Coral reefs represent some of the most biologically diverse and productive ecosystems on Earth. These massive underwater structures provide habitat for approximately one-fourth of all marine species. The foundation of these complex ecosystems is symbiosis, a close and long-term interaction between two different species. Without this fundamental, mutually beneficial partnership, the rapid growth and immense scale of reef-building corals in nutrient-poor tropical waters would not be possible.
The Essential Partners
The core of this relationship involves two distinct organisms: the coral polyp and microscopic algae. The coral polyp is a small, sessile animal that lives in colonies and secretes a hard, calcium carbonate skeleton. This animal serves as the host for its partner, providing the physical structure of the reef.
The partner is a single-celled dinoflagellate alga, commonly referred to as zooxanthellae, which lives within the tissues of the coral polyp. These algae reside in membrane-bound vacuoles inside the host’s gastrodermal cells. This arrangement is an obligate mutualism, meaning both species gain a survival advantage, and the coral’s long-term survival is dependent on the presence of the symbiotic algae.
The Exchange
The success of coral reefs stems from the efficient exchange of compounds between the coral host and its algal symbiont. This exchange is powered by the algae’s ability to capture light energy through photosynthesis. The zooxanthellae convert sunlight and carbon dioxide into organic molecules, acting as an internal food factory for the coral.
The primary compounds produced by the algae are simple sugars, glycerol, and amino acids. An estimated 90% of the organic material created by photosynthesis is transferred directly to the coral host. This influx of energy drives the coral’s high metabolism and rapid growth rate.
In return, the coral polyp provides the algae with a stable, protected environment within its tissues. The coral also supplies the essential raw materials the algae need to fuel photosynthesis. These materials include carbon dioxide, a waste product of the coral’s respiration, which the algae immediately recycle.
The coral excretes nitrogen and phosphorus compounds, which are scarce in the clear tropical waters where reefs thrive. These waste products are readily absorbed by the algae and utilized as nutrients for their growth. This tight, internal recycling loop conserves materials that would otherwise be lost to the surrounding water.
The consistent energy supply from the algae enables the coral to build its massive skeletal structure. This energy powers the calcification process, where the coral precipitates calcium carbonate to form its rigid limestone skeleton. Corals with their symbiotic algae can deposit calcium carbonate up to ten times faster than corals lacking the symbionts. This accelerated skeletal growth allows reefs to become immense, complex geological structures.
Consequences of Symbiotic Breakdown
The symbiotic relationship is highly sensitive to environmental change. When corals are subjected to stress, the balance of the partnership quickly breaks down. The most common stressors are elevated sea temperatures and intense solar radiation.
Under these conditions, the photosynthetic machinery of the zooxanthellae malfunctions. This failure leads to the overproduction of toxic compounds called reactive oxygen species (ROS) within the algae’s cells. These species are harmful to both the algae and the host coral tissue.
In response to this internal toxicity, the coral host expels the zooxanthellae from its gastrodermal cells. This expulsion is a defense mechanism that defines coral bleaching. Because the zooxanthellae contain the pigments that give coral its color, their loss causes the coral tissue to become transparent, revealing the white of the underlying calcium carbonate skeleton.
A bleached coral is not immediately dead, but it has lost the source of up to 90% of its nutritional energy. This state is one of starvation, leaving the coral vulnerable to disease and reducing its growth and reproductive capacity. If the stressful conditions are temporary, the coral may reacquire new symbionts and recover. However, if the stress persists, the coral will starve and the colony will die, leading to the structural degradation of the reef.