Mediated electron transfer efficiencies of Se(IV) bioreduction facilitated by meso-tetrakis (4-sulfonatophenyl) porphyrin

Abstract

The redox-active sulfonated porphyrin meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS) was novelly proposed to facilitate the mediated electron transfer (MET) of the otherwise time-consuming selenite (Se(IV)) bioreduction. TPPS (0.2 mM) increased the Se(IV) (39.5 ± 1.0 mg∙L−1) bioreduction rate by Shewanella oneidensis·MR-1 by 4- to 5-fold. UV–vis, redox potential, cyclic voltammetry, and electrochemical impedance spectroscopy showed that the underlying mechanism was the redox-active transformation of the porphyrin ring, which led to a 25% decrease of the activation energy. Furthermore, the Michaelis-Menten fitting and the experiments on specific respiratory chain inhibitors suggested that TPPS enhanced the MET efficiencies of NADH-CoQ, FAD-CoQ, and Cyt bL-Cyt bH with stronger affinity (Km−1TPPS = 50.0 mM−1 > Km−1control = 0.8 mM−1) between the Se(IV) and microorganisms. Interestingly, TPPS enhanced the MET efficiencies by increasing the contents of redox-active compounds in extracellular polymeric substances. This study offers a realistic application and further mechanistic understanding of porphyrins as redox mediators for the Se(IV) bioreduction.