The Iron and Manganese Cycles

Abstract

The chapter discusses the processes regulating the transformations of iron and manganese in nature and the relationship between the cycling of these and other biologically active elements. Both the oxidation and the reduction of iron and manganese in natural environments is, to a large extent, promoted by microbial catalysis, but abiotic transformations are also important and may compete with the biological processes. In the Earth's crust, iron and manganese are mainly found as minor components of rock-forming silicate minerals such as olivine, pyroxenes, and amphiboles. Iron has a high abundance of 4.3% by mass in the continental crust. At a 50-fold lower crustal abundance than iron, manganese is the second most abundant redox-active metal. There are many similarities between iron and manganese in terms of both geochemistry and microbiology. Microbes play an important role in the oxidation of reduced iron and manganese. Dissimilatory iron- and manganese-reducing microorganisms catalyze the reduction of Fe (III) to Fe (II), and of Mn (III) or Mn (IV) to Mn(II). The chapter discusses the microbial manganese and iron reduction in aquatic environments. Three basic conditions—absence of oxygen and presence of electron donors and oxidized manganese or iron in an appropriate form—are required to fulfill for microbial iron or manganese reduction to thrive in normal aquatic environments of near neutral pH. The chapter discusses the two experimental approaches used to determine rates of microbial Fe reduction in aquatic sediments. The first is a direct quantification of changes in Fe(III) or Fe(II) pools during sediment incubations. The second approach determines rates of dissimilatory iron and manganese reduction by comparing the depth distribution of total carbon oxidation, based on production of dissolved inorganic carbon, to measured rates of sulfate reduction.