The total concentration of a metal in solution or in sediments is a poor predictor of its potential effect. In most cases, the actual effect is not correlated with the chemically determined total metal content yet this is the criterion that is often used for regulatory purposes. A better estimation of effect relies on estimating metal speciation in solution, (i.e., how metal species interact with inorganic or organic ligands and how these species affect cellular targets); typically, this is achieved using electrochemical methods, multicomponent thermodynamic equilibrium speciation modelling, or a combination of both. These data are subsequently included in models such as the Biotic Ligand Model that is then used to predict metal toxicity. Following sequential extraction and chemical analysis, two experimental approaches are typically used to obtain information on the metal fraction that is bioavailable: i) uptake assays using algae or invertebrates; and ii) the use of microbial bioreporters.

Microbes have evolved for billions of years in the presence of essential and toxic metals and have finely tuned strategies to maintain metal homeostasis, i.e., maintain an optimal intracellular level of essential metals, while removing toxic ones. At the cellular level, transcription regulators (i.e., metal sensing proteins controlling gene expression) and operator and promoter regions are part of the regulatory circuitry that controls microbial metal homeostasis and resistance. Bioreporters can be designed based on this regulatory circuitry and usually provide early detection, require little pre-treatment, offer increased sensitivity, can be tailored to be site-specific and are very cost effective. Numerous bacterial reporters have been created for the assessment of bioavailable metals and metalloids. Despite their great potential, however, the application of a biosensor developed in the laboratory to an environmentally relevant situation is rarely undertaken. This is often due i) to the lack of portability of the instruments required to detect and quantify the signal produced by the biosensors, ii) the inability of the biosensor physiology to adapt to environmental conditions relevant to metal speciation (e.g., anoxic environments) and iii) the presence of potential interferences in complex environmental mixtures due to variations in the affinity of metals to the sensor proteins.

In this talk, I propose to review recent advances in the application of whole cell biosensors to environmental samples, offer alternatives to some of the issues currently preventing proper application of biosensors to environmental samples and use data generated in my laboratory to explore As and Hg biogeochemical cycles.