The mysterious culprit behind a deadly starfish disease is not an infection, as scientists have thought.
In contrast, several types of bacteria that live millimeters from the skin of starfish deplete oxygen from water and effectively suffocate animals, the researchers report on Jan. 6 in Frontiers in Microbiology. These microbes thrive when there are high levels of organic matter in warm water and create a low-oxygen environment that can cause starfish to melt into a puddle of drool.
The disease that wastes starfish, causing lethal symptoms such as decaying tissue and limb loss, first gained notoriety in 2013 when starfish living on the U.S. Pacific coast died in massive numbers. Outbreaks of the disease also occurred before 2013, but never on such a large scale.
Scientists suspected that a virus or bacterium could be making starfish sick. This hypothesis was supported by a 2014 study in which it was found that unhealthy animals may be infected with a virus (SN: 19/11/14). But the link disappeared when later studies found no relationship between the virus and dying starfish, leaving researchers perplexed (SN: 5/5/16).
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The new discovery that a boom in nutrient-loving bacteria can drain oxygen from water and cause disease waste "challenges us to think there can't always be a single pathogen or a smoking gun," says Melissa Pespeni, a biologist at the University of Vermont in Burlington who did not participate in the work. Such a complex environmental scenario for killing starfish "is a new kind of idea for (disease) transmission."
There are certainly many red herrings during the hunt for the reason why starfish along the Pacific coast of North America melted into goo, says Ian Hewson, a marine biologist at Cornell University. In addition to the original hypothesis of a viral cause of the disease that wastes starfish – which Hewson's team reported in 2014 in Proceedings of the National Academy of Sciences but later denied – he and colleagues analyzed a number of other explanations, from differences in water temperature even exposing animals to bacteria. But nothing reliably triggered the waste.
Afterwards, the researchers examined the types of bacteria that live with healthy starfish compared to those that live among animals with wasting diseases. “That’s when we had our moment,” Hewson says.
Hewson and colleagues found that the types of bacteria known as copiotrophs, which thrive in nutrient-rich environments, were around starfish at higher-than-normal levels long before animals developed lesions. Bacterial species that survive only in environments with little or no oxygen have also thrived. In the lab, starfish began to waste when researchers added phytoplankton or a common bacterial growth ingredient to the warm-water baths in which microbes and starfish lived.
Experimental depletion of water oxygen had a similar effect, causing injury in 75 percent of the animals, while none succumbed in the control group. Starfish breathe by diffusing oxygen over small external projections called gills of the skin, so the lack of oxygen as a result of the blooming copiotrophs leaves starfish fighting through the air, according to the data. It is unclear how the animals degrade under low-oxygen conditions, but it could be due to a massive cell death.
Although the disease is not caused by a contagious pathogen, it is transmissible in the sense that dying starfish generate more organic matter that stimulates bacteria to grow in nearby healthy animals. “It’s kind of a snowball effect,” Hewson says.
The team also analyzed starfish tissues that had succumbed to the 2013 mass extinction, which followed a large algae bloom on the west coast of the United States, to see if such environmental conditions could explain that outbreak. In the fast-growing appendages that help them move, the starfish that perished had high amounts of a form of nitrogen that was in low-oxygen conditions, a sign that those animals died from lack of oxygen.
The problem may get worse with climate change, Hewson says. "Warmer waters can't have as much oxygen (compared to colder water) just because of physics." Bacteria, including copiotrophs, also bloom in warm water.
But identifying the likely cause could help experts better treat diseased starfish in the lab, Hewson says. Some techniques include increasing oxygen levels in a water tank to make gas more readily available to starfish or getting rid of extra organic matter with ultraviolet light or water exchange.
“There is still a lot to discover with this disease, but I think (this new study) takes us a long way to understand how it occurs,” Pespeni says.