To most people, the ‘biosphere’ begins with green plants and ends with bacteria. Everything in it is part of a food chain that depends on organic carbon initially captured and packaged by photosynthesis. Most geoscientists, though, are aware that this view is too restrictive, as it does not include carbon fixation that is fuelled by chemical energy.

It is becoming increasingly obvious that energy and carbon cycling in the deep ocean and subseafloor are potentially important issues in solving redox and carbon budgets. Yet, quantification of the magnitude and activity of this dark and deep biosphere and its organic versus inorganic energy and carbon sources is difficult. Photosynthesis and its products are pervasive, and it is hard to identify environments that are unaffected by their presence to at least some degree.