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Compared to farms on land, raising crops underwater comes with unique challenges. For one, aquafarmers can’t spray pesticides to help their crops deal with disease. “Aquafarmers are busted when a disease hits,” says biologist Florian Weinberger, of the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany. “Any pesticide would immediately wash away.” Without any way to prevent their spread, parasites and infections can take a huge toll.
Now, researchers are working on a new way to protect farmed seaweed: by stimulating algal immune systems so the organisms can defend themselves.
Since seaweed is gaining popularity—as food, as biofuel, or as an additive in many products from foods to cosmetics to pharmaceuticals—vast underwater farms stretch for dozens of kilometers along coastlines in Asia, and increasingly in Europe and North America. Globally, algae farms grow 30 million tonnes annually, worth almost US $7-billion, with China being the biggest producer. By 2050, experts predict that production might reach 500 million dry tonnes, potentially replacing up to 20 percent of fossil fuels.
Like most other living things, algae employ an immune response to keep themselves healthy. In land plants, biologists have sometimes managed to harness this innate power using an approach that resembles the ways vaccinations boost human immunity. By exposing a plant to disease residues, such as pathogen-produced proteins, its natural self-defenses become activated, allowing it to fend off a real attack more easily. Several so-called “resistance inducers” are already being marketed to protect crops such as garlic, melon, or tobacco.
But can the same approach work for seaweed?
To study this, Weinberger collaborated with scientists at the Ocean University of China in Qingdao. Weinberger and his team exposed commercially cultured kelps to a substance that the algae themselves produce. When the seaweeds are attacked by pathogens, they naturally release non-toxic saccharides, or sugars. Nearby algae sense these chemicals, which prompts them to boost their own defenses by, for example, producing proteins and other compounds that are antimicrobial and help the algae fight off bacteria and fungi.
The scientists found that just one weekly dose of a particular saccharide reduced losses to disease. The seaweeds’ heightened immune responses also made them less susceptible to parasitic microalgae.
“The approach is very interesting, and I appreciate the environmental friendliness,” says Claire Gachon, a molecular plant pathologist with the Scottish Association for Marine Science, who was not involved in the study. “The intensification of algal farming is rapidly increasing the economic burden for farmers. I was told that diseases have become so bad in Korean farms that the national TV broadcasts their status.”
So far, the resistance-inducing treatment works best in algae seedlings, which are often kept in crowded nursery ponds. With fewer destroyed by pathogens, treated algae grew about twice as dense as untreated ones.
Another pleasant surprise was that Weinberger’s resistance-induced seedlings grew just as fast as their untreated peers. It was unexpected because scientists think organisms normally refrain from keeping their immune defenses constantly ramped up because this uses energy that could be used for other purposes—such as to grow. “I was astonished,” Weinberger says.
Mature algae also fended off microorganisms better when treated, but they attracted more barnacles and other pests, too.
Weinberger suspects that revving up the seaweeds’ immune systems changed which bacteria grew on their surfaces. The stronger immune response may have caused the seaweeds to lose the beneficial microorganisms, such as those that protect them from barnacles, as well as harmful ones.
“Apparently, we overshot our goal,” Weinberger says.
Weinberger hopes to fine-tune the approach by finding substances that will lead the algal immune system to just target harmful microbes. But first, researchers will need to better understand the seaweed microbiomes themselves.