Polarization (open squares) and power (closed squares) curves for the methanol-fed MFC.

<p>Polarization (open squares) and power (closed squares) curves for the methanol-fed MFC.</p

Metagenomic Analyses Reveal the Involvement of Syntrophic Consortia in Methanol/Electricity Conversion in Microbial Fuel Cells

<div><p>Methanol is widely used in industrial processes, and as such, is discharged in large quantities in wastewater. Microbial fuel cells (MFCs) have the potential to recover electric energy from organic pollutants in wastewater; however, the use of MFCs to generate electricity from methanol has not been reported. In the present study, we developed single-chamber MFCs that generated electricity from methanol at the maximum power density of 220 mW m⁻² (based on the projected area of the anode). In order to reveal how microbes generate electricity from methanol, pyrosequencing of 16S rRNA-gene amplicons and Illumina shotgun sequencing of metagenome were conducted. The pyrosequencing detected in abundance <i>Dysgonomonas</i>, <i>Sporomusa</i>, and <i>Desulfovibrio</i> in the electrolyte and anode and cathode biofilms, while <i>Geobacter</i> was detected only in the anode biofilm. Based on known physiological properties of these bacteria, it is considered that <i>Sporomusa</i> converts methanol into acetate, which is then utilized by <i>Geobacter</i> to generate electricity. This speculation is supported by results of shotgun metagenomics of the anode-biofilm microbes, which reconstructed relevant catabolic pathways in these bacteria. These results suggest that methanol is anaerobically catabolized by syntrophic bacterial consortia with electrodes as electron acceptors.</p></div

Catabolic pathway for methanol/acetate conversion in the methanol-fed MFC predicted from the metagenome data (A), and phylum-level distributions of genes assigned to each catabolic step (B).

<p>Step I, methanol:THF methyltransferase; II, acetyl-CoA synthase (EC.2.3.1.169); III, carbon monoxide dehydrogenase (EC.1.2.7.4); IV, acetyl-CoA synthetase (EC.6.2.1.1); V, phosphate acetyltransferase (EC.2.3.1.8); and VI, acetate kinase (EC.2.7.2.1).</p

Comparison of the total lengths of large contigs affiliated with different phyla as determined by MEGAN or BLSOM analyses of the metagenome data.

<p>Comparison of the total lengths of large contigs affiliated with different phyla as determined by MEGAN or BLSOM analyses of the metagenome data.</p

Typical time courses of cell voltage (A), methanol concentration (B), and acetate concentration (C), after supplementation of the MFC with 10 mM methanol.

<p>In panels B and C, data are means ± SD (n = 3), and error bars are shown when they are larger than symbols.</p

Gene clusters containing putative methanol:THF methyltransferases (black arrows) in metagenome contig NODE_348 and the genome of <i>Sporomusa ovata</i>.

<p>Possible genes encoding transcriptional regulators for methyltransferases are indicated with gray arrows. Results of BLAST search for the genes are described in the table.</p

Summary of numerical data for the metagenomic analyses of microbes associated with the anode biofilm in the methanol-fed MFC.

<p>Summary of numerical data for the metagenomic analyses of microbes associated with the anode biofilm in the methanol-fed MFC.</p

Distribution of microbes in the methanol-fed MFC.

<p>(A) Total protein contents showing amounts of microbes associated with the anode biofilm, cathode biofilm, and electrolyte. (B) Results of an anode-exchange experiment, in which cell voltages of methanol-fed MFCs were measured after the microbe-bearing anode was transferred to a reactor containing fresh electrolyte (black line) and a new anode was placed in the spent electrolyte of the initial reactor (gray line).</p

Metagenomic insights into the genus <i>Geobacter</i> obtained by MEGAN analyses.

<p>(A) Sub-genus level distribution of genes affiliated with the genus <i>Geobacter</i> based on BLAST homology. ‘<i>Geobacter</i>’ in the figures represents genes that were assigned to <i>Geobacter</i> at the genus level, but not to any genome-sequenced strain. (B) A partial MEGAN tree showing taxonomic distribution of genes encoding the acetate-catabolizing enzymes acetyl-CoA synthetase (EC 6.2.1.1), phosphate acetyltransferase (2.3.1.8), and acetate kinase (2.7.2.1), assigned to the class <i>Deltaproteobacteria</i>. LCA parameters: Min score, 50; top percent, 1.0; Min support, 2.</p

Comparative Metagenomics of Anode-Associated Microbiomes Developed in Rice Paddy-Field Microbial Fuel Cells

<div><p>In sediment-type microbial fuel cells (sMFCs) operating in rice paddy fields, rice-root exudates are converted to electricity by anode-associated rhizosphere microbes. Previous studies have shown that members of the family <i>Geobacteraceae</i> are enriched on the anodes of rhizosphere sMFCs. To deepen our understanding of rhizosphere microbes involved in electricity generation in sMFCs, here, we conducted comparative analyses of anode-associated microbiomes in three MFC systems: a rice paddy-field sMFC, and acetate- and glucose-fed MFCs in which pieces of graphite felt that had functioned as anodes in rice paddy-field sMFC were used as rhizosphere microbe-bearing anodes. After electric outputs became stable, microbiomes associated with the anodes of these MFC systems were analyzed by pyrotag sequencing of 16S rRNA gene amplicons and Illumina shotgun metagenomics. Pyrotag sequencing showed that <i>Geobacteraceae</i> bacteria were associated with the anodes of all three systems, but the dominant <i>Geobacter</i> species in each MFC were different. Specifically, species closely related to <i>G. metallireducens</i> comprised 90% of the anode <i>Geobacteraceae</i> in the acetate-fed MFC, but were only relatively minor components of the rhizosphere sMFC and glucose-fed MFC, whereas species closely related to <i>G. psychrophilus</i> were abundantly detected. This trend was confirmed by the phylogenetic assignments of predicted genes in shotgun metagenome sequences of the anode microbiomes. Our findings suggest that <i>G. psychrophilus</i> and its related species preferentially grow on the anodes of rhizosphere sMFCs and generate electricity through syntrophic interactions with organisms that excrete electron donors.</p> </div