Virus harnessed to destroy harmful algal blooms

By Will Jarrett

Along the Western coast of Japan, it is not uncommon to see swathes of ocean turn rust red.

This discoloration, known as the “red tide,” is caused by several species of algae growing in massive blooms. These blooms suck up oxygen, block out light, and clog fishes’ gills so they cannot breathe. Many release biotoxins that kill marine life and some can even cause severe health problems in humans.

For decades, marine biologists across the globe have been trying to find ways to fight harmful algal blooms, with limited success. But a recent study published in Aquaculture reveals a possible solution. Researchers have managed to kill off a bloom of Heterocapsa circularisquama, one of the main culprits behind the Japanese red tide, in its early stages of growth – by spraying it with a virus.

“It was a really huge trial,” said lead researcher Natsuko Nakayama of the National Research Institute of Fisheries and Environment of Inland Sea. Conducted on the saltwater Lake Kamo, the field test was the culmination of several years of smaller trials and nearly as long spent convincing locals that the method was safe.

Blooms of H. circularisquama tend to die off when they become infected by a virus called HcRNAV, which occurs naturally in the marine environment. As the algae dies, it sinks and deposits the virus on the seabed. Nakayama’s innovation is to use this virus-infused sediment to speed up the natural process of infection. Her team scooped up the sediment, froze it to kill off any unwanted marine life (but not the cold-resistant virus), and then sprayed it on a bloom as it began to develop. This allowed the virus to attack the algae earlier than it would have done in the wild.

The idea of controlling algae with viruses has actually been around, at least in theory, since the 1960s, but this is the first time that naturally occurring viruses in sediment have been used. Nakayama and her team hope that this method will work better at scale than using lab-grown viruses, because there is no need to painstakingly culture them. The new approach may also be more acceptable to a public that might balk, understandably, at spraying lab-grown viruses into the ocean.

The results of the experiment seem promising. Five days after spraying, the size of the test bloom was reduced by 99%.

Two small 'cages' from the Lake Kamo experiment.

The "cages" in Lake Kamo used to test spraying HcRNAV | Courtesy of Natsuko Nakayama

The need for effective control of algal blooms is becoming increasingly urgent all over the world, according to many experts. Larry Brand, who studies algal ecology at the University of Miami, said that the red tide off the west coast of Florida has “increased about fifteenfold” in the last 30 years.

The main driver of the increase is a higher concentration of nutrients in the water – fertilizer from farming and sewage from an expanding population means more for the algae to feed on.

Climate change is exacerbating the problem. “With the increase in temperature, there is more moisture in the air, and so we are getting more intense rainfall,” said Brand. “With that increased rainfall, you are pushing the nutrients and sewage into the coastal waters faster and faster.”

The warming of the oceans may also increase the algae’s growth rate and help dangerous blooms thrive. Broadly speaking, toxic algae tend to prefer warmer waters.

An increase in blooms could be ecologically disastrous. Algal blooms can cover hundreds of miles, and they can kill seagrass (an excellent carbon store), fish, seabirds, and mammals like manatees in huge numbers. They can ravage aquaculture as well; multiple clam farms around Florida’s West coast were forced to close during a bad bloom in 2018 and have not reopened.

Some blooms can even be a serious problem for human health. Studies suggest that high rates of neurodegenerative diseases may be related to a toxin produced by a species of green-blue algae that enters the food supply via shellfish. In China, studies have linked freshwater algal blooms to a spike in liver cancer. And the red tide has made people sick in Florida too.

“Officially, they say that no-one has died from it,” said Brand. “I’ve talked to a number of people, however, and they will say something like, ‘My grandmother had a lot of heart problems and asthma and all that, and then the red tide came along and she died.’”

“If you’re already in bad shape in your health, I can certainly see how you could be affected by the red tide, by these neurotoxins. It could put you over the edge.”

A mass fish death in West Florida.

Mass fish deaths are not uncommon on the Florida coast | Courtesy of Larry Brand

Despite the many problems marine algal blooms cause, “there are very few tools in the toolbox” to fight them, said Leanne Flewelling of the Florida Fish and Wildlife Conservation Commission.

One popular control method in China and South Korea is spraying clay to smother the algae and pull it to the seafloor, but this can be prohibitively expensive and the environmental impacts are not clear. “If your species is a toxin producer, the clay is bringing all those toxins down,” said Flewelling. “If you are doing this over a shellfish farm, where there is stuff at the bottom to harvest, that is a concern.”

There is also uncertainty around what happens to the sunken algae. “Do they die when they’re down there? Or do they get released after some period of time?” Flewelling asked. “Every species probably reacts a little bit differently.”

Nakayama’s technique aims to minimize these issues. The HcRNAV virus kills the algae quickly and does not drag toxins down to the seabed. It is selective, infecting just one species of algae, so spraying is unlikely to have unintended ecological impacts. And only a small amount of sediment needs to be sprayed before the virus can start spreading rapidly on its own, keeping costs low.

Even so, Nakayama said that the method has limitations. “HcRNAV can only infect the Heterocapsa circularisquama. They don’t infect other algae,” she said.

Further research is required to see if a similar technique could work for other species of algae, such as those found off the Florida coast. It is also unknown how the sediment spray will fare in choppy coastal areas, in different temperatures, and at a larger scale. Trials have only moved into open water in the last couple of years and they were conducted in a small area on the quiet waters of Lake Kamo.

“Now I am looking for other viruses for other algae,” said Nakayama, who is pushing forward in her research and hopes to be able to control blooms across the whole of Lake Kamo within two years. “Baby steps!”

Header photo credit Linda Xu