JOLEAH LAMB was scuba diving on the Great Barrier Reef, with a dazzling palette of colours spread out beneath her. Admittedly, a few of those were pieces of plastic. That was a shame, but she didn’t give it much thought during the long hours she spent cataloguing the reef’s health.
Then her research took her to the waters around Indonesia. There, the plastic problem was far worse, and she started logging it wherever she found it. “Sometimes a fishing line was so entwined in the coral you would cut yourself or hurt the coral if you tried to get rid of it,” says Lamb. Once her notes read: “diaper, gross!”.
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It was only later, back in her lab at Cornell University in New York, that she noticed there was a strange connection between the plastic she recorded and patches of diseased coral. Besides strangling seabirds and poisoning fish, plastic may have another surreptitious way of harming the oceans – and perhaps our health too. Now the race is on to find out exactly how this connection works, in the hope that we can sever it.
Reefs face many threats these days, not least bleaching, which occurs when stresses like heat cause corals to expel the symbiotic algae that live in their tissues. Coral can recover, though, if the algae recolonise. More deadly are a collection of diseases known as white syndrome. They are some of the most serious to have plagued coral recently and they nearly always kill, stripping the coral’s tissue to leave bare skeletons.
White syndrome is a problem the world over, particularly so off Florida. In 2016, a few years after the diseases began hitting coral, marine biology consultant William Precht and his colleagues at the University of Miami and Florida Institute of Technology the disease was “arguably one of the most lethal ever recorded on a contemporary coral reef”.
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It has been tricky to work out what causes white syndrome and how to treat it. We know that in one type of Hawaiian coral, three different species of bacteria, including one named Vibrio owensii, are to blame for one of the diseases. The same symptoms occur in other coral species that are dying elsewhere, but the causes are unknown.
Lamb and her colleagues have now studied 159 reefs around Thailand, Myanmar, Indonesia and Australia. Their results, published a few months ago, showed that when there was no plastic in contact with the coral, about 4 per cent of it had one of six common diseases, including white syndrome. But .
We know that plastic waste is causing a widespread environmental catastrophe in the oceans. Plastic makes up about 70 per cent of the rubbish in the sea, and has turned up everywhere, even in the deepest ocean trenches. In one instance, researchers found 18 tonnes of it washed up on a single small Pacific island. We also know that birds and animals can become entangled in plastic and that it can harm fish that eat it. But Lamb’s work was the first to connect plastic with infectious disease in the ocean.
The hitch-hiker hypothesis
What could explain the link? On one level, the answer might be simple: the plastic could be damaging the coral, allowing microbes easy entry. One of the diseases Lamb studied was skeletal eroding band disease, which occurs when cuts to the coral polyp tissue – easily caused by sharp-edged plastic pieces – become infected. Another condition that Lamb looked at, black band disease, is caused by a hotchpotch of bacteria that thrive in low-oxygen conditions. A coral smothered in a plastic bag would be an ideal home for them.
But Lamb thinks there could be something else going on. That is partly because the incidence of white syndrome was 17 per cent higher in the presence of plastic, and these diseases are not thought to be initiated by cuts or other tissue damage. She also says that the amount of diseased coral did not depend on how much plastic there was in the surroundings, suggesting that the mere presence of plastic was significant. What if, she wondered, the bacteria that cause white syndrome are arriving on fragments of polystyrene cups or water bottles? They could come from far-off waters, hitching a ride across the ocean on plastic.
“Bacteria communities living on plastic are unique – and uniquely dangerous”
In truth, we already knew that bacteria floating free in the ocean can travel huge distances. In May, a team led by Kim Prather at the University of California, San Diego, published evidence that . These droplets travel all around the world and play an important role in forming clouds.
Still, it stands to reason that bacteria living on plastic could be more problematic for coral than the free-living type. Solid surfaces provide a place for nutrients to accumulate so colonies can grow. Plus, plastic is buoyant and notoriously hard to break down. Microbial colonies may well hang around for longer and travel further on plastic than they would on other ocean detritus.
Here’s the rub. Research is beginning to show that the microbe ecosystems that live on plastic – the so-called plastisphere – are unique, and uniquely dangerous. “Although the ocean is full of microbial organisms, plastic waste may be concentrating particular pathogens that infect corals following contact,” says Lamb.
