Research M Watch Increased plant density in northern latitudes
Some of the same chemical compounds that interfere with hormone-signaling pathways in animal cells can also interfere with important signaling pathways in plants, according to new evidence reported by John McLachlan and colleagues at Tulane and Xavier Universities in New Orleans. The finding adds to an already long list of concerns over the impacts of these so-called endocrine disrupters in the environment. Phytochemically activated pathways, such as those necessary for nitrogen fixation, are of particular concern in plants exposed to hormone-mimicking chemicals. For nitrogen fixation to occur, chemical signals must be exchanged between plants and rhizobial bacteria. When the researchers exposed alfalfa (Medicago sativa) to a variety of endocrine-disrupting chemicals, they found that planar phenolic compounds, including the insecticide DDT, the plasticizer bisphenol A, and the herbicides 2,4-D and 2,4,5-T, inhibited the symbiotic signaling pathway between the alfalfa and rhizobial bacteria (Sinorhizobium meliloti). Nonplanar compounds, including endosulfan, methoxychlor, aldrin, dieldrin, dursban, vinclozolin, and diazinon, which do not contain a free hydroxyl group, however, were found to have no effect on the signaling pathway that leads to nitrogen fixation. The researchers believe that the planar phenolic chemicals interfere with alfalfa’s ability to initiate symbiosis with S. meliloti by antagonizing the induction of common nodulation (nod) genes. By increasing the concentration of luteolin, a flavonoid that activates nod genes, they found that inhibition of the phytochemically activated pathway could be overcome. (Nature 2001, 413, 128–129)
Plants growing above 40 degrees north latitude, from New York to Madrid to Beijing, have experienced increasingly lush growth over the past two decades, according to satellite data obtained by Liming Zhou and colleagues at Boston University and NASA Goddard Space Flight Center. The rise in temperatures, linked to an increase in greenhouse gases in the atmosphere, is one possible explanation for the observed increase in northern vegetation. Although the area of vegetation does not appear to be expanding, the density of existing plant life in north-
ern latitudes is increasing. This increase is more pronounced in Eurasia than in North America, which correlates well with observed warming trends. In Eurasia, which has seen more warming than North America since the 1970s, the average growing season is now about 18 days longer than in 1981, whereas in North America the growing season has been extended by about 12 days, the researchers report. If northern forests continue to grow more vigorously, they would be expected to absorb more carbon dioxide from the atmosphere, which could have a significant impact on the global carbon cycle. (J. Geophys. Res. 2001, 106 (D17), 20,069–20,083)
NASA GODDARD SPACE FLIGHT CENTER
Nitrogen fixation inhibited by endocrine disrupters
Above 40degreesnorth latitude,plantshave experienced increasinglylush grow th overthe past20years,particularlyin Eurasia. NOVEMBER 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Antibiotics, pesticides, exotic species, and even fishing nets are causing rapid evolution in organisms at unprecedented rates. The overall impact of human activities on evolutionary processes is so marked that it may be the world’s dominant evolutionary force, affecting everything from viruses to snails to weeds, reports Harvard University biologist, Stephen Palumbi. Impacts are well documented, not likely to decline, and must be addressed, Palumbi contends. He notes, for example, that some bacteria, such as Staphylococcus aureus, are increasingly common in hospitals and are already unresponsive to all but the most powerful antibiotics. In addition, some insect pests have become so insecticide-tolerant that chemical control is useless. Plans for introducing new drugs, health policies, pesticides, and biotechnology products ought to include evolutionary processes as an important consideration in analyses of potential impacts, he suggests. Palumbi believes that recognizing the speed and pervasiveness of such evolutionary changes, predicting evolutionary trajectories, and advance planning of control mechanisms are key to slowing evolutionary changes and reducing impacts on species. Such planned actions could also reduce the economic and social costs of unintended evolutionary processes, he says. Not responding to these challenges will result in playing an expensive catch-up game when chemical control agents and medications fail, he notes. Palumbi has a valid point. The annual cost of this human-accelerated evolution is about $33–$50 billion in the United States alone. (Science 2001, 293, 1786–1790).
Bacteria detect organic mercury Finnish researchers Marko Virta and colleagues at the University of Turku have constructed a sensor that uses recombinant whole cell bacteria for detecting organic mercury compounds. The high-throughput sensor measures the biologically available fraction, as opposed to the total amount of mercury present, and it is inexpensive, portable, and far less labor-intensive than traditional methods. 444 A
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MINNA-LIISA ÄNKÖ
Evolutionary consequences of human activities
A 96-w ellplate containing three differentconcentrationsofbacteria glow sin the presence of organic mercury.The tw o brightestupperrow shave the highestnumberofbacteria,and the concentration ofmethylmercuryincreasesfrom leftto right.
A mercury-sensitive regulatory part of the bacteria, Serratia marcescens, acts as a receptor, and a firefly luciferase gene serves as a reporter. In the presence of organic mercury compounds, the receptor activates the expression of luciferase, which is measured as glowing luminescence. Ideally, the sensor would distinguish between types of inorganic and organic mercury by also including bacteria sensitive to only inorganic mercury. The detection limit of the sensor is somewhat higher than liquid and gas chromatography, but according to the researchers, should be sensitive enough for detecting methylmercury in contaminated water and sediment samples. (Anal. Chem. 2001, 73, 5168–5171)
Predicting an ecosystem’s future An updated method for predicting species adaptation in changing ecosystems has been developed that accounts for interactions between many different species. Traditionally, a researcher might use a “snapshot view” of the system at a particular point in time to predict the average response of the species to ecosystem changes. That is a static type of model, which assumes that an ecosystem, or a species in it, will remain in equilibrium and never evolve in response to environmental changes. Jon Norberg of Stockholm University, Simon Levin of Princeton
ENVIRONMENTAL SCIENCE & TECHNOLOGY / NOVEMBER 1, 2001
University, and their colleagues have now developed a complex mathematical model that represents an ecosystem as they see it: one that is dynamic and in which many different species interact and adapt. To do this, they look at species adaptation within “functional groups”—groups of species that interact together within a given environment. For example, a species of tree might be damaged by environmental changes that reduce available sunlight. Those changes might, however, allow other species in the same ecosystem to thrive. The new model allows scientists to manipulate various environmental parameters within an ecosystem, such as temperature, humidity, and nutrient availability. This approach, Norberg and Levin believe, will lead to a more precise prediction of evolutionary processes under changing conditions. Their predictive ecosystem modeling approach uses schemes developed for quantitative genetics problems. The model’s principal framework suggests that phenotypic variance within functional groups is linearly related to the ability of flora and fauna to respond to environmental changes. Thus, the long-term productivity of a group of species with high phenotypic variance may be greater than the productivity of the best single species, despite shortterm decreases in productivity through high phenotypic variance. (Proc. Natl. Acad. Sci. USA, 2001, 98 11,376–11,381)
