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

<i>Desulfuromonas acetexigens</i> 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 <i>D. acetexigens</i> were compared with the model organism <i>Geobacter sulfurreducens</i> 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, <i>D. acetexigens</i> biofilms established faster, generating current after ~4 days, whereas <i>G. sulfurreducens</i> exhibited a lag phase of ~8 days. Limitations of extracellular electron transfer already occurred at lower average biofilm volumes for <i>D. acetexigens</i> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo> <mml:msub> <mml:mrow> <mml:mover><mml:mrow><mml:mi>B</mml:mi> <mml:mi>V</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> <mml:mrow><mml:mover><mml:mi>J</mml:mi> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> <mml:mo>)</mml:mo></mml:mrow> </mml:math> ≈ 16 ± 6 μm³ μm^-2) than for <i>Geobacter</i> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mover><mml:mrow><mml:mi>B</mml:mi> <mml:mi>V</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> <mml:mrow> <mml:mover><mml:mrow><mml:mi>J</mml:mi></mml:mrow> <mml:mo>¯</mml:mo></mml:mover> <mml:mo>max</mml:mo></mml:mrow> </mml:msub> </mml:math> ≈ 40 ± 7 μm³ μm^-2). One monocultural <i>D. acetexigens</i> 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 <i>Sporomusa sphaeroides</i>.