Algae don't belong to a single kingdom. They are a diverse group of organisms that are classified into several different kingdoms, including:

* Protista: The majority of algae, including most single-celled algae, belong to this kingdom.

* Plantae: Some algae, like green algae (Chlorophyta), are considered part of the plant kingdom.

* Chromalveolata: This kingdom includes some types of algae like diatoms and brown algae (Phaeophyceae).

The classification of algae can be complex and is still evolving as scientists learn more about these organisms.