Introduction

Interactions with microorganisms have long been recognized as playing a key role in determining the cycling and ultimate fate of metals in the environment. On the one hand, bioleaching from naturally occurring ores or synthetic sources may result in the release and dispersion of metals while, on the other hand, microbial sorption or accumulation processes concentrate and tend to remove metal species from the surrounding environment. The bioconcentration occasioned by the latter processes may also represent an entry path into the food chain, with potentially fatal consequences for higher organisms.

Microbial metal sorption or accumulation processes may be classified as either dependent or independent of metabolism (Blackwell, Singleton & Tobin, 1995). The former occurs in most, if not all microbial forms, sorption depending on the physicochemical nature of the microbial cell wall. Metal sorption or uptake (typically from the surrounding solution) results from chemical and/or physical binding of metal ions to cell wall functional groups and is, in the main, unchanged if the cells are living, denatured or dead. Metabolism-dependent processes are generally slower and involve active metal transport into and localization within the cell interior (Blackwell & Tobin, 1999). In many instances non-active binding occurs first and it is the initially bound metal that is subsequently transported to the cell interior.

The term biosorption has variously been applied to both the overall process of metal uptake by biological materials and the non-metabolic sorption process.