Metal-Bacteria Interactions at both the Planktonic Cell and Biofilm Levels

Geomicrobiology is a relatively new field that aims to establish how microorganisms interact with the surrounding inorganic environment. Most of what is known so far has been derived from laboratory experiments using free-swimming (or planktonic) bacteria, however, throughout nature, nutrient limitations and other environmental challenges encourage bacteria to live in a complex and dynamic surface-associated community known as a biofilm. These assemblages exhibit micron-scale variations in environmental chemistry (such as pH and redox potential), bacterial cell physiology, and are exemplified by a viscous extracellular matrix that also shows significant heterogeneity over very small scales. The inherent complexity of these systems leaves us with very little knowledge about how microbes drive geochemical cycles in the natural environment in which they are found. Fortunately, recent developments in instrumentation in the laboratory (particularly in microscopy and other analytical techniques) have generated a greater understanding of how microbial biofilms influence the geochemistry of their surroundings. This chapter discusses several properties of bacteria that make them efficient mediators of metal ion chemistry, the range of metalmicrobe interactions found in natural environments, and a number of physico-chemical differences between the planktonic and biofilm modes of growth. To conclude, the chapter outlines the recent developments in the field that have allowed us to gain a greater appreciation of the added controls biofilm communities exert on the fate and speciation of metal ions in the natural environment.