This study reveals that reduction of carbon dioxide (CO2) to commodity chemicals can be functionally compartmentalized in bioelectrochemical systems. In the present example, a syntrophic consortium composed by H-2-producers (Rhodobacter sp.) in the biofilm is combined with carboxidotro phic Clostridium species, mainly found in the bulk liquid. The performance of these H-2-mediated electricity-driven systems could be tracked by the activity of a biological H-2 sensory protein identified at cathode potentials between -0.2 V and -0.3 V vs SHE. This seems to point out that such signal is not strain specific, but could be detected in any organism containing hydrogenases. Thus, the findings of this work open the door to the development of a biosensor application or soft sensors for monitoring such systems. (C) 2016 Elsevier Ltd. All rights reserved.