Kinetics of Fe(III) and Mn(IV) reduction by the Black Sea strain of Shewanella putrefaciens using in situ solid state voltammetric Au/Hg electrodes

Solid state gold-amalgam (Au/Hg) microelectrodes were used in situ to analyze dissolved Fe2+ and Mn2+ release from Fe(III) (oxy)hydroxides and MnO2 during reduction by strains of Shewanella putrefaciens, in particular the Black Sea strain, MR-4. Different strains (common to freshwater or seawater environments) showed a lag time in the release of Mn2+ but a similar first-order rate constant for Mn2+ production. These data suggest that all strains use a common enzyme system. An increase in the initial amount of cells added to the solid phase showed an increase that was not linear with initial cell concentration, indicating surface limitation effects. Fe(III) citrate was reduced faster than Fe(OH)3 than α-FeOOH. Addition of nitrate to the cultures inhibited both Mn(IV) and Fe(III) reduction. These data indicate that metal oxide reduction should be an important process when nitrate and oxygen concentrations are low. Calculations were performed using both first-order kinetic plots for data after the lag phase and a two-parameter exponential growth equation ([Mn]=AeBt). The slope B gives similar information to kobs found from first-order log [Mn] vs. time plots whereas A, the pre-exponential factor, gives information on the lag phase commonly observed in culture experiments. A is shown to be a function of several variables depending on the experiment performed.