Spatially heterogeneous propionate conversion towards electricity in bioelectrochemical systems

Bioelectrochemical technologies have been found promising to electricity conversion and accelerate anaerobic digestion. However, as an important end product in anaerobic digestion and potential power source in wastewater, the mechanism of propionate conversion from electroactive biomass is still ambiguous. Here we find that the propionate degradation and extracellular electron transfer are conducted separately at the biofilm and bulk. The responsible player is found to be a trilateral mutualism of Geobacter, Arcobacter and Azospirillum, which can be torn apart by the selection pressure of spatial heterogeneity. The biofilm community contains 66–70% of Geobacter while the Arcobacter and Azospirillum take 26–56% in the planktonic communities. Besides, microbial communities of the same generations are more similar than the same inoculum. These results hint the importance of both planktonic bacteria and immobile electroactive bacteria during propionate degradation, which has broader implications on bioelectrochemical systems and anaerobic digesters designs. [Display omitted] •Propionate degradation by electroactive community can be spatially heterogeneous.•Exoelectrogens and propionate degraders are in the biofilm and the bulk.•The mutualism of Geobacter (biofilm), Arcobacter and Azospirillum (bulk) is found.•Planktonic microorganisms are important for propionate degradation in BESs.