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 analy ze dissolved Fe2+ and Mn2+ release from Fe(III) (oxy)hydroxides and MnO2 du ring reduction by strains of Shewanella putrefaciens, in particular the Bla ck Sea strain, MR-4. Different strains (common to freshwater or seawater en vironments) showed a lag time in the release of Mn2+ but a similar first-or der rate constant for Mn2+ production. These data suggest that all strains use a common enzyme system. An increase in the initial amount of cells adde d to the solid phase showed an increase that was not linear with initial ce ll concentration, indicating surface limitation effects. Fe(III) citrate wa s reduced faster than Fe(OH)(3) than alpha-FeOOH. Addition of nitrate to th e 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 firs t-order kinetic plots for data after the lag phase and a two-parameter expo nential growth equation ([Mn] = Ae(Bt)). The slope B gives similar informat ion to k(obs) found from first-order log [Mn] vs. time plots whereas A, the pre-exponential factor, gives information on the lag phase commonly observ ed in culture experiments. A is shown to be a function of several variables depending on the experiment performed. (C) 2000 Elsevier Science B.V. All rights reserved.