MOBILISATION AND SEDIMENTARY DEPOSITION OF IRON BY MICROBIAL PROCESSES

The mobilisation of iron from granite and its subsequent deposition are part of an investigation into a biofilm-forming microbial consortium enriched from oligotrophic Shield groundwaters in the Lac du Bonnet Batholith, eastern Manitoba. Although the concentration of iron dissolved in the groundwater is minimal, the biofilm bacteria are able to extract it directly from the granite accessory iron-bearing minerals, biotite and magnetite. The main energy-producing reaction of this consortium is dissimulatory reduction of iron, and this reaction appears to be both constitutional and widespread. Since iron is only soluble at low pH, it must be chelated to be in solution at the Shield groundwater pH of 8.5 to 9.0. In the laboratory, iron, chelated with citrate, is rapidly precipitated by the consortium as the citrate is metabolised. The form of this precipitate depends upon the ratio of iron to carbon. If the ratio is greater that 1:6, ferrous minerals such as siderite, Fe(OH)2 and vivianite are formed, whilst if the ratio is less than this, ferric minerals are precipitated, mainly ferrihydrite that alters to hematite. Recent 1H NMR studies have shown that, where there is an excess of the citrate required to chelate the iron or when reactions are slowed by the spectra experimental conditions, the iron is reduced whilst chelated and forms ferrous minerals. However, if less citrate is present or the reactions occur more rapidly, then the iron is precipitated in the ferric state before it is reduced. Primary features retained in ironformations are found in many shallow water forms such as widespread microbands representing fossil algal mats. Commonly there is no nearby source of iron, which suggests that it originates in terrestrial weathering, and is transported in dilute waters before being microbially concentrated in iron-rich sediments. We suggest, therefore, that ironformations are primarily due to the ability of bacteria to cycle iron from igneous rocks and hydrothermal vents to sedimentary deposits, and that in oligotrophic environments a consortium is most efficient in utilising the fluctuating availability of nutrients.