Courtesy of Steven Rock
Humankind has passed a remarkable environmental milestone: People now consume more of Earth's natural resources than the planet can replace.1 In light of this, pollution abatement technologies, coupled with development of renewable energy resources, seem destined to become big business during the 21st century.
What is unfolding is a multidisciplinary, biology-led wave of discovery and innovation, emanating from myriad collaborations of scientists who are working with what the environment has to offer--from microbes to plants to even the oceans themselves.
CLEARING THE AIR Burning fossil fuels releases carbon dioxide (CO2) into the atmosphere where it, along with other greenhouse gases, contributes to global warming. Since the US Industrial Revolution began about 200 years ago, the amount of anthropogenic CO2 (the principal greenhouse gas) emitted to the atmosphere--primarily due to the expanding use of fossil fuels for energy--has risen from about 280 parts per million to 370 ppm, according to the US Environmental Protection Agency (EPA) and the Department of Energy (DOE). Estimates for the future do not predict lower numbers: A recent EPA report, for example, estimates that US greenhouse gas emissions will increase 43% by 2020.2
What to do with it all is a looming question. One idea is to capture excess CO2 and se-quester it somewhere until natural processes can slowly recycle it, or until a better plan is created. Some researchers have experimented with pumping excess gaseous CO2 into deep underground geologic repositories and into forests, while other investigators have experimented with fertilizing the oceans with iron to stimulate the growth of phytoplankton, which, theoretically, would consume greater amounts of carbon dioxide. Arguably, the most promising possibility is directly injecting liquid CO2 into the ocean, 1,000 meters or deeper, from either shore stations or tankers at sea. The notion of burying excess CO2 in the deep ocean, though more than 20 years old, caught on after the Kyoto Protocols in 1997.
The world's oceans serve as the planet's primary carbon dioxide sink. Oceanic plants and microbes use the CO2 as their main carbon source, recycling it through photosynthesis. "Over the last century, the amount of carbon dioxide--not carbon but carbon dioxide itself--that has been put into the ocean is more than 300 billion tons," says ocean chemist Peter Brewer, senior scientist at the Monterey Bay Aquarium and Research Institute (MBARI) in northern California. "Right now, about 20 million tons a day of carbon dioxide goes into the surface waters of the ocean, globally, and it's getting slowly mixed down into the deep." As the Co2 penetrates the surface, it dissolves into the water and appears as a 'tracer' field, a highly concentrated stream of CO2 that is circulated and diluted over time, becoming mixed and transported via the general oceanic circulation. For example, he continues, "The mean ventilation age of the Atlantic Ocean is about 250 years, [meaning] carbon dioxide absorbed into the surface waters of the far North Atlantic will be mixed into deep waters, and will be reexposed to the air in the Antarctic about 250 years later."
At shallow depths, injected liquid carbon dioxide will rise to the surface. Researchers initially hypothesized that liquid CO2 put into the sea's cold, extremely high-pressure environment would react with the water to form a solid ice-like compound, called a clathrate hydrate, that would remain as a stable layer on the seafloor. "The early ideas of storage as a hydrate were somewhat confused," says Brewer, who in 1999 with his colleagues at MBARI and Stanford University designed and executed some of the first experiments to investigate fundamental principles of direct injection.
Courtesy of Steven Rock
They placed liquid CO2 in beakers, at varying depths, off the northern California coast. The CO2 reacted rapidly with the seawater, forming a gas hydrate at the bottom of the beaker and expanding the volume until it overflowed in gaseous liquid, which then floated along the seafloor without penetrating the sediment.3 Based on their observations and chemical thermal dynamics, Brewer and crew hypothesized that CO2 hydrates would dissolve like any mineral exposed to unsaturated water, and their subsequent studies measured that.4 "We were able to make accurate measurements of this because of MBARI's special access [via submersibles] to the deep ocean," says Brewer, who has been studying oceanic carbon dioxide for more than 20 years.
