Algae and multicellular plants are autotrophs because they produce their own food through photosynthesis.

Here's why:

* Autotrophs: Organisms that can make their own food from inorganic sources.

* Photosynthesis: The process by which plants and algae use sunlight, carbon dioxide, and water to produce glucose (sugar) for energy and oxygen as a byproduct.

In essence, algae and plants use the energy from sunlight to convert carbon dioxide and water into food (sugars). This is what makes them autotrophs, self-feeders.