The microbial community structure of anodic biofilms plays a key role in bioelectrochemical systems (BESs). Bacteria composing these biofilms confer different capabilities to the system to convert organic matter into electrical current, using the anode as sole electron sink. As complex ecosystems are generally used as inoculum, many bacterial species having interconnected ecological interactions are present which makes more complex the elucidation of their exact role in these systems. Thus, the aim of the present study was to identify the key bacteria for the conversion of single short chain acids into electrical current. Here, two-compartment labscale microbial electrolysis cells were inoculated with the same activated sludge. Batch tests were operated in quadruplicate, at the same time, and were fed with four different substrates: acetate, lactate, butyrate and propionate, at 80 mMe- equivalents. Biofilm and planktonic communities were analyzed at the end of the batch operation. Mainly, the anodic biofilms were dominated by the Geobacter genus (62.41% of the total sequences) known to be electroactive. At the species level, Geobacter sulfurreducens dominated in presence of lactate and acetate, while other species such as Geobacter toluenoxydans and Geobacter pelophilus were present when fed with butyrate and propionate. These results indicate for the first time a specificity within the Geobacter genus compared to the electron donor, suggesting in one hand that a competitive process occurred for electrode colonization and in the other hand that syntrophic interactions are necessary for the complete oxidation of substrates such as propionate and butyrate. All together, these results provide a new insight into the ecological relationships within electroactive biofilms and suggest eco-engineering perspectives to improve the performances of BESs.