Research▼Watch Oceanic sink for fixed nitrogen Bacterial conversion of ammonium to nitrogen may be a more important sink for fixed nitrogen than previously thought, according to two new studies. The findings suggest that denitrification—the reduction of nitrate to nitrogen by bacteria—is not the only significant process in anoxic waters that converts fixed inorganic nitrogen to gaseous nitrogen. Because primary production is often limited by the availability of fixed nitrogen (nitrate, nitrite, and ammonium), processes that remove such forms of nitrogen on a global scale can have a large effect on ecosystem function and biogeochemical cycles. The anaerobic oxidation of ammonium with nitrite to nitrogen, known as the anammox process, has been previously shown to produce significant amounts of gaseous nitrogen in some marine sediments. In the new studies, researchers show that the process is widespread in anoxic environments and could account for 30– 50% of oceanic nitrogen production. Tage Dalsgaard of the National Environmental Research Institute in Denmark and colleagues at the University of Southern Denmark and Universidad de Costa Rica found evidence of the anammox process in anoxic waters of Golfo Dulce, a 200-mdeep coastal bay in Costa Rica. Their experiments indicate a strong coupling between the ammonium produced by denitrification and further conversion of that ammonium to gaseous nitrogen by the anammox process. Because the chemistry of Golfo Dulce waters is similar to that of oxygen-depleted zones of the oceans, the researchers expect the process to be an important sink for oceanic nitrogen on a global scale. In a different study, Marcel Kuypers and colleagues at the Max Planck Institute for Marine Microbiology in Bremen, Germany, along with researchers in the Netherlands, found evidence that bacteria in the Black © 2003 American Chemical Society
Sea, the world’s largest anoxic basin, perform the anammox process. For the first time, the researchers identified the bacteria as belonging to the order Planctomycetales. They found
Figure Not Available for Use on the Web
the bacteria to be abundant in the Black Sea and directly linked to the removal of ammonium in both suboxic waters and sediments. (Nature 2003, 422, 606–608; 608–611)
“Safe” lead levels harmful to children Lead can be harmful to children even at levels typically thought to be safe, according to two new studies. In the first, scientists found that lead causes intellectual impairment in children at levels below the current acceptable blood-lead level of 10 micrograms per deciliter (µg/dL), and in a second report, researchers discovered that girls with blood-lead levels of 3 µg/dL showed signs of delayed growth and puberty. The findings suggest that the acceptable blood-lead level for children should be lowered. In the first study, Richard Canfield of Cornell University and colleagues measured blood-lead concentrations in 172 children at the ages of 6, 12, 18, 24, 36, 48, and 60 months and administered IQ tests to those children when they reached the ages of 3 and
5 years. After adjusting for factors such as maternal IQ and quality of the home environment, the researchers found that blood-lead concentrations were inversely associated with IQ. The results showed that the relationship between lead levels and IQ is nonlinear, and the effects are the greatest in children with blood-lead levels below 10 µg/dL. When lifetime average blood-lead levels increased from 1 to 10 µg/dL, the researchers saw a decrease in IQ of 7.4 points. When blood-lead levels increased from 10 to 30 µg/dL, they saw a decline in IQ of only 2–3 points. In a second, unrelated study, Sherry Selevan of the U.S. EPA and colleagues examined the relationship between blood-lead levels and pubertal development in 8- to 18-year-old girls enrolled in a nationally representative survey conducted by the U.S. Centers for Disease Control and Prevention. Girls with blood-lead levels of 3 µg/dL were shorter in height than those with levels of 1 µg/dL, and African-American and MexicanAmerican girls with blood-lead levels of 3 µg/dL had significant delays in breast and pubic hair development compared with girls with levels of 1 µg/dL. In non-Hispanic white girls, however, there were no significant delays in pubertal measures associated with blood-lead levels of 3 µg/dL. The study indicates that blood-lead levels commonly encountered in the U.S. population are affecting fundamental developmental processes in girls and raises the possibility that lead can also affect other hormonally mediated processes. (N. Engl. J. Med. 2003, 348, 1517–1526; 1527–1536)
Uncertainty analysis points to big CO2 cuts “It is even worse than we thought” could be the theme of this analysis. Ken Caldeira and colleagues at Lawrence Livermore National Laboratory, the University of Illinois at
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% carbon emissions–free primary power
Urbana–Champaign, and New York University ask the long-term question: What will it take to stabilize the CO2-induced warming of the atmosphere to just a 2 °C increase from preindustrial levels by the year 2150? Despite tremendous uncertainties in key values, the best-case prediction is that by the end of this century, more than 75% of primary power sources would have to be free of CO2 emissions, following a “business-as-usual” economic scenario.
