CoachAn organism is autotrophic when it builds its organic matter from (Chapter 1). It needs a source of energy and a source of electrons. Photoautotrophs take energy from light: plants, algae and cyanobacteria take their electrons from water and release oxygen (oxygenic); purple and green bacteria take them from hydrogen sulfide or organic acids and release none. Chemoautotrophs take both energy and electrons from the oxidation of inorganic compounds — ammonia, nitrite, sulfide, hydrogen, ferrous iron — with no light at all: the nitrifying bacteria of the soil, and the bacteria that feed the ecosystems of deep-sea vents. All use the Calvin cycle, or a related cycle, to fix the carbon; all reduce it with NADPH and pay with ATP; they differ only in where the ATP and the electrons come from.
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Example 14.15 (The global budget)
Photosynthesis fixes about of carbon a year on land and in the oceans, half of it by single-celled algae and cyanobacteria invisible to the eye; it turns over the carbon dioxide of the atmosphere every seven years and its oxygen every two thousand. The oxygen came the same way: for two billion years cyanobacteria released it into an atmosphere that had none, and every aerobic organism, from the mitochondrion up, is built on the leak of one enzyme system that splits water.