Microbial Metal Cycling in Aquatic Environments

Abstract

This chapter focuses on microbial metal redox metabolism, with an emphasis on iron (Fe) and manganese (Mn) cycling in the water column and surface sediments. It deals exclusively with metal cycling in circumneutral pH environments. The majority of the Fe and Mn that enters aquatic systems comes in the form of insoluble oxides (Fe and Mn) and silicate phases (Fe only; Mn-rich silicates are uncommon), which are produced during weathering of rock-forming silicate minerals (e.g., olivines, pyroxenes, and amphiboles) in the terrestrial environment and transported to coastal marine environments and lakes by rivers and streams. Although insoluble oxide phases are by far the most abundant forms of Fe(III) and Mn(IV) in neutral-pH aquatic environments, several recent studies suggested that small but significant quantities of soluble Fe(III) exist in circumneutral sediment pore fluids. A review by Emerson provides an overview of the history of research on circum-neutral bacterial Fe(II) oxidation as well as the physiology and systematics of Fe(II)-oxidizing bacteria (FeOB). In light of the foregoing analysis of the role of microbes in the oxidative side of the Fe and Mn cycles in aquatic environments, it is clear that Fe and Mn redox cycling is an example of microbial syntrophy analogous to the well-known syntrophic relationships among N- and S-oxidizing and -reducing microorganisms in natural systems. Both the water column and sediment chemical profiles illustrate several key aspects of the Fe-Mn redox cycling systems in aquatic environments.