Hydrologic flow controls on biologic iron(III) reduction in natural sediments

Bacterial reduction of a hematite-rich natural coastal sand was studied in flow-through column reactors at flow rates which varied from 0.62 to 11 pore volumes d^-1. Sand columns were wet-packed with the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32, and a PIPES-buffered, lactate-containing growth medium was pumped through the columns for over 20 days. Soluble Fe(II), acetate and lactate concentrations measured in the column effluents showed that steady-state conditions were established after a few days with every flow rate. The steady-state effluent Fe(II) concentration was directly controlled by the flow rate where [Fe(II)]_SS decreased as the flow rate increased. Increased flow rate increased biologic activity based on the steady-state flux of soluble Fe(II) and total Fe(II) production (included Fe(II) extracted from sand at the conclusion of the experiment), decreased the fraction of lactate oxidized for energy that likely increased cell synthesis, and decreased the concentration of sorbed Fe(II) that, in turn, decreased the relative percentage of Fe(II) retained by the column materials. Increased biologic activity was likely promoted by greater reactant delivery (i.e., lactate, N, P) and greater advective removal of Fe(II). These results demonstrate that biologic Fe(III) reduction, cell growth, and abiotic Fe(II) sorption are all coupled to the hydrologic flow rate.