(i) Methanogens: These are microorganisms that produce methane as a byproduct of metabolism in anoxic conditions. They belong to class Archaea.(ii) Halophiles: These are microorganisms that grow and thrive in high salt concentrations. (iii) Thermoacidophiles: These are microorganism that are both thermophilic and acidophilic, that is they can grow under conditions of high temperature and low pH.