|Heterocysts are not found.||Heterocysts are usually found.|
|The photosynthetic pigment is bacteriochlorophyll.||The photosynthetic pigments are chlorophyll-a and phycobilins.|
|They may or may not have many flagella.||They do not have flagella.|
|During photosynthesis, oxygen is not released as a by-product.||During photosynthesis, oxygen is released as a by-product.|
|The cell wall is one to two layered.||The cell wall is four layered.|
|Sexual reproduction takes place by conjugation.||The conjugation has not been reported so far.|
The most primitive and ancient bacteria is known as archaebacteria. These are supposed to be the oldest of the living fossils.
Characteristic of Archaebacteria
Based on ecological properties, the archaebacteria can be classified into three groups:
They are aerobic forms of bacteria found in hot sulphur springs. They are high temperature and acid loving cyanobacteria. Under aerobic conditions, these sulphur-dependent bacteria are called thermoacidophiles. They can reduce sulphur to H2S under anaerobic conditions.
These are anaerobic archaebacteria which occur in marshy places and convert the formic acid or CO2 to methane. In biogas plants, they produce the methane gas, which is used as a fuel.
These are anaerobic bacteria which occur at salt rich in soils. They prepare pigmented membrane in the presence of sunlight. This process is different from that of eukaryotic photosynthesis.