Research on deep injection, however, is only beginning. The unknowns include impact on marine life, and feedback mechanisms of the climate and the oceanic carbon cycle. "All researchers agree that as we move forward ... by midcentury we will face a much higher carbon dioxide [concentration in the] ocean, and there is an appalling ignorance of what a low-pH, high-CO2 ocean will do to marine life," says Brewer. As part of an international team, he will be conducting another experiment this fall to address that and other issues. "Whether we put [the excess CO2] into the deep ocean deliberately or whether we continue on our present course of air deposition ... the carbon dioxide is going into the ocean."
In other parts of the global ocean, the sub, so to speak, has left the station. Since the late 1990s, Norway's state-owned oil company has been injecting carbon dioxide from a platform in the North Sea deep into an aquifer, to avoid paying the country's carbon tax that was instituted to reduce CO2 emissions. A globally unique project, Statoil is pumping about one million tons annually into the aquifer, which lies beneath a layer of shale caprock, 80 meters thick. Company officials, according to Statoil's Web site, estimate that the formation, which extends for several hundred kilometers in length and about 150 kilometers in width, can store 600 billion tons of the excess CO2.
On the East Coast, armed with a $15 million grant, geneticists and biologists at Harvard Medical School, the Massachusetts Institute of Technology (MIT), and two Harvard-affiliated hospitals are studying the proteomics and genomics of Prochlorococcus, an oceanic group of bacteria that removes much of the CO2 deposited into the ocean from the air and fixes it into carbon.
Prochlorococcus, discovered in the late 1980s by MIT collaborator Sallie (Penny) Chisholm, is important because it is the Earth's major carbon fixer and is ubiquitous, says Harvard geneticist George Church, who heads the research team. "We want to create a blueprint--to have a parts list of the RNAs and proteins--and know how the various components interact to make a functional network within a cell and between cells in a community, and in a community in the ocean," he says. "The better the models are, the better you can debug a disaster after it happens or prevent it from happening."
They also will "blueprint" Pseudomonas and Caulobacter. The former is a versatile infectious family of microbes that can degrade benzene and naphthalene and consume the carbon that Prochlorococcus produces; the latter is a bacterial group, common in freshwater streams, that can degrade chemical wastes and other pollutants.
REMEDIATING THE LAND The worldwide number of contaminated land sites is unknown. In the United States, the DOE estimates that about 220,000 sites need remediation. The conventional decontamination method is to remove the soil and sediments and bury them elsewhere. For many sites, the enormity of the contaminated areas, and hence the cleanup costs, makes this approach unfeasible. Other areas, used as testing grounds for nuclear and military weapons assessments, have been so heavily contaminated with toxic cocktails that they are deemed "technically impractical" to clean up.
During the last decade, more scientists have focused attention on using microbes that can consume chemicals or transform heavy metals, and plants that can accumulate and degrade organic pollutants or contain and stabilize metal contaminants by acting as filters or traps. "There's been a lot more interest in bugs in the last several years," confirms microbial ecologist Terry Hazen, who works with microbes to bioremediate various contaminated US sites. In 1994, while working for the Westinghouse Savannah River Company, he and his colleagues remediated several areas at the Savannah River Site (SRS) in South Carolina. This facility, where tritium and plutonium for the US nuclear stockpile have been produced for more than 50 years, is heavily contaminated with radioactive waste and many other pollutants; it resides on the EPA's list of Superfund sites, areas deemed an immediate threat to human health and the environment.
In one SRS area contaminated with trichloro-ethylene (TCE), a hazardous solvent used to remove oil and grease from metal parts, the researchers used a process called biosparging. They injected air mixed with methane into the ground through horizontal wells, reaching the area containing naturally occurring methane-oxidizing bacteria (Methylosinus trichosporium, for example), which can degrade TCE by turning it into carbon dioxide and salt. The technique increased the microbial population 10 billion times, and in less than two years, the biodegradation was complete and the TCE was gone.5 This patented approach, Hazen says, is being used in the United Kingdom, South America, Asia, and Africa.
