Some of the key characteristics of these organisms are as follows: Algae can either be unicellular or multicellular.They are mostly aquatic organisms. They are found in places where there is a sufficient amount of moisture available. Algae can reproduce both sexually, as well as asexually. They lack true plant structures like roots, stems, leaves, and specialised reproductive structures of plants that are multicellular in nature. There are various types of algae like green algae, blue-green algae, yellow-green algae, brown alga, etc.