Study of extracellular electron transfer in engineered methanogenic systems
Author(s)
Lam, Yu Chung
Type
Thesis
Abstract
Anaerobic digestion of low-strength wastewater is often limited by slow hydrolysis, accumulation of fermentation intermediates, and poor energetic efficiency. Although extracellular electron transfer (EET) and direct interspecies electron transfer (DIET) are recognised as important for methanogenesis, their wider roles in complex anaerobic treatment systems remain poorly understood, particularly at pilot scale. This thesis examines the role of EET in a staged anaerobic fluidised-bed membrane bioreactor (SAF-MBR) treating domestic wastewater, combining reactor performance data with metagenomic, metatranscriptomic, and pangenomic analyses. Conductive granular activated carbon (GAC) was applied to promote biofilm-associated electron exchange, and a rule-based, gene-centric framework was used for functional inference, enabling resolution of EET mechanisms in largely uncultured communities beyond taxonomic enrichment alone.
The results show that EET is not confined to methanogenesis but is linked to improved upstream carbon conversion. Distinct functional guilds were identified, including Bacteroidota enriched in multi-haem cytochromes associated with polymer hydrolysis and acidogenesis, and Desulfobacterota expressing electrically conductive pili linked to syntrophic propionate oxidation. Under sulphate-limited conditions, sulphate-reducing bacteria engaged in DIET with Methanothrix, indicating cooperative rather than competitive interactions and challenging conventional models of sulphate–methane competition. Pangenomic analysis of Methanothrix revealed a previously uncharacterised MspA-like amyloid sheath protein, proposed as a putative electron uptake module. A bespoke amyloid protein prediction framework was required due to the lack of close homologues in existing databases, highlighting limitations in standard annotation approaches for EET-related functions. No clear evidence of horizontal transfer of canonical EET genes was found, although phage-associated transfer of antibiotic resistance genes was detected, suggesting incidental genetic exchange within EET-enabled biofilms. Overall, this work reframes EET as a system-level mechanism supporting hydrolysis, syntrophic stability, and resilient carbon conversion in advanced anaerobic treatment processes.
The results show that EET is not confined to methanogenesis but is linked to improved upstream carbon conversion. Distinct functional guilds were identified, including Bacteroidota enriched in multi-haem cytochromes associated with polymer hydrolysis and acidogenesis, and Desulfobacterota expressing electrically conductive pili linked to syntrophic propionate oxidation. Under sulphate-limited conditions, sulphate-reducing bacteria engaged in DIET with Methanothrix, indicating cooperative rather than competitive interactions and challenging conventional models of sulphate–methane competition. Pangenomic analysis of Methanothrix revealed a previously uncharacterised MspA-like amyloid sheath protein, proposed as a putative electron uptake module. A bespoke amyloid protein prediction framework was required due to the lack of close homologues in existing databases, highlighting limitations in standard annotation approaches for EET-related functions. No clear evidence of horizontal transfer of canonical EET genes was found, although phage-associated transfer of antibiotic resistance genes was detected, suggesting incidental genetic exchange within EET-enabled biofilms. Overall, this work reframes EET as a system-level mechanism supporting hydrolysis, syntrophic stability, and resilient carbon conversion in advanced anaerobic treatment processes.
Version
Open Access
Date Issued
2025-02-01
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Lee, Po-Heng (Henry)
Sponsor
The Government of Hong Kong
UK Research and Innovation
Grant Number
MRP/071/20X
IDRIC Project MIP 5.6
97/2019
Publisher Department
Department of Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)