Phyloplankton and Marine plants are like the "forests" of the ocean. Phytoplankton are single-celled plants that serve as food to many animals and also regulate how much carbon is in the atmosphere. Algae are photosynthetic eukaryotes that live in the ocean too. They are unicellular, colonial, or multicellular. Seagrasses are flowering marine plants and there are about 50 species of seagrasses. Kelp grow in cold temperatures around the world and they grow really fast if there are the right conditions. Kelp grow close to the surface so they can get some sunlight.