Respiration and dissolution of iron(III) containing clay minerals by bacteria

A quantitative study of microbial clay mineral reduction coupled to the oxi dation of organic carbon was carried out using an Fe(III)-reducing bacteriu m (Shewanella putrefaciens strain MR-1). Total CO2 production, organic acid depletion, and Fe(III) reduction were measured in the same cultures with f ormate or lactate as the carbon source and clay as the sole electron accept or. Mean ratios of 1.6:1 and 4.9:1 were observed for structural Fe(III) red uction coupled to formate oxidation and lactate oxidation, respectively. Wh en organic ligands were added under similar culture conditions, the extent of clay reduction was enhanced up to 2-fold in the order of nitrilotriaceti c acid (NTA) > oxalate > citrate > malate. Further, dissolution of the clay mineral structure was inferred as dissolved Fe(II) comprised up to 50% of the total clay-hound Fe reduced in cultures to which organic ligand was add ed. Here we provide the first direct measurements which show that (1) bacte ria may couple the respiration of Fe(III) bound in smectite clay minerals t o carbon cycling, (2) organic ligands increase the bioavailability of Fe(II I) bound in clay minerals, and (3) bacterial Fe(III) reduction in the prese nce of organic ligands may lead to clay mineral dissolution. These discover ies have important implications for the biogeochemistry of soils where Fe(I II)-bearing clay minerals are abundant.