Most reef-building corals contain Zooxanthellae, a photosynthetic microscopic algae, in their tissues. Zooxanthellae produce sugar for the corals from photosynthesis; in return, corals obtain the energy to build reefs. These microscopic algae are also great indicators of the ocean's health and global warming. Ocean warming is the major cause for many reefs dying around the world, as it damages and kills corals. Zooxanthellae.
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