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Bacteria are prokaryotic organisms that require nutrients and energy for their growth and development like other living organisms. They require carbon, hydrogen, oxygen, nitrogen, metals and water for their biochemical processes. On the basis of their energy source and the carbon, bacteria are divided into two major classes: autotrophs and heterotrophs.
Nutritional Types of Bacteria
The classification of bacteria based on nutrition is as follows:
Organisms that obtain carbon from carbon dioxide and use light energy or inorganic chemical compounds to produce complex organic compounds are known as autotrophs. These are bacteria that can synthesise their own food from inorganic compounds.
Autotrophic bacteria are further classified into two:
Photoautotrophic bacteria use CO2 as their carbon source to convert it into carbohydrates in the presence of sunlight. These bacteria have bacteriochlorophyll and bacterioviridin pigments in their photosystems. Example: cyanobacteria, purple sulphur bacteria and green sulphur bacteria.
Note: Green sulphur bacteria use H2S as the hydrogen donor.
Read: Autotrophic Nutrition
Chemoautotrophs are organisms that use inorganic sources to synthesise organic compounds in the absence of light. These bacteria lack any pigments and carry out only the dark phase of photosynthesis. Example: sulphur bacteria that oxidise elemental sulphur to gain energy, Hydromonas (hydrogen bacteria) that convert hydrogen into water, iron bacteria that obtain energy by oxidising dissolved ferrous oxides, methanogens and nitrifying bacteria.
Heterotrophs are organisms that cannot make their own food but instead obtain nutrition from other organic sources that may be living or dead. Heterotrophs can be divided into two:
Photoheterotrophic bacteria are those that use light as their source of energy but cannot use carbon dioxide as the carbon source. Instead they obtain nutrition from organic compounds found in the environment such as alcohols, carbohydrates and fatty acids. Examples: purple non-sulphur bacteria, heliobacteria and green non-sulphur bacteria.
Chemoheterotrophs are organisms that derive their energy as well as their carbon source from organic compounds such as carbohydrates and lipids. Example: saprophytic bacteria
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