REDUCTION OF STRUCTURAL FE(III) IN SMECTITE BY A PURE CULTURE OF SHEWANELLA-PUTREFACIENS STRAIN MR-1

Shewanella putrefaciens is a species of metal-reducing bacteria with a versatile respiratory metabolism. This study reports that S. putrefac iens strain MR-1 rapidly reduces Fe(III) within smectite clay minerals . Up to 15% of the structural Fe within fermginous smectite (sample SW a-1, Source Clays Repository of the Clay Minerals Society) was reduced by MR-1 in 4 h, and a range of 25% to 41% of structural Fe was reduce d after 6 to 12 d during culture. Conditions for which smectite reduct ion was optimal, that is, pH 5 to 6, at 25 to 37 degrees C, are consis tent with an enzymatic process and not with simple chemical reduction. Smectite reduction required viable cells, and was coupled to energy g eneration and carbon metabolism for MR-1 cultures with smectite added as the sole electron acceptor. Iron(III) reduction catalyzed by MR-1 w as inhibited under aerobic conditions, and under anaerobic conditions it was inhibited by the addition of nitrate as an alternate electron a cceptor or by the metabolic inhibitors tetrachlorosalicylanilide (TCS) or quinacrine hydrochloride. Genetic mutants of MR-1 deficient in ana erobic respiration reduced significantly less structural Fe than wild- type cells. In a minimal medium with formate or lactate as the electro n donor, more than three times the amount of smectite was reduced over no-carbon controls. These data point to at least one mechanism that m ay be responsible for the microbial reduction of clay minerals within soils, namely, anaerobic respiration, and indicate that pure cultures of MR-1 provide an effective model system for soil scientists and mine ralogists interested in clay reduction. Given the ubiquitous distribut ion and versatile metabolism of MR-1, these studies may have further i mplications for bioremediation and water quality in soils and sediment s.