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tering novel bacterial species that causeillnesses in people.

Scientists suspect that many microbialspecies responsible for human illnessesfirst preyed upon amoebas, unicellularorganisms found in aquatic environ-ments. Though microscopic, amoebasnormally are shaped like “a piece ofchewing gum that’s been chewed,” saysSharon G. Berk of Tennessee Techno-logical University in Cookeville. But afterthey’ve been infected by bacteria, theybecome spherical. Finally, they burst andrelease the bacteria, she says.

Well known among those pathogens areLegionella pneumophila bacteria, whichcause Legionnaires’ disease. Cooling tow-ers—structures that cool water streams forair conditioning, power generation, or otherpurposes—are a source of Legionella-infected amoebas. These towers cool waterby exposing it to outside air and then blow-ing out warm air. That airflow carries finewater droplets, which can release bacteria.

Berk and her colleagues wonderedwhether infected amoebas would be moreprevalent in cooling towers than in natu-ral water environments. They took sam-ples from cooling towers on hospitals andindustrial buildings and also from riversand lakes.

The researchers found infected amoebasin 22 of 40 cooling tower samples but inonly 3 of 40 natural samples. A statisticalanalysis revealed that cooling towers were16 times as likely to contain infected amoe-bas as were natural aquatic environments,the researchers report in an upcomingEnvironmental Science & Technology.

The scientists then attempted to growthe amoeba-infecting bacteria on labora-tory plates for identification but succeededfor only a small number of the bacteria. Ofthe strains identified from the cooling towersamples, three were L. pneumophila, threemore were new strains related to knownbacteria, and one was novel, notes Berk.

Berk says that she expects that manymore unidentified pathogens were living inthe amoebas. The researchers don’t knowwhy amoeba-infecting bacteria flourish inthe cooling tower environment.

The study “adds some insight relating toboth the environmental complexity andthe potential for emergence of new diseasefrom cooling towers,” says microbiologistRichard Bentham of Flinders Universityin Adelaide, Australia. “The implication ofcooling towers as ‘pathogen accelerators’makes them an intriguing research focus.”

James M. Barbaree, a microbiologist atAuburn University in Alabama, agrees thatconditions in cooling towers appear con-ducive to the growth of infected amoebas.But considering how many different waterenvironments there are, he adds, “I don’tthink you would say that it explicitly is goingto occur more in all cooling towers than[in] all natural samples.” —A. CUNNINGHAM

Mercury RisingNatural wildfires release pollutant

Fires in high-latitude forests and peaty soilsof the Northern Hemisphere may loft hun-dreds of tons of mercury into the atmos-phere each year, much more than scientistshad expected, a new analysis suggests.

Much of the world’s industrial emis-sions of this toxic pollutant originatesfrom the burning of coal contaminatedwith the element. “When it comes backto the ground, mercury forms strongchemical bonds with organic material,so it often gets locked away in rich for-est soils and in peat,” says Merritt R.Turetsky, an ecologist at Michigan StateUniversity in East Lansing.

Scientists estimate that industrialsources together with natural ones suchas volcanoes annually send between 4,400and 7,500 tons of mercury into the atmos-phere. Previous studies suggested thatwildfires in upland forests of the North-ern Hemisphere release about 23 tons ofthe pollutant each year. However, soil dataand new computer models now indicatethat wildfire emissions of mercury couldbe much higher, Turetsky and her col-

leagues report in the Aug. 28 Geophysi-cal Research Letters.

In fires in Alaskan and Canadian forests,much of the material that burns is twigs,moss, and other organic material on theground and in the soil, says Turetsky. Eachsquare meter of forest soil contains about3.4 milligrams of mercury. Concentrationsare even higher in peaty soils, where thedry surface layers hold about 11.5 mg/m2

of mercury, the team finds.Scientists had generally considered peat

lands not susceptible to fire, says Turet-sky. However, by examining the carboncontent of peat, she and her colleaguesrecently found that those areas burn, onaverage, once every century or so—a ratethat’s similar to that in northern forests.The scientists estimate that more than10,000 square kilometers of forest andpeat lands burn worldwide each year,sending more than 340 tons of mercuryinto the atmosphere.

“This is a sizable pool of mercury,” saysRichard Bindler, an environmental scien-tist at Umeå University in Sweden. Theamount of the pollutant returned to theatmosphere by fires is much larger thanthe quantity that leaves the forests andpeat lands via runoff, he notes.

The acreage burned each year in NorthAmerican boreal forests has approxi-mately doubled in the past few decades,says Eric S. Kasischke, a fire ecologist atthe University of Maryland at CollegePark. Moreover, increased droughtsanticipated in arctic regions could causewater tables to drop, leaving more mer-cury-tainted peat vulnerable to fire.“When water levels are low enough, asubstantial amount of peat can be con-sumed [by fire],” thereby boosting mer-cury emissions, he notes. —S. PERKINS

SCIENCENEWSThis Week

TAINTED SMOKE Plumes lofted from wildfires in Northern Hemisphere peat lands andforests, such as this Alaskan blaze in June 2003, annually release about 340 tons of mercury. TU

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