Besides strangling seabirds and sea turtles, poisoning fish and clouding the waters with unsightly debris plastic, pollution also stresses coral through light deprivation, toxin release and anoxia—and by giving pathogens a foothold for invasion.
Drifting plastic makes ideal vessels for carrying infectious disease across vast stretches of water. Common household plastic items made out of polypropylene has been shown to become heavily inhabited by bacteria—including those associated with the globally devastating group of coral diseases known as white syndromes—plus, it is buoyant and notoriously hard to break down.
When plastic meets coral
Cornell researchers surveyed 159 coral reefs in the Asia-Pacific region to assess the influence of plastic waste on disease. According to their estimates, about 11.1 billion plastic items were entangled in the reefs across the region. When plastic debris meets coral, the scientists found the likelihood of disease increases 20 times—from 4 to 89 percent.
Structurally complex corals are eight times more likely to be affected by plastic, suggesting that microhabitats for reef-associated organisms and valuable fisheries will be disproportionately affected. The more spiky the coral species, the more likely they were to snag plastic.
One of the diseases studied, skeletal eroding band disease, occurs when cuts into the coral tissue—something that is easily caused by stringy or sharp-edged plastics—become infected. Another condition, black band disease, is caused by bacteria that thrive in low-oxygen conditions such as under a plastic bag smothering a coral.
What researchers noted was that the prevalence of disease did not depend on how much plastic there was in the surroundings, but merely whether plastic was present. This was a clue that perhaps the bacteria had hitched a ride across the ocean on fragments of water bottles or polystyrene cups.
Dutch researchers studying microbial communities on plastic flotsam found drifting in the North Atlantic found that these communities were quite distinct from those normally found in the open ocean. More importantly, they also found traces of genes from bacteria linked to white syndrome in Hawaiian corals.
Furthermore, ongoing research is beginning to show that the microbial ecosystems are unique and uniquely dangerous. While the ocean is full of all sorts of bacteria, plastic waste may be concentrating pathogens that infect corals on impact.
Plastic levels on coral reefs correspond to estimates of terrestrial mismanaged plastic waste entering the ocean. Plastic waste management is critical for reducing diseases that threaten ecosystem health and human livelihoods.
The scientists forecast that by 2025, plastic going into the marine environment will increase to roughly 15.7 billion plastic items on coral reefs, which could lead to skeletal eroding band disease, white syndromes and black band disease.
This study demonstrates that reductions in the amount of plastic waste entering the ocean will have direct benefits to coral reefs by reducing disease-associated mortality.
— Joleah Lamb, Cornell University