MOBILISATION, METABOLISM AND REPRECIPITATION OF IRON BY A MICROBIAL BIOFILM

Although the oligotrophic Shield groundwaters of the Lac du Bonnet Batholith of eastern Manitoba contain only a minimal concentration of dissolved iron, ferrihydrite and siderite have been found precipitated within a subsurface biofilm. An indigenous microbial consortium had mobilised iron from the biotite and magnetite in the surrounding granite. This consortium has various metabolic activities, but the main energy-producing reaction is bacterial dissimulatory iron reduction. Iron is only soluble at low pH, so that at the Shield groundwater pH of 8.5 to 9.0 it has to be chelated to be in solution. Using citrate for chelation in the laboratory, the iron is rapidly precipitated as the citrate is metabolised by the consortium. The ratio of iron to available carbon determines the form of this precipitate. Ferrous minerals such as siderite, ferrous hydroxide and vivianite are formed when the ratio is greater that 1:5, but ferric minerals are precipitated, mainly as ferrihydrite, if the ratio is less than this. Recent NMR proton studies have shown that with an excess of citrate, the iron may be reduced whilst still chelated, and precipitated in the ferrous form. When less citrate is present, the iron is mainly precipitated in the ferric state before reduction. Microscopic investigation has shown that the iron is precipitated around certain bacteria, which are then ingested by protozoa, ensuring that the iron remains within the biofilm, and thus increasing the probability of its incorporation into sediments. Ironformations retain many shallow water forms such as widespread microbands that represent fossil microbial mats. When there is no nearby source the iron has to be transported, possibly by these iron-rich biofilm concentrates. We suggest from this work that, the deposition of ironformations are primarily due to the ability of microbial consortia to cycle iron from igneous and metamorphic rocks and hydrothermal vents to sedimentary deposits.