These vast deposits indicate not just life but photosynthesis. Not the familiar form of photosynthesis that we see around us in the green leaves of plants and algae, but a simpler precursor. In all forms of photosynthesis, the energy of light is used to strip electrons from an unwilling donor. The electrons are then forced on to carbon dioxide to form organic molecules. The various forms of photosynthesis differ in their source of electrons, which can come from all kinds of different places, most commonly dissolved (ferrous) iron, hydrogen sulphide, or water.
