Understanding the Details of Coral Symbiosis: Why Are Zooxanthellae Important to Corals?
Most people have only fleetingly heard about zooxanthellae and the role that this unique type of algae plays in maintaining corals and helping them survive, but why are zooxanthellae important to corals anyway? Couldn’t coral polyps simply thrive by eating plankton and capturing food particles with their tentacles? While the full understanding of corals’ symbiotic relationship with zooxanthellae is not easy to fully understand, scientists are now fairly confident of having a more precise understanding of the relationship between these two marine creatures. The extent of that relationship goes well beyond what most of us would assume.
What Are Zooxanthellae?
Zooxanthellae are single-celled eukaryotes from the phylum Dinoflagellata that are known primarily for their ability to maintain a symbiotic relationship with various marine invertebrates. Aside from coral, zooxanthellae are also associated with sea anemones and jellyfish. Zooxanthellae contain chlorophyll a and chlorophyll c to facilitate the process of photosynthesis, through which they obtain the carbon products that they and their hosts need in order to survive. Although they can also be free-living, most populations of zooxanthellae typically reside in a host’s tissue, where their adaptive properties help them thrive and multiply.
The Process of Interaction Between Corals and Zooxanthellae
Zooxanthellae are not just an “add-on” that coral polyps find useful in their struggle for survival. They play an essential role in that respect, and corals are actually completely dependent on them for their survival. The zooxanthellae present in the tissue of coral polyps are sometimes responsible for providing up to 90% of the host’s energy, which is used to help the coral grow, build coral reef structures, reproduce and exercise metabolic control. Zooxanthellae are primarily associated with reef-building corals because of the added protection these organisms can offer. Their higher energy requirements also play out to the benefit of both parts of the symbiotic circle, especially since reef-building corals are equipped to house a greater number of zooxanthellae than other species.
Water Conditions and the Symbiosis Between Corals and Zooxanthellae
There is a reason why hard coral can’t be found below a depth of about 300 feet or why they require specific water pH, salinity and clarity. The conditions have to be ripe for the algae to be able to survive, otherwise the symbiotic cycle will be broken, and the coral will die as well. Zooxanthellae are highly sensitive when it comes to low salinity levels, and they also require clear enough water to allow enough sunlight for photosynthesis to occur. At greater depths, where the sun’s radiation can’t reach, the zooxanthellae are unable to produce enough nutrients to sustain their coral hosts. So, as you can see, the question “why are zooxanthellae important to corals” is much more complex than we can first assume.