Like a priceless painting, the beautiful blue and green swirl in a lake or pond presents a look-don’t-touch kind of situation. This is the work of proliferating cyanobacteria, also known as blue-green algae, which produce toxins that are toxic to humans and other animals, especially when blooms corrupt freshwater supplies. These toxins, which the microbes developed to deter herbivores, are linked to ALS and Parkinson’s disease, plus muscle paralysis and liver and kidney failure. One of the toxins, anatoxin-a, is known as Very quick death factorin case you doubt that toxicity.
It seemed a shame, then, that a highly nutritious fern called Azolla—which eats green carpet ducks on ponds—long ago struck up a deal with a species of cyanobacteria, an “endocyanobiont.” If they live in the fern, the microbes get shelter and in return provide the plant with essential nitrogen. Lately, scientists have been campaigning to turn the fast-growing Azolla into a food of the future. Others envision it becoming both a sustainable biofuel and a fertilizer that captures carbon. But these ideas probably won’t get very far if the cyanobacteria living inside end up being highly toxic.
A new paper suggests that Azolla may one day find its way to plates: An international team of researchers has discovered that endocyanobiont are no typical cyanobacteria. “The cyanobacteria living in Azolla don’t produce any of these toxins, and they don’t even have the genes needed to create those toxins,” said Daniel Winstead, a Penn State research technologist and co-author of the paper. . “So it removes one of those barriers to using it as food or even animal feed.”
This is not to say that one should find a local pond, discard the Azolla and eat it by the handful. Other research groups must confirm that Azolla is fully non-toxic and safe for consumption before an industry can develop and produce the plant for food. Winstead’s previous research found that while some species of Azolla are high in harmful polyphenols, a species native to the southeastern United States called Carolina Azolla has much lower levels that are further reduced to safe amounts by cooking. Azolla is also high in protein and nutrients such as potassium, zinc, iron and calcium.
Azolla and the cyanobacteria that host it have developed a mutually beneficial relationship together. Other cyanobacteria species that float in the open synthesize toxins to ward off hungry fish. “For the cyanobacteria to live inside the Azolla, it can’t produce those toxins or it will also kill the plant,” Winstead said. “So at some point it no longer had those genes, and that’s unique among cyanobacteria.”
In exchange for providing the microbe housing, the Azolla gets an extremely valuable resource: nitrogen. Plants need that element to thrive, but not many species can extract it from the atmosphere themselves. So-called “nitrogen fixers”, such as beans and clovers, rely on bacteria in their roots to break down process nitrogen to something the plant can use to grow. The endocyanobiont does the same for Azolla, helping to fuel the growth that allows the plant to double its biomass as fast as every two days.
Winstead said that some small farmers already use Azolla as fertilizerand now that the cyanobacteria are confirmed to be non-toxic, perhaps the technique can spread. With that natural source of nitrogen, farmers would be less dependent on synthetic fertilizers, of which the production and use syringes greenhouse gases and pollute rivers and lakes. Azolla can also be used as animal feed, like some farmers already do if they cannot afford traditional feed for cattle and poultry.
In the 1980s and ’90s, farmers in China succeeded in exploiting Azolla for both purposes. They grew Azolla in their flooded rice fields, added fish that fed on the plants, and then ate the fish. But it was a difficult process. It was labor intensive to grow, as farmers had to separate the fish before applying herbicides or pesticides. When the fields drained, crews worked the Azolla into the soil as fertilizer, but this was also labor intensive.
While Azolla can fix its own nitrogen thanks to its cyanobacteria, it needed applications of phosphorus to actually grow in rice fields. “There is no free lunch,” said Jagdish Ladha, a soil scientist and agronomist at the University of California, Davis, who was not involved in the new paper. Those farmers in China have switched to using cheap synthetic fertilizers instead. But the idea behind industrializing the production of Azolla would be to produce the plant on a larger scale and then conveniently package it as fertilizer or animal feed.
Beyond its potential in farming, Azolla could also become a biofuel, according to Winstead, just as corn has been used to make biodiesel. That fuel will be close to carbon neutral: As the plant grows, it sequesters carbon; burning the biofuel will then release that carbon back into the atmosphere. By incorporating Azolla into soil as fertilizer, farmers will put even more carbon into the soil.
Humans can also shape Azolla as they have modified other crops such as wheat and corn, selectively breeding the most desirable traits, such as larger grains. “There’s a lot of potential for Azolla to go through that process,” Winstead said, “whether it’s creating a variation of Azolla that tastes the best, or it’s a variation of Azolla that has the most vitamins or the most protein has, or perhaps the best nitrogen-fixing ability.”
