Propionate is a valuable platform chemical, in particular as a precursor for the production of sustainable jet fuel. However, its biological production remains a challenge, particularly in mixed-culture waste fermentation, where a mixture of volatile fatty acids is typically generated with a low propionate selectivity. This study investigates a two-stage fermentation process to convert food waste into propionate using two different inocula. The strategy relies on driving microbial metabolism toward lactate production in the first-stage fermentation, which will serve as the intermediate for propionate production during the second step of the fermentation. With both inocula, lactate was effectively consumed and preferentially converted into propionate and acetate, with propionate concentrations reaching up to 17.90 +/- 0.35 g L-1, representing 45.51 +/- 1.81% of total COD metabolites. This production was associated with the growth of Anaerosporobacter, Tyzzerella and some Clostridium genera, in both inocula, suggesting their pivotal role in the lactate-propionate pathway. Overall, these results demonstrate that a two-stage fermentation strategy can efficiently target high propionate concentrations and reasonable selectivity using mixed cultures, by specifically targeting the lactate-propionate pathway, regardless of the initial inoculum.

The production of biofuels from lignocellulosic biomass is one of the most promising alternatives for generating clean energy. However, various challenges arise when using this recalcitrant biomass in biotechnological processes for the production of biofuels (ethanol, biogas, biohydrogen, and methane), such as low yields due to the structural complexity of lignocellulose. In this sense, this study evaluated a biological pretreatment of a lignocellulosic waste mixture consisting of fruit peels (FRP) and brewer's spent grain (BSG), through solid-state fermentation (SSF) using the white-rot fungi Trametes versicolor and Pleurotus ostreatus. Different pretreatment conditions were assessed, including duration (15, 25, and 35 days), temperature (28 and 32 degrees C), and fungal strain (T. versicolor and P. ostreatus), to determine their impact on reducing sugars concentration and delignification and improving the bioaccesibility of the biomass in further bioenergy obtention process. The most suitable pretreatment condition was to carry out the SSF with T. versicolor at 28 degrees C for 25 days, obtaining a delignification of 15 +/- 0.14 % and a reducing sugars consumption of 24 +/- 0 %. The substrate pretreated under these conditions (TV-H) resulted in a 2.06-fold improvement in hydrogen production (mLH2 L-1) compared to the raw substrate as well as a shift in microbial communities and metabolic pathways involved in comparison to the raw substrate in dark fermentation, demonstrating an improvement in the process when biomass is pretreated with Trametes versicolor.

In anaerobic digestion, high ammonia concentration and thermophilic conditions inhibit acetoclastic methanogens, favoring syntrophic oxidation of volatile fatty acids. In the well-known ADM1 model, however, syntrophic oxidation of acetate is not included. In this study, we estimated and validated kinetic parameters of syntrophic acetate oxidizing bacteria (SAOB) and associated syntrophs (syntrophic propionate oxidizing bacteria (SPOB), hydrogenotrophic methanogen (HM)) using data from dedicated enrichment experiments. Syntrophic interactions are inherently constrained by thermodynamics, requiring tight cooperation between partners to make methanogenesis possible. We thus compared a classical ADM1-based approach (MAMD1) with a thermodynamically constrained version (MTh) that includes estimation of growth yields and inhibition directly from thermodynamic principles. Both modeling approaches enabled successful parameter estimation, but MTh had several advantages: by reducing the number of empirical parameters and enforcing thermodynamic feasibility, it improved parameter identifiability and provided more realistic growth yields, although uncertainties in half-saturation constants (KS) remain relatively high. The analysis further revealed that, unlike SPOB and HM, SAOB cannot generate ATP through substrate oxidation alone yet still exhibit growth. This paradox points to missing or poorly understood metabolic pathways (e.g., alternative electron shuttle or energy conservation mechanisms). Overall, the study provides validated parameter ranges for syntrophic partners under thermophilic and high-ammonia conditions and demonstrates the added value of incorporating thermodynamic constraints in ADM1-type models to improve robustness and reveal knowledge gaps in microbial energy metabolism.

