Biofilms and metal geochemistry: the relevance of micro-organism-induced geochemical transformations

INTRODUCTION This chapter is intended to provide a brief overview of the key concepts underlying the emerging area of environmental microbial metal geochemistry, rather than an exhaustive synthesis. The reader is referred to the following more comprehensive reviews on the biogeochemistry of metals (Warren & Haack, 2001), metal-mineral reactions (Brown & Parks, 2001), emerging molecular-level geochemical techniques (O'Day, 1999; Brown & Sturchio, 2002) and a recent synthesis of how genetic expression in the environment can underpin geochemical reactions (Croal et al., 2004). The relevance of micro-organisms to metal behaviour arises from the overlap of the biosphere with the geosphere and the transformations that occur because of their interactions. Microorganisms have evolved in intimate association with the rocks, soils and waters (i.e. geosphere) in which they find themselves. In order to grow and survive, they have adapted to these environments and use the inorganic components to drive their metabolic machinery; the myriad functional pathways by which they do so ensure that they influence a number of key elemental cycles in the process. As a consequence, many important geochemical processes are ultimately shaped by life, rather than strict geochemical equilibria, a fact that is increasingly recognized as strict geochemical principles fail to constrain observed environmental behaviour. Trace-metal behaviour in the environment is of increasing global concern as water and soil contamination with these toxic substances continues and the detrimental effects on ecosystems and human health emerge.