Multiple paths of electron flow to current in microbial electrolysis cells fed with low and high concentrations of propionate

Abstract

Microbial electrolysis cells (MECs) provide a viable approach for bioenergy generation from fermentable substrates such as propionate. However, the paths of electron flow during propionate oxidation in the anode of MECs are unknown. Here, the paths of electron flow involved in propionate oxidation in the anode of two-chambered MECs were examined at low (4.5 mM) and high (36 mM) propionate concentrations. Electron mass balances and microbial community analysis revealed that multiple paths of electron flow (via acetate/H2 or acetate/formate) to current could occur simultaneously during propionate oxidation regardless of the concentration tested. Current (57-96 %) was the largest electron sink and methane (0-2.3 %) production was relatively unimportant at both concentrations based on electron balances. At a low propionate concentration, reactors supplemented with 2-bromoethanesulfonate had slightly higher coulombic efficiencies than reactors lacking this methanogenesis inhibitor. However, an opposite trend was observed at high propionate concentration, where reactors supplemented with 2-bromoethanesulfonate had a lower coulombic efficiency and there was a greater percentage of electron loss (23.5 %) to undefined sinks compared to reactors without 2-bromoethanesulfonate (11.2 %). Propionate removal efficiencies were 98 % (low propionate concentration) and 78 % (high propionate concentration). Analysis of 16S rRNA gene pyrosequencing revealed the dominance of sequences most similar to Geobacter sulfurreducens PCA and G. sulfurreducens subsp. ethanolicus. Collectively, these results provide new insights on the paths of electron flow during propionate oxidation in the anode of MECs fed with low and high propionate concentrations.

Set anode potentials affect the electron fluxes and microbial community structure in propionate-fed microbial electrolysis cells

“…In the O.C reactors, complete removal of propionate was facilitated through microbial partnerships between fermenters ( Smithella and Syntrophobacter ) and hydrogenotrophic and acetoclastic methanogens. In our earlier study30, we demonstrated that propionate degradation in MECs occurred via a microbial partnership between fermenters and Geobacter , and low propionate removal (80% removal) was observed at 36 mM (similar to the concentration used in the current study) compared to complete removal at 4.5 mM30. Methanogenesis was not an important electron sink in our earlier study at both propionate concenrations tested, whereas methanogenesis (via hydrogenotrophic methanogens) was an important sink in the current sudy.…”

“…Based on the aforementioned studies, it seems that the complete degradation of propionate at elevated concentrations in MECs requires microbial partnerships between fermenters, hydrogenotrophic methanogens and Geobacter . Whereas, at low propionate concentration microbial partnership between Geobacter and fermenters was sufficient to achieve complete removal of propionate in MEC30.…”

“…Methanogenesis was not an important electron sink in our earlier study at both propionate concenrations tested, whereas methanogenesis (via hydrogenotrophic methanogens) was an important sink in the current sudy. In our previous study30, MEC anodes were initially enriched (30 days) in single chamber air-cathode MFCs, where oxygen intrusion through the cathode might have affected the growth of methanogens. Whereas in the current study, the reactors were operated under MEC (anaerobic) mode from the start of the experiment, and this provided a suitable environment for the enrichment of hydrogenotrophic methanogens.…”

“…Two-chambered, cube-shaped MECs with a working volume of 40 mL (anode chamber) and 20 mL (cathode chamber) were constructed as previously described30. The two chambers were separated by an anion exchange membrane (5 cm 2 ; AMI 7001, Membranes International, Glen Rock, NJ).…”

“…Recently, Hari et al . demonstrated that propionate degradation at the anode of MECs operated at an applied voltage of 0.7 V using a power source occurs via a microbial partnership between fermenters and Geobacter 30. They showed that multiple paths of electron flow to current (via acetate/H 2 or acetate/formate) could occur simultaneously during propionate oxidation regardless of the propionate concentration tested.…”