| Microorganisms are abundant in the atmosphere and can be transported globally on winds. In general, the upper atmosphere is an understudied, extremely harsh environment where the limits of life can be tested unlike anywhere else on our planet. However, the challenge of collecting samples (and historic reliance upon culture-based characterization methods) has prevented a comprehensive understanding of microbes at high altitudes. The goal of my dissertation is to examine the airborne transport of microorganisms across the Pacific Ocean using surface-, satellite-, aircraft-, and model-based observations. A variety of techniques were employed to achieve maximum sensitivity to airborne microbes (including dead and non-cultivable cells). Air was collected at the summit of the Mt. Bachelor Observatory (2.7 km above sea level in Oregon, USA) where free tropospheric plumes routinely arrive from distant, transpacific sources. Microbial abundance and richness values were derived from qPCR measurements and 16S rRNA microarray analysis. Thousands of distinct bacterial taxa (and 6 archaeal species) were detected in Asian air samples, spanning a wide range of phyla and surface environments. Viable cells (including presumptive plant pathogens) were identified by rRNA gene sequencing. Next, we flew a high-altitude (20 km) sampling mission over the Pacific Ocean in a period that coincided with intercontinental dust plumes. Several bacterial and fungal isolates were recovered and later exposed to a series of stratosphere simulations to evaluate survivability during transport. Most (99.9%) of cells were killed by ultraviolet irradiation within several hours, indicating that the upper atmosphere can be a critical barrier to long-distance microbial dispersal. Overall, we discovered the upper atmosphere contains a surprising degree of microbial richness and that atmospheric winds can bridge populations (containing living and dead cells) between Asia and North America. Our results inform a number of important environmental and population health problems on both sides of the Pacific. Furthermore, we demonstrate that microbial biogeography is a useful tool for validating aerosol source regions and global transport models. Ultimately, studying life in the upper atmosphere redefines our concept of the biosphere and can provide new insights about the diversity, distribution, and evolution of life on Earth. |
