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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.
            Look! There it has drifted under the bridges, and out into the ocean, where it hovers over the reef.

Algal is the adjective:
            Red indicates unusually high algal growth.