Mineral transformation associated with the microbial reduction of magnetite

Although dissimilatory iron reducing bacteria (DIRB) are capable of reducin g a number of metals in oxides and soluble forms, the factors controlling t he rate/extent of magnetite reduction and the nature of the mineral product s resulting from magnetite reduction are not well understood. This study wa s carried out to investigate mechanisms and biogeochemical processes occurr ing during magnetite reduction by the DIRE, Shewanella putrefaciens strains CN32 and MR-1. Reduction experiments were pel formed with biogenic and syn thetic magnetite in well-defined solutions. Biogenic magnetite was generate d via microbial reduction of hydrous ferric oxide (HFO). Biogenic magnetite in solutions buffered with either bicarbonate (HCO3-) or 1,4-piperazinedie rhanesulfonic (PIPES), with or without P, was inoculated with strain CN32 a nd provided with lactate as the electron donor. Synthetic magnetite in a ba cteriological growth medium (M1) was inoculated with either aerobically or anaerobically grown cells of strain (CN32 or MR-1). Fe(II) production was d etermined by HCl extraction of bioreduced samples in comparison to uninocul ated controls, and the resulting solids were characterized by X-ray diffrac tion (XRD), Mossbauer spectroscopy, scanning and transmission electron micr oscopy (SEM and TEM). The extent and rate of biogenic magnetite reduction i n the bicarbonate-buffered medium was higher than that in the PIPES-buffere d medium, via complexation of bioproduced Fe(II) with HCO3- (or PO43-) and formation of siderite (vivianite). S, putrefaciens CN32 reduced more synthe tic than biogenic magnetite with differences attributed mainly to medium co mposition. In the HCO3--buffered solutions, Fe(III) in the biogenic magneti te was reduced to Fe(II), and siderite precipitated. In the PIPES-buffered medium, Fe(III) in biogenic magnetite was also reduced to Fe(II), but no se condary mineral phases were observed. Vivianite formed in those solutions c ontaining P and in all synthetic magnetite treatments where there was suffi cient supply of P from the M1 medium. Electron microscopy and Mossbauer spe ctroscopy results suggest that the reduction process involves dissolution-p recipitation mechanisms as opposed to solid state conversion of magnetite t o vivianite or siderite, The aqueous medium, pH, strain type, and bacterial growth conditions all affected the extent of magnetite reduction. The abil ity of DIRE to utilize Fe(III) in crystalline magnetite as an electron acce ptor could have significant implications for biogeochemical processes in se diments where Fe(III) in magnetite represents the largest pool of electron acceptor. (C) 2000 Elsevier Science B.V. All rights reserved.