Electroactive biofilms are capable of extracellular electron transfer and are therefore relevant to geochemistry, biocatalysis, and energy production. Although electroactive biofilms are widely applied in microbial fuel cells (MFCs) for renewable electricity production, where they transfer electrons to solid electrodes, the extracellular electron transfer chain is not completely understood. This lack of knowledge is an obstacle to the development of efficient bioelectrochemical devices, such as biosensors, biocatalyst and innovative MFCs. In order to solve this problem, we will grow known electroactive biofilms in multiple electrochemical cells with potentiostat control and determine the limiting steps in the extracellular electron transfer chain. We will use a combination of electrochemical and biochemical methods to characterize the electrode-biofilm interface. We will then screen environmental samples from metal-contaminated subsurface environments using a combination of enrichment methods, metal-reduction assay and electrochemical methods. By this token, we expect to identify microbial consortia which possess higher efficiency in extracellular electron transfer. The best electroactive biofilms will be grown in MFCs. We expect to increase 10-fold the electrical power output and to make MFCs applicable to a wider range of electrical and electronic systems. Lastly, we will develop the first amperometric biosensor for environmental monitoring of acetate based on electroactive biofilms. The proposed project will produce innovative research in two strategic European areas of applied biotechnology, namely renewable energy sources and industrial biosensors. During the four years of research, we will collaborate with engineers, microbiologists, and electrochemists from research institutions across Europe and the USA. In the last part of the project we expect to patent our innovative biosensor technology and find industrial partners interested to its realization.