Cyanobacteria is a large and diverse phylum in the kingdom Bacteria. The phylum Cyanobacteria includes many different species, but all these species have one thing in common that makes them different than all other bacteria - they are oxygenic phototrophs.

Let's break this term down further: 'Oxygenic' means that they are able to generate and release oxygen. 'Phototroph' refers to the ability of cyanobacteria to use light photons ('photo-') for nourishment ('-troph').

Putting it together, cyanobacteria use photosynthesis to acquire energy from light, generating oxygen as a byproduct. The energy is then used to enzymatically convert carbon dioxide from the air into the nutrients needed for growth.

The cyanobacteria contain a specific type of chlorophyll called chlorophyll a and other pigments that are used for photosynthesis. The chlorophylls and pigments efficiently capture specific wavelengths of light, transferring the light energy to the cell.

One common pigment is phycocyanin, which is a blue color ('cyan' means dark blue). This is the origin of the 'cyano' in the name 'cyanobacteria,' and this pigment, in conjunction with the green chlorophyll, is the source of the common name 'blue-green algae.' It is important to note, however, that cyanobacteria are not a species of algae.

Characteristic Features of Cyanobacteria:

1. They are omnipresent, and occur in all possible kinds of habitats.

2. The pigments found in this group are-chlorophyll a, P-carotene, Antheraxanthin, Aphanicin, Aphanizophyll, Flavacin, Lutein, Myxoxanthophyll, Oscilloxanthin, Zeaxanthin, Allophycocyanin, Phycocyanin, Phycoerythrin.