Reviewed by Lexie CornerNov 15 2024
According to a study published in Frontiers in Marine Science, researchers found that a mixture of chlorine and cocoa butter paste effectively controls the disease that causes stony coral tissue loss, offering an alternative to antibiotic treatments.
The study’s co-authors Argel Horton and Laura Arton apply the chlorine paste to a large coral (Orbicella annularis). The bright white area is where the treatment has already been applied. Image Credit: Dr. Graham Forrester.
Diseases that cause tissue loss are common on coral reefs. While treatment options exist, their effectiveness varies, and some can have negative environmental impacts.
This non-antibiotic therapy could represent the first step toward improving reef environmental conditions, potentially allowing corals to better combat infections on their own in the future.
In the tropical Atlantic, stony coral tissue loss disease (SCTLD) is particularly severe due to its rapid spread, high infection rates, and significant mortality. Antibiotics are the most commonly used treatment to prevent the spread of SCTLD, but they come with drawbacks, including increased antibiotic resistance.
An international team of researchers has now investigated whether an antibiotic-free treatment for diseased corals could help reduce antibiotic contamination in the oceans while improving coral health.
Our study shows that direct treatment of SCTLD can help corals survive a disease outbreak and highlights that active management of disease is possible in the field. Antibiotic pollution is a problem worldwide, so we are working to develop a non-antibiotic treatment that would slow down tissue loss diseases.
Dr. Greta Aeby, Study Senior Author and Researcher, Qatar University
Antibiotics vs Chlorine
The researchers assessed the effectiveness of several treatments by applying them to diseased corals on Horseshoe Reef, near the British Virgin Islands. Some corals were treated with the antibiotic amoxicillin, while others received a paste made from chlorine and cocoa butter.
Aeby added, “In this mix, the active ingredient is sodium hypochlorite, an antiseptic commonly used to kill bacteria or viruses. The chlorine powder we used in our treatment is the same used to kill germs in swimming pools. The cocoa butter was just the delivery mechanism allowing us to spread the chlorine on the coral lesions.”
The researchers applied both therapies directly to the corals, returning to the reef every four to five weeks to assess and document the lesions and reapply the treatments as necessary. After approximately 80 days, the median percentage of tissue destroyed was 17.6 % in the chlorine-treated colonies and 1.7 % in the amoxicillin-treated colonies.
Environmentally Friendly
While the antibiotic treatment was more effective at containing SCTLD, the researchers suggested that using chlorine could help reduce undesired side effects, such as the growth of drug-resistant bacteria. They noted that antibiotic-resistant bacteria not only harm corals. “Any organism – crabs, fish, even humans – in that same environment has a higher risk of encountering bacteria that are now antibiotic-resistant,” says Aeby.
In contrast to the environmental impact of antibiotics, the chlorine and cocoa butter paste mixture biodegrades quickly, with the chlorine naturally deactivating within a day.
Another advantage is its lower cost; the ingredients are readily available at drug and hardware stores.
The antibiotic paste is not only tedious to produce, it also is often too expensive for conservationists, who operate with minimal funding on Caribbean island.
Argel Horton, Marine Biologist, Ministry of Environment, Natural Resources and Climate Change, British Virgin Islands
Protecting Oceans Vital to Coral Health
The researchers noted that not all corals responded equally to the treatments, and several coral species from different locations were not included in the study. They anticipate that future research will explore the effectiveness of various treatment methods in other regions.
“Disease treatment can help manage a disease, but it will not remove the disease from the coral populations,” Aeby stated.
While direct treatments can reduce pathogen load in the environment, treating coral colonies individually, as done in this study, is not a long-term solution and is not practical on a large scale.
Aeby concluded, “The best strategy would be to improve environmental conditions so that corals have a better chance of fighting disease themselves. This includes cleaning up water pollution and rebalancing the ecosystem.”
Journal Reference:
Forrester, G. E. et. al. (2024) The relative effectiveness of chlorine and antibiotic treatments for stony coral tissue loss disease. Frontiers in Marine Science. doi.org/10.3389/fmars.2024.1465173