Microbes can generate electricity in microbial fuel cells and transform contaminants in nature, utilizing extracellular electron transfer (EET). Fungi exist widely in environment but few studies have concerned an associated EET mechanism and so their application is limited. In this study electrochemical techniques were employed to study the extracellular reduction capability of fungi using a representative model viz. Pachysolen tannophilus; a solid electrode and soluble Cr(VI) were used as extracellular electron acceptors. Despite a thick cell wall, the yeast was proved to be electrochemically active and an obvious redox peak was observed at -0.41 V in differential pulse voltammetry. Analysis of electrochemical and HPLC data led to the proposal of an EET with riboflavin as the electron transfer mediator. Riboflavin secreted by the yeast was also responsible for the transformation of Cr(VI) to less mobile and less toxic Cr(III). Further results demonstrated that the acidophilic P. tannophilus secreted riboflavin to a concentration of 0.26 mu mol/g biomass at pH 3, but only to 0.15-0.16 mu mol/g biomass at pHs between 4 and 7. The findings contribute to the understanding of biogeochemical processes and further contribute to innovative remediation of polluted environments. (C) 2016 Elsevier Ltd. All rights reserved.