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Smoke's Surprising Secret
When high school student Sarah Mims set up an experiment in her family's yard in the spring of 2002, she was just expecting to continue her research from the previous summer. Instead, she made a new discovery.

In her project from the previous summer, she demonstrated that some of the dust in the air around her hometown of Seguin, Texas, had blown across the Atlantic Ocean from Africa's Sahara Desert, more than 4,000 kilometers away. This time around, she set up microscope slides and a continuous-operation air filter on a deer stand-turned observation tower in the hopes of collecting dust from Asia that arrives in Texas in the spring. Mims was going to check the dust samples for the presence of fungus and bacteria.
Model data of Aerosols July 26, 2001

Dust, smoke, and other aerosols move all around the world. This map of aerosol optical depth (calculated by computer models) shows the dust storms in the Sahara Desert that brought dust all the way to the United States on July 26, 2001. Click on the image to see the movements of aerosols in June, July, and August of 2001. Credit: NASA

Sarah set out her microscope slides along with Petrifilms--strips of gel and nutrients that can be used to grow microorganisms like bacteria and mold. Within a few days, the gel strips were practically a jungle of different colored splotches, each one a separate fungal colony. Large numbers of fungal spores were also visible on the microscope slides Sarah had exposed to the air almost every day between April 25 and May 17. There was just one problem, at least from her "Asian-dust-carrying-microbes" hypothesis: the slides didn't show any Asian dust!

Photograph of red sunset

Spectacular sunsets are caused by small particles in the air called aerosols. In the summer of 2001, dust blown from the Sahara Desert frequently contributed to brilliantly colored evening skies over Texas. (Photograph by Sarah A. Mims)

What Sarah did see on the slides was a lot of black carbon particles, in other words, soot. To find out why, Mims turned to satellite images. Observations from the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) during that time showed huge plumes of smoke from forest fires in southern Mexico and Central America crossing the Gulf of Mexico and blowing over Texas. Could the Central American smoke be bringing the fungal spores with it?

Sarah counted the number of carbon particles on her slides and compared that to the number of spores. She found that when the number of carbon particles (smoke) went up, the number of certain kinds of spores--but not all kinds--went up, too. This is exactly what you would expect to see if some of the spores came from local sources and some came along with the smoke.

To demonstrate that smoke can harbor fungal spores, Mims set fire to several different kinds of organic material and held Petrifilms out in the smoke to collect any spores that might be released. Her spore-covered films were clear evidence that spores can be released in the smoke from burning vegetation.

Unpredictable fungi

Micrograph of dust

Using a microscope Mims found that the particles she captured in Texas matched published images of Saharan dust. (Micrograph by Sarah A. Mims)

"I think it's a fascinating possibility," says Gene Shinn, a scientist with the U.S. Geological Survey's Coastal and Marine Geology Program, based in Florida. Shinn and his colleagues have been culturing bacteria from African dust blowing over the Caribbean for many years, linking the decline of Caribbean coral reefs in the 1970s, 80s, and 90s to bacterial infections brought over the sea with Saharan dust. While the air samples they collect almost always contain some bacteria that have hitched a ride across the Atlantic on the dust particles, Shinn says the presence or absence of fungi is unpredictable.

"Sometimes there are fungi in the samples, and sometimes not. We really had no idea why this was the case. We never considered smoke as a possibility, but there is periodic widespread biomass burning that takes places in the African Congo, and that smoke could be coming across the Atlantic mixed with dust. Here in Florida, we also get a lot of the smoke from fires in Central America. We have felt all along there had to be some additional factor explaining the fungi, and it's intriguing to think this could be it."

Gathering More Evidence

Photograph of Sarah Mims

Sarah Mims, a teenager from Seguin, Texas, is studying the smoke and dust that arrive over her hometown from thousands of miles away. By her senior year in high school, she had already made some surprising discoveries. (Photograph by Forrest M. Mims III) Credit: NASA

Sarah's senior year project, carried out this past summer, was designed to gather stronger evidence that the spores came from Central American smoke, not from local fungi. To reduce the chances that the spores were from Texas, Sarah needed to collect air samples as close to the open waters of the Gulf of Mexico as possible, and from high up in the air. To solve the first problem, she chose to collect samples at Padre Island, on the Texas coast. To solve the second problem, she made her own device for collecting samples. "I took a plastic cup and cut holes in the rim, where you drink, and then attached it to the kite with string. Then I pressed my microscope slides into the bottom of the cup."

In the summer of 2003, she analyzed the two main kite samples and found both spores and soot. Back-trajectories maps (maps that show where air has been) showed that the air over Padre Island the day she was flying her kite had originated in the Yucatan two days earlier, where plenty of fires were burning. These latest observations--collected far from land and way up in the air—once again linked smoke and spores, making Sarah a lot more confident that the spores are from Central America. Her amazing discovery is sure to be the start of this young scientist's promising career.

For more information and images on Sarah Mims' research see NASA's Earth Observatory

Rebecca Lindsey
NASA Earth Observatory