can use arsenate as a terminal electron acceptor to reduce As+5 to As+3 (Jackson and Dugas; 2003), conserving energy from the oxidation of organic carbon in anaerobic environments but producing the more toxic form of arsenic. Other microbes (or even the same organism, e.g., Thermus HR-1: see Gihring and Banfield, 2001) oxidize arsenite to arsenate, sometimes using arsenite as substrate and conserving energy as a chemoautotroph (e.g., Oremland et al., 2002). Oxidized arsenic (arsenate) is accidentally taken up by micro-organisms as part of the phosphate transport system, due to the similarity of the As+5 oxyanion species to inorganic ortho-phosphate, and the effects of arsenic toxicity can be increased or decreased by pH, temperature, and coexposure to other metals. A variety of bacteria have developed resistance to extreme arsenic concentrations, reducing arsenate to arsenite intracellularly and pumping out arsenite (Silver and Keach, 1982). The microbial response to toxic arsenic is largely to change it to the most toxic and mobile species, which are then available to be taken up in crops or infiltrated to groundwater. Both As+5 and As+3 are taken up by rice (Abedin et al., 2002) and vegetables (Queirolo et al., 2002).
Interdisciplinary collaboration will be essential to advance our understanding of the complex interrelationships at the intersection of agriculture, soils, microbiology, and public health. Conceptually, the soil environment controls the variety and quantity of elements and nutrients taken up by plants and therefore the elemental composition of plants and their nutritional status. Ultimately, this manifests itself in terms of what is eaten by humans, and therefore biogeochemical cycling in soils strongly impacts what people ingest. Soil, the easily disturbed interface between humans and the geological substrate, constitutes a ripe area of research for the earth science and public health communities. High-priority collaborative research activities are:
To determine the influence of biogeochemical cycling of trace elements in soils as it relates to low-dose chronic exposure via toxic elements in foods and ultimately its influence on human health. For example, it is well known that zinc and cadmium compete for plant uptake in soils and that zinc protects against excess cadmium uptake. Similar protective mechanisms influence the bioavailability of cadmium in the human body. However, in general, little is known about these elemental interactions and the influence of mixtures of elements on bioavailability in both soils and the human body. Similarly, little is known about low-dose chronic exposure via toxic elements in foods.