Answer:
Chemoautotrophs are the organisms which derive energy from oxidation of electron donating elements in their environment i.e. they get their energy from chemical reactions rather than sunlight (organisms which derive their energy from sunlight are called photoautotrophs.)
Chemoautotrophs use elements like hydrogen sulphide, ferrous oxide, elemental sulphur, hydrogen and ammonia as their electron donating elements.
They usually synthesize required organic compounds from carbon dioxide.
Mostly, they are archaea or bacteria, extremophiles and are found in extreme conditions such as deep sea vents, extreme saline areas, areas of high temperature(such as lava beds), areas of low pH etc. In addition, they are the primary producers of such ecosystems.
Different types of chemoautotrophs are thermophiles, halophiles, methanogens, sulphur oxidizers, thermoacidophiles etc.