INFLUENCE OF THE MICROBIAL IRON CYCLE ON BIF DEPOSITION

Microbial mediation of the extraction and precipitation of iron and silica may be able to explain the accumulation of the vast amounts of these minerals found in BIF (banded iron formations), since it is difficult to account for this accumulation solely through abiological chemical and physical processes. The genesis of BIF has two main problems: the source and mobilization of the iron, and the deposition of the sediment. A microbial consortium, enriched from a biofilm discovered in an underground research laboratory at 400 m, is used to illustrate the relevant microbial reactions. Laboratory experiments show that microbial mediation can readily extract iron from biotite and magnetite, as well as from iron chelated with citrate in solution. Notably, this consortium preferentially obtains energy through dissimulatory iron reduction, although in the absence of iron, fermentation occurs. In both cases the consortium is able to maintain a low redox, indicating that an anaerobic atmosphere is unnecessary. The iron is rapidly precipitated in either the ferrous or ferric form, depending on the relative concentrations of iron and carbon. The major minerals precipitated are siderite and ferrihydrite. However, under low temperature diagenesis ferrihydrite is found to be considerably more labile than siderite. These minerals are securely trapped within the biofilm matrix, due both to the direct metabolic reduction of iron, and to the indirect influence of the microbial environment. Sedimentation of this iron-rich biofilm would be similar to that of present day algal mats.

It has not been possible to demonstrate unequivocally that earth’s BIF cannot be formed without microbial mediation, but if this were so, then remote sensing of iron formations on other celestial bodies, such as Mars, might be able to tell us whether life exists, or indeed had ever existed.