Seaweed: You find it in chocolate milk, ice cream and sherbet. Mayonnaise, cheese and instant pudding. Latex and cosmetics. Sushi and fertilizers. Now seaweed may have another use: cleaning up soil polluted by the pesticide DDT.
Ian Singleton of the University of Newcastle upon Tyne in England and Ravi Naidu of the University of South Australia experimented with a concoction of powdered green and red seaweed mixed into DDT-contaminated soil. They found that 80 percent of the toxic chemical disappeared within six weeks. The reason, Singleton and Naidu discovered, is that small amounts of seaweed facilitate the breakdown of DDT by microbes that live in soil.
The polluted soil was from a stockpile at the Southern Waste Depot in Maslins, South Australia, that had come originally from an orchard. The seaweed came from Glenelg Beach, a popular strand where Naidu often walks.
The shoreline there is covered with so much washed-up seaweed that the local government is reviewing plans to remove tons of green and red wrack that drift ashore with the tides, Naidu said.
"It was a eureka moment," he said, "when one morning I stood ankle-deep in seaweed and realized here's a substance that's got the right combination of elements" to work in DDT bioremediation -- cleaning up pollutants by biological means.
"Some of the best discoveries indeed are found by serendipity," said microbiologist Peter Adriaens of the University of Michigan. "DDT tends to be found in huge contamination sites, acres and acres in size. This research on seaweed could be scaled up at low cost, and it would be much easier to do than other technologies that have been tried."
Soil contaminated with DDT is a major worldwide problem, Naidu said, because concentrations of the banned pesticide continue to build up in the environment for years after its use. "DDT's toxicity, and bioaccumulation and biomagnification along the food chain cause damage to ecological systems and threaten human health."
Once called "the miracle insecticide," DDT came into widespread agricultural and commercial use in the late 1940s. In 1948, Switzerland's Paul Hermann Mueller was awarded the Nobel Prize for medicine for developing it. During the 30 years it was widely applied in the United States, more than 675,000 tons were sprayed onto cotton and other crops. The peak year was 1958, when nearly 80 million pounds of DDT blanketed American farmlands.
The picture changed in 1962 with Rachel Carson's best-selling book, "Silent Spring." Her portrait of a world where no birds sang made the public aware of the perils of DDT. Over the next decade, research confirmed that DDT was responsible for thinning the eggshells of birds and killing many. Numbers of eagles, hawks and other birds plummeted. Carson's book also raised questions about DDT's harmful effects on people. Scientists later confirmed a link between DDT and human ills such as cancer, infertility and various nervous system disorders. In 1972, the pesticide was outlawed in the United States.
In 2001, DDT was banned internationally under the United Nations' Stockholm Convention on Persistent Organic Pollutants, but about 25 countries including South Africa still use it. The pesticide is essential, say officials in these nations, to fight rampant malaria.
DDT kills mosquitoes; malaria is transmitted to humans via mosquito bites. According to U.N. estimates, malaria kills one child every 30 seconds and more than a million people each year. "Half of the world's population lives in the 103 countries where the whine of a mosquito can herald sickness and death," Pascoal Mocumbi, former prime minister of Mozambique, wrote in an editorial in the journal Nature. African countries are pushing for development of less toxic pesticides and more effective drugs to combat malaria, but, in the meantime, they say they cannot afford to give up DDT.
That dismays biochemical engineer and DDT expert Malcolm Sumner, retired from the University of Georgia, who believes "one can't condone the use of DDT in today's world. We now know that it takes decades for it to disappear. South Africa, for example, has been bombarded over and over again with DDT since just after World War II. This very persistent pesticide has become part of the landscape in soils there, by the tons."
He called Naidu's and Singleton's work "a breakthrough: the development of a concept that could work on a large scale to accelerate the natural breakdown of DDT."
Although there has been no new use of DDT in the United States for more than 30 years, American soils still bear witness to its use, especially in formerly heavily treated areas, such as those along Georgia's Savannah River.
It is as though soil is sealed in an impenetrable box, says Naidu, who, along with Singleton and others, reported the research results in the April 2004 issue of the Journal of Chemical Technology and Biotechnology: "DDT gets into this 'box,' so the microbes that would normally break it down can't get at it. Seaweed has sodium in it. Sodium opens that box. It separates the tightly bound matrix that holds soil particles together and allows microbes to get in."
Seaweed is also a source of carbon, Singleton said, that stimulates the microbes' growth. "With millions of microbes living in every square foot of soil, they play a major role in the fate of DDT in the environment," he said. With the addition of seaweed, "the pesticide degrades much faster than its usual long process."
The initial breakdown of DDT depends on microbes that function best without oxygen. The scientists wet the soil to encourage these "anaerobic" microbes, Naidu said. "They're very particular about their working conditions." In fact, too much seaweed added to the soil hindered the bacteria's efforts. "When a lot is used," Naidu said, "the excess carbon and sodium get in the way of the process."
By trying varying amounts of seaweed, "we were able to find the right ratio of seaweed to soil to break down the most DDT," Naidu said. "Exactly 0.5 percent."
Now that they have found the magic number, Naidu and Singleton are ready to test their results on large samples of DDT-contaminated soil.
"We'd like to do another pilot run, but this time on five or 10 tons of soil," Singleton said. "If we can enlist the aid of seaweed to clean up a site the size of a football field, for example, we'll have taken a major leap forward in dealing with the world's DDT problem."