Algae are simple water plants with no true leaves or roots. Green algae, an algae bloom, algae on the glass.
They run from single cells to seaweed metres long, and they produce a large share of the oxygen in the air. Algae is the plural, with alga the rarely used singular, though almost nobody says it.
1nounalgaeSimple water plants with no true leaves or roots.
“Green algae covered the surface of the pond in the hot summer.”
algae is a plant.