In the muck

Randall Kerstetter shows off the duckweed collection at the Waksman Institute at Rutgers.
Courtesy of Wesley M Jackson

Duckweed first appeared in satellite images of Venezuela in 2004 as a mysterious swirl of green on the surface of Lake Maracaibo, doubling in size with each passing day. Maracaibo is one of South America’s largest bodies of water, but with brackish water and few nutrients, it had never harbored this rapid-growing aquatic plant. Local scientists speculated that heavy rains washed sewage and nutrients into the lake along with duckweed colonies from neighboring ponds. By June, they estimated that the world’s smallest flowering plant covered 18 percent of the lake’s surface before it began receding.

The invasion could have been an ecological disaster, but for Rutgers geneticist Randall Kerstetter, it was a sign of hope. In the taxonomic showdown over the ideal organism for producing biofuel, Kerstetter is putting his mental energy into duckweed. “Duckweed is the fastest growing plant,” he says. “If you’re interested in biofuels, you’re interested in biomass.”

On an early fall morning, Kerstetter dips his hand into the muck on the edge of a canal near his office at the Waksman Institute of Microbiology in Piscataway, New Jersey, and pulls up what looks to be a clump of algae. Upon closer inspection, however, the greenery is not pond scum, but a complex organism with buoyant fronds and threadlike rootlets. In the winter, Kerstetter explains, duckweed produces starch-rich organs called turions that sink to the bottom of the water and could easily be harvested as feedstock for ethanol and other fuels.

Today, marine algae have captivated scientists and companies like Exxon as a future biofuel, an eco-friendly substitute to petroleum. Although the duckweed infestation frightened some Venezuelan scientists, Kerstetter says they have little to worry about. Unlike algae, duckweed is unlikely to make the water go anoxic, smothering life below. “At night, algae suck all the oxygen out of the pond, but duckweed does all its gas exchange with the air. All the fish, frogs, and even the algae, don’t mind having duckweed around.”

Kerstetter claims duckweed could produce up to 6 tons of biomass per hectare—about twice that produced via switchgrass, another biofuel source. Unlike switchgrass, the turions contain little cellulose and lignin, which makes them readily convertible to ethanol, and unlike corn, duckweed agriculture would not compete with a traditional food species. Indeed, duckweed can be grown in remediation ponds filled with hog waste or soak up excess fertilizer from agricultural runoff. Christoph Benning, a biochemist who studies algae-based biofuels and is not involved in the duckweed project, says that duckweed has an impressive potential but its freshwater dependence will limit it to niche applications, such as those conducted in conjunction with bioremediation efforts. “One solution is not going to fit the bill for every application,” he explains.

In 2008, Kerstetter and his collaborators convinced the Joint Genome Institute in Walnut Creek, Calif., to sequence greater duckweed (Spirodela polyrhiza) for its potential role in both biofuel production and bioremediation. With just 170 megabases, the duckweed genome is only slightly larger than the Arabidopsis genome, and half the size of rice’s. It will also be the first sequence of a monocot plant outside the grass family, which Kerstetter says would aid in understanding the evolution of plant genomes.

In a chilly chamber in the basement of Rutgers, one of Kerstetter’s undergraduates is working under a fume hood on gene insertion experiments, designed to one day help the team control the turion “switch,” getting the plants to continuously produce the starchy organs that they normally just make in winter. It’s been just over a year since the project began, but the student, Paul Yan, has already improved the transfection rate—the frequency with which he can successful insert genes for herbicide resistance. He holds up an herbicide-laden Petri dish to the light, displaying patches of pale, dead fronds mixed in with the verdant, new growth, the green representing the scientists’ hopes for a cleaner future.