INTRODUCTION

Both basic and applied science issues drive our interests in the microbiology of the deep terrestrial subsurface. As an environment that is disconnected from the Earth's surface, the deep subsurface is less subject to variations in temperature and light and, in unsaturated zones, to intense gradients across interfaces created at the microscale level. These characteristics dictate an average growth rate that is very slow, up to thousands of years per cell division (Kieft & Brockman, 2001), and an ecosystem where change occurs over very long time scales (Fredrickson & Onstott, 2001). Thus, the subsurface is one of the most extreme environments on Earth, and identifying what limits life in the subsurface has value as a model for life on other planets (Chapelle et al., 2002; Nealson & Cox, 2002). The inadvertent release of contaminants from industrial processing plants and storage tanks, as well as the possibility of permanently depositing nuclear wastes deep below the Earth's surface (Pedersen, 2001), raise questions about how microbial activities might exacerbate or mitigate contamination problems in the subsurface.

The terrestrial subsurface is the habitat for diverse microbial communities that, together with the oceanic subsurface, may be the habitat for the largest proportion of Earth's biomass (Whitman et al., 1998). As subsurfaces are characterized by a range of physical and chemical properties, from fully aerated sedimentary shallow aquifers to deep igneous rocks devoid of oxygen and elevated in temperatures, their microbial communities are equally varied (Fredrickson & Fletcher, 2001).