Low electrode potentials enhance current generation by Geobacter sulfurreducens biofilms: A high-throughput study

Abstract

The microbial species Geobacter sulfurreducens uses different extracellular electron transfer (EET) pathways depending on the potential of the final electron acceptor, yet a complete understanding of EET mechanisms and the impact of thermodynamically limiting potentials remains elusive. Here, we employ a custom-designed high-throughput system that enables the simultaneous and continuous execution of 128 parallel experiments to investigate the complete spectrum of potentials ([-0.25 to 0] V vs. SHE) impacting the metabolic energy generation in axenic G. sulfurreducens electroactive biofilms (EABs). These were grown for 500 h in three consecutive stages and characterized electrochemically. The EABs grown on electrodes poised below the apparent midpoint potential ([-0.18 to -0.16] V) grew slower than those grown at conventional, non-limiting potential (0 V), developing 50% smaller biofilms and 2.4-fold higher anodic plateau currents on average ([0.1 vs. 0.04] mA cm^-2). These also exhibited enhanced charge transport coupled to higher average concentrations of charge carriers ([1.6 vs. 0.4] mM_e^-), the latter impacting linearly the anodic plateau current. Low- and high-potential redox pools were discriminated with the former comprising 50%–70% of storable charge. Overall, these findings strongly suggest an overexpression of charge carriers in G. sulfurreducens EABs cultivated at lower potentials and highlight the useful contribution of high-throughput tools for boosting research in electromicrobiology.