Similar to the idea of a kidney transplant, blood transfusion, or skin graft; scientists are testing the waters of the possibility of transplanting zooxanthellae to bleached corals. Now, the first thought I had when I read this title from Reefbuilders was “How brilliant!”.  The world is becoming increasingly aware of the grave phenomenon happening to coral reefs around the world. Due to climate change and pollution coral organisms are being robbed of their symbiotic counterpart, zooxanthellae, and dying off because of it at an alarming rate.

Coral bleaching is a big issue for corals in the ocean but also any corals kept in aquariums. While it is mostly thought that coral bleaching is a one way street, but is it possible to “nurse” a bleached coral organism back to its zooxanthellae stocked glory? The short answer is yes; it is possible but extremely unlikely without special techniques and precise attention. In order to understand the techniques used to bring corals back from the brink of death, let’s go over a little coral 101 and better understand the act of coral bleaching itself.


Coral Bleaching 101

The terms “coral bleaching” and “bleaching events” get thrown out in the media a lot and the sad part is that not many people actually understand what is happening to coral. As it alludes to, yes coral loses its color and appears a very dismal “bleached” white color, but it is not as simple as coral becoming ghost like.

Fundamentally, coral is living; it is an organism that lives and dies just like we do. It is not just a rock or structures like many people think. Coral bleaching is so much more than just coral organisms losing their color. This appearance is of course how the event earned its name, but is a much more complex process than what appears and this process begins with coral’s relationship with zooxanthellae.

Coral has a very important relationship with zooxanthellae. Zooxanthellae are dinoflagellates that are present in the cytoplasm of many marine invertebrates like coral. They are photosynthetic algae that live in the tissues of coral organisms and many times give coral their vibrant hues. Zooxanthellae and coral have a mutualistic symbiotic relationship, thus they benefit each other by a cooperative existence. Coral gives the zooxanthellae a safe environment and compounds they need for photosynthetic events while zooxanthellae produces oxygen and helps the coral remove waste. Zooxanthellae also provide coral with essential products of photosynthesis like amino acids, glucose, and glycerol which is critical for the coral in order to make essential molecules such as fats, proteins, and carbohydrates. They also use the products zooxanthellae provide to produce calcium carbonate used to build their hard exterior. The relationship between zooxanthellae and coral reefs is so important since it drives the growth and productivity of coral reefs. This relationship is responsible for a close knit web of recycling nutrients. Since coral often live in nutrient poor tropical ocean waters, this recycling is extremely important to the whole system. In fact is so important in the ocean, that 90% of organic matter that is photosynthetically produced in seawater is produced by zooxanthellae and is transferred to a host coral tissue.

Besides the critical productivity and recycling of nutrients, zooxanthellae are also the component of coral that give stony or hard coral species their beautiful, vibrant colors.  Zooxanthellae is not a true part of coral; that is in stressful or less than optimal conditions it leaves or expels from the coral tissue. These conditions could include any range of water quality or a taxi the zooxanthellae is exposed too.  This could include but is not limited temperature, pH, dissolved oxygen, salinity, light, and water movement.  When an exodus of zooxanthellae occurs from a coral, the coral becomes a white or bleached color. This of course gave rise to the nickname “coral bleaching”.

Coral decline or die-off is often associated with coral bleaching or coral appearing bleached or less bright. While coral bleaching brings the coral closer to mortality, it does not kill the coral instantaneously.  In other words, the coral survives the bleaching events but it leaves the coral very vulnerable and at high risk of death. While the symbiotic relationship can be re-established, it often takes too long to actually save the coral organism.


Zooxanthellae transplant

 Scientists have been trying to figure out ways to transplant zooxanthellae to bleached corals to return them to health. The obvious first step to doing this is to remove the coral from the stress that caused the bleaching in the first place. In aquarium corals this should be no problem as an experienced aquarist can adjust parameters to best suit the coral. The temperature should be stable, not too high or low and the intensity of light should also be considered (dependent of the species). Other water quality parameters like pH and salinity should also been considered.

However, in the ocean you can see how this causes a major snag. As nice as it would be to adjust water quality on a whim out in the great, blue ocean it is simply not feasible. Often for corals out in the ocean, facing coral bleaching due to stress from water parameters, the only practical way to restore the coral is to transplant it to a place where water parameters can be controlled.

Once the stressors are under control and the coral is in an ideal environment, zooxanthellae must be introduced. Now in the ocean, zooxanthellae is abundant. This is how bleached corals out in the ocean are able to recover in the event that stressors are removed and the parameters for zooxanthellae and corals relationship are met. In aquariums, filtration limits the amount of zooxanthellae in the water to little or none. There are products that include phytoplankton like cultured Symbiodinium. These could work but the next step would be directly introducing healthy zooxanthellae to a bleached coral (think like a transplant) and like many transplants there are different methods.

Testing out the idea

To test out the idea, we follow the trials of an aquarist at Reefaholic who is trying the concept with a carpet anemone. To start, just one tentacle, full of viable anemone zooxanthellae, was removed from a healthy ritteri anemone, and this tentacle was then wrapped in a shrimp to be fed directly to the bleached carpet anemone. This is a great idea since it is common for coral to directly ingest zooxanthellae. Another method being discussed and tested is using a syringe to directly into the organism.

A lot of the testing is being done with anemones since they are a great model to learn more about corals. The jury is still out on whether this technique will work or not but it sure is a great start to proactively stopping coral decline.

References and photos courtesy of: