Final answer:
Coral reefs are unable to survive below the euphotic zone primarily due to the lack of sunlight, which is necessary for the photosynthesis of symbiotic algae that provide vital nutrients to the corals.
Step-by-step explanation:
Coral reefs cannot survive below the euphotic zone primarily due to the lack of sunlight. This region of the ocean is too deep for sunlight to penetrate, making photosynthesis impossible for the symbiotic algae, known as zooxanthellae, which live in the corals. These algae provide essential nutrients to the coral through photosynthesis, which is a process crucial for the survival of coral reefs. Without these nutrients, the coral polyps cannot produce the calcium carbonate skeletons that form the reef's structure. Factors like ocean acidification and rising temperatures cause coral bleaching, further adding to the challenges for coral survival. However, below the euphotic zone, the absence of light makes photosynthesis and coral growth unsustainable, thereby inhibiting the survival of the coral reef ecosystem.