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Purple bacteria, also known as purple photosynthetic bacteria, are gram-negative prokaryotes that can make their own food by converting light energy into chemical energy in anoxic (free of oxygen) conditions.

They contain the pigments bacteriochlorophylls (a and b), along with carotenoids that give them purple, orange, red and brown colours. These anoxygenic phototrophs are found widely in nature, specifically in aquatic conditions, where anoxic conditions are maintained.

Division of Purple Bacteria

The purple bacteria are divided into purple sulphur bacteria and purple non-sulphur bacteria based on their physiological properties, their tolerance and utilisation of sulphide.

Purple Sulphur Bacteria

The purple sulphur bacteria are anaerobic or microaerophilic organisms that are found in stagnant water and hot springs. These organisms do not employ water as their reducing agent (hence, no oxygen production) but rather use sulphur in the form of thiosulphate or sulphides as the electron donor in the photosynthetic pathway.

The sulphide gets oxidised to form elemental sulphur that may be further oxidised into sulphuric acid. Almost 25 genera of purple sulphur bacteria are known that includes species which produce sulphur granules intracellularly as well as extracellularly.

Examples of purple sulphur bacteria include Allochromatium and Thiocapsa, which are most commonly studied.

Purple Non-sulphur Bacteria

Previously, purple non-sulphur bacteria were distinguished from the purple sulphur bacteria on the basis that the purple non-sulphur bacteria could not tolerate even millimolar levels of sulphide, while the purple sulphur bacteria could. However, this basis of the division was refuted by many scientists.

Currently, it is believed that the purple non-sulphur bacteria can tolerate millimolar (0.5mM) levels of sulphide and can oxidise it into S₄O₆^2-, SO₄^2- or S⁰. Another distinction is that the S⁰ formed by purple non-sulphur bacteria is always stored extracellularly.

Examples of non-sulphur bacteria include Rhodobacter and Rhodospirillum.

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Also Read:

• Difference between Green and Purple Sulphur Bacteria
• Difference between Bacterial Photosynthesis and Plant Photosynthesis
• Autotrophic Bacteria: Definition, Types and Examples