Lignocellulosic biomass (LCB) is the world’s most abundant renewable carbon source, yet its potential to drive a circular bioeconomy remains largely untapped. Microbial electrochemical technologies (METs) offer a promising route for converting this complex feedstock into electricity or valuable chemicals. However, LCB-MET advancement is hindered by a fundamental challenge: LCB recalcitrance necessitates depolymerization, a process mismatched with the metabolic capabilities of most electroactive microorganisms (EAMs). While EAMs excel at oxidizing simple substrates, most lack the hydrolytic machinery to break down LCB, creating a critical performance bottleneck. Addressing this requires a multi-disciplinary approach. At the heart of the biological challenge lie two core paradigms,each drawing on microorganisms sourced from nature or artificially engineered: (i) specialized strains capable of both hydrolytic and electrogenic functions, or (ii) synthetic consortia establishing division of labor between fermentative microbes and EAMs. These strategies do not operate in a vacuum; their performance is constrained by materials and environment. To evaluate the progress and potential of these interdependent biological, material, and engineering strategies, this review examines the landscape of LCB utilization in METs. It synthesizes recent advances in coupling depolymerization with extracellular electron transfer, critically evaluate microbial players from pure strains and mixed communities to genetically modified organisms and synthetic consortia, and assesses the key operational parameters, challenges, and potential solutions that define this field. Moving beyond, the review provides graphic representations and statistical analyses of recent publications to establish quantitative performance benchmarks.

Microbial electrolysis cells (MECs) are conventionally used for renewable biohydrogen production, yet their potential to steer carbon fluxes toward value-added molecules remains underexplored. Here, continuous dark fermentation (DF) of food waste (FW) operated at increasing organic loading rates (30–60 g VS/L·d) at neutral pH. The resulting effluents were enriched in acetate (26–31%), ethanol (3–15%), propionate (10–14%) and succinate (8–22%) on a chemical oxygen demand (COD) basis, while butyrate decreased from 25% to 8%. The effluent produced at 60 g VS/L·d was selected for MEC operation because of its high content of readily degradable substrates (acetate and ethanol), low butyrate level, and increased succinate concentration (a known propionate precursor), making it suitable for propionate enrichment and hydrogen production. MECs developed stable electroactive biofilms achieving H2 productions of 0.54–0.59 L H2/g COD, cathodic H2 recoveries up to 90%, and Coulombic efficiencies of 86 ± 2%, with current densities up to 11.5 A/m². Propionate selectivity increased from 12 ± 3% in the DF effluent to 59 ± 5% after MEC operation, as a result of preferential consumption of readily degradable metabolites, particularly acetate, together with succinate conversion into propionate. Microbial community analyses revealed enrichment of hydrolytic and acidogenic taxa during DF, and dominance of Geobacter-like electroactive bacteria (up to 66 ± 5%) alongside fermentative populations during MEC operation, consistent with the observed metabolite transformations. These findings highlight MECs as selective polishing units for simultaneous energy recovery and targeted carbon valorization, advancing the DF–MEC concept toward sustainable bio-based chemical production.

This review covers recent advances in the fixing of dinitrogen in microbial bioelectrochemical systems (BES) where bacteria release or accept electron to/from electrodes for their respiratory metabolism, either directly or indirectly. We discuss how BES may be interesting platforms for producing ammonium or biomass from N2 fixation. The potential for N2-fixation in BES is first discussed with a focus on possible metabolism and different mechanism that may lead to an increase of fixed dinitrogen. We then review recent examples where dinitrogen is fixed at the cathodes of BES, either by pure cultures of hydrogenotrophic and/or diazotrophic bacteria using cathodic H2 or reduced redox mediators as the electron, or by mixed enriched consortia. A section is then devoted to the special case of nitrogen fixation at anodic microbial electrode where organic matter oxidation also occurs. Finally, a comparison of the reported current performance of nitrogen fixation in BES with other biotic (anerobic digestion) or abiotic (Haber-Bosch process, electrochemical N2 reduction) is provided together with a perspective on possible optimization and application of this emerging microbial electrochemical and technological process.

