Inside Iowa State

March 30, 2007

Investigating the purifying potential of plants

by Barbara McBreen, Ag Communications

The experiment started with a giant pickle jar.

"We put several maize leaves in a jar, introduced a phenol gas and let it sit for a while. We tested the air in the jar and the phenol was all gone. There was none left," said Gwyn Beattie, professor of plant pathology.

That was good news. Phenol is a gas that causes the foul odors found in automobile exhaust, cigarette smoke, decomposing manure and emissions from burning wood, coal and municipal waste.

In her experiment with the pickle jars, Beattie needed to find a measurable amount of phenol in order to test how the leaves remove it from the air. So, she began reducing the number of leaves in the jar.

Holding her thumb and finger two inches apart, Beattie said, "We put three little leaves about this big in the jar and we finally had a measurable amount of phenol at the end of 24 hours. The plants really took it up."

Life on a leaf

To a microbe, a leaf is a vast landscape. To Beattie, those leaf-dwelling microbes are the basis of her research. In 1995, she came to Iowa State to study leaf-associated bacteria, including how they can be used to reduce air pollutants and odors.

"The leaf surface is considered a nutrient-limited environment and I'm researching whether bacteria on a leaf can utilize the organic compounds that are air pollutants," said Beattie, who was named the Robert Earle Buchanan Distinguished Professor of Bacteriology for Research and Nomenclature in 2006.

Beattie said the process is called phylloremediation, a natural cleaning process that uses the leaves and its microflora to clean the air.

"Phylloremediation is similar to rhizoremediation, which works because plants draw water to the roots and bring soil pollutants into the area where large numbers of bacteria live. Rhizoremediation has been studied extensively and works, but phylloremediation is relatively uncharted territory," Beattie said.

Beattie and Janice Seibel, now a research associate in veterinary microbiology at Iowa State, began looking at how plants filter phenol out of the air eight years ago. They used phenol as an indicator compound because it's nontoxic and a component of the odor emitted from livestock facilities. It's also a compound that is easily degraded by most microorganisms.

Phenol is common in the air but usually is at low levels, Beattie said. When you are near a source - such as a livestock facility - and you can smell an odor, it's often an indication of higher levels of phenol.

"Your nose is a great test of air pollutants like phenol," Beattie said. "In Iowa, it's commonly emitted from livestock and municipal waste facilities, but in other parts of the world, it's produced from burning coal or chemical industries."

How does it do that?

To understand how much phenol microorganisms can degrade or mineralize, Beattie and Seibel needed to determine how the microbes and plants work together to take phenol out of the air.

"We needed to measure how much phenol the plant absorbed before we could look at what the bacteria could mineralize," Beattie said. "We didn't have any measurements because it hasn't been studied much."

Plants remove phenol from the air by moving it into the plant through the stomata (pores). They also found that phenol adheres to the leaf surface where the microbes may be able to use it.

"One mystery we wanted to solve was whether the bacteria could find the phenol that attached to the leaf because bacteria are not found everywhere on the leaf," Beattie said.

To track the phenol, Amarjyoti Sandhu, a graduate student in microbiology, used a biological reporter that turned the microbes fluorescent green when they came in contact with phenol. That produced some interesting results.

"The plant was like a sponge. It held the phenol until the bacteria could detect it," Beattie said. "A plant could be harmed by high levels of this stuff, thus the bacterial degradation of phenol could help protect a plant."

Microbe tracker

Sandhu also used a radioactive label to track the phenol molecules. Using this method, she could compare how much phenol was being taken into the plant and how much was being mineralized by the microbes.

"It was the first conclusive demonstration that bacteria could take an airborne compound and degrade it on the leaf," Beattie said. "This means that both the plant and the microbes are working together to purify the air."

Beattie said researchers are just beginning to understand the potential of phylloremediation. Studies like this could provide tools to help eliminate air pollutants near livestock and industrial facilities.

"It may mean that the more plants growing around a facility, the better. They influence the aerodynamics and act as a filter for particulates and airborne molecules," Beattie said.

"I can also envision an air outtake system that would filter through a whole canopy of leaves," she added. "We recently found that phenol-degrading microbes are part of the natural flora on leaves, so it's conceivable that we can find ways to increase natural populations of these microbes, such as planting species that favor them."

"The plant was like a sponge. It held the phenol until the bacteria could detect it."

Gwyn Beattie