Duckweed Quacks Volumes of Potential

Since the late 1960s, scientists have studied duckweed for animal and human consumption because of its high protein content. Researchers are now tapping into the plant's innate environmental benefits, from desalinating wastewater to exploring its potential as a viable starch-based feedstock for ethanol production.
By Bryan Sims
The drive to develop sustainable nonfood, starch-based ethanol feedstocks and more efficient conversion processes is intensifying as the U.S. attempts to reduce ethanol's carbon footprint by transitioning from corn to cellulosic ethanol. That has prompted researchers at North Carolina State University to take a closer look at plants, such as duckweed, that could be a potential feedstock for ethanol production.

Duckweed has traditionally been studied because of its inherently rich protein content at 30 percent to 35 percent on a dry-weight basis. The purpose was to explore whether duckweed could be a protein source for animal and human food. A growing interest in sustainable ethanol feedstock development, however, has researchers exploring the plant's starch content.

North Carolina State University researchers Anne-Marie Stomp, associate professor of forestry, Jay Cheng, professor of biological and agricultural engineering, and Mike Yablonski, post-doctoral research associate, are discovering that duckweed can be used to clean up animal waste at industrial hog farms and could be used to make ethanol. They have determined that duckweed grown on swine wastewater can produce five to six times more starch per acre than corn, according to Stomp, who co-authored the research with Cheng.

The research, funded by the Biofuels Center of North Carolina, was presented at the annual conference of the Institute of Biological Engineering in March in Santa Clara, Calif.

"The original investigations focused pretty much entirely on the protein side," Stomp says. "At the time all of that work was being done, there was no compelling economic reason to domesticate this plant because we had plenty of other plant protein sources in grain and legumes. Back then, the prices of those grains and legumes were low and the market was fully supplied."

The one challenge that has impeded duckweed's progress in becoming a sustainable, dedicated energy crop for biofuels production or being used as a bioremediator for farm or city wastewater treatment operations is the fact that it wasn't domesticated. "The trick to domesticating duckweed is going to be how much it will cost per ton to grow this stuff," Stomp says, adding that data on economic feasibility will be released later this year. "That number provides a threshold for commercial viability," she adds.

Cheng and Stomp are currently developing a pilot-scale project to further investigate the best way to establish a large-scale system for growing duckweed in animal wastewater, and then harvesting and drying the plant. "We're actually exploiting a lot of existing technology used in the food industry, because duckweed is like a slurry," Stomp says. "You can pump it, sieve it and do other things."

In the meantime, duckweed will remain of interest to scientists as a viable synergistic component to the renewable fuels/energy sectors, possibly even being used with corn in existing ethanol operations, according to Stomp. "We're not saying we're going to replace corn," she says. "It's just another option out there for ethanol producers. It's the idea that if we're going to solve this energy crisis we're going to need a bunch of ideas. One idea isn't going to save us."

Water Purification Potential
Propagated in agricultural and/or municipal wastewater, duckweed naturally extracts nitrogen and phosphate pollutants. This could benefit large-scale hog farms where animal waste is stored in large lagoons for biological treatment. Duckweed's bioremediation properties allow it to capture pollutants and prevent their release into the air. The plant could save farmers money because they wouldn't have to purchase expensive desalination equipment for their lagoons. "Duckweed is exquisitely good at recovering low levels of nutrients from water," Stomp says. "It gets the water clean enough for reuse naturally, and it's virtually cost-free for farmers."

Duckweed can also reduce algae growth (by shading), coliform bacteria counts and mosquito larvae on ponds, while concentrating heavy metals, capturing or degrading toxic chemicals and encouraging the growth of other aquatic animals such as frogs or fowl. Additionally, duckweed is one of the fastest growing plant species on the planet. Scientists are also beginning to unlock duckweed's potential as a player in carbon cycling and carbon sequestration.

Duckweed bioaccumulates about 99 percent of the nutrients contained in wastewater and produces a valuable protein-rich biomass as a byproduct, which can be fed to certain fish and added to poultry feed. Duckweed can also assimilate small hydrocarbons such as glucose and sucrose and, as a result, perform heterotrophic growth from the wastewater. The nutrients can be removed permanently from the system as the plants are harvested.

Due to its high affinity for absorbing pollutants in wastewater, Stomp posed a hypothetical scenario where the use of duckweed by farmers could mutually benefit a city willing to provide wastewater effluent for fresh water reuse. For example, a farmer could pay the city or municipality for its wastewater and have it transported to his farm, a concept that some people refer to as duckweed-based wastewater treatment, Stomp says. The farmer could take that wastewater and mix it with his livestock wastewater to dilute it so that it can be used to grow duckweed, which would clean the water and the farmer could sell it back to the municipality for reuse.

Calling on Genome Sequencing
The duckweed family, or Lemnaceae, is a family of flowering plants. Specifically, Lemnaceae is an aqueous monocot-similar to grasses and palms-and is divided into five genera: Lemna, Spirodela, Wolffia, Landoltia and Wolffiella. Of these five genera, Spirodela is the largest and Wolffiella is the smallest.

Researchers at Rutgers University's Waksman Institute of Microbiology-a research facility on the Busch Campus of Rutgers University in New Jersey-channeled its resources in July 2008 into sequencing the Spirodela polyrhiza genome. The Spirodela has the least DNA per cell compared with its genera counterparts. The team aims to investigate duckweed's potential for sustainable sequestration of carbon dioxide, ecosystem carbon cycling and biofuel production.

In related research, the U.S. DOE's Joint Genome Institute announced in July 2008 that its Community Sequencing Program would support the genomic sequencing of Spirodela polyrhiza as one of its priority projects this year directed toward new biomass and bioenergy programs.

Preliminary findings by the Rutgers research team found that, through high-throughput sequencing, specific duckweed varieties obtained from 50-year-old sterile duckweed cultures shipped from Switzerland are comprised of Bradyrhizobium-a nitrogen-fixing bacteria that forms nodules on host plants. Bradyrhizobium also have symbolic relationships with legume plants, which can't live without the bacteria's essential nitrogen-fixing processes, according to Todd Michael, a member of the Waksman Institute and an assistant professor of plant biology and pathology.

"This was really surprising to us because we didn't expect to find this microorganism," Michael says. "There's still a chance this could be one level of contamination. Now the question is: Is this bacteria DNA actually affixing nitrogen for the Spirodela? This is a totally new relationship that we didn't expect, but this could explain why the plant grows so fast."

Michael said the research team is also developing methods to exploit the Wolffia gene. "This would give us the potential to do things like enhancing the DNA of genes to uptake specific types of heavy metals or modulate how fast we control growth rate," he says.

By harnessing the naturally occurring ability of plants to transform genes by way of various DNA interactions in their own natural habitats, Michael's research team intends to continue researching these specific duckweed varieties in sterile culture environments to determine what the ecology of each looks like in different locations and the behavior of each with an array of other microorganisms. "We can then use that information to go back and maybe suggest specific varieties for different locations, and maybe utilize the genetic makeup we already have to target specific applications," Michael says.

Bryan Sims is a Biomass Magazine associate editor. Reach him at [email protected] or (701) 746-4950.