A quantitative study of microbial clay mineral reduction coupled to the oxidation of organic carbon was carried out using an Fe(III)-reducing bacterium (Shewanella putrefaciens strain MR-1). Total CO2 production, organic acid depletion, and Fe(III) reduction were measured in the same cultures with formate or lactate as the carbon source and clay as the sole electron acceptor. Mean ratios of 1.6:1 and 4.9:1 were observed for structural Fe(III) reduction coupled to formate oxidation and lactate oxidation, respectively. When organic ligands were added under similar culture conditions, the extent of clay reduction was enhanced up to 2-fold in the order of nitrilotriacetic 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-bound Fe reduced in cultures to which organic ligand was added. Here we provide the first direct measurements which show that (1) bacteria may couple the respiration of Fe(III) bound in smectite clay minerals to carbon cycling, (2) organic ligands increase the bioavailability of Fe(III) bound in clay minerals, and (3) bacterial Fe(III) reduction in the presence of organic ligands may lead to clay mineral dissolution. These discoveries have important implications for the biogeochemistry of soils where Fe(III)-bearing clay minerals are abundant.

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