Microorganisms harness biochemical energy from their surroundings and shuttle electrons out of cells to external terminal electron acceptors. Extracellular electron transport (EET) can take place via diffusive exchange of a dissolved electron donor (e.g., H2, formate, acetate) between microbes (i.e. mediated electron transfer) and direct electron transfer, which requires physical contact between microbes and the terminal electron acceptors. There is increasing evidence for the important role of direct EET in many microbial communities; yet the exact biophysical mechanisms remain not fully resolved. A primary goal of this dissertation is to investigate the mechanisms underlying EET in the process of anaerobic oxidation of methane as well as anode respiration carried out by Geobacter sulfurreducens biofilms. In light of high-resolution metabolic activity acquired using FISH and nanoSIMS, in this thesis I have developed reactive transport models that provide mechanistic understanding of the EET process that can be adapted to a broader microbial community.