New research has revealed that the deadly stony coral tissue loss disease that is ravaging the hard-coral population around Florida, Grand Cayman and other Caribbean islands is most likely caused by a virus, rather than by bacteria, as had been previously thought.
A report, written by Thierry M. Work, a wildlife disease specialist with the US Geological Survey’s National Wildlife Health Center, states that SCTLD is probably a viral disease that is infecting the tiny plant-like organisms, called Zooxanthellae, which have a symbiotic relationship with the hard corals which are their hosts.
Stony coral tissue loss disease was first documented in 2014 near Miami harbour in Florida. It has been found to impact some 25 species of hard coral, such as mountain and star coral, and has led to the functional extinction of at least one species – pillar coral – in Florida.
The disease was first spotted in Cayman last summer, just off Rum Point. Since then, it has spread in both directions along the coastline, and stretches from North West Point in West Bay to the edge of Frank Sound, as well as at isolated dive sites off Seven Mile Beach and Smith Cove.
According to the report, the disease was assumed to be caused by bacteria, based on three factors. Firstly, the presence of different bacteria found in lesioned versus normal tissues; secondly, the apparent spread of the disease among colonies; and, thirdly, cessation of the spread of lesions in individual colonies of coral treated with antibiotic paste.
However, researchers found that microscopic examinations of tissues of corals affected with SCTLD have not revealed the presence of bacteria associated with tissue death.
Instead, after examining microscopic evidence of dead and dying coral cells, as well as the symbiotic organisms, researchers found that in colonies affected with SCTLD, there was a breakdown in the host cell/Zooxanthellae symbiosis.
“It is unclear whether host cells die first leading to death of Zooxanthellae or
vice versa,” the report states. “Based on microscopy, hypotheses as to possible causes of SCTLD include infectious agents not visible at the light microscopy level or some sort of toxicosis, perhaps originating from Zooxanthellae.”
The researchers then examined corals affected with SCTLD, using electron microscopy which allowed them to see details not visible on light microscopy. Electron beams have a shorter wavelength than light beams, thus allowing electron microscopes to produce higher-resolution images than standard light microscopes, and reveal details at a subcellular level not visible on light microscopy.
This closer examination revealed that the Zooxanthellae in SCTLD-affected corals appeared similar to viruses found in infected plants.
“Based on these findings, we hypothesize that SCTLD is a viral disease of Zooxanthellae leading to coral host death,” the report states.
It added, “Efforts to confirm the presence of a virus associated with SCTLD through other means should be a priority.”
Tammi Warrender, the lead coordinator in the Cayman Islands Department of Environment’s SCTLD Response Project, said the findings may show why SCTLD seems to slow down during periods of coral bleaching when the Zooxanthellae are temporarily separated from the coral.
“This is an important puzzle piece and big step towards finding future treatments for this disease,” she told the Cayman Compass.
Asked why antibiotics, which are used to treat bacteria rather than viruses, seem to halt the spread of lesions on corals if they are infected with a virus, report author Thierry Work told the Compass, “There is no evidence at the microscopy level that bacteria are involved with SCTLD in Florida. As to why SCTLD lesion progression is sometimes halted by antibiotic application, I do not have an answer to that. No one has made the link between actually killing bacteria and stopping of lesions.
“Antibiotics, like many pharmaceuticals, have actions above and beyond their intended target. For instance, some deworming medicines used to treat animals for worms also have important effects on the immune system of the animal being treated. Perhaps that is what is occurring in corals.”
The researchers noted that the logical next steps would be to confirm the presence of viral infections in Zooxanthellae by growing and propagating the virus in laboratories, to allow for a better understanding of the virus and the host response, and eventually to “possibly setting the stage for management options and interventions”.
SCTLD manifests on affected corals as white lesions, as the cells die. The only weapon in the arsenal against the disease is an antibiotic paste that divers have been applying to the infected corals. While the antibiotic paste can stop the spread of the existing lesion, it does not prevent new lesions from appears on a coral.
It is still unknown why SCTLD suddenly showed up in 2014, but the researchers suggested that perhaps the virus had been present historically in Florida, but “somehow environmental conditions have changed sufficiently to allow it to flourish. For instance, climate change is expected to significantly alter the dynamics and spread of plant pathogens, so conceivably, the same could apply to Zooxanthellae.”