Food waste fermentation offers a promising approach for the production of volatile fatty acids (VFAs). However, controlling the VFA profile, particularly enriching propionic acid, remains a challenge due to the complexity of microbial communities involved. This study investigates the influence of key operating parameters-initial pH, temperature, and substrate concentration-on VFA production and microbial community dynamics. Batch fermentation experiments using reconstituted food waste were conducted under varying conditions. The highest propionic acid selectivity was achieved at an initial pH of 9, a temperature of 35 degrees C, and a substrate concentration of 7.8 g VS/L, with statistically significant improvement over other conditions. Microbial community analysis based on 16S rDNA sequencing revealed distinct profiles shaped by the operational settings. Notably, increased relative abundances of Enterobacteriaceae, Lachnospiraceae, and Exiguobacterium spp. were associated with higher propionic acid production. These results highlight the strong interplay between fermentation conditions, microbial ecology, and metabolite profiles, providing insights for optimising food waste valorisation towards selective VFA production.

Over the past decades, biodiesel production has sharply increased worldwide and has led to an overproduction of glycerol, as by-product. Therefore, glycerol is not only produced at low cost with a wide availability but is also a versatile precursor of useful value-added chemicals such as1,3-propanediol. At an industrial scale, glycerol conversion into 1,3-propanediol is almost entirely carried out by fermentation processes as they have shown the best economic and environmental performances. The aim of this article is to provide an up-to-date state of the art on the fundamentals and fermentation process strategies for the microbial conversion of glycerol into 1,3-propanediol. Glycerol fermentation metabolism is detailed and strategies concerning microbial inoculum (i.e., pure cultures of natural or genetically modified strains vs. mixed cultures or artificial consortia), process configuration (i.e., batch, fed-batch and continuous reactors, biomass immobilisation) and related operational parameters (i.e., temperature, pH, oxido-reduction potential) are discussed for the optimisation of 1,3-propanediol production by fermentation.

AIMS:In previous studies, it was demonstrated that co-culturing Clostridium pasteurianum and Geobacter sulfurreducens triggers a metabolic shift in the former during glycerol fermentation. This shift, attributed to interspecies electron transfer and the exchange of other molecules, enhances the production of 1,3-propanediol at the expense of the butanol pathway. The aim of this investigation is to examine the impact of fumarate, a soluble compound usually used as an electron acceptor for G. sulfurreducens, in the metabolic shift previously described in C. pasteurianum.METHODS AND RESULTS:Experiments were conducted by adding along with glycerol, acetate, and different quantities of fumarate in co-cultures of G. sulfurreducens and C. pasteurianum. A metabolic shift was exhibited in all the co-culture conditions. This shift was more pronounced at higher fumarate concentrations. Additionally, we observed G. sulfurreducens growing even in the absence of fumarate and utilizing small amounts of this compound as an electron donor rather than an electron acceptor in the co-cultures with high fumarate addition.CONCLUSIONS:This study provided evidence that interspecies electron transfer continues to occur in the presence of a soluble electron acceptor, and the metabolic shift can be enhanced by promoting the growth of G. sulfurreducens.

The production of nitrogen fertilizers in modern agriculture is mostly based on the Haber-Bosch process, representing nearly 2% of the total energy consumed in the world. Low-energy bioelectrochemical fixation of N2 to microbial biomass was previously observed but the mechanisms of microbial interactions in N2-fixing electroactive biofilms are still poorly understood. The present study aims to develop a new method of enrichment of autotrophic and diazotrophic bacteria from soil samples with a better electron source availability than using H2 alone. The enrichment method was based on a multi-stage procedure. The first enrichment step was specifically designed for the selection of N2-fixing bacteria from soil samples with organic C as electron and carbon source. Then, a polarized cathode was used for the enrichment of autotrophic bacteria using H2 (hydrogenotrophic) or the cathode as electron source. This enrichment was compared with an enrichment culture of pure diazotrophic hydrogenotrophic bacteria without the use of a microbial electrochemical system. Interestingly, both methods showed comparable results for N2 fixation rates at day 340 of the enrichment with an estimated average of approximately 0.2 mgNfixed/L.d. Current densities up to -15 A/m² were observed in the polarized cathode enrichments and a significant increase of the microbial biomass on the cathode was shown between 132 and 214 days of enrichment.These results confirmed an enrichment in autotrophic and diazotrophic bacteria on the polarized cathode. It was hypothesied that autotrophic bacteria were able to use either the H2 produced at the cathode or directly the cathode through direct electron transfer (DET) as more biomass was produced than with H2 alone. Finally, the analysis of the enriched communities suggested that Desulforamulus ruminis mediated microbial interactions between autotrophic anaerobic and heterotrophic aerobic bacteria in polarized cathode enrichment. These interactions could play a key role in the development of biomass in these systems and on N2 fixation. Based on these findings, a conceptual model on the functioning of mixed cultures N2-fixing electroactive biofilms was proposed.