Now chair of the Field Research Advisory Panel of the Natural and Accelerated Bioremediation Research (NABIR) program in the DOE's Office of Biological and Environmental Research, Hazen is tackling more complexly contaminated sites. Microbes cannot fully digest elements in these sites, where the state of the art is containment, he says, "and that takes more work, more modeling, and a lot more understanding of the ecology, the hydrology, and the biogeochemistry."
At Area 100H on the Hanford Nuclear Reservation in the state of Washington, Hazen is attempting to enclose seepage of chromium waste. (Chromium was used in the nuclear reactors and treatment systems to help reduce corrosion.) The plan is to inject lactate, in a process known as biostimulation, to enhance the population of microbes, such as iron reducers, that are already present. The microbes alone cannot fully digest chromium 6, but they can, he says, reduce the less toxic chromium 3. "It will stay immobilized in the subsurface in this condition as long as the environment does not become strongly oxidizing."
Studies also are underway into how these microbes consume these toxins. Investigators want to learn what is preventing them from fully digesting radio nuclides and other pollutants, such as polychlorinated biphenyls (PCBs), for which microbes have yet to fully acquire a taste. At the Lawrence Berkeley National Laboratory, Hazen is working at the bench alongside biologist and bioinformaticist Adam Arkin, an assistant professor at University of California, Berkeley, on a multi-institutional, $36.6 million, five-year grant to examine stress response pathways in Desulfovibrio vulgaris, a metal- and radionuclide-reducing bacterium. The goal, he says, is to develop new strategies for both natural attenuation (where the microbes take their own course) and bioremediation (where humans assist the microbes).
Elsewhere, biologists Jeffrey T. Bolin, Purdue University, and Lindsay Eltis, University of British Columbia, and colleagues have identified a major stumbling block that Burkholderia strain LB400 encounter while attempting to decompose PCBs.6 "The process of digestion requires a long chain of chemical steps ... and what we have done is isolate one of those steps that causes problems for the bacteria, that cause a clog in the biochemical pipeline," says Bolin. The hope is that further research will lead to a new breed of microbes that can learn to consume PCBs.
GREEN MACHINES An estimated 350 species of plants naturally take up various metals and trace elements, including arsenic, cadmium, chlorine, and selenium. Some species hyperaccumulate various noxious substances. Sunflowers, for example, can remove radionuclides from water and have been used to soak up radioactive elements near Chernobyl in the Ukraine. Alpine herbs take in zinc. Grass and clover eat oil. Environmental engineers refer to a stand of poplar trees as "a self-assembling, solar-powered, pump-and-treat system" to remove solvents such as TCE.
"By metabolizing available nutrients in toxic substances, these plants convert the substances to less toxic metabolites, and also stimulate the degradation of organic chemicals in the rhizo-sphere by the release of root exudates," explains Steven Rock, an environmental engineer based at EPA's National Risk Management Research Laboratory in Cincinnati, Ohio. "Phytotechnologies are still in their infancy," he says, "but the concept has been field-tested now at 12 Superfund sites." Trees and grasses foster ecosystem restoration, as well as indicate a watershed's health and resilience. Plants and trees also can be used as riparian corridors to prevent pollution from running off fields and into streams or rivers, says Rock, who for nine years has been part of a team tracking research into ways plants can abate pollution.
"The concept [of phytoremediation] is so simple; farming the metal out of the soil," adds David Salt, associate professor of plant molecular physiology at Purdue University. "The nice thing is that it's cheap and you're left with a soil at the end of the remediation that can be used for other things."
Exactly how these plants do what they do remains, for the most part, a mystery; why they do it, no one knows for sure. Theories include the obvious: They take up toxins for self-defense (so pests don't find them tasty) or for nutrition. "You've heard of the black box; well, this is sort of like a green box--you can tell what goes in and what comes out and you're not entirely sure what happens inside," says Rock.
