INTRODUCTION

Prokaryotes are the Earth's smallest life form and, yet, have the largest surface area : volume ratio of all cells (Beveridge, 1988, 1989a). They are also the most ancient form of life and have persisted on Earth for at least 3.6 × 109 years, even in some of the most extreme environments imaginable, such as the deep subsurface. Most of these early primitive (and today's modern) natural environments possess reasonably high amounts of metal ions that are capable of precipitation under suitable pH or redox conditions. Deep-seated in such geochemical situations is the likelihood of suitable interfaces that lower the local free energy, so that interfacial metal precipitation is promoted. Bacteria, being minute and having highly reactive surfaces (interfaces), are exquisitely efficient environmental particles for metal-ion adsorption and mineral nucleation. Metal ions interact with available reactive groups (or ligands) on the bacterial surface and precipitates grow as environmental counter-ions interact with more and more metal at the site (Beveridge & Murray, 1976, 1980; Beveridge et al., 1982; Ferris & Beveridge, 1986; Fortin et al., 1998). Once formed, these precipitates are under the influence of natural geochemical and additional microbially mediated conditions (Lee & Beveridge, 2001) that instigate the development of fine-grain minerals, usually via dehydration, so that crystalline phases are eventually developed (Beveridge et al., 1983). These minerals commence as so-called ‘nano-mineral phases’ and grow with time to become larger and larger.