# Relationship of electrochemical performance and biofilm development of Desulfuromonas acetexigens and Geobacter sulfurreducens in microbial electrolysis cells

## TL;DR

This study compares the electrochemical performance and biofilm development of two bacteria in microbial electrolysis cells, finding that D. acetexigens starts producing current faster and achieves high efficiency.

## Contribution

The study introduces a novel comparison of D. acetexigens and G. sulfurreducens using OCT for non-invasive biofilm analysis in MECs.

## Key findings

- D. acetexigens biofilms generate current faster than G. sulfurreducens under identical conditions.
- D. acetexigens achieves a high Coulombic efficiency of ~96% with no detectable hydrogen utilization.
- Extracellular electron transfer limitations occur at lower biofilm volumes for D. acetexigens compared to G. sulfurreducens.

## Abstract

Desulfuromonas acetexigens has gained attention as a biocatalyst in microbial electrolysis cells (MECs) due to its inability to utilize hydrogen as an electron donor, which favors beneficial Coulombic efficiencies (CE). In this study, the electrochemical performance and biofilm morphology of D. acetexigens were compared with the model organism Geobacter sulfurreducens in flow cell MECs. Biofilm morphology was assessed non-invasively via optical coherence tomography (OCT), providing insight into quantitative parameters, including spatially resolved thickness, biovolume and anode surface coverage. While both species achieved similar maximum current densities when cultivated under identical conditions, D. acetexigens biofilms established faster, generating current after ~4 days, whereas G. sulfurreducens exhibited a lag phase of ~8 days. Limitations of extracellular electron transfer already occurred at lower average biofilm volumes for D. acetexigens ((BV¯J¯max) ≈ 16 ± 6 μm3 μm−2) than for Geobacter (BV¯J¯max ≈ 40 ± 7 μm3 μm−2). One monocultural D. acetexigens cultivation revealed a CE of ~96%, consistent with no detectable hydrogen utilization under the tested condition, while some cultivations showed net acetate increases. Phylogenetic analyses of the latter indicated niche dominance of the target EAM despite homoacetogenic and clostridial contaminants. Production of short-chain fatty acids suggested interspecies metabolic interaction and led to the hypothesis of an electrode-mediated ethanol to acetate fermentation by electroactive microorganisms and ethanol-utilizing contaminants such as the homoacetogen Sporomusa sphaeroides.

Central labeled diagram of an MEC-flowcell shows cathodes, anode, and OCT device for biofilm morphology measurement. Left section depicts D. acetexigens with a color-coded biofilm heightmap and a current density versus time graph peaking before stabilizing. Right section illustrates G. sulfurreducens with its own heightmap and a current density versus time graph showing a rise, peak, and decline.