Continuous thermophilic hydrogen production from an enzymatic hydrolysate of agave bagasse: Inoculum origin, homoacetogenesis and microbial community analysis

Mixed culture biotechnology and its versatility in dark fermentative hydrogen production

Over the years, extensive research has gone into fermentative hydrogen production using pure and mixed cultures from waste biomass with promising results. However, for up-scaling of hydrogen production mixed cultures are more appropriate to overcome the operational difficulties such as a metabolic shift in response to environmental stress, and the need for a sterile environment. Mixed culture biotechnology (MCB) is a robust and stable alternative with efficient waste and wastewater treatment capacity along with co-generation of biohydrogen and platform chemicals. Mixed culture being a diverse group of bacteria with complex metabolic functions would offer a better response to the environmental variations encountered during biohydrogen production. The development of defined mixed cultures with desired functions would help to understand the microbial community dynamics and the keystone species for improved hydrogen production. This review aims to offer an overview of the application of MCB for biohydrogen production.

Sustainable biohydrogen production from lignocellulosic biomass sources - metabolic pathways, production enhancement, and challenges

Hydrogen production from biological processes has been hailed as a promising strategy for generating sustainable energy. Fermentative hydrogen production processes such as dark and photofermentation are considered more sustainable and economical than other biological methods such as biophotolysis. However, these methods have constraints such as low hydrogen yield and conversion efficiency, so practical implementations still need to be made. The present review provides an assessment and feasibility of producing biohydrogen through dark and photofermentation techniques utilizing various lignocellulosic biomass wastes as substrates. Furthermore, this review includes information about the strategies to increase the productivity rate of biohydrogen in an eco-friendly and sustainable manner, like integration of dark and photofermentation techniques, pretreatment of biomass, genetic modification of microorganisms, and application of nanoadditives.

Biohydrogen from organic wastes as a clean and environment-friendly energy source: Production pathways, feedstock types, and future prospects

Microbial fermentation of organic matter under anaerobic conditions is currently the prominent pathway for biohydrogen production. Organic matter present in waste residues is regarded as an economic feedstock for biohydrogen production by dark and photo fermentative bacteria. Agricultural residues, fruit wastes, vegetable wastes, industrial wastewaters, and other livestock residues are some of the organic wastes most commonly used for biohydrogen production due to their higher organic content and biodegradability. Appropriate pretreatments are required to enhance the performance of biohydrogen from complex organic wastes. Biohydrogen production could also be enhanced by optimizing operation conditions and the addition of essential nutrients and nanoparticles. This review describes the pathways of biohydrogen production, discusses the effect of organic waste sources used and microbes involved on biohydrogen production, along with addressing the key parameters, advantages, and difficulties in each biohydrogen production pathway.

Knowing the enemy: homoacetogens in hydrogen production reactors

One of the bottlenecks of the hydrogen production by dark fermentation is the low yields obtained because of the homoacetogenesis persistence, a metabolic pathway where H₂ and CO₂ are consumed to produce acetate. The central reactions of H₂ production and homoacetogenesis are catalyzed by enzyme hydrogenase and the formyltetrahydrofolate synthetase, respectively. In this work, genes encoding for the formyltetrahydrofolate synthetase (fthfs) and hydrogenase (hydA) were used to investigate the diversity of homoacetogens as well as their phylogenetic relationships through quantitative PCR (qPCR) and next-generation amplicon sequencing. A total of 70 samples from 19 different H₂-producing bioreactors with different configurations and operating conditions were analyzed. Quantification through qPCR showed that the abundance of fthfs and hydA was strongly associated with the type of substrate, organic loading rate, and H₂ production performance. In particular, fthfs sequencing revealed that homoacetogens diversity was low with one or two dominant homoacetogens in each sample. Clostridium carboxivorans was detected in the reactors fed with agave hydrolisates; Acetobacterium woodii dominated in systems fed with glucose; Blautia coccoides and unclassified Sporoanaerobacter species were present in reactors fed with cheese whey; finally, Eubacterium limosum and Selenomonas sp. were co-dominant in reactors fed with glycerol. Altogether, quantification and sequencing analysis revealed that the occurrence of homoacetogenesis could take place due to (1) metabolic changes of H₂-producing bacteria towards homoacetogenesis or (2) the displacement of H₂-producing bacteria by homoacetogens. Overall, it was demonstrated that the fthfs gene was a suitable marker to investigate homoacetogens in H₂-producing reactors. KEY POINTS: • qPCR and sequencing analysis revealed two homoacetogenesis phenomena. • fthfs gene was a suitable marker to investigate homoacetogens in H2 reactors.

