Researchers at the University of Washington report they've found evidence of 2,100 unique microbial species hitching rides across the Pacific Ocean in the upper troposphere.
"The long-range transport and surprising level of species richness in the upper atmosphere overturns traditional paradigms in aerobiology," David J. Smith, who recently earned a doctorate in biology and astrobiology, said. "It's a small world. Global wind circulation can move Earth's smallest types of life to just about anywhere."
An estimated 7.1 million tons of aerosols -- dust, pollutants and other atmospheric particles, including microorganisms -- cross the Pacific each year, researchers said, carried by wind storms into the upper reaches of the troposphere.
The troposphere, the layer of air closest to earth up to about 11 miles, is where almost all weather occurs.
Large plumes of aerosols in the troposphere can make the trans-Pacific trip in just seven to 10 days, the researchers said. Most of the microorganisms originated from soils in Asia and were either dead on arrival in North American or harmless to humans, they said.
However, while most of the species are already present in low background levels on the West Coast, the atmospheric plumes can bring elevated levels of the organisms, researchers said.
"I was very surprised at the concentrations. One might expect the concentrations of cells to decrease with altitude based on fallout and dilution," Smith said. "But during these plume events, the atmosphere was pooling these cells just as it does with other kinds of air pollution."