100 4.5 °C 80
3 °C 2 °C
60
1.5 °C
40 20 0 2000
2020 2040 2060 Year
2080 2100
Regardless of the climate sensitivity value (over the curves), this analysis predicts that a significant percentage of the world’s power will have to come from carbon-free emitting sources by 2050 to hold climate warming to 2 °C. (Adapted with permission. Copyright 2003 American Association for the Advancement of Science)
The analysis centers on the climate sensitivity value, which is the global mean temperature change resulting from a doubling of atmospheric CO2 levels. Values for this variable range from 1.5 to 4.5 °C, which means that a 2 °C stabilization would have to hold CO2 levels at 700 to 380 ppm, respectively; the current CO2 atmospheric concentration has already reached 370 ppm. In the business-as-usual economic model, some energy is already generated by nuclear and renewable sources. Nevertheless, the authors find that action will be needed soon to stabilize the climate. According to their analysis, 900 ± 500 MW of new power generation that is free of carbon emissions would have to be brought online every day for the next 50 years to reach the 2 °C goal. Even with a more radical 4 °C stabilization goal, approximately 410 MW of new carbon-free power would be required every day. (Science 2003, 299, 2052– 2054)
PFOS is an endocrine disrupter For the first time, researchers have shown that perfluorooctane sulfonate (PFOS) crosses the blood–brain barrier and disrupts endocrine function, increasing levels of stress hormones and disrupting reproductive cycles. Although products such as 3M’s Scotchgard that contain compounds that degrade to PFOS have been withdrawn from the market, the mechanisms behind the toxic effects of PFOS may well pertain to other perfluorinated compounds whose use has been steadily increasing over the past three decades. Michigan State University neuroendocrinologist Sheba MohanKumar and colleagues injected groups of rats with either 1 or 10 milligrams of PFOS/kg body weight for 14 days. Both low and high doses of PFOS affected the reproductive cycles of female rats. At the low dose, 34% of the rats had irregular cycles, and at the high dose, 58% of the rats had irregular cycles. PFOS levels in the tissues of animals from the low-dose group were comparable to levels in people who are occupationally exposed. At the end of the treatment, the researchers dissected the rats’ brains, sampled regions of the hypothalamus, and measured neurotransmitters in the regions that regulate stress and reproduction. They also collected blood serum to measure the stress
Structure of perfluorooctane sulfonate
hormone corticosterone and leptin, a protein hormone involved in appetite regulation. Treatment with the high dose of PFOS increased blood corticosterone levels by about 75% and increased norepinephrine levels in the hypothalamus, indicating that PFOS treatment stimulated the stress system; however, the low dose had no effect. Previous studies in rats, mice, guinea pigs, rabbits, and monkeys have shown that exposure to PFOS affects the liver. PFOS has also been
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shown to decrease body weight, serum cholesterol, and triglycerides. Rats exposed to the high dose of PFOS in this study also lost weight and ate less. In addition, the highdose treatment decreased serum leptin levels, but the significance of this finding is unclear, according to the researchers. (Environ. Health Perspect. 2003, DOI: 10.1289/ehp. 6128)
Accidental exposure reveals low-level effects The eggs of female mice exposed to low levels of bisphenol A (BPA), a chemical used to make some common plastics and resins and a suspected endocrine disrupter, show “highly significant” increases in chromosomal abnormalities, according to new research by Patricia Hunt of Case Western Reserve University and colleagues. The findings raise questions about human exposure to low levels of the chemical. The research was inspired by an accident. The wrong agent was used to clean the plastic cages housing mice involved in a study on age-related changes in the viability of mammalian eggs, or oocytes. The cleansing agent caused BPA to leach out of the plastic cages, exposing the mice to the chemical. As a result, the incidence of eggs exhibiting the chromosomal defects characteristic of aneuploidy—the leading cause of miscarriages, congenital defects, and mental retardation— rose dramatically for the exposed mice. The effects were observed at or below BPA levels currently considered to be safe for humans. In a later phase of the project, the researchers intentionally exposed mice to similar levels of BPA. “Relatively modest” doses of 0.02 milligrams of BPA per kilogram of body weight during the crucial period when the mouse eggs are ripening altered the eggs’ ability to divide properly after fertilization. The results suggest that low-level BPA exposure can impact future generations, although the researchers stress that important questions remain, such as when the critical exposure period may be. They say that methods for evaluating the incidence of aneuploid mammalian eggs could one day become the basis of new tests to detect endocrine disruption. (Current Biology 2003, 13, 546–553)
