Microbes in the atmosphere: prevalence, species composition, and relevance to cloud formatio
De Leon, Natasha Isabel
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The composition and prevalence of microorganisms in the upper troposphere and their role in aerosol-cloud-precipitation interactions represent important, unresolved questions for biological and atmospheric sciences. Most studies to date were restricted to samples taken near the Earth’s surface and/or to laboratory incubations that do not simulate well in-situ conditions. Further, the ability of most microbial taxa to serve (or not) as cloud condensation nuclei (CCN) remains uncharacterized. Therefore, the major objectives of this research effort were to characterize the composition and relative abundance of airborne bacteria in the troposphere, and measure their CCN activity under different growth conditions. To this end, low- and high-altitude air masses were sampled before, during, and after two tropical storms, and the microorganisms present in the samples were assessed based on quantitative PCR and microscopy. Viable bacterial cells represented on average around 20% of the total particles in the 0.25- to 1-μm diameter range, revealing that bacteria represent an important and underestimated fraction of coarse mode aerosols. Twenty bacterial isolates were recovered from these and additional rain samples, and the degree of their cell hygroscopicity was measured based on the contact angle of the bacterial cells with water. A wide range of contact angles was observed, with isolates ranging from very hydrophilic to very hydrophobic. The CCN activity of each isolate was studied by introducing aerosolized bacteria into a continuous flow stream-wise thermal gradient CCN counter. Hydrophilic bacteria were found to have a critical supersaturation of 0.1% compared to hydrophobic bacteria, which showed a critical supersaturation of 0.2% or higher. These supersaturation conditions are relevant for cloud formation in continental areas. Collectively, these findings suggested that airborne bacterial cells represent an underappreciated aspect of the troposphere, with potentially significant impacts on the hydrological cycle, clouds, and climate.