The chemoautotrophs are a special class of bacteria that can synthesize carbohydrates like the photosynthetic bacteria do, but which obtain the energy for doing so from inorganic reactions rather than from the sun.

Some of them obtain energy by oxidizing ammonia to nitrite or nitrate, others convert H2S to elemental sulfur, thiosulfate, or sulfate, and still others oxidize Fe (II) to Fe (III).

One might think that chemosynthesis, which uses inorganic reactions for energy, is an older mechanism than photosynthesis.

This is unlikely, since all chemoautotrophic bacteria have well-developed respiratory chains and use O2 as an oxidant. It is more likely that these chemoautotrophs are specially adapted forms, which found an alternative means to solar radiation to power their carbohydrate syntheses. They can be neglected in a search for the origin of life.

Chemoautotrophs and the inorganic materials from which they obtain their energy

o Nitrifying bacteria Nitrosomonas, Nitrococcus - Ammonia

o Nitrifying bacteria Nitrobacter, Nitrococcus - Nitrite

o Thiobacillus - Hydrogen Sulfide, Sulfur, Sulfate

o Ferrobacillus, Gallionella - Ferrous Iron Salts