Salt and other researchers are peering into these green boxes. Along with Northern Arizona University's Michael W. Persans and Ken Nieman, Salt has identified and cloned genes from the wild mustard Thlaspi goesingense, native to the Austrian Alps, where it hyperaccumulates nickel. "From experiments with yeast, we suggest that members of the CDF [cation diffusion facilitator] family of genes in Thlaspi goesingense are involved in putting the metals into [a] vacuole, a kind of storage compartment," says Salt, the principal investigator.7
Currently, Salt's focus is on validating that hypothesis, and on engineering a nonmetal-accumulating model organism, Arabidopsis thaliana, also a member of the mustard family, to hyperaccumulate nickel, zinc, and cadmium. "Right now, we have both members of the CDF gene family and as yet unpublished genes encoding certain metabolic enzymes from T. goesingense over-expressed in Arabidopsis," informs Salt. "We have confirmed that at least one of these groups of genes definitely confers enhanced metal tolerance. We are still investigating the other set of genes."
While research efforts into the inner workings of hyperaccumulating plants continue, the work on bioengineering fast- growing plants to clean up a whole host of pollutants has begun. At the University of Georgia, molecular geneticist Richard B. Meagher and colleagues have engineered several plant species to extract and detoxify ionic mercury and the more hazardous methylmercury from soil and groundwater.8 Last November, Meagher announced that his lab had developed the first transgenic system for removing arsenic from the soil.9 The team inserted two genes, arsC (arsenate reductase) and ECS (gamma-glutamylcysteine synthetase), from the common bacterium Escherichia coli, which allows Arabidopsis to tolerate arsenic (an element usually lethal to these plants), remove it from the soil, and transport it to the plant's leaves, where it is trapped. "Our data," Meagher says, "demonstrate the first significant increase in arsenic tolerance and hyperaccumulation by genetically engineered plants." Meagher and his teams have discovered hundreds of plant and bacterial genes. "We're just scratching the surface," he says. "There is so much more to be learned."
Nobody knows that better than Lena Q. Ma, a soil and environmental chemist from the University of Florida. Several years ago, she searched for trees that can hyperaccumulate arsenic at an abandoned wood site contaminated with chromated copper arsenate (a solution used to treat wood) but found none. She expanded her initial objective to all of the vegetation growing on the site. She and her students tested 12 species of plants (including the trees) and discovered a remarkable hyperaccumulator in the Chinese brake fern (Pteris vittata).
"It is a primitive plant that doesn't just take up arsenic, it thrives on arsenic," says Ma. "We added 100 ppm arsenic--that's pretty high--and in one week, it doubled its biomass."10 Ma's objectives now are to "fully understand its uptake, translocation, distribution, and detoxification" of arsenic.
Courtesy of Terry Hazen
PURIFYING THE WATERS Monitoring and testing native species to assess the level of contaminants in water bodies is now considered a necessity, not just an adjunct to chemical analyses. "For many years, we've been rather chemistry-driven, because very sensitive chemical analytical techniques came along well before sensitive biological techniques, with biological monitoring largely being tacked on almost as an afterthought," says ecotoxicologist Peter Matthiessen, director, UK Natural Environment Research Council's Center for Ecology and Hydrology, Windermere, Cumbria.
Scientists are using integrated biomonitoring systems, combining chemical techniques with new in vitro biological detection kits to assess more quickly what chemicals or pharmaceutical metabolites and steroids sentinel species have taken up, and to address the often elusive questions of how and why. If the monitored animals show there is a pollution problem, "then you use integrated chemical and biological techniques to tell you what the causes of that problem may be," Matthiessen says.
