GOLDSCHMIDT CONFERENCE TOULOUSE 1998 Enzymatic reduction of iron oxides by anaerobic bacteria

Dissimilatory microbial iron(III)-reduction has been proposed as an important chemical change that takes place in the development of certain anaerobic soils and sediments. Several studies have demonstrated that iron(III)-reduction accounts for the oxidation of 35 to 65 % of organic matter in anaerobic sediments (Lovley, 1991). Microbial oxidation coupled to iron(III)-reduction is an important natural mechanism for contaminant removal from groundwater or shallow aquifers polluted with landfill leachates (Albrechtsen and Christensen, 1994). Futhermore, iron(liD-reduction can influence the biogeochemical cycles of important nutrients such as nitrogen, carbon, sulphur and phosphorous (Lovley, 1991). Up to now, numerous iron(III)-reducing bacteria have been described in the literature (Lonergan et al., 1996). These bacteria re versatile towards electron donors and acceptors which can be used for growth. Although iron(III)-reducing bacteria re studied for many years, little is known about the biochemistry and the mechanism of iron(IIl)-reduction. Iron(III)-reducing bacteria are able to utilize Fe(III) either chelated or present in minerals uch as ferrihydrite, goethite and others. Important minerals such as magnetite, siderite and vivianite are formed during the activity of those bacteria. The question arises whether Fe(IIl) is taken up and reduced in the periplasm or cytoplasm or whether electrons are transferred to Fe(IIl) of the minerals. Evidence has been presented that direct contact between the cells and the metal oxide is necessary for generation of energy (Arnold et al., 1988). Our experimental data obtained with Geobacter sulfurreducens indicate that a peripheral c-type cytochrome localized on the outside of the outer membrane is the terminal reductase of the iron oxide respiration. Methods Geobacter sulfurreducens was cultivated with Fe(III)-citrate (50 raM) and acetate (30 mM) in a medium composed as described (Lovley and Phillips, 1993). Cell harvest and preparation of subcellular fractions were carried out under anaerobic ondi-TABLE 1. Iron(III) reductase activity in different fractions of Geobacter sulJurreducen