Introduction to the Biogeochemical Cycling of Arsenic

Although arsenic is highly toxic to humans, and indeed most other forms of life, some microorganisms have evolved to tolerate relatively high concentrations of the metalloid, while specialist examples even thrive on the element, using it as a source of energy for growth. This surprising array of microbial processes, together with inorganic and physical processes, constitutes the global arsenic cycle. Even at the low concentrations found in seawater and freshwater (see Vaughan this issue), microorganisms can accumulate arsenic, often to concentrations many times those encountered in the environment they inhabit. For example, in the marine environment, arsenic is normally present as arsenate, and can be taken up by a range of organisms (including phytoplankton, algae, crustaceans and molluscs), methylated and further metabolised into organic compounds, some of which are passed up through the marine food chain. Indeed, the major organoarsenic compound in marine animals is arsenobetaine, but it can be degraded eventually by sediment microorganisms, returning arsenic to the seawater and closing the marine cycle for this element (reviewed in Mukhopadhyay et al. 2002 ). In addition to the trace concentrations of arsenic encountered in most environments, there are areas where arsenic can reach much higher concentrations, due to either anthropogenic pollution or a specific geological setting (e.g. evaporative, hydrothermal, sulphidic or Fe3+-oxyhydroxide–dominated environments). In these environments, specialised bacteria that gain their energy requirements from redox transformations of arsenic have been discovered. The key to generating energy from arsenic lies in its redox chemistry, which is characterised by at least four oxidation states: -3, 0, +3 and +5. However, the predominant forms of inorganic arsenic are +5 (arsenate: H2AsO4 and HAsO4) and +3 (arsenite: H3AsO3 and H2AsO3). Energy is made available to support microbial life from the oxidation of As3+ to As5+, with the electrons from this transformation transferred to a suitable electron acceptor such as nitrate or oxygen. Conversely, under anaerobic conditions, the microbial reduction of As5+ to As3+ can be coupled to the oxidation of organic matter or inorganic electron donors (e.g. hydrogen, sulphide) or both, which also yields energy for growth. A synopsis of these processes is illustrated in FIGURE 1, with the underlying biochemistry (discussed later) also shown.