Matthiessen and colleagues used this approach to find out why males in certain fish species were suffering serious gender identity issues. "In some of the rivers in Britain, 100% of the fish are feminized to some extent," he explains. "In the Aire in Yorkshire, and the Nene in East Anglia, for instance, all of the males in roach (Rutilus rutilus), a common fish in Britain, have eggs in their testes, which is obviously abnormal." So, the investigators took water samples from the areas where the fish were found, and fractionated those samples using an EPA-developed technique called toxicity identification and evaluation. Then, they bioassayed each fraction using a sensitive bioassay, a genetically modified yeast cell line that contains the human estrogen receptor gene. They screened each fraction and identified which were estrogenic. "We could then subfractionate those and continuously simplify," he says. The strategy paid off. They found that the male fish were being exposed to both natural and synthetic estrogenic substances through sewage.11
At other benches, researchers are working on new methods for decontaminating water. Hossein Rostami, assistant professor, mathematics and science at Philadelphia University, has developed a system using fly ash, a waste product from burning coal, to remove heavy metals from contaminated water. Combining the fly ash with sand, activating chemicals, and a secret ingredient that Rostami calls "foo-foo dust," he and his colleagues are experimenting with reactive barriers that can either change or break down contaminants as the water flows through them.
Courtesy of Jeff Bolin
"We are finding that we can remove 80 to 90% of the heavy metals, and what's left [retained in the barrier] is a lot less damaging and can be disposed of or cleaned at much less cost than conventional treatments," says Rostami, who has worked on recycling fly ash since the early 1990s and holds two related US patents.12 Every year, 110 million tons of fly ash is generated in the United States, and less than 30% is reused or recycled. "This is [an example of] one environmental problem being used to solve another."
While research in many of these areas is only beginning, virtually all these investigators agree that nature's capabilities can be humbling. "There is a sort of an irony in turning back to nature to see how it accomplishes cleanup," reflects Salt. "There are a lot of things we can learn if we just look around us."
A.J.S. Rayl (email@example.com) is a freelance writer in Malibu, Calif.
1. M. Wackernagel et al., "Tracking the ecological overshoot of the human economy," Proc Natl Acad Sci, 99:9266-71, 2002.
2. "US climate action report--2002," US Department of State, Washington, DC, 2002, p. 71-80.
3. P.G. Brewer et al., "Direct experiments on the ocean disposal of fossil fuel CO2," Science, 284:943-5, 1999.
4. P.G. Brewer, "Contemplating action: Storing carbon dioxide in the ocean," Roger Revelle Commemorative Lecture, Oceanography, 13:84-92, 2000.
5. T.C. Hazen, "Case study: Full-scale in situ bioremediation demonstration (methane biostimulation) of the Savannah River Site Integrated Demonstration Project," in Bioremediation of Contaminated Soils, D.C. Adriano, J.M. Bollag, editors, Monograph of the Soil Science Society of America/American Society of Agronomy, 1999.
6. S. Dai et al., "Identification and analysis of a bottleneck in PCB biodegradation," Nat Struct Biol, 9:934-9, December 2002.
7. M.W. Persans et al., "Functional activity and role of cation-efflux family members in Ni hyperaccumulation in Thlaspi goesingense," Proc Natl Acad Sci, 98:9995-10000, 2001.
8. R.B. Meagher, "Phytoremediation of heavy metal pollution: Ionic and methyl mercury," in OECD (Organization for Economic Cooperation and Development) document: "Biotechnology for water use and conservation: The Mexico '96 workshop," p. 305-21, 1997.
9. O. P. Dhankher et al., "Engineering tolerance and hyperaccumulation of arsenic in plants by combining arsenate reductase and gamma-glutamylcysteine synthetase expression," Nat Biotech, 20:1140-5, December 2002.
10. L.Q. Ma et al., "A fern that hyperaccumulates arsenic," Nature, 409:579, 2001.
11. P. Matthiessen et al., "The impact of oestrogenic and androgenic contamination on marine organisms in the United Kingdom--summary of the EDMAR Programme," Mar Environ Res, 54:645-9, 2002.
12. H. Rostami et al., "Removal of cesium from contaminated water using alkali fly ash permeable reactive barrier (AFA-PRB) material," Conference on Selected Catalytic & Non-Catalytic Reduction for NOx Control, Pittsburgh, Pa., 2000.