Impact of electron scavenging during electric current generation from propionate by a Geobacter co-culture

Abstract

Propionate accumulates and inhibits anaerobic biodegradation processes. Technologies such as bioelectrochemical systems (BES) are being developed to remove this volatile fatty acid and convert it into bioenergy. Geobacter metallireducens is an electrogenic bacterium capable of oxidizing propionate, but slowly and inefficiently. Here, G. metallireducens was submitted to adaptive laboratory evolution (ALE) to improve its propionate metabolism. The adapted G. metallireducens, which harbors 62 mutations, reduced Fe(III) 3.6 times faster than the wild type with propionate as substrate. During growth, the adapted strain generated H2 at a rate of 25.4 μM h−1. To determine if quick H2 removal could accelerate propionate oxidation further, the adapted G. metallireducens was co-cultivated with a H2-oxidizing autotrophic Geobacter sulfurreducens strain. With Fe(III) as the electron acceptor, the co-culture oxidized propionate 2.2 times faster than the adapted G. metallireducens pure culture. In a BES, the co-culture outperformed the pure culture and generated a maximal current density of 2.34 A m−2. The anode of the co-culture-driven BES was coated by a thick biofilm reaching 116 μM made of both Geobacter species uniformly distributed. These results demonstrate that the combination of ALE and co-cultivating is an efficient strategy to optimize the metabolism of G. metallireducens for propionate-driven bioprocesses.