Final answer:
Seaweeds, also known as algae, are plant-like protists that perform photosynthesis and are autotrophs. They lack true plant tissues and organs and come in various colors such as green, red, and brown.
Step-by-step explanation:
Seaweeds are often misconceived as plants but are actually plant-like protists or algae. They obtain food through a process known as photosynthesis, which makes them autotrophs. They are capable of synthesizing their own food using light energy, usually from the sun, carbon dioxide, and water.
Algae contain chloroplasts, much like land plants, and some even have the same photosynthetic pigments, such as chlorophyll a and b plus carotene in the same proportions as plants. However, seaweeds lack true tissues and organs that define true plants and do not possess roots, stems, or leaves. Regarding their color, seaweeds can be red, brown, or green, influenced by the types of photosynthetic pigments they contain.
While some algae, especially green algae, share common characteristics with land plants and are considered by some to be plants, this classification is not unanimously agreed upon due to the presence of certain plant characteristics in only green algae (Chlorophytes and Charophytes), but not in other types.