Iron Transformations by Freshwater Bacteria

Abstract

Although the bacteria involved in the iron cycle have been recognized since the last century, they have received scant attention compared with those responsible for the cycling of carbon, nitrogen, and sulfur. This is hardly surprising; although iron forms 5{%} by weight of the earth's crust and is of considerable economic importance, the involvement of bacteria in the global iron cycle is of little quantitative significance (Nealson, 1983). In the presence of oxygen and at near neutral pH, conditions which prevail over much of this planet, the oxidation of iron and its precipitation and deposition as the ferric form, Fe(III), is essentially a chemical process. The reaction is, however, dependent on pH, ferrous iron [Fe(II)] concentration, temperature, and ionic strength of the solution. In a freshwater system where the last two components were relatively stable, Davison and Seed (1983) found no evidence for biological mediation of the reaction. Given a solubility product of 10−38 M for Fe(OH)3 and therefore a probable maximum concentration of free Fe(III) at neutrality of 10−17 M what, then, is the likely involvement of bacteria in the iron cycle of freshwater systems?