Published online by Cambridge University Press: 10 December 2009
Introduction
Exploration of the microbial world got off to a slow start some 350 years ago, when Leeuwenhoek and his contemporaries focused their microscopes on very small life forms. It was not until about 20 years ago, however, that exploration of the world of intra-terrestrial microbes gathered momentum. Until then, it was generally assumed that life could not persist deep underground, out of reach of the sun and a photosynthetic ecosystem base. In the mid 1980s, the drilling of deep holes for scientific research started. Holes up to thousands of metres deep were drilled in hard as well as sedimentary rock, and up came microbes in numbers equivalent to what could be found in many surface ecosystems (Pedersen, 1993). The deep subterranean biosphere had been discovered.
Defining the boundary between the ground-surface biosphere and the subterranean biosphere is problematic: various scientists define it differently, and there is no general consensus. For our purposes the main criterion is that the subterranean biosphere begins where contact with the surface biosphere is lost. This lies beneath soil and root zones, beneath the ground-water table, and beneath sediment and crust surfaces. A long time should have elapsed since last surface contact, ‘long time’ in this respect being at least several decades, preferably hundreds of years or more. In our view it is not depth per se that defines a subterranean ecosystem; rather, it is the duration of isolation from the surface.
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