Which role for minority microbial species in bioelectrochemical devices?

In the majority of studies interested in the microbiology of bioelectrochemical systems (BESs), greater importance is assigned to most abundant species settled in electroactive biofilms (EABs). Here, experiments in three-electrode devices involved conservation or removal of BESs’ planktonic bacteria and addition of exogenous species. The impact of those bacterial communities on both the electroactivity and the microbiology of EABs was investigated. Deep analysis of experimental data (chronoamperometric monitoring and high-throughput sequencing) followed by statistical calculations allowed to propose explanations regarding the microbial ecology of EABs. Best coulombic efficiency (CE - 34%) was obtained with the conservation of planktonic bacteria. Joint reading of electrochemical data and species relative abundances highlighted two distinct communities settlement: predominance of Desulfuromonas in EABs (42 to 53%), concomitant with the presence of planktonic species belonging to Mollicutes and Actinobacteria classes; or coinciding settlement of Geobacter and Clostridium in EABs. This study showed that exogenous species brought into contact with established biofilms were not necessarily likely to induce the loss their electrochemical activity. Relationships which exist between strictly speaking electroactive bacteria and other species present in BESs were identified as key for the sustainability of efficient biofilms.