What are algae?

Algae are plant-like living organisms. They live in the seas, freshwater, or damp land areas. They range from tiny single-celled organisms, which can be seen only under a microscope, to larger plant-like planktons, used as food by blue whales or flamingoes. 

Large, leaf-like algae, known as 'seaweed, grow extensively in shallow waters to form dense underwater forests.