Deciphering the Role of Cell Envelop Proteins of Electrotrophic Methanogens in Extracellular Electron Transfer (EET) Mechanism 

Description

Attaining sustainable development is the grand challenge of this century. Water, energy,and carbon emission are prioritized under the United Nations Sustainable DevelopmentGoals. Similarly, Hong Kong has aligned her efforts towards these goals. Methanogens,which are microbes capable of methane production, play an important role in anaerobicwastewater treatment and renewable bioenergy production. Recently, some methanogenswere found to consume electron directly, in addition to traditional electron carriersubstrates (e.g., hydrogen), for CO2 biomethanation, resulting in enhanced anaerobictreatment kinetics and methane yield. This has also created new possibilities for biogasupgrading and energy offset in CO2-emiting industries, further contributing towardssustainability goals. Key to facilitating these applications and goals realization is thecomprehensive understanding of methanogen extracellular electron transfer (EET)mechanism.  Electrically conductive materials such as, granular activated carbon (GAC), have beenwidely reported to induce EET in electrotrophic methanogens. These materials conductelectrons, produced by exoelectrogens, to methanogens, increasing electron transfer rateby 10 times when compared to substrate-based diffusion transfer. Our previousmicrobiome studies have revealed co-occurrences of exoelectrogens and electrotrophicmethanogens (e.g., Methanothrix) on GAC, which were absent in non-conductivematerials. However, EET mechanism remains ill-defined and is extremely difficult tostudy for many methanogens due to technical challenges.  Since cell envelop is the first point of contact between the methanogen and an incomingelectron, this research aims to decipher the role of cell envelop proteins in EET andidentify EET protein candidates in electrotrophic methanogens. Methanothrix soehngeniiand Methanosarcina mazei are chosen for this study owing to their propensity todominate in anaerobic EET environments. With this knowledge, we could selectivelytarget putative EET proteins for further in-depth study to delineate the elusive EETmechanism. Our preliminary data showed that known genes involved in EET (Hdr andFpo complexes) and a putative EET gene (encoding for a multi-heme cytochrome c) wereidentified and active in a Methanothrix metagenome bin recovered from GAC. Buildingupon these findings, this project plans to (1) determine the main cell envelop layer(s) forEET, (2) purify cell envelop proteins for proteomics analysis and characterization, and(3) determine the involvement of cell envelop proteins and decipher their roles in EETmechanism. The outcome of this project is expected to yield new possibilities foranaerobic waste treatment and waste CO2 biomethanation, and on the broader level,generate new research directions in microbial EET for various biotechnologicalapplications including biorefinery and bioremediation.