The term plastisphere was coined in 2013 by Linda Amaral-Zettler at the Royal Netherlands Institute for Sea Research. That year, she and her colleagues looked at microbes living on plastic flotsam in the North Atlantic. They found that the communities were quite distinct from those in the open ocean. They also found one gene fragment that couldn’t be pinned on any specific species, but probably belongs to one of several Vibrio bacteria, one of which is the species known to cause white syndrome in Hawaiian corals.
All this supports Lamb’s idea that hitch-hiking bacteria are a problem for coral. Her study is “perhaps some of the most compelling information that speaks to the relationship”, says Amaral-Zettler.
It may not be just coral at risk. In 2016, when researchers at Old Dominion University in Virginia examined microplastic debris collected from Chesapeake Bay, they detected one bacteria species that can cause gastroenteritis in humans and another that can cause blood poisoning. The same year, microbial ecologist Inga Kirstein at the Alfred Wegener Institute in Germany studied plastic from the Baltic Sea and .
It is easy to envisage these tiny hitch-hikers slipping into our food. Over 180 marine species – including things we eat, such as farmed mussels, oysters, clams and many fish – are known to ingest microplastic. In the process, they do not take in just chemicals like hydrocarbons and polymers, but also the microbes that live on these fragments.
We could also be gulping them down when swimming in the sea. It is well established that swimming near waste water outflows makes people more likely to contract stomach illnesses. But in 2016, Anisha Keswani at the University of Stirling, UK, and her colleagues floated the idea that . Keswani seeded plastic bags and bottles with E. coli and found that they still bore colonies of the bacteria three weeks later. “This suggests to me that we definitely should consider this a potential risk to human health,” she says.
Still, the hitch-hiker hypothesis is young and we don’t know for sure whether itinerant microbes really are causing disease. The evidence is most compelling for corals, but even there things are not clear-cut. In Florida, white syndrome is spreading even though the corals are not inundated with plastic as they are in Indonesia. Precht thinks that the syndromes may have multiple causes. “Everybody used to think that coral disease, like with the flu, was from a certain strain of pathogen,” he says. “But with coral it can be many different pathogens working together.”
Coral probiotics
If plastic is indeed exacerbating the problem, at least in some places, are there measures we can take? Karen Bohnsack, who coordinates a reef resilience programme at the Florida Department of Environmental Protection, says her organisation is considering the use of bacteriophages – viruses that infect and kill bacteria. But that is some way off. First, researchers need to identify all the bacteria that can cause coral disease. Then they must find some safe way of distributing suitable phages over wide areas of ocean.
Another potential approach stems from a slightly different way of seeing disease. In this view, the hitch-hikers are not inherently damaging, but become problematic when they outcompete the coral’s “good bacteria”, weakening them. “We are starting to think about coral disease as a loss of microbial diversity,” says Lamb.
That aligns with the thinking that the balance of gut microbes plays a huge role in human health. There’s no clear evidence for this in coral as yet, but if true it points squarely to a solution that might help reduce the risk of infections getting into human food: probiotics. Ocean farms routinely give probiotics to their fish, and in theory these formulations could be tweaked to help suppress the bad bacteria delivered by plastic.
For her part, Lamb is getting back out into the water. She has recently returned from setting up multiple studies – of oysters in Myanmar, clams in Indonesia, and most recently mussel beds near Seattle – to track how much microplastic, and potentially disease, enters the food web. “If mussels have a lot of plastic, we’re eating that,” she says. And these are hitch-hikers that you really don’t want to pick up.
The plastisphere evolves
No one doubts that there is a huge amount of plastic in the ocean. But based on how much we produce, we have found only between a hundredth and a tenth as much as we would expect. Biologist Ricard Solé at Pompeu Fabra University in Barcelona thinks he can explain why.
According to modelling carried out by Solé and his colleagues, there are no known natural processes that can explain the disparity. So in 2017, they proposed as an yet unknown and unproven idea: perhaps bacteria are evolving the ability to eat plastic.
That isn’t as far-fetched as it may sound. After all, ocean plastic is an isolated ecological niche, the sort of place where new traits might evolve. Biologist Linda Amaral-Zettler at the Royal Netherlands Institute for Sea Research surveyed ocean plastic a few years ago, and found the communities of bacteria living on them were quite distinct from those in the ocean. She also suggested they might be living off chemicals leaching from the plastic.
Seen in that light, perhaps the idea that plastic is spreading bacteria that cause disease in coral isn’t so surprising.
This article appeared in print under the headline “The other plastic plague”
