Chemosynthesis is a process by which certain organisms synthesize carbohydrates by using energy obtained by the oxidation of inorganic substances.
Most of the bacteria obtain their food materials from external sources and they cannot synthesize their food by themselves.
These are called heterotrophic organisms.
Whereas, some bacteria are capable of synthesizing their food either by photosynthesis or chemosynthesis.
Organisms which use sunlight energy for synthesis of food materials are called photosynthetic organisms or photoautotrophs.
Those organisms which use chemical energy for the synthesis of carbon compounds are called chemosynthetic organisms.
There are two groups of chemosynthetic organisms namely, chemosynthetic autotrophs and chemosynthetic heterotrophs.
Examples for chemosynthetic autotrophs are Nitrosomonas, Beggiatoa.
Nitrosomonas oxidizes ammonia into nitrite.
The energy liberated during this process is used for the synthesis of carbohydrates.
2NH3 +3O2 → 2NO2– +2H2O + 2H+ + Energy
Beggiatoa oxidises H2S to sulphur and water.
During this, energy is released and used for its growth.
Sulphur is stored as granules inside cell.
H2S + [O] → H2O + S + Energy
Examples for chemosynthetic heterotrophs are fungi, most bacteria, animals and man.
These organisms cannot prepare their food materials, hence they are heterotrophs.
They obtain the energy for growth by chemi- cal reactions ie. by oxidizing the organic compounds.
For example, en- ergy is released when glucose is oxidised in the process of respiration.
Thus, these organisms are chemosynthetic heterotrophs.