Ferrous hydroxy carbonate is a stable transformation product of biogenic magnetite

An ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10–15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO4 at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fewas rapid; with 60% of the total Fe (FeTOT) reduced in one day. Only an additional 10% of FeTOT was reduced over the next three years. A fi ne-grained (~10 nm), cation-excess (CE) magnetite with an Fe/FeTOT ratio of 0.5–0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micrometer-sized ferrous hydroxy carbonate [FHC; Fe2(OH)2CO3(s)], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe/FeTOT ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of the stoichiometric magnetite (0.33). To the best of our knowledge, this is the fi rst report of biogenic FHC, and was only reported twice in literature but in a very different context. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe-excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of fi ne-grained, biogenic magnetite in anoxic sediments. KUKKADAPU ET AL.: FORMATION OF FERROUS HYDROXY CARBONATE 511 of persistence in soil and sediment environments where dissimilatory Fe reduction has occurred may result from its excess Fe content and small particle size. In this communication, we report the rapid production of CE magnetite from the bioreduction of a poorly crystalline Fe oxide mixture at circumneutral pH by a single culture DIRB (Shewanella putrefaciens). We then followed the stability of this phase in the presence of microbial oxidation products of lactate (carbonate and acetate). Our specifi c objectives were to identify whether changes in the Fe/FeTOT ratio of the biogenic CE magnetite would occur upon aging, and whether CE magnetite would exhibit instability and transform to other mineral phases. CE magnetite was found to be unstable in our experimental system and transformed to another poorly known crystalline Fe phase. Our fi ndings provide new insights on the long-term stability of DIRB-produced magnetite, and partially explain why this specifi c biosynthetic phase is infrequently observed in anoxic environments. MATERIALS AND METHODS