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Chapter 44 : Microbial Metal Cycling in Aquatic Environments *
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.
Schematic representation of the cycling of metals in aquatic ecosystems. Note that when the water column is well-oxygenated, metal redox cycling is localized at the sediment-water interface. Modified from Fig. 1 in reference 206 with permission of Routledge/Taylor & Francis Group LLC.
Pathways of Fe and Mn oxidation and reduction in circumneutral-pH aquatic environments. Modified from Fig. 8.4 in reference 28 with permission of the publisher.
Depth profiles of dissolved Fe(II) and Mn(II), particulate Fe(III) and Mn(IV), and other constituents in a stratified lake (Paul Lake, Mich.). Particulate Fe(III) and Mn(IV) may include small amounts of sorbed Fe(II) and/or Mn(II). Dashed lines indicate approximate boundaries across which metal redox cycling takes place. Redrawn from Fig. 1 in reference 127 and Fig. 1 and 7 in reference 230 with permission of the publishers.
Depth profiles of aqueous and solid-phase Fe, Mn, and other constituents and rates of Fe(III) reduction (FeR), sulfate reduction (SR), and methanogenesis (MG) in the sediments of a freshwater wetland (Talladega National Forest) (top panels) and a shallow coastal embayment (Aarhus Bay, Denmark) (bottom panels). The Mn(IV) values shown in panels C and G may include small amounts of sorbed or precipitated Mn(II). Data are redrawn from Fig. 1 in reference 211 , Fig. 2 in reference 189 , Fig. 3 in reference 50 , Fig. 2 , 4, and 6 in reference 243 , Fig. 2 in reference 244 , and Fig. 3 in reference 239 with permission of the publishers.
Fe(III) oxide morphologies produced by O2-dependent FeOB activity. (A) Stalk-like structures characteristic of Gallionella spp. from a freshwater spring rich in Fe(II); (B) sheath structures of Fe(III) oxides, also from a freshwater spring; (C) filamentous Fe(III) oxides from a marine hydrothermal vent. In all cases, the structures are composed of amorphous ferrihydrite-like minerals that dissolve readily in ammonium oxalate. Although these structures were all formed by FeOB, the bacteria are not visible unless stained with a fluorescent dye. Bar = 10 µm.
Overview of metal biochemistry in aquatic environments a
Summary of reactions associated with Fe and Mn redox cycling in natural water and sediments a
Studies which have enumerated the abundance of culturable Fe- and Mn-reducing and -oxidizing microorganisms in aquatic environments a
Molecular biological studies of metal-reducing and metal-oxidizing microorganisms in aquatic environments a
Overview of connections between Fe and Mn redox transformations and the behaviors of other metals or metalloid elements in aquatic systems