alga, algae

Alga is the singular noun and requires a singular verb:

            A colony of Licmophora fans itself across another alga, Bonnemaisonia.

Algae is the plural form and requires a plural verb:

            When the algae die, they fall to the bottom.

Occasionally algae may be considered a collective noun and treated as singular:

            A map shows how algae periodically blooms in the bay.

Algal is the adjective:

            Red indicates unusually high algal growth.