VIAccording to scientists, about 2-3 billion years ago, ancient cyanobacteria produced oxygen. This bacterium increases the amount of oxygen in the earth’s atmosphere. They performed photosynthesis in the cell. The numerous chlorophyll (photosynthetic organ) seeds of all plants are produced by evolved cyanobacteria. Plant’s cells have attracted cyanobacteria to themselves through an intrinsic symbiotic mechanism. In contrast, cyanobacteria cling to plants for protection. During the process of parasitic, cyanobacteria perform the function of converting sunlight into energy for plants.
(Source: https://www.scientificamerican.com/)