Free Tropospheric Transport of Microorganisms from Asia to North America
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Microorganisms are abundant in the troposphere and can be transported vast distances on prevailing winds. This study measures the abundance and diversity of airborne bacteria and fungi sampled at the Mt. Bachelor Observatory (located 2.7 km above sea level in North America) where incoming free tropospheric air routinely arrives from distant sources across the Pacific Ocean, including Asia. Overall deoxyribonucleic acid (DNA) concentrations for microorganisms in the free troposphere, derived from quantitative polymerase chain reaction assays, averaged 4.94 × 10−5 ng DNA m−3 for bacteria and 4.77 × 10−3 ng DNA m−3 for fungi. Aerosols occasionally corresponded with microbial abundance, most often in the springtime. Viable cells were recovered from 27.4 % of bacterial and 47.6 % of fungal samples (N = 124), with 49 different species identified by ribosomal DNA gene sequencing. The number of microbial isolates rose significantly above baseline values on 22–23 April 2011 and 13–15 May 2011. Both events were analyzed in detail, revealing distinct free tropospheric chemistries (e.g., low water vapor, high aerosols, carbon monoxide, and ozone) useful for ruling out boundary layer contamination. Kinematic back trajectory modeling suggested air from these events probably originated near China or Japan. Even after traveling for 10 days across the Pacific Ocean in the free troposphere, diverse and viable microbial populations, including presumptive plant pathogens Alternaria infectoria and Chaetomium globosum, were detected in Asian air samples. Establishing a connection between the intercontinental transport of microorganisms and specific diseases in North America will require follow-up investigations on both sides of the Pacific Ocean.
KeywordsMicrobial Abundance Free Troposphere Brevibacillus Airborne Bacterium Microbial Concentration
Research funding was provided by the National Science Foundation (NSF) Integrative Graduate Education and Research Traineeship (IGERT) program at the University of Washington (UW) Graduate Program in Astrobiology, the National Geographic Society/Waitt Grants Program (W177-11), the NASA Astrobiology Institute Director's Discretionary Fund, the Washington NASA Space Grant Consortium, and the UW Biology Department (Sargent Award). The Mt. Bachelor Observatory atmospheric chemistry measurements were funded by NSF Grant ATM-0724327. Critical sampling support was made possible by Bryan Hicks, Patrick Ball, Carol Higginbotham, Tom Lomax, and the staff at Mt. Bachelor Ski Resort. The authors are grateful to Victoria Long, Clara Wright, and Phil Howard (NASA KSC) for assistance with SEM imaging and John Catechis, Gerard Newsham, and Martin Hayes (NASA KSC) for help with sequencing. We would also like to thank Ray Wheeler (NASA KSC) and the anonymous manuscript reviewers for their time and feedback. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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