Electricity from methane by reversing methanogenesis

Abstract

Given our vast methane reserves and the difficulty in transporting methane without substantial leaks, the conversion of methane directly into electricity would be beneficial. Microbial fuel cells harness electrical power from a wide variety of substrates through biological means; however, the greenhouse gas methane has not been used with much success previously as a substrate in microbial fuel cells to generate electrical current. Here we construct a synthetic consortium consisting of: (i) an engineered archaeal strain to produce methyl-coenzyme M reductase from unculturable anaerobic methanotrophs for capturing methane and secreting acetate; (ii) micro-organisms from methane-acclimated sludge (including Paracoccus denitrificans) to facilitate electron transfer by providing electron shuttles (confirmed by replacing the sludge with humic acids), and (iii) Geobacter sulfurreducens to produce electrons from acetate, to create a microbial fuel cell that converts methane directly into significant electrical current. Notably, this methane microbial fuel cell operates at high Coulombic efficiency.

Figure 1. Voltages generated in the MFCs with combinations of micro-organisms.

Micro-organisms in the MFCs include combinations of the following: the air-adapted M. acetivorans host containing pES1-MATmcr3 (‘AA/Mcr3'), the air-adapted M. acetivorans host containing the empty plasmid pES1(Pmat) (‘AA/Empty'), M. acetivorans C2A containing pES1-MATmcr3 (‘C2A/Mcr3'), G. sulfurreducens and sludge. The absence of the air-adapted M. acetivorans strain is indicated as ‘no AA/no Mcr3,' the absence of G. sulfurreducens is indicated as ‘no G. sulfurreducens', the absence of sludge is indicated as ‘no sludge' and the replacement of the methane headspace with a nitrogen headspace is indicated as ‘no methane'. Sludge was added to the indicated MFCs once the voltage of each MFC decreased to a threshold value of 150 mV or below, at time 0. For MFCs not including sludge, time 0 here is indicated as 132 h after inoculation and set-up. All values (voltages between the anode and cathode across a 1 kΩ fixed resistance) are represented as means±s.e.m. from at least three replicate MFCs. (a–e) The red voltage values for ‘AA/Mcr3/G. sulfurreducens/sludge' are repeated to aid in comparing results to values for (a) ‘AA/Empty/G. sulfurreducens/sludge', (b) ‘AA/Mcr3/G. sulfurreducens/no sludge', (c) ‘AA/Mcr3/no G. sulfurreducens/sludge', (d) ‘C2A/Mcr3/G. sulfurreducens/sludge' and (e) ‘no AA/no Mcr3/G. sulfurreducens/sludge'. (f) Values from ‘AA/Mcr3/no G. sulfurreducens/no sludge' is compared to values from ‘AA/Empty/no G. sulfurreducens/no sludge', (g) values from ‘no AA/no Mcr3/no G. sulfurreducens/no sludge' are compared to values from ‘no AA/no Mcr3/no G. sulfurreducens/sludge', and (h) values from ‘AA/Mcr3/G. sulfurreducens/sludge/no methane' are displayed.

Figure 2. Genera identified in sludge and in the MFCs.

Relative compositions are displayed based on the number of identified 16S rDNA reads compared to the total reads and are characterized to the genus taxonomic level. Genera consisting of at least 1% of the relative compositions of each sample are displayed, and DNA reads belonging to these genera are considered for the total reads in each sample. ‘Original sludge' is the initial sludge 811 days after isolation, and the ‘Acclimated sludge' is the pooled seven Round 3 methane-acclimated sludge cultures (Supplementary Table 1) after 567 days of incubation, with both of these sludge samples used to inoculate MFCs. MFCs included the air-adapted M. acetivorans host containing pES1-MATmcr3 (‘AA/Mcr3') and G. sulfurreducens. Sludge was added to the indicated MFCs once the voltage of each MFC decreased to a threshold value of 150 mV or below. All MFCs included methane in the headspace except the ‘AA/Mcr3/G. sulfurreducens/sludge/no methane' sample. Samples from MFCs were taken after 40 or 65 days of incubation for the two ‘AA/Mcr3/G. sulfurreducens/sludge' samples, or after 16 days of incubation for the ‘AA/Mcr3/G. sulfurreducens/sludge/no methane' sample.

Figure 3. Proposed model of biological production of electricity from methane in MFCs.

The air-adapted strain producing Mcr from ANME (‘AA/pES1-MATmcr3') consumes methane to produce acetate and excess electrons. Acetate is further oxidized completely to carbon dioxide, producing more excess electrons by G. sulfurreducens or sludge that can be substituted to a significant extent by P. denitrificans (‘Sludge (Paracoccus spp.)'). The sludge has a second role in providing humic acid-like electron shuttles, and G. sulfurreducens has a second role in providing optimized electron transfer to electron shuttles via multi-haem cytochromes (grey circles) that are located in the outer membrane or S-layer in bacteria and archaea, respectively.