Coral reefs are renowned for their breathtaking colors, which result from a complex biological partnership between an animal and an alga. Coral itself is a marine invertebrate, a colony of tiny, soft-bodied polyps that secrete a hard, protective calcium carbonate skeleton. The hues displayed by these organisms are derived from two distinct sources: microscopic algae living within the coral’s tissues and pigments produced by the coral animal itself.
The Symbiotic Coloring Agent
The primary source of color for most healthy corals comes from symbiotic algae known as zooxanthellae. These single-celled organisms reside within the coral polyps’ tissues in a mutually beneficial relationship. The algae contain photosynthetic pigments, primarily chlorophyll and peridinin, which give the coral host colors typically appearing as browns, yellows, and sometimes greens.
Zooxanthellae use sunlight to produce sugars, lipids, and amino acids through photosynthesis. They transfer up to 90% of this organic material to the coral polyp, supplying the majority of the animal’s energy needs for growth and skeleton creation.
The Coral’s Intrinsic Pigments
Separate from the symbiotic algae, the coral polyps produce pigment molecules that contribute to the striking, neon colors seen on reefs. These pigments are proteins, broadly categorized as fluorescent proteins (FPs) and non-fluorescent chromoproteins (NFPs). Fluorescent proteins absorb light at one wavelength (e.g., blue or ultraviolet light) and re-emit it at a longer, lower-energy wavelength, which we perceive as a bright, glowing color.
These intrinsic pigments create intense blues, purples, reds, and cyan colors. A primary function of these proteins is photoprotection, acting as a natural sunscreen for the coral and its symbiotic algae. In deeper, low-light environments, these fluorescent proteins also act as light manipulators, capturing available light and shifting its wavelength to one that the zooxanthellae can use more efficiently for photosynthesis.
Environmental Influence on Color Expression
The exact colors a coral displays are dynamically regulated by the external environment, particularly the quality and intensity of light. Corals living in shallow water, exposed to high levels of sunlight, often increase the production of intrinsic fluorescent proteins to shield their symbionts from intense UV radiation. This light-induced pigment synthesis results in corals that are highly saturated with bright, protective colors.
Conversely, in deeper water where light is scarce, corals adjust the concentration and type of their pigments to maximize light harvesting. Some deep-water species produce red fluorescent proteins that convert the dominant blue light into longer, red wavelengths, which can boost the algae’s photosynthetic efficiency.
The Loss of Color: Coral Bleaching
The color of a coral serves as a clear indicator of its overall health, and the most dramatic loss of color is known as coral bleaching. This phenomenon is a stress response, typically triggered by rising ocean temperatures, though pollution or extreme low tides can also be a cause. When the water temperature exceeds the coral’s tolerance for an extended period, the symbiotic relationship breaks down.
The stressed coral polyp expels the zooxanthellae from its tissues. Without the pigmented algae, which provide the coral with its primary food source and brown-green coloration, the coral tissue becomes transparent, revealing the stark white of its underlying calcium carbonate skeleton. Although a bleached coral is not immediately dead, it is severely stressed and vulnerable to starvation and disease.