Aquatic plants turn pollutants into green products

With tiny leaf-like fronds, and the smallest flower and fruit in the plant world, duckweed may sound like delicate botanic specimen. However, this bright green denizen of ponds across Asia teams up with an army of symbiotic microbes to supercharge its robust growth.

Researchers at the Duckweed Holobiont Resource & Research Center (DHbRC) of the Faculty of Science, Kasetsart University in Bangkok, Thailand, now plan to harness the plant for green economic development.

Helped by microbes, duckweed nourished by factory wastewater and carbon emissions could be harvested for use in sustainable bioproducts including protein-rich feeds, green fuels and biodegradable plastics, recent research suggests.

For more than ten years, Masaaki Morikawa, an environmental molecular biologist at Hokkaido University, Japan, has explored how duckweed can contribute to a greener world. The object of his study has been the duckweed ‘holobiont,’ the symbiotic organism made up by the plant and its microbiome.

Just as humans have a microbiome, a vital colony of bacteria in the gut and on the skin, so too are plants colonized by microorganisms. As a small plant with a high relative surface area, duckweed is particularly heavily influenced by the microbes colonizing its surfaces. Morikawa’s research shows that the growth of the tiny aquatic plant may be enormously amplified by these even smaller organisms.

Making a splash

The DHbRC has been established to create a biobank of duckweed holobionts to further explore the plant’s green economy credentials. The centre was founded through a cooperative research agreement between the Thai and Japanese governments signed in 2021 and includes scientists from 19 Thai and 8 Japanese research departments. It was proposed by Morikawa and is part-funded by Japan’s Scientific and Technology Research Partnership for Sustainable Development (SATREPS) Be-HoBiD programme of Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA).

The duckweed holobiont could be “an ideal technology” for Thailand’s budding bio-circular-green economy, says Morikawa. In the right climate, the holobiont grows well all year. “It has potential applications in nutrition, bioremediation, biofuel and bioplastics production,” he says.

In Thailand, people traditionally eat a duckweed species called Wolffia globosa. It is easy to grow and harvest, inexpensive, and has an extraordinary nutritional profile.

Wolffia is a valuable source of amino acids, vitamins, antioxidants and minerals, and it is a rare non-animal source of vitamin B12. In addition, the whole plant body contains up to a remarkable 40% protein per measure of dry weight.

Soybean — another plant regarded as being high in protein — contains just 30–40% protein, much of which is only in its seeds. In a world where meat production is increasingly costly to the environment, Morikawa points out that increased production of Wolffia could see it become a ‘superfood’ for both humans and livestock.

Other species of duckweed that are less palatable to humans also promise to be useful. Duckweed thrives on high concentrations of nitrogen and phosphorous, which are common pollutants in industrial wastewater.

“Wastewater becomes a free form of duckweed fertilizer,” Morikawa says. As it grows, the duckweed consumes the minerals and cleans the water, which can then be returned to the natural environment. The harvested duckweed can be processed into nutritious, inexpensive animal feed or other bioproducts.

Perfect partnerships

In their early experiments, Morikawa and his colleagues discovered the key role the duckweed’s microbial partners play in growth. In 2010, they isolated the world’s first known plant growth promoting bacteria (PGPB) for duckweed. This Acinetobacter was called P23.¹

PGPBs can produce plant growth compounds, aid nutrient uptake or fight off plant pathogens. By inoculating sterile duckweed plants with P23, the researchers increased the plant’s biomass.

A two-step production process, where plants are first colonized and then cultivated, resulted in up to 2.3 times more biomass after a week compared to plants not given P23.² The researchers established that the larger experimental duckweed biomass also effectively removed more nitrogen and phosphorous from the water than a control plant.

Duckweed holobionts could offer key environmental advantages over other forms of wastewater bioremediation that have been trialed in Thailand, Morikawa notes. ‘Activated sludge' treatments using microorganisms to break down contaminants require a lot of energy to aerate the water. Aquatic bioremediation by water hyacinth has also been tried, but when the plant dies it decays to become its own sludgy pollutant.

Building a biobank

The key research focus at DHbRC is to explore the symbiotic relationship between the plant and its microbes, looking at parameters such as growth rate, protein levels and starch content. While high protein is desired for animal feed applications, high starch levels are the ideal starting point for turning duckweed into biofuels, or for making sustainable bioplastics. These products are all being targeted by dedicated Be-HoBiD projects at DHbRC. As the plant absorbs CO₂ as it grows, these processes could be close to carbon neutral.

One challenge is to find ways to prolong the presence of PGPBs in duckweed. These select microbes gradually decline as rival microbes muscle in.

Interactions between the duckweed holobiont and minerals in the water are also not yet fully understood. Morikawa and his colleagues recently showed that two PGPB strains, P23 and P6, inhibited duckweed growth in low nitrogen wastewater, because the microbes competed with the plant for the nutrient. They identified a different PGPB, with a limited nitrogen metabolism, that was more effective in this environment.³ The DHbRC aims to build a duckweed holobiont biobank for different applications. Duckweed is also a useful host organism that provides an easy-to-use platform for promoting holobiont research.

As a method for treating water, duckweed promises to significantly reduce the carbon footprint and energy consumption of pollution remediation in Thailand, while sustainably generating various valuable green products. With the collaborative efforts of DHbRC researchers, ambitions for the little plant in the sustainable economy are huge.