Algae These plant-like protists all carry on photosynthesis. Although most algae are single-cell organisms, some are large multicellular organisms. The largest algae are the seaweeds known as kelp that are hundreds of feet long. All algae, no matter their size, lack tissue differentiation. Most aquatic algae compose the phytoplankton , the foundation of aquatic food chains.
Phytoplankton produces almost half the world's carbohydrates and oxygen.
These heterotrophic organisms spend a part of their lives in a mobile, amoeba-like form. They produce a reproductive structure that makes them look like a fungus. They appear as a glistening, viscous mass of slime. Most are yellow or red, but some are white. While most water molds are aquatic, some live in the soil, and others are plant parasites.