To combat the spread of a mysterious disease affecting coral in the Caribbean, researchers are smearing antibiotic goo on thousands of corals by hand. Others are now dosing corals with probiotics with the hope of bolstering their resilience to the disease.
Stony coral tissue loss disease (SCTLD) was first spotted off the coast of Miami, Florida, in 2014 and scientists have been struggling to understand the mysterious pathogen ever since. The disease infects only hard corals 鈥� the reef-building kind with rigid calcium carbonate skeletons.
When a coral is infected with SCTLD, it sloughs off chunks of its colourful soft tissue, exposing its white skeleton underneath. The infection can spread to and kill the surrounding reef within weeks, leaving a ghostly coral graveyard in its wake. In the eight years since its discovery, SCTLD has spread across reefs from Florida to Grenada with mortality rates close to 100 per cent in some coral species.
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鈥淚t sounds very morbid, but the animal鈥檚 tissue is melting away, essentially,鈥� says at the University of North Carolina Wilmington. 鈥淚t鈥檚 already changing the entire Caribbean.鈥�
Researchers don鈥檛 know if the pathogen responsible for SCTLD is a bacterium or a virus, which has made developing treatments for the disease arduous. Marine ecologists have been testing different remedies on corals in both aquarium tanks and the wild with varied success.
One promising method is applying an antibiotic paste directly to the corals, but the process is time and labour-intensive. Using a caulk-gun-like tube, scuba divers smear the thick, amoxicillin-laced paste on the edge of a lesion with the goal of halting the disease鈥檚 spread. Even though amoxicillin targets bacteria, it may still be effective at slowing the progression of lesions caused by a virus.
A on the effectiveness of the antibiotic paste found that across five different coral species in Florida, the treatment stopped the progression of the disease in 67 to 90 per cent of corals depending on the species. A found the amoxicillin treatment healed 95 per cent of coral disease lesions, though some needed follow-up treatments.
鈥淚t鈥檚 like a cauterisation of the tissue and the disease at that point in time,鈥� says at Nova Southeastern University in Florida.
Since 2018, more than 21,000 corals in the state have been treated with amoxicillin paste.
But one major concern about using amoxicillin paste in the ocean is inadvertently creating antibiotic resistance in other marine animals.
That led Ushijima and his former colleague, Valerie Paul, at the Smithsonian Marine Station in Florida to look at another potential solution: probiotics that could make corals more resilient to the disease.
They started by isolating bacteria growing on corals that appeared most resistant to infection, and then smeared a paste containing those bacteria onto both sick and healthy corals to give them a boost. In addition to the probiotic paste, the researchers are also testing a method where they cover corals in a clear plastic garbage bag. They then inject the probiotic slurry into the tented cover, dosing the coral with bacteria over the next 2 hours before removing the bag.
鈥淚t鈥檚 kind of like creating our own little mini aquarium,鈥� says Paul.
The researchers started field-testing the probiotic paste in early 2020, but their work was stalled by the covid-19 pandemic. But in the dozens of corals they have treated so far, Paul says they have seen the disease slow or stop in some cases. Until the researchers complete more field tests with probiotics, amoxicillin paste will remain the go-to treatment.
鈥淭here鈥檚 some controversy as to whether you should be using antibiotics in the natural environment,鈥� says Walker. 鈥淏ut the alternative is to watch everything die.鈥�
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