Anaerobic oxidation of methane coupled with extracellular electron transfer to electrodes

Abstract

Anaerobic oxidation of methane (AOM) is an important process for understanding the global flux of methane and its relation to the global carbon cycle. Although AOM is known to be coupled to reductions of sulfate, nitrite, and nitrate, evidence that AOM is coupled with extracellular electron transfer (EET) to conductive solids is relatively insufficient. Here, we demonstrate EET-dependent AOM in a biofilm anode dominated by Geobacter spp. and Methanobacterium spp. using carbon-fiber electrodes as the terminal electron sink. The steady-state current density was kept at 11.0 ± 1.3 mA/m² in a microbial electrochemical cell, and isotopic experiments supported AOM-EET to the anode. Fluorescence in situ hybridization images and metagenome results suggest that Methanobacterium spp. may work synergistically with Geobacter spp. to allow AOM, likely by employing intermediate (formate or H₂)-dependent inter-species electron transport. Since metal oxides are widely present in sedimentary and terrestrial environments, an AOM-EET niche would have implications for minimizing the net global emissions of methane.

Figure 1

Current generation from anaerobic oxidation of methane in a microbial electrochemical cell (MxC). Methane was the sole electron donor, and no exogenous electron acceptor was provided except for the carbon fiber electrodes. (a) Current density in response to alternate methane and nitrogen gas. This alternating gas test was conducted with the MxC in the acclimation period. (b) Current density over time in the MxC operated inside the anaerobic chamber (MxC_AC). Connection and disconnection of the electrodes during the supply of methane caused abrupt increases in current density due to biofilm capacitance effects. Triangles indicate the points of interruption.

Figure 2

Community composition of bacteria (top) and archaea (bottom) using Best Hit Classification based on the SILVA Small Subunit (SSU) rRNA database in MG-RAST. The total abundance of SSU rRNA gene reads was 27,084: 22,946 reads were bacterial, and 982 reads were archaeal. The category “Others” indicates these genera of archaea and bacteria had relative abundance below 1%.

Figure 3

Relative abundances of genes for AOM-related enzymes according to taxonomic annotations by BLAST. The category “Others” indicates genera with relative abundance below 1%.

Figure 4

Relative abundances of genes for EET-related enzymes according to taxonomic annotations. The category “Others” indicates genera with relative abundance below 1%.

Figure 5

Fluorescence in situ hybridization (FISH) images of intact biofilms on carbon fibers from the microbial electrochemical cell anaerobic chamber (MxC_AC) anode. (a) shows the 3D structure of the colonies formed by Methanobacteriaceae family (green) and Geobacter genus (red) on the surfaces of a single carbon fiber; yellow indicates the overlap of green and red colors. (b) shows biofilm structure in 2D. Image a is obtained from a confocal microscope, and image b is from an epifluorescence microscope.