Algae are important in the formation of coral reefs.
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red
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Red algae

Explanation:

Coral reefs are the structure which are one of the some most important underwater ecosystems of the world. They are characterized by the formation of roof-building corals which are basically the colonies of dead or living polyps(corals) attached together through deposition of calcium carbonate.

Red algae has an important role in the formation of coral reefs  because their thread like structure traps the particles present in the sand of water which ultimately gives the algae a mechanical support for their growth as well as the formation of coral reefs.

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