Metabolic division of labor between Acetivibrio thermocellus DSM 1313 and Thermoanaerobacterium thermosaccharolyticum MJ1 enhanced hydrogen production from lignocellulose

A novel microbial community restructuring strategy for enhanced hydrogen production using multiple pretreatments and CSTR operation

To achieve rapid enrichment of the targeted hydrogen-producing bacterial population and reconstruction of the microbial community in the biological hydrogen-producing reactor, the activated sludge underwent multiple pretreatments using micro-aeration, alkaline treatment, and heat treatment. The activated sludge obtained from the multiple pretreatments was inoculated into the continuous stirred tank reactor (CSTR) for continuous operations. The community structure alteration and hydrogen-producing capability of the activated sludge were analyzed throughout the operation of the reactor. We found that the primary phyla in the activated sludge population shifted to Proteobacteria, Firmicutes, and Bacteroidetes, which collectively accounted for 96.69% after undergoing several pretreatments. This suggests that the multiple pretreatments facilitated in achieving the selective enrichment of the fermentation hydrogen-producing microorganisms in the activated sludge. The CSTR start-up and continuous operation of the biological hydrogen production reactor resulted in the reactor entering a highly efficient hydrogen production stage at influent COD concentrations of 4000 mg/L and 5000 mg/L, with the highest hydrogen production rate reaching 8.19 L/d and 9.33 L/d, respectively. The main genus present during the efficient hydrogen production stage in the reactor was Ethanoligenens, accounting for up to 33% of the total population. Ethanoligenens exhibited autoaggregation capabilities and a superior capacity for hydrogen production, leading to its prevalence in the reactor and contribution to efficient hydrogen production. During high-efficiency hydrogen production, flora associated with hydrogen production exhibited up to 46.95% total relative abundance. In addition, redundancy analysis (RDA) indicated that effluent pH and COD influenced the distribution of the primary hydrogen-producing bacteria, including Ethanoligenens, Raoultella, and Pectinatus, as well as other low abundant hydrogen-producing bacteria in the activated sludge. The data indicates that the multiple pretreatments and reactor's operation has successfully enriched the hydrogen-producing genera and changed the community structure of microbial hydrogen production.

Composition of Lignocellulose Hydrolysate in Different Biorefinery Strategies: Nutrients and Inhibitors

The hydrolysis and biotransformation of lignocellulose, i.e., biorefinery, can provide human beings with biofuels, bio-based chemicals, and materials, and is an important technology to solve the fossil energy crisis and promote global sustainable development. Biorefinery involves steps such as pretreatment, saccharification, and fermentation, and researchers have developed a variety of biorefinery strategies to optimize the process and reduce process costs in recent years. Lignocellulosic hydrolysates are platforms that connect the saccharification process and downstream fermentation. The hydrolysate composition is closely related to biomass raw materials, the pretreatment process, and the choice of biorefining strategies, and provides not only nutrients but also possible inhibitors for downstream fermentation. In this review, we summarized the effects of each stage of lignocellulosic biorefinery on nutrients and possible inhibitors, analyzed the huge differences in nutrient retention and inhibitor generation among various biorefinery strategies, and emphasized that all steps in lignocellulose biorefinery need to be considered comprehensively to achieve maximum nutrient retention and optimal control of inhibitors at low cost, to provide a reference for the development of biomass energy and chemicals.

Electron syntrophy between mixed hydrogenogens and Geobacter metallireducens boosted dark hydrogen fermentation: Clarifying roles of electroactive extracellular polymeric substances

To modulate the electron transfer behavior of hydrogen-producing bacteria (HPB) for enhanced hydrogen production, Geobacter metallireducens culture (GM) was introduced as an electron syntrophy partner and redox balance regulator in dark fermentation systems with hydrogen-producing sludge (HPS) as inoculum. The highest hydrogen yield was 306.5 mL/g-COD at the GM/HPS volatile solids ratio of 0.08, which was 65.2 % higher than the HPS group. The multi-layered extracellular polymeric substances (EPS) of GM played a significant role in promoting hydrogen production, with c-type cytochromes probably serving as electroactive functional components. The addition of GM significantly improved the NADH/NAD+ ratio, electron transport system activity, hydrogenase activity, and electrochemical properties of HPS. Furthermore, the microbial community structure and metabolic functions were optimized due to the potential syntrophic interaction between Clostridium sensu stricto (dominant HPB) and Geobacter, thus promoting hydrogen production. This study provided novel insights into the interactions among exoelectrogens, electroactive EPS, and mixed HPB.