounds. However, existing oxygen dosing techniques are imprecise, which has led to inconsistent results between studies. At the same time, real-time oxygen fluxes cannot be reliably quantified due to the complexity of most bioreactor systems. Thus, there is a pressing need for robust monitoring and process control in applications where oxygen serves as an operating parameter or an experimental variable. This review summarizes and evaluates the available methodologies for oxygen measurement and dosing as they pertain to anaerobic microbiomes. The historical use of (micro-)aeration in anaerobic digestion and its potential role in other anaerobic fermentation processes are critiqued in detail. This critique also provides insights into the effects of oxygen on these microbiomes. Our assessment suggests that oxygen dosing, when implemented in a controlled and quantifiable manner, could serve as an effective tool for bioprocess engineers to further manipulate anaerobic microbiomes for either bioenergy or biochemical production.

Bioaugmentation combined with biochar to enhance thermophilic hydrogen production from sugarcane bagasse

In this study, Thermoanaerobacterium thermosaccharolyticum MJ2 and biochar were used to enhance thermophilic hydrogen production from sugarcane bagasse. MJ2 bioaugmentation notably increased the hydrogen production by 95.31%, which was further significantly improved by 158.10% by adding biochar. The addition of biochar promoted the degradation of substrate, improved the activities of hydrogenase and electron transfer system, and stimulated microbial growth and metabolism. Microbial community analysis showed that the relative abundance of Thermoanaerobacterium was significantly increased by bioaugmentation and further enriched by biochar. PICRUSt analysis showed that MJ2 combined with biochar promoted metabolic pathways related to substrate degradation and microbial metabolism. This study provides a novel enhancement method for hydrogen production of the cellulolytic microbial consortium by exogenous hydrogen-producing microorganism combined with biochar and deepens the understanding of its functional mechanism.