To look at a coral reef is to see a fantastic world of color and life. But what is perhaps most interesting lies within the chaos. Beneath the busy surface of a coral reef lies a symbiotic relationship unknown to most. This is the world of algae, specifically the algae that live within the coral themselves. It is this partnership that drives energy production within the reef and keeps alive the diverse species found therein. They are the primary producers of this underwater world, using light energy to produce nutrients that in turn feed not just themselves but the entire reef. Without these algae, coral reefs—and much other marine life—would cease to exist.

The majority of corals in the taxonomic phylum Cnidaria host within their tissues species of symbiotic algae from the genus Symbiodinium.2 The algae capture light and perform photosynthesis, giving nutrients to their coral hosts in return for shelter and protection.2 These algae are not born into the coral; rather coral polyps ingest them throughout their lives.2 This allows for the change of different species of Symbiodinium, which are separated into groups called clades, within a particular coral.2 As it turns out, different clades may be responsible for many different phenomena in corals separated by geographical distance or depth.

An interesting phenomenon is that many corals have the ability to change out their dominant Symbiodinium species symbionts after an event called bleaching, in which a coral or group of coral expel all of their symbiotic algae and lose their color, appearing ‘bleached’ afterwards.1 This appears to be a way for coral to adapt to changing ocean conditions. For instance, in recent years as the ocean acidifies, coral that can more easily change their clades may have an evolutionary advantage over those that cannot. At the same time this results in the use of fat stores to sustain the coral, a process that over the space of just a few weeks can lead to serious damage or death.2 Because they cannot physically move to change their environment, changing symbiont clade may be one of the few ways a coral can adapt to changing environmental pressures. In addition to ocean acidification, water pollution from factories and tourism spots have caused massive amounts of light-scattering debris to accumulate in coastal areas where reefs are found. If long-term ocean debris clouds the water around a coral, it may expel its symbionts in favor of a clade with different photosynthetic capabilities. Recently, these changes due to human interaction with the environment have led many coral species, sometimes in huge conglomerates, to bleach.6 This could be the corals’ attempt to change their symbiotic algae in hopes to adapt to the warmer climate, as increases in temperature also stress the coral metabolically.7 Changing the type of symbiont may allow regulation of the uptake of oxygen or the usage of light to create energy at a different rate than would otherwise be possible. However, it also appears that coral will expel their algae in response to external stressors that may or may not have anything to do with photosynthesis, including salinity changes, bacteria, and chemicals.2 This points to a much more complex relationship between coral and their algae symbionts. Although Symbiodinium species in general have a positive effect on their hosts, it is possible that upkeep of the symbiotic relationship could become energetically unfavorable for the coral, causing the coral to expend energy in times of stress. This could result in bleaching, allowing the coral to save on some of its energy sources.

Light does not pass as freely through water as it does through air. Certain light waves do not effectively penetrate the water’s surface; this is why water appears blue. Because of this, coral and their symbiotic algae cannot use the same mechanisms for light absorption that land plants use. Another consequence is that at different depths, different spectra of light dominate. In areas near the surface, red, yellow, and green light waves still reach the organisms that live there. However, at the deeper reaches of the ocean the majority of light is blue. This means that coral must adapt their choice in algal symbionts to the depth at which they live. In fact, it appears that the major species or clade within a particular coral may be a result of competitive exclusion in which different algal species fight over the limited space within the coral’s tissues until the best suited species or clade effectively outcompetes all other species.1 This points to the ability of coral to change the major clade that resides within them through bleaching in response to new external conditions like pollution. That said, there are several documented coral species that show a high specificity of symbiont preference.1 If some coral species have a specific clade with which they are always associated, that is, if they cannot as easily switch clades, this could allude to a relative advantages of certain coral species over others in rapidly changing environmental conditions like those facing our oceans today.

The dynamic relationship between coral and algae is of upmost importance for the continued balance of reef and ocean ecosystems. Corals not only put large amounts of energy into the ocean, but they also provide a home for reef fish and other organisms.2 As corals bleach and die, their skeletons are dissolved by the water and degraded by other organisms. Without fresh growth, this ends in the loss of habitat for countless fish and invertebrates. Coral is the glue that holds a reef ecosystem together, and symbiotic algae are similarly the glue that holds coral together. Without symbiotic algaes, reef ecosystems would cease to exist, affecting millions of human lives and eliminating one of the most amazing and diverse examples of speciation on the planet. Keeping this relationship alive and well is crucial to the continued wellbeing of our oceans. Currently, the reef ecosystem is under threat from ocean acidification, a direct consequence of increased carbon dioxide in the atmosphere from human consumption.7 This makes it harder for coral to grow which in turns slows reef growth as a whole.7 If this trend is not reversed, it could spell disaster for coral reefs around the world.

The reality of bleaching is obvious, regardless of one’s stance on global warming. The masses of bleaching, if anything, offer a concrete proof that our global environment is rapidly changing at the cost of the stability of one of the world’s most important ecosystems. It seems that a large part of the destabilization comes from the breakdown of this relationship between coral and algae. As the coral expel their symbionts in favor of other clades, they stress themselves to the point of starvation and death. Mass death of coral leads to a domino effect throughout the rest of the reef, with all organisms affected in some way or another. In a sense, coral reefs affect just about every ecosystem on the planet. They provide energy to other reef organisms that are harvested by marine and land animals alike, including humans. Beyond that, they help to block coastal areas from large waves, which can protect these areas from flooding. Coral reefs are an extremely important part of our world, and if we continue to pollute the oceans and allow corals to acidify, we risk losing them.


  1. Baker, A. C. Annu. Rev. Ecol. Evol. Syst. 2003, 34, 661-689. 
  2. Borneman, E. H. Aquarium Corals: Selection, Husbandry, and Natural History. Charlotte, VT: Microcosm: Charlotte, Vermont, 2001. 
  3. Favia favus. The IUCN Red List of Threatened Species. details/133569/0 (accessed Apr. 1, 2015). 
  4. Levy, O. et al. J. Exp. Biol. 2003, 206, 4041-4049. 
  5. The IUCN Red List of Threatened Species. (accessed Apr. 1, 2015). 
  6. Working Together Today for a Healthier Reef Tomorrow. Australian Government Great Barrier Reef Marine Park Authority; 2011. 
  7. Doney, S. C. et al. Annu. Rev. Mar. Sci. 2009, 1, 169-192.