Zooxanthellae and Coral

[Originally posted to MBA Student Oceanography Club (SOC)]

Zooxanthellae featured heavily in Dr. Webster’s talk during February's meeting, and the mutualism between corals and zooxanthellae deserves special attention.  In the sunlight-rich but nutrient-poor tropical waters where they occur, reef forming corals and zooxanthellae need one another to survive.  Corals provide zooxanthellae with a comfortable environment, protection from predators, and access to concentrated nutrients (especially carbon and nitrogen that the corals get by eating other organisms).  In exchange, the zooxanthellae photosynthesize, and share the sugars and carbohydrates they produce with their hosts.  Many corals derive as much as 90% of their energy needs from the zooxanthellae and would die without them.  Zooxanthellae are therefore critically important for coral reef health.

Unfortunately, we know very little about zooxanthellae.  Zooxanthellae all belong to the genus Symbodinium, but there is debate as to whether Symbodinium represents a single diverse species or several closely related species.  What is known is that there are several different types of Symbodinium called clades.  Some clades are closely associated with corals and are rarely found outside of them.  Others are usually free living and only rarely found in corals. Clades also vary in how useful they are to the corals; some give lots of energy to the corals, whereas others give relatively little.  Scientists speculate that different forms perform best in different environments, and that different clades will maximize the benefits realized by corals depending on the environmental conditions.

For their part, the corals seem to have some ability to choose which clades they harbor.   Corals in different environments may house different types of Symbodinium, and interestingly, large corals often host different types within the same colony.  For example, many colonies will have one clade on their top surface, while having another clade along their sides. 

Promoting the growth of clades specialized to your environment works remarkably well for corals – that is until the environment changes.  If conditions change, a particular clade of Symbodinium may no longer be beneficial, and the coral must seek a new partner.   Corals stressed by high temperatures, for example, may expel all of their zooxanthellae, a process known as bleaching (so named because the corals often appear white without their symbionts; the first image below is an example of a healthy coral, while the second image shows a stressed coral that appears to be losing its color).  Scientists believe that this may be a coral’s emergency effort to be recolonized by heat-tolerant symbionts.  Sometimes corals are able to find new zooxanthellae and survive (there is some evidence that corals acclimatized to warm waters do not bleach, giving support to the idea that the proper symbiont may make all the difference).  Often, however, bleaching results in death.  Many scientists consider increased bleaching due to climate change a major threat to the world’s coral reefs.