Enhanced start-up of anaerobic facultatively autotrophic biocathodes in bioelectrochemical systems

Abstract

Biocathodes in bioelectrochemical systems (BESs) can be used to convert CO2 into diverse organic compounds through a process called microbial electrosynthesis. Unfortunately, start-up of anaerobic biocathodes in BESs is a difficult and time consuming process. Here, a pre-enrichment method was developed to improve start-up of anaerobic facultatively autotrophic biocathodes capable of using cathodes as the electron donor (electrotrophs) and CO2 as the electron acceptor. Anaerobic enrichment of bacteria from freshwater bog sediment samples was first performed in batch cultures fed with glucose and then used to inoculate BES cathode chambers set at −0.4V (versus a standard hydrogen electrode; SHE). After two weeks of heterotrophic operation of BESs, CO2 was provided as the sole electron acceptor and carbon source. Consumption of electrons from cathodes increased gradually and was sustained for about two months in concert with a significant decrease in cathode chamber headspace CO2. The maximum current density consumed was −34±4mA/m2. Biosynthesis resulted in organic compounds that included butanol, ethanol, acetate, propionate, butyrate, and hydrogen gas. Bacterial community analyses based on 16S rRNA gene clone libraries revealed Trichococcus palustris DSM 9172 (99% sequence identity) as the prevailing species in biocathode communities, followed by Oscillibacter sp. and Clostridium sp. Isolates from autotrophic cultivation were most closely related to Clostridium propionicum (99% sequence identity; ZZ16), Clostridium celerecrescens (98–99%; ZZ22, ZZ23), Desulfotomaculum sp. (97%; ZZ21), and Tissierella sp. (98%; ZZ25). This pre-enrichment procedure enables simplified start-up of anaerobic biocathodes for applications such as electrofuel production by facultatively autotrophic electrotrophs.