Facilitating intracellular electron bifurcation by mediating flavin-based extracellular and transmembrane electron transfer: a novel role of pyrogenic carbon in dark fermentation for hydrogen production

Authors: TIAN, WENJING - The Hong Kong Polytechnic University; TANG, YANFEI - The Hong Kong Polytechnic University; Ducey, Thomas; KHAN, EAKALAK - University Of Nevada; TSANG, DANIEL - The Hong Kong Polytechnic University

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2024
Publication Date: 9/24/2024
Citation: Tian, W., Tang, Y., Ducey, T.F., Khan, E., Tsang, D. 2024. Novel roles of biochar for dark fermentation in strengthening links between hydrogen evolution and energy conservation by facilitating flavins-based transmembrane and extracellular electron transfer. Journal of Environmental Science and Technology. 58(40):17766-17776. https://doi.org/10.1021/acs.est.4c05994.
DOI: https://doi.org/10.1021/acs.est.4c05994

Interpretive Summary: Anaerobic digestion is a widespread technology used for the breakdown of organic matter in the absence of oxygen. During this process, methane is a common byproduct, and is useful as a combustible fuel. Manipulation of anaerobic digestion, in a process known as dark fermentation, can also result in the production of hydrogen gas, that likewise can be used as a combustible fuel. Hydrogen is a green, carbon-free, energy source, making it a highly desirable fuel. However, the dark fermentation process suffers from poor efficiency, resulting in low hydrogen yields. Recently however it has been demonstrated that biochar can stimulate microbial activity and increase dark fermentation efficiency. This study focused on two objectives: first, to understand the role that biochar plays in this process; and 2) optimize dark fermentation efficiency using biochars produced at differing temperatures. Results indicate that biochar both enriches microbial species capable of performing dark fermentation, and serves as an electrical conduit to transfer electrons within the dark fermentation-related pathways, enhancing hydrogen production. These findings should allow for more efficient and greener anaerobic digestion-related, energy production technologies.

Technical Abstract: Biochar is deemed as a promising enhancer to H2-yielding dark fermentation (DF), but little is known about how biochar regulates extracellular electron transfer (EET), and influences transmembrane respiratory chains, and intracellular metabolism. This study addressed these knowledge gaps through a series of assays, and demonstrated that biochar could notably improve DF performance, and that BC800 (produced by pyrolysis at 800 °C) addition increased H2 yields by 371.9%. Electrochemical tests showed BC800 facilitated electron transfer of membrane-bound flavins by working as conductive conduits to promote extracellular electron shuttling of riboflavin. Comparative metagenome/metatranscriptome analyses indicated flavin-containing Rnf complex was the potential transmembrane respiratory enzymes associated with BC800-mediated EET. Based on NADH/NAD+ circulation, the promoted Rnf complex (1.68-fold upregulation) could stimulate function of electron bifurcating Etf/Bcd complex (2.63-fold upregulation) and startup of glycolysis. The promoted glycolysis could lead to NADH accumulation, and further stimulate Etf/Bcd complex to balance NADH/NAD+ level, which would provide more reduced ferredoxin for group A1 [FeFe]-hydrogenases (2.10-fold upregulation). This proton-energy linked mechanism could achieve coupling production of ATP and H2. This study verified the roles of biochar in mediating EET and transmembrane/intracellular pathways and emphasized the crucial roles of electron bifurcation in DF.