Sorption of Fe (hydr)oxides to the surface of Shewanella putrefaciens: Cell-bound fine-grained minerals are not always formed de novo

Shewanella putrefaciens, a gram-negative, facultative anaerobe, is active i n the cycling of iron through its interaction with Fe (hydr)oxides in natur al environments. Fine-grained Fe precipitates that are attached to tile out er membranes of many gram-negative bacteria have most often been attributed to precipitation and growth of the mineral at the cell surface. Our study, of the sorption of nonbiogenic Fe (hydr)oxides revealed, however, that lar ge quantities of nanometer-scale ferrihydrite (hydrous ferric oxide), goeth ite (alpha -FeOOH), and hematite (alpha -Fe2O3) adhered to the cell surface . Attempts to separate suspensions of cells and minerals with an 80% glycer in cushion proved that the sorbed minerals were tightly attached to the bac teria. The interaction between minerals and cells resulted in the formation of mineral-cell aggregates, which increased biomass density and provided b etter sedimentation of mineral Fe compared to suspensions of minerals alone . Transmission electron microscopy observations of cells prepared by whole- mount, conventional embedding, and freeze-substitution methods confirmed th e close association between cells and minerals and suggested that in some i nstances, the mineral crystals had even penetrated the outer membrane and p eptidoglycan layers. Given the abundance of these mineral types in natural environments, the data suggest that not all naturally occurring cell surfac e-associated minerals are necessarily formed de novo on the cell wall.