While algae are often grouped with plants, they are actually in a separate kingdom called Protista.

Here's why:

* Photosynthesis: Both plants and algae use photosynthesis to convert sunlight into energy.

* Kingdom Difference: Algae are single-celled or multicellular organisms that lack the specialized tissues and organs found in plants.

* Habitat: Algae can live in a wider variety of environments than plants, including saltwater, freshwater, and even on land.

Therefore, algae are the living things that use photosynthesis but are not in the plant kingdom.