Here's the breakdown:

* Some algae are single-celled: These are called microalgae, and they include common types like diatoms, dinoflagellates, and some green algae. These tiny organisms are incredibly important to the ecosystem, as they form the base of many food chains.

* Other algae are multicellular: These are called macroalgae, and they are the familiar "seaweed" we often see in the ocean. Macroalgae are composed of many cells organized into tissues and structures.

So, the answer is:

* Some algae are single-celled, and some are multicellular.