Co-fermentation can differently impact H2 production, with positive or negative interactions observed. Positive interactions were usually attributed to a balanced composition and improved buffer capacity. However, the impact of co-fermentation on microbial communities (H2-producing and H2-consuming bacteria) remains unexplored. This work aimed to deepen the interaction mechanisms observed in co-fermentation targeting microbial communities with an innovative focus on H2-consuming bacteria (homoacetogens). The H2 production of seven mixtures (food waste fractions and rye silage) and individual performances were compared by Biochemical Hydrogen Potential (BHP). Final microbial communities were characterized by sequencing and qPCR. A positive correlation between H2 yield and Soluble and Easily Extractible Sugars (SEES) content was observed for all the substrates (R² = 0.79), confirming this correlation not only for individual substrates but also mixtures. Positive interactions were observed for most of the mixtures, with a H2 yield significantly higher (16-37%) than expected. The faster H2 production observed in mixtures (2.6 times faster) was correlated to the selection of Clostridiaceae_1 family (final relative abundance of 90 - 98%) and decline of homoacetogens. It is therefore essential to further understand the microbial communities’ dynamics in co-fermentation to develop efficient fermentation systems.

With the escalating global demand for food and increasing concerns about environmental sustainability, finding innovative and efficient solutions for managing organic waste has become an urgent necessity. Anaerobic digestion (AD) is a biological process that converts organic matter into biogas in the absence of oxygen. The fundamental principles and key factors influencing the AD process, including substrate characteristics, microbial communities, temperature and pH are reviewed. Some insights into the history and recent development of anaerobic digestion in France and in Europe are given before the main effluents and waste treatment technologies are described. Finally, the valorization of AD products is presented as well as perspectives towards higher value-added products.

Groundwater is an important resource that can help in climate change adaptation. However, the pollution of these aquifers with nitrate is a widespread problem of growing concern. Biological denitrification using inorganic electron donors shows significant advantages in treating nitrate-polluted groundwater where organic matter presence is negligible. However, mass transfer limitations and secondary contamination seem to be the major hinderance to spread the use of these technologies. This could be solved by the use of bioelectrochemical systems (BES), which emerge as an attractive technology to solve these problems due to the reported low energy demand and high denitrification rates. However, technical and operational issues must be considered to replicate these results at full-scale. This review summarizes the biological basis of autotrophic denitrification and the key aspects of its application in bioelectrochemical systems. In addition, an estimation of the capital costs required for the implementation of a BES considering different population sizes and initial nitrate concentration in the groundwater is made.

The biodegradable and renewable nature of lignocellulosic biomass (LCB) has gained significant interest in recent years. This study explores the lignocellulolytic and electrogenic potential of Shewanella oneidensis MR-1, Cellulomonas fimi ATCC 484, and Cellulomonas biazotea NBRC 12680 on LCB. Two strategies were tested: assessing strains LCB degradation ability under non-electrochemical and electrochemical conditions. Strain selection was based on literature, and bioinformatical analyses were conducted to predict CAZymes and carbohydrate degradation pathways. Cellulomonas strains have a potential to degrade LCB due to high CAZyme count and specific metabolic pathways. Strains growth capacity on LCB was evaluated by culturing without electrodes on LCB for 12 days, showing superior growth on wheat bran compared to wheat straw. Enzymatic assays indicate laccase activity in all strains, highest in C. biazotea NBRC 12680 (11.66 IU). The strains ability to form electrogenic biofilms on carbon cloth anodes polarized at +0.2 V (vs Ag/AgCl) was evaluated. The results indicate that bioanodes can function with wheat bran (max current density: 14.92 mA m(-2)), with voltammograms showing redox activities. Attenuated total reflection Fourier transform infrared spectroscopy shows lignin and protein degradation in both electrochemical and non-electrochemical experiments. These findings suggest potential use of these strains in electro-microbial systems with LCB. (c) 2024 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited. All rights, including fortext and data mining, AI training, and similar technologies, are reserved.