Protists are eukaryotic cells that cannot be classified as plants, animals, or fungi. Most are unicellular (such as euglena, amoeba, diatoms, and paramecium). However, there are a few multicellular protists such as algae and kelp (also known as seaweed).
Photosynthetic protists, such as phytoplankton, produce large quantities of oxygen for oxygen-breathing (both aquatic and terrestrial) organisms to use.
Aquatic protists serve as a food source. Many fish and zooplankton eat protists. Even animals, such as whales and molluscs, rely on protists as a part of their diet. For example, some sea urchins will often remove kelp forests by removing their holdfasts. Some algae are edible for human consumption (think sushi!).
In addition to being a food source, kelp provides shelter and a habitat for many aquatic organisms.