Ecotoxicology and Environmental Safety 56 (2003) 140–147 Issues underlying use of biosensors to measure metal bioavailability Christopher Rensing à and Raina M. Maier Department of Soil, Water, and Environmental Science, University of Arizona, Room 429, Shantz Boulevard # 38, Tucson, AZ 85721, USA Received 20 March 2003; accepted 20 March 2003 Abstract Heavy metal-mediated toxicity in the environment is dependent on bioavailable metal concentrations both internal and external to microbial cells. Both internal and external metal bioavailability are influenced by multiple factors in the soil environment. External factors include pH, redox potential, ionic strength, organic matter and clay content. The internal bioavailable metal concentration is dependent on both the aforementioned external factors, as well as metal uptake and efflux activities that are specific for each microorganism. The metal-specific biosensors discussed in this article can be used to measure internal metal bioavailability. r 2003 Elsevier Inc. All rights reserved. Keywords: Bioavailability; Transition metals; Heavy metals; Uptake; Efflux; Biosensor 1. Metal bioavailability in the environment Metal speciation and the resulting bioavailability rather than total metal concentration determines the overall physiological and toxic effects of a metal on biological systems (Bernhard et al., 1986; Hughes and Poole, 1989; Morrison et al., 1989; Roane et al., 1996). Total metal refers to all metal present in a given environment. In contrast, one can define external bioavailable metal as the soluble, ionic form of the metal. This is the metal that can interact with surrounding microbial cells or other biota. To under- stand the difference between total and external bioavail- able metal, one must understand metal speciation in the system. The speciation of a metal refers to the various forms of the metal present including the soluble, exchangeable, carbonate-bound, oxide-bound, organic matter-bound, and residual fractions (Tessier et al., 1979; Davis et al., 1993; Brown et al., 1999). At any given time in an environmental system, the external bioavailable metal is considered to be the soluble fraction. However, if equilibrium conditions change, specifically if the soluble metal fraction is removed, some metal forms will quite readily become soluble and bioavailable. Metal forms that are most subject to this type of change are the exchangeable, and the carbonate-, oxide-, and organic matter-bound fractions. In contrast, the residual fraction is composed of stable, low- solubility crystalline metal forms that do not readily become bioavailable. Metal speciation and the resulting formation of the various fractions is dependent on the combined effects of time, pH, redox potential, ionic strength, and in the case of soils, organic matter and clay content (Sposito, 1989; Alloway, 1990; Brierley, 1990; Moore, 1994). 1.1. pH and redox potential At high pH, metals tend to form insoluble metal mineral phosphates and carbonates, whereas at low pH they tend to be found as free ionic species or as soluble organometals. Consider a medium containing phos- phate, perhaps the most common buffer constituent used in microbiological media. Even a small change in pH can decrease metal solubility and hence metal bioavailability by several orders of magnitude. For example, according to the MINEQL+ geochemical speciation model, the solubility of cadmium at pH 6 in the presence of 1.3mM phosphate is 88mM. Increasing the pH to 7 reduces cadmium solubility to 10 mM. Redox potential also influences speciation. The redox potential (E h ) of an environment is established by oxidation–reduction reactions that tend to be relatively slow, particularly in soil environments. However, microbial activity can dramatically influence the rate ARTICLE IN PRESS à Corresponding author. Fax: +520-621-1647. E-mail address: rensingc@ag.arizona.edu (C. Rensing). 0147-6513/03/$-see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0147-6513(03)00057-5