Algae diversity.. Single or multiple algae cells. Phytoplankton. The base of the food network provides oxygen to the atmosphere resulting from photosynthesis

Algae are not different in color only. There are single-celled algae, or several large cells with a length of 65 meters.
Some single-cell algae are called plankton, and are meant as phytoplankton.
Plankton plays an important role in the environment. It forms the base of the food network, supplying the atmosphere with oxygen from photosynthesis.