Investigating Stony Coral Tissue Loss Disease
How have FWRI scientists been investigating SCTLD back in the laboratory?
What does the disease look like on a microscopic level?
Coral Tissue Sampling
When it became clear that the disease outbreak was spreading rapidly past its starting point in Miami, scientists took tissue samples from infected corals in southeast Florida in 2015. Samples continued to be collected along Florida’s Coral Reef as the disease spread north and south. These samples allowed FWC Fish and Wildlife Health scientists to investigate what is causing the disease and how coral tissue changes in response to it on a cellular level. To collect the tissue samples, staff used a 1-inch diameter stainless steel biopsy corer (see photo). After coring, the punched holes made in the coral are plugged with epoxy to reduce harm to the coral. Samples are then promptly fixed (that is, preserved) with formalin base fixative.
Coral Tissue Processing
The samples are then brought back to the laboratory where they are photographed and analyzed. Observations are made about the condition of the diseased corals, and if there are any abnormalities or external parasites. Because the disease impacts the soft tissues, scientists need to remove the coral skeleton to be able to better see the tissue. Decalcified tissues with their skeletons dissolved are ready for “routine histology”, a process used to cut very thin slices of tissue, which are then stained on slides and viewed under a light microscope. Different special stains can be applied to the tissue slide to help see specific cells, tissue types, or organisms (like bacteria).
Photo: Diseased Montastraea cavernosa. Note: scattered black spots found at the exposed white skeleton are ciliates (a type of parasite).
Light and Electron Microscopy
The slides are then examined under a light microscope to see if there are any pathogens or pathological changes (i.e., unusual cell damage). Tissues are also prepared for transmission electron microscopy. The electron microscope can be used to magnify coral cells up to 100,000 times. Scientists can look inside the cells to see if there are any changes to cell structures (called organelles), or if any viruses or other microbes are present.
Healthy Polyps and Diseased Coral
The photo on the right is a close-up view of a typical stony coral tissue loss disease lesion. Within the box are unaffected polyps next to the disease boundary. How the exposed skeleton looks is an indication of how recently that part of the coral died – the stark white parts of the skeleton are the most recently dead. The skeleton covered by a thin green band of algae died at least a few weeks prior. The center of the lesion is where the mortality started and is overgrown with turf algae and other organisms, obscuring the features of the skeleton.
This research has shown that corals affected with stony coral tissue loss disease first show symptoms internally, with no outward signs of the disease. The beneficial algae that live in coral tissues and gives them their color – zooxanthellae – are the first to succumb to the disease. Coral cells nearby the affected zooxanthellae then also start to rupture, disintegrate, and die. When the affected tissue and cell death reaches the outer edge of the polyps and surface tissues, it can be seen by the naked eye as it creates the tissue loss lesions you see in the photos above. Although we now know what the disease causes at the cellular level, scientists are still working to determine the overall cause and mechanisms of the disease.
Do bacteria play a role in stony coral tissue loss disease?
As research into the disease progressed, some evidence pointed to a bacterial culprit behind the disease. In a multiagency study, scientists from the University of Miami, Mote Marine Laboratory, and the FWRI Coral Research Program wanted to learn what role, if any, bacteria are playing in the disease outbreak. They identified bacteria taken from samples of healthy and diseased corals, water, and sand from three different zones. These zones included where the outbreak was active (epidemic), where the outbreak was past its peak (endemic), and areas where the disease had yet to arrive.
They found different types of bacteria depending on the zone and coral species sampled. They also found that some bacteria in the orders Rhodobacterales and Rhizobiales were always higher in abundance in diseased coral tissue in all coral species studied. These bacteria were also present in higher numbers in water and sand samples from diseased reefs. Although these findings are intriguing, Rhodobacterales and Rhizobiales may not be the main cause of the disease. For example, they may be taking advantage of an already sick coral and making it worse. Only more research will tell!
With more answers comes more questions…
How exactly are the bacteria impacting the diseased corals? How are they spreading through the environment? Are they causing the disease or simply taking advantage of already weakened corals? How did this outbreak start in the first place? Researchers are working on answering these and many more questions regarding this disease. FWC scientists continue to process and evaluate samples collected between 2016-2019, as well as working with partners to continue to monitor, study, and treat the disease. Please see the Publications page to read more about the science behind stony coral tissue loss disease.