Research▼Watch Anthropogenic sources of ozone and “well-mixed” greenhouse gases are responsible for ~80% of the recent growth in the height of the tropopause—the boundary between the troposphere and the stratosphere, according to the first quantitative analysis of the factors that determine tropopause height. The analysis, which was reported in the July 25 issue of Science, shows that increasing tropopause height is another indicator of climate change and that anthropogenic ozone emissions are an important factor. Since 1979, researchers have observed a several-hundred-meter increase in the tropopause. To understand this phenomenon, B. D. Santer and colleagues from the Lawrence Livermore National Laboratory, the Lawrence Berkeley National Laboratory, the National Center for Atmospheric Research, Deutsches Zentrum für Luft- und Raumfahrt (Germany), and the University of Birmingham (U.K.) used sophisticated atmospheric models to evaluate the effects of sulfate aerosols, changes in solar irradiance, volcanic aerosols, greenhouse gases, and ozone on tropopause height. They found that tropospheric warming, predominately due to greenhouse gases, and stratospheric cooling by ozone are responsible for the tropopause growth, but the relative importance of the two factors is not yet known. Tropospheric warming was determined to be 0.07 °C per decade since 1979. Data from actual climate observations further support these results when compared with the model’s predictions. (Science 2003, 301, 479–483)
Plants that hoard heavy metals Researchers interested in understanding the molecular mechanisms that allow plants to hyperaccumulate metals should consider focusing their efforts on the pennycress, Thlaspi caerulescens, according to a new analysis by scientists at Purdue Univer-
sity and Massey University in New Zealand. The study, which was published in the August issue of New Phytologist, is the first step toward finding a model plant species for use in large-scale efforts to uncover the genes involved in metal phytoremediation. Wendy Ann Peer and colleagues collected different wild metal hyperaccumulators from 20 areas throughout Austria, France, Turkey, and the
about 88% of A. thaliana’s genes, which have already been sequenced. Peer and colleagues are currently testing another 30 hyperaccumulators and eventually plan to pit T. caerulescens against the winner of the current evaluation. It would be fitting if pennycress comes out on top for genetic studies because it was the first hyperaccumulating plant to be identified in 1885. (New Phytologist 2003, 159, 421–430) SCOTT BAUER, USDA AGRICULTURAL RESEARCH SERVICE
Measuring climate change
Leaves of the pennycress, Thlaspi caerulescens, accumulate more heavy metals than other plant leaves, making it a good model for studying the genes involved in phytoremediation.
United States during the spring and summer of 2001. All of the specimens were members of the cabbage family and relatives of Arabidopsis thaliana, a plant whose genes are known and easy to manipulate. The scientists evaluated the plants’ ability to accumulate metals and considered factors that make the plants easy to grow, analyze, and manipulate genetically. T. caerulescens was the most promising because it grows quickly and easily in the lab, sets several thousand seeds, and is self-fertile. It also has a compact diploid genome and shares
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Dioxin in Vietnamese food Although the spraying ended more than three decades ago, remnants of the herbicide Agent Orange are still showing up in foods purchased in “hot spot” cities in Vietnam, according to the most recent analysis of dioxins in the Vietnamese food supply. The findings, which were published in the August issue of the Journal of Occupational and Environmental Medicine, provide new evidence that elevated dioxin levels in residents of Vietnamese cities such as Bien Hoa City, where intense spraying occurred between 1962 and 1971, can be attributed to present-day contamination of some commonly consumed foods. In a recent study, 95% of 43 selected residents of Bien Hoa City were found to have elevated blood levels (>5 parts per trillion [ppt]) of the most toxic dioxin congener—2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD)— but not other dioxins. For comparison, typical blood TCDD levels in the Vietnamese population are 2 ppt. TCDD was a major contaminant in Agent Orange, and the elevated TCDD levels found in the Bien Hoa City residents are characteristic of exposure to the contaminated herbicide. To determine whether food is the primary source of TCDD, Arnold Schecter of the School of Public Health at the University of Texas, Houston, and colleagues analyzed 16 food samples purchased from markets in Bien Hoa City in 2002. The researchers found high levels of TCDD © 2003 American Chemical Society
in several food products, including ducks, chickens, fish, and a toad, but surprisingly, they did not find elevated TCDD levels in beef and pork. Free-ranging ducks were found to have the highest TCDD levels (up to 331 ppt), followed by the snakehead fish, which had the highest TCDD level (65 ppt) of any fish. In addition to TCDD, the researchers found elevated levels of PCBs as well as the insecticide DDT and its metabolites in some food samples. They also found hexachlorocyclohexanes and hexachlorobenzene, although the levels varied widely. Because these compounds can contribute to adverse health outcomes, with or without elevated dioxin levels, the researchers suggest that public health work focused on dioxins needs to be expanded to include these other toxic chemicals. (J. Occup. Environ. Med. 2003, 45, 781–788)
