Coralalgae

Coralalgae refers to the microscopic algae that live in symbiosis with reef‐building corals. The most common type is the dinoflagellate genus Symbiodinium, often called zooxanthellae. These photosynthetic organisms reside within the tissues of coral polyps, providing the coral with products of photosynthesis such as glucose and oxygen. In return, the coral offers the algae a protected environment and access to waste nutrients including nitrogen and phosphorus. This mutualistic relationship is essential for the construction of coral reefs, as the energy supplied by the algae fuels the growth and calcification of the coral skeleton.

The term “coralalgae” can also encompass other photosynthetic symbionts such as cyanobacteria and green algae that are found in some coral species. While Symbiodinium are the dominant partners in tropical reef systems, the presence of multiple algal types can influence a coral’s tolerance to temperature, light, and other environmental stresses. Some corals host more than one type of Symbiodinium, which can shift in relative abundance over time, a phenomenon known as symbiont shuffling.

Coralalgae are sensitive to changes in water quality, temperature, and acidity. Elevated temperatures can damage the photosynthetic apparatus of the algae, leading to the breakdown of the symbiosis and the expulsion of the algae by the coral, a condition called coral bleaching. Bleached corals are more vulnerable to disease and mortality. Ocean acidification also reduces the availability of carbonate ions needed for coral skeleton formation, indirectly affecting the energetic balance between coral and algae.

Scientists study coralalgae to understand reef resilience, to develop restoration techniques, and to monitor the health of marine ecosystems. Techniques such as pigment analysis, genetic sequencing, and fluorescence imaging are employed to identify algal types and assess their functional roles. Ongoing research aims to discover more about how various algal symbionts contribute to coral adaptation in a changing climate.