Syntrophic interactions improve power production in formic acid fed MFCs operated with set anode potentials or fixed resistances

Formic acid is a highly energetic electron donor but it has previously resulted in low power densities in microbial fuel cells (MFCs). Three different set anode potentials (-0.30, -0.15, and +0.15 V; vs. a standard hydrogen electrode, SHE) were used to evaluate syntrophic interactions in bacterial communities for formic acid degradation relative to a non-controlled, high resistance system (1,000 Ω external resistance). No current was generated at -0.30 V, suggesting a lack of direct formic acid oxidation (standard reduction potential: -0.40 V). More positive potentials that allowed for acetic acid utilization all produced current, with the best performance at -0.15 V. The anode community in the -0.15 V reactor, based on 16S rDNA clone libraries, was 58% Geobacter sulfurreducens and 17% Acetobacterium, with lower proportions of these genera found in the other two MFCs. Acetic acid was detected in all MFCs suggesting that current generation by G. sulfurreducens was dependent on acetic acid production by Acetobacterium. When all MFCs were subsequently operated at an external resistance for maximum power production (100 Ω for MFCs originally set at -0.15 and +0.15 V; 150 Ω for the control), they produced similar power densities and exhibited the same midpoint potential of -0.15 V in first derivative cyclic voltammetry scans. All of the mixed communities converged to similar proportions of the two predominant genera (ca. 52% G. sulfurreducens and 22% Acetobacterium). These results show that syntrophic interactions can be enhanced through setting certain anode potentials, and that long-term performance produces stable and convergent communities.