Reef-building corals must live in the photic zone due to a unique biological partnership that relies entirely on sunlight. The photic zone is the uppermost layer of the ocean, typically extending down to about 200 meters, where sunlight supports photosynthesis. These hard-bodied species, known as scleractinian corals, construct massive coral reefs. Without sufficient light, the biological mechanisms fueling these organisms cannot function.
Defining the Symbiotic Relationship
The dependency on light begins with a mutualistic relationship between the coral polyp and microscopic algae called zooxanthellae. The coral polyp is an animal, a soft-bodied cnidarian that secretes a hard calcium carbonate skeleton. Millions of these single-celled algae, which are dinoflagellates of the family Symbiodiniaceae, live within the coral’s tissue.
The coral provides the algae with a protected environment and necessary compounds for photosynthesis, such as carbon dioxide and metabolic waste products. In return, the zooxanthellae produce and transfer organic compounds that the coral uses for nutrition. This internal recycling system allows the coral reef ecosystem to flourish in nutrient-poor tropical waters.
Light as the Primary Energy Source
The fundamental requirement for light stems from the photosynthetic process carried out by the zooxanthellae. These algae convert light energy, carbon dioxide, and water into usable organic compounds, including sugars, lipids, and amino acids. This energy transfer is efficient, providing the coral with between 90% and 95% of its total daily energy requirements.
This influx of internally produced nutrients allows the coral to sustain a high metabolic rate and grow rapidly. Without adequate light, the zooxanthellae cannot photosynthesize effectively, causing the energy transfer to drop dramatically. If light intensity falls below a certain threshold, the coral starves, as external feeding on plankton cannot meet its vast energy demand. This need for a constant, high-volume energy supply restricts scleractinian corals to shallow, clear waters where sunlight is abundant.
Accelerated Skeletal Growth
Beyond providing energy for survival, light is necessary to enhance building the massive calcium carbonate skeleton, a phenomenon called Light Enhanced Calcification. The growth of the coral skeleton requires the deposition of calcium carbonate, a process that is thermodynamically challenging in seawater. The zooxanthellae’s photosynthetic activity helps overcome this challenge by consuming carbon dioxide during the day.
This removal of carbon dioxide from the coral’s internal fluid raises the local pH and the concentration of carbonate ions. An increased concentration of carbonate ions makes it easier and faster for the coral polyp to precipitate calcium carbonate. This light-driven acceleration of calcification allows reef-building corals to create vast, complex reef structures that grow vertically toward the sunlit surface.
Deep-Water Corals: The Key Distinction
The existence of deep-water corals provides a contrast to the shallow-water reef-builders. These deep-sea counterparts are known as azooxanthellate corals because they do not host symbiotic algae. They can exist in dark, cold waters, often at depths exceeding 200 meters.
Since they lack the internal energy source from zooxanthellae, these corals rely entirely on capturing plankton and organic matter from the water column for nutrition. Their reliance on external food sources results in a much slower growth rate. Consequently, deep-water corals form solitary structures or smaller, scattered aggregations, unlike the immense, rapidly growing reefs characteristic of their light-dependent relatives.