Kinetics of microbial and chemical reduction of humic substances: implications for electron shuttling

Humic substances (HS) are redox-active natural organic compounds and serve as electron shuttles between microorganisms and iron(III) minerals. Here we demonstrate that electron shuttling is possible only at concentrations of dissolved HS of at least 5-10 mg C/L. Although such concentrations can be found in many rivers, lakes, and even in some aquifers there are also many marine and freshwater systems with DOC < 5 mg C/L where consequently electron shuttling is not expected to happen. We found that in the case of HS concentrations which do not limit electron shuttling, Geobacter sulfurreducens transfers electrons to HS at least 27 times faster than to Fe(III)hydroxide. Microbially reduced HS transfer electrons to ferrihydrite at least 7 times faster than cells thereby first demonstrating that microbial mineral reduction via HS significantly accelerates Fe(III) mineral reduction and second that electron transfer from reduced HS to Fe(III) minerals represents the rate-limiting step in microbial Fe(III) mineral reduction via HS. Microbial reduction of HS transfers as many electrons to HS as chemical reduction with H2 indicating that all redox-active functional groups that can be reduced at a redox potential of -418 mV (Eh(0) of H2/H+ redox couple at pH 7) can also be reduced by microorganisms.