Post by Ben Cook, Biology undergraduate student

Corals are in danger

All around the world we are hearing about how climate change is affecting various environments and creatures. One such environment that has been getting a lot of attention is coral. If you’ve ever gone snorkeling, you are familiar with coral – they are what form the reefs along the coast. Coral is important because it provides a habitat for many different types of fish, which we to see when we go snorkeling.

Coral reefs are bleaching

The same reef in American Samoa before, during, and after a coral bleaching event. Photographed by The Ocean Agency / XL Catlin Seaview Survey / Richard Vevers. Credit: Yale Climate Connections.

Coral reefs around the world are dying at unprecedented rates, partly because of coral bleaching. When coral bleaches, the coral expels all of the photosynthetic algae, or zooxanthellae, that it has inside of it. These phytoplankton live inside of the coral and provide energy for the coral through photosynthesis, while the coral gives the algae a safe environment and nutrients for photosynthesis. The reason that these symbiotic phytoplankton are being expelled is because the water surrounding the coral is too warm, and the stressed coral begins bleaching. When these phytoplankton are purged, the coral loses its main source of food, which leaves them more susceptible to disease. Many people believe that this kills the coral, but the coral is still alive, and if it can recover the zooxanthellae that it purged – or new zooxanthellae floating in the water can recolonize the coral – they will recover. However, reefs all over the world are not recovering, leaving the reef fish that we so love without a home, forcing them to find another area to can live in.

Coral reefs are more diseased

A coral colony with the black band disease showing dead coral, the disease lesion (or wound), and live coral, photo credit: C. Runyon (UH). From Hawaii Department of Land and Natural Resources.

Increased temperatures leave the coral vulnerable to the transmission of illnesses throughout populations, including as white band disease which is becoming more virulent. Coral populations often form a large connected body, making disease transmission easy and causing massive damage that often kills the coral.

To combat this problem, scientists have been looking into cures for diseases affecting coral populations. However, the most effective approach recommended by scientists is to prevent the illness from even infecting the coral in the first place by protecting the coral and making sure that it is healthy enough to fight off any illnesses.

The most common form of treatment for coral after being infected is to fragment the coral bodies and get rid of diseased fragments. This is hard to do on a large scale across many reefs, but can be a way of keeping coral from spreading if infected fragments are caught early and removed before the disease spreads too far. This is similar to amputating diseased limbs to stop the spread of an infection throughout the human body, but generally it is a last-resort option. Without a better solution for coral though, this is all that can be done.

Researchers are also looking into the effects of introducing healthy pro-biotic bacteria into the bodies of coral. The probiotic bacteria could possibly interfere with the pathogens using cell to cell signaling to exclude the pathogen and increase the production of antimicrobials, which kills or inhibits the growth of microorganisms and causes little to no damage to the host they are introduced in. Researchers suggest inoculating bodies of coral with beneficial bacteria in order to protect them from possible pathogens.

The most recent option being studied by researchers is phage therapy, which “uses lytic bacteriophage viruses to highly selectively target specific coral pathogens.” Basically, they introduce viruses that target pathogens affecting coral to kill that pathogen without harming the coral, similar to a vaccine in human beings. While this method is very effective in treating some pathogens in coral, it can only be used when the causes of the illness have been determined. It also only protects against specific diseases, so if another pathogen is introduced into the coral, the phages will only defend against the original pathogen. While there are many options being studied, it is difficult for scientists to advise managers on how to treat coral without a greater understanding of diseases affecting coral and how these treatments will work with the coral. If more is not done to understand how to treat these diseases affecting coral, or how to prevent these illnesses, we could see large losses of coral in the near future.