Estrogenic activity of cadmium Low levels of cadmium can mimic the effects of estrogen in vivo, according to new research published in the August issue of Nature Medicine. The findings, which were observed in rats, suggest that environmentally relevant concentrations of the heavy metal can act as an endocrine disrupter and have wide-ranging effects in the uterus and mammary gland. Mary Beth Martin and colleagues at Georgetown University, Hampton University, and the University of Pittsburgh removed the ovaries from female rats and, after a 3-week recovery period, exposed the rats to cadmium concentrations of 5 micrograms per kilogram (µg/kg) body weight. The researchers found that 4 days after treatment, there was a 1.9-fold increase in the rats’ uterine wet weight. When they exposed the rats to an antiestrogen, the effect was blocked, suggesting that the effects of cadmium are mediated by the estrogen receptor. The researchers also observed that exposure to cadmium promoted growth and development of the rats’ mammary glands. In a positive control group of rats, which received estradiol at a daily dose of 60 µg/kg, they saw a 3.8-fold increase in uterine wet weight and enhanced mammary gland growth. The researchers also found that in utero exposure to cadmium mimicked the effects of estrogen. In fe-
male offspring of rats exposed to cadmium, the onset of puberty was earlier and mammary gland development was enhanced compared with controls that were not exposed to cadmium or estradiol. Previous studies have shown that heavy metals, such as nickel and cadmium, form a high-affinity complex with the estrogen receptor and can act like a steroidal estrogen in breast cancer cells. The new results are the first to provide evidence that cadmium also has potent estrogenic activity in whole organisms. (Nat. Med. 2003, 9, 1081–1084)
in their shell hinge, and had increased death rates. Nearly a third of the larvae that survived developed into hermaphrodite adults. In contrast, there were no hermaphrodites in the control group. “These results are worrying,” says team leader Helen Nice, “not only because of the damage to the oysters, but also because this chemical may well be affecting other organisms sharing their environment. It is vital when chemicals are screened that the effects on vulnerable developmental stages be considered.” (Mar. Ecol. Prog. Ser. 2003, 256, 293–300)
Nonylphenol effects on oysters
Infant mortality linked to air pollution
British researchers have found that nonylphenol causes sexual deformities in oysters and produces large numbers of hermaphrodites at levels 10 times lower than currently recommended limits. The findings, which were published in Marine Ecology Progress Series on July 17, join other
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reports raising concerns about this widely used compound (Environ. Sci. Technol. 2001, 35, 2909–2916; Environ. Sci. Technol. 2002, 36, 10A–11A). Nonylphenol is a breakdown product from a surfactant commonly used in detergents, dispersing agents, herbicides, spermicides, and cosmetics. The U.K. Environment Agency’s recommended safe level in fresh and marine water is 1.0 microgram per liter (µg/L). Using concentrations of nonylphenol as low as 0.1 µg/L, scientists with the University of London found that developing embryos and larvae of the Pacific oyster—a commercially valuable bivalve—experienced delays in development, showed abnormalities
An estimated 2500 fewer infant deaths occurred during the U.S. recession of 1981–1982 because of a reduction in total suspended particles (TSPs) in the air, report economists Kenneth Chay and Michael Greenstone in the August issue of The Quarterly Journal of Economics. The analysis finds that reducing particulate pollution has substantial effects on infant mortality, particularly within one month of birth, even at levels below the U.S. EPA standard of 75 micrograms per cubic meter. TSPs are believed to be the most harmful form of air pollution. During the 1981–1982 economic recession, some areas experienced large reductions in TSPs, presumably because of reduced manufacturing. Using a series of complex mathematical and statistical analyses, the researchers found a strong correlation between reductions in TSPs and reduced health-related infant mortality. They were able to rule out effects from confounding factors, including natality variables, weather, income, and Medicaid payments, because compared with the large differences in pollution reductions from one county to the next, these other factors did not change as sharply or abruptly. The decline in mortality occurred primarily within one month of birth, a result suggesting that maternal exposure to TSPs can affect the fetus. In addition, the researchers saw a significant correlation between the decline in TSPs and increased birth weight; however, the difference was too small to account for the observed decline in mortality. (Quart. J. Econ. 2003, 118, 1121–1167)
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