INTRODUCTION

Exploration of the microbial world started slowly about 350 years ago, when van Leeuwenhoek and his contemporaries first focused their microscopes on extremely small living things. It is only during the last 20 years, however, that exploration of the world of intraterrestrial microbes has gathered momentum. Previously, it had generally been assumed that persistent life could not exist deep underground, out of reach of the sun and a photosynthetic ecosystem base. In the mid-1980s, scientists started to drill deep holes, from hundreds to a thousand metres deep, in both hard and sedimentary bedrock, and up came microbes in numbers equivalent to those found in many surface ecosystems. The world of intraterrestrial microbes had been discovered.

Intraterrestrial ecosystems have been reviewed elsewhere and the content of those reports need not be repeated here (Ghiorse & Wilson, 1988; Pedersen, 1993a, 2000; Bachofen, 1997; Bachofen et al., 1998; Fredrickson & Fletcher, 2001; Amend & Teske, 2005). Instead, this chapter will focus on characteristics that distinguish the intraterrestrial from the terrestrial world.

Most ecosystem environments have specific, distinguishing characteristics. The environments of intraterrestrial microbial ecosystems occupy a special position, differing substantially in many respects from those of most surface-based ecosystems. In many ways, underground ecosystems must be approached quite differently from the way in which those on the surface would be approached.