Research▼Watch Lead linked to infertility
Researchers in Germany have found that an anaerobic bacterium, which is known to degrade chlorobenzenes, can also break down dioxins, including extremely toxic congeners. Although bacterial mixtures have been reported to transform selected dioxins via reductive dechlorination, this is the first pure culture shown to do so.
Exposure to lead damages sperm function and may contribute to declining sperm counts, according to a new study by fertility researchers. In light of these findings, the team believes environmental lead exposure limits should be re-evaluated. Susan Benoff of the Fertility Research Laboratories at the North Shore-Long Island Jewish Research Institute in Manhasset, N.Y. and colleagues investigated lead levels and sperm function in semen from the partners of 140 women undergoing their first in vitro fertilization cycle. The researchers found a wide range of lead levels and a significant association between high lead levels and low fertilization rates. “We have evidence that higher lead levels interfere both with the ability of the sperm to bind to the egg and with its ability to fertilize the egg,” says Benoff. The team obtained similar results when they exposed healthy sperm from nine fertile donors to increasing doses of lead. Although increased seminal plasma lead levels did not appear to affect male reproductive hormone function, they were associated with decreased sperm concentration, and sperm shape, form, and movement, suggesting that lead also acts in the testis. The findings were unexpected, as none of the men were in occupations likely to produce high lead exposure. Smoking and alcohol consumption were associated with high lead levels in only 29 of the patients. Benoff suggests other contributory factors such as lack of exercise or a diet low in calcium or high in lactose or fat, which can enhance lead accumulation. (Human Reproduction 2003, 18, 374–383)
LORENZ ADRIAN
Bacteria break down dioxins
A bacterium called CBDB1, which is shown here, thrives on chlorobenzenes and can also make a meal of dioxins.
Michael Bunge of the MartinLuther-Universität Halle-Wittenberg and colleagues identified strains of bacteria from sediments contaminated with polychlorinated dibenzo-pdioxins and dibenzofurans using polymerase chain reaction. Among the bacteria that were capable of dechlorinating dioxins were two strains belonging to the genus Dehalococcoides. One of the strains, called CBDB1, is the only bacterium known to dechlorinate chlorobenzenes, and the other has been shown to convert tetrachloroethene to ethene. The researchers confirmed that Dehalococcoides is indeed involved in dioxin dechlorination by studying the capability of CBDB1. They found that CBDB1 can transform several toxic dioxin congeners into less harmful ones, even without the addition of chlorobenzenes, indicating that the bacterium can use dioxins as respiratory electron acceptors. The results suggest that microorganisms of the Dehalococcoides cluster could be useful in bioremediating dioxin-contaminated sites. (Nature 2003, 421, 357–360) © 2003 American Chemical Society
Bacterial growth limited by phosphorus Recent efforts to regulate phosphorus runoff may be long overdue. In a study of bacterial growth in a pristine coastal salt marsh in South Carolina, re-
searchers found that phosphorus, not nitrogen, was the limiting nutrient. Moreover, as the levels of phosphorus dropped, microbial heterotrophs produced more N2O, suggesting that limiting phosphorus could reduce excess nitrogen in these marshes. Extensive earlier research has shown that nitrogen is the limiting nutrient in coastal areas, including salt marshes. This research confirms those findings for plants (macrophytes). However, when P. V. Sundareshwar, J. T. Morris, and their colleagues at the University of South Carolina and Coastal Carolina University added nitrogen or phosphorus to unfertilized marsh plots, bacterial growth was much higher with the latter nutrient. In fact, overall bacterial growth was limited by phosphorus and secondarily by labile carbon. These results, say the authors, show that ecosystems cannot be managed just by regulating a single nutrient. For example, although hypoxia in temperate coastal waters can be linked to high levels of nitrogen, phosphorus enrichment of “blackwater” rivers has been shown to increase biological oxygen demand. Moreover, phosphorus’s relationship with nitrogen proved to be complex. As expected, plants grew best when both nitrogen and phosphorus were present. Phosphorus is known to stimulate nitrogen fixation in temperate coastal water columns. However, in the marsh sediments, the nutrient reduced heterotrophic nitrogen fixation unless more carbon (in the form of glucose) was added, suggesting that phosphorus loading can limit carbon for microbial processes. On the other hand, limiting phosphorus increased the loss of nitrogen through the production of N2O. Overall, the researchers report that adding both nitrogen and phosphorus increased soil respiration and carbon turnover, which has important ramifications for climate change models factoring in carbon fixation, storage, and release. (Science 2003, 299, 563–565)
APRIL 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 135 A
