Abstract
Electromethanogenesis is the bioreduction of carbon dioxide (CO2) to methane (CH4) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F420-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp.
Highlights
The term electromethanogenesis was first coined by Cheng and co-workers to indicate the reduction of carbon dioxide to methane mediated by Methanobacterium palustre using an electrode as electron donor [1]
The electromethanogenic system was operated for 53 days feeding CO2 as the sole carbon source
Hydrogen was only recorded at the beginning of the experiment, 1.78 mmols (73.5% v/ v in the headspace) and 1.33 mmols (54.8% v/v) at days 3 and 5, respectively, and remained below detection limits (< 2% v/v) for most of the time (Fig 1)
Summary
The term electromethanogenesis was first coined by Cheng and co-workers to indicate the reduction of carbon dioxide to methane mediated by Methanobacterium palustre using an electrode as electron donor [1]. Changes from closed and open electric circuits were used to analyze the relative expression of the selected genes.
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