Shewanella secretes flavins that mediate extracellular electron transfer. This study shows that Shewanella species, such as Shewanella oneidensis MR-1 and Shewanella sp. MR-4, use flavins, particularly riboflavin, as soluble redox mediators to transfer electrons to electrodes. These flavins are secreted by the bacteria and adsorb to electrode surfaces, especially in older biofilms. The presence of riboflavin significantly enhances electron transfer to electrodes, with riboflavin being the dominant component in biofilm supernatants after prolonged incubation. The study also demonstrates that riboflavin can bind to Fe(III) and Mn(IV) oxy(hydr)oxides, suggesting its role in metal chelation and electron transfer. The findings indicate that flavins are not only redox shuttles but also contribute to metal reduction and surface binding. The ability of Shewanella to secrete flavins and bind them to surfaces challenges previous assumptions about the role of soluble redox mediators in geochemical processes. The study highlights the complex interactions between Shewanella and its environment, including the use of flavins for electron transfer, metal chelation, and surface binding, which may influence biogeochemical cycles and microbial interactions. The results suggest that flavins play a crucial role in the extracellular respiration of Shewanella, contributing to its ability to reduce metals and interact with various surfaces. The study also provides insights into the metabolic costs of flavin production, showing that it is energetically efficient compared to other redox mediators. Overall, the findings emphasize the importance of flavins in the electron transfer processes of Shewanella and their potential impact on environmental and biogeochemical systems.Shewanella secretes flavins that mediate extracellular electron transfer. This study shows that Shewanella species, such as Shewanella oneidensis MR-1 and Shewanella sp. MR-4, use flavins, particularly riboflavin, as soluble redox mediators to transfer electrons to electrodes. These flavins are secreted by the bacteria and adsorb to electrode surfaces, especially in older biofilms. The presence of riboflavin significantly enhances electron transfer to electrodes, with riboflavin being the dominant component in biofilm supernatants after prolonged incubation. The study also demonstrates that riboflavin can bind to Fe(III) and Mn(IV) oxy(hydr)oxides, suggesting its role in metal chelation and electron transfer. The findings indicate that flavins are not only redox shuttles but also contribute to metal reduction and surface binding. The ability of Shewanella to secrete flavins and bind them to surfaces challenges previous assumptions about the role of soluble redox mediators in geochemical processes. The study highlights the complex interactions between Shewanella and its environment, including the use of flavins for electron transfer, metal chelation, and surface binding, which may influence biogeochemical cycles and microbial interactions. The results suggest that flavins play a crucial role in the extracellular respiration of Shewanella, contributing to its ability to reduce metals and interact with various surfaces. The study also provides insights into the metabolic costs of flavin production, showing that it is energetically efficient compared to other redox mediators. Overall, the findings emphasize the importance of flavins in the electron transfer processes of Shewanella and their potential impact on environmental and biogeochemical systems.