Chapter 5 : Microbial Transformations of Arsenic in the Subsurface

Abstract:

This chapter describes the biochemical basis of microbial-metalloid interactions with emphasis on energy-yielding redox biotransformations that cycle between the As(V) and As(III) oxidation states. A particular focus is the microbial reduction of As(V) that is sorbed onto subsurface sediments, and the subsequent mobilization of As(III) into water that is abstracted for drinking and irrigation. The reduction of As(V) by Geobacter uraniireducens clearly has a number of important environmental implications. This organism is the first member of the family Geobacteraceae to be shown to reduce As(V) and this activity is supported by the identification of genes potentially encoding a respiratory arsenate reductase. Genetic studies are needed to confirm the role of these genes in arsenic metabolism in G. uraniireducens, and explore the diversity of As(V)-reducing Geobacter species and the ecophysiology of organisms in arsenic-impacted subsurface sediments. Recent studies highlight that metal-reducing bacteria, especially dissimilatory As(V)-respiring bacteria, may play an important role in the microbial process. The microbiological focus of research in this area is shifting now toward the unequivocal identification of the organisms involved, which will in turn lead to studies on the underlying mechanisms at a molecular (genetic) level and the factors that result in the activation of these physiological processes in situ. Recent studies suggest that sulfate reduction can be supported even in very low carbon aquifer sediments that contain low concentrations of electron donor, resulting in removal of arsenic mobilized by dissimilatory metal reduction.