Explain the role of zooxanthellae in coral reef ecosystems and how light and temperature affect this symbiosis.

• A. Zooxanthellae are parasitic bacteria in coral polyps; light reduces their activity; warm water helps them.
• B. Zooxanthellae are photosynthetic algae living within coral tissues; they provide corals with sugars and enhance calcification; adequate light supports photosynthesis; elevated water temperatures disrupt the symbiosis, causing coral bleaching.
• C. Zooxanthellae are free-living algae in reef waters that do not interact with coral; light has no effect on corals.
• D. Zooxanthellae are animals that graze on corals; temperature has no effect.

Answer: (B)