Metal adsorption onto bacterial cell walls represents a gateway for metal bioavailability to the cell, and the rate and extent of metabolic processes that require metal bioavailability can be related to the concentration and speciation of metal adsorbed onto bacterial cell envelope functional groups. Although bacterial metal bioavailability can be correlated to aqueous metal speciation, the concentration and speciation of metals on bacterial cell walls represents a better and more mechanistic foundation for bioavailability models. The Biotic Ligand Model (BLM) accounts for metal toxicity to organisms by invoking a generic ‘biotic ligand’ which is the site of introduction of a metal to an organism. The BLM has been used successfully to account for metal toxicity. However, the weakness of the BLM is that the biotic ligand is merely a generic, fictional site. Binding is not represented by a balanced chemical reaction, so great inaccuracies may develop in extrapolating these predictions to systems not directly studied in the laboratory. In addition, there is growing evidence that metal binding onto organisms can involve more than one type of site, and different metals can exhibit different affinities for different site types. The complexity of binding environments on bacteria relative to the simple one-site BLM does not mean that the whole BLM approach is a failure, but it does mean that a new generation of BLMs must be developed to account for this complexity.<br/>       Recent research has dramatically improved our understanding of the mechanisms involved in metal binding onto bacterial cell binding sites. Our improved understanding of bacterial metal binding puts us in a prime position to transform bacterial biotic ligand models of metal bioavailability to include realistic, mechanistically-sound metal binding reactions, thereby dramatically improving the accuracy and predictability of these models<i>. </i>For the first time, we are currently poised to be able to relate cell wall metal speciation to observable metabolic effects. In this talk, we describe several initial studies that illustrate that metal adsorption onto bacteria represents a controlling process in metal bioavailability for a range of metabolic processes, and that using surface complexation modeling of metal adsorption onto bacteria can yield advanced rate laws for these processes, thereby enabling more accurate quantitative predictions of bioavailability in complex geologic and engineered systems.

Modeling metal bioavailability to bacteria using surface complexation modeling

251st National Meeting (2016)

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