Promoting direct interspecies electron transfer and acetoclastic methanogenesis for enhancing anaerobic digestion of butanol octanol wastewater by coupling granular activated carbon and exogenous hydrogen

Butanol octanol wastewater (BOW) generated from syngas conversion of coal contained abundant toxic organic pollutants. Anaerobic digestion is a promising technology for BOW, but abundant toxic substances would inhibit the activity of anaerobic microorganisms. Granular activated carbon (GAC) and exogenous hydrogen (EH₂) were employed to enhance anaerobic digestion of BOW. The results indicated that methane production increased to 289.55 ± 17.43 mL CH₄/g COD in EH₂/GAC group, which was 1.07, 2.04, and 1.98 times of that in GAC, EH₂, and control groups, respectively. In EH₂/GAC group. The relative abundance of Geobacter and Methanosaeta increased rapidly to 25.36% and 52.81%, respectively, and the relative abundance of Clostridium was 9.78%. The underlying mechanism might be that GAC promoted the enrichment of Geobacter, and EH₂ changed metabolic mechanism of Clostridium, stimulating the enrichment of Methanosaeta. Direct interspecies electron transfer was promoted by EH₂/GAC, thus improving the methane production rate of BOW.

On-site CO2 bio-sequestration in anaerobic digestion: Current status and prospects

The advantages of anaerobic digestion (AD) technology in organic solid waste treatment for bioenergy recovery are evidenced in worldwide. Recently, more attention has been paid to on-site biogas research, as well as biogenic CO₂ sequestration from AD plant, to promote "carbon neutral". Single-phase and two-phase AD system can be incorporated with various CO₂ bioconversion technologies through H₂ mediated CO₂ bioconversion (in-situ and ex-situ biogas upgrading), or other emerging strategies for CO₂ fixation without exogenous H₂ injection; these include in-situ direct interspecies electron transfer reinforcement, electromethanogenesis, and off-gas reutilization. The existing and potential scenarios for on-site CO₂ bio-sequestration within the AD framework are reviewed from the perspectives of metabolic pathways, functional microorganisms, the limitations on reaction kinetics. This review concluded that on-site CO₂ bio-sequestration is a promising solution to reduce greenhouse gas emissions and increase renewable energy recovery.

Deep insights into the network of acetate metabolism in anaerobic digestion: focusing on syntrophic acetate oxidation and homoacetogenesis

Acetate is a pivotal intermediate product during anaerobic decomposition of organic matter. Its generation and consumption network is quite complex, which almost covers the most steps in anaerobic digestion (AD) process. Besides acidogenesis, acetogenesis and methanogenesis, syntrophic acetate oxidation (SAO) replaced acetoclastic methanogenesis to release the inhibition of AD at some special conditions, and the importance of considering homoacetogenesis had also been proved when analysing anaerobic fermentations. Syntrophic acetate-oxidizing bacteria (SAOB), with function of SAO, can survive under high temperature and ammonia/ volatile fatty acids (VFAs) concentrations, while, homoacetogens, performed homoacetogenesis, are more active under acidic, alkaline and low temperature (10°C-20°C) conditions, This review summarized the roles of SAO and homoacetogenesis in AD process, which contains the biochemical reactions, metabolism pathways, physiological characteristics and energy conservation of functional bacteria. The specific roles of these two processes in the subprocess of AD (i.e., acidogenesis, acetogenesis and methanogenesis) were also analyzed in detail. A two phases anaerobic digester is proposed for protein-rich waste(water) treatment by enhancing the functions of homoacetogens and SAOB compared to the traditional two-phases anaerobic digesters, in which the first phase is fermentation phase including acidogens and homoacetogens for acetate production, and second phase is a mixed culture coupling syntrophic fatty acids bacteria, SAOB and hydrogenotrophic methanogens for methane production. This review provides a new insight into the network on production and consumption of acetate in AD process.