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

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 b(L)-Cyt b(H) with stronger affinity (Km(TPPS)(-1) = 50.0 mM(-1) > Km(control)(-1) = 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.