Final answer:
Seaweeds are plant-like protists known as algae that perform photosynthesis like plants but lack true plant tissues and organs, which is why they are classified as protists.
Step-by-step explanation:
Seaweeds are characteristic of plant-like protists, specifically a type of algae. They are often mistaken for plants due to their similar appearance and the fact they perform photosynthesis, a process where they convert light energy into chemical energy to produce their food. The green pigment responsible for their color in green algae is chlorophyll, which is also found in plants. Unlike plants, however, seaweeds lack true tissues or organs such as roots, stems, or leaves, and they do not have a waxy cuticle to prevent water loss. This lack of true plant characteristics is what classifies them as protists rather than plants. Some types of algae, such as kelp, can form large structures similar to forests in the ocean. Algae play essential roles in their ecosystems, providing food and oxygen through their photosynthetic activities.