Abstract
The composition and prevalence of
microorganisms in the middle-to-upper troposphere (8–15 km altitude)
and their role in aerosol-cloud-precipitation
interactions represent important,
unresolved questions for biological and atmospheric science. In
particular, airborne microorganisms
above the oceans remain essentially
uncharacterized, as most work to date is restricted to samples taken
near the Earth’s
surface. Here we report on the microbiome
of low- and high-altitude air masses sampled onboard the National
Aeronautics and
Space Administration DC-8 platform during
the 2010 Genesis and Rapid Intensification Processes campaign in the
Caribbean Sea.
The samples were collected in cloudy and
cloud-free air masses before, during, and after two major tropical
hurricanes, Earl
and Karl. Quantitative PCR and microscopy
revealed that viable bacterial cells represented on average around 20%
of the total
particles in the 0.25- to 1-μm diameter
range and were at least an order of magnitude more abundant than fungal
cells, suggesting
that bacteria represent an important and
underestimated fraction of micrometer-sized atmospheric aerosols. The
samples from
the two hurricanes were characterized by
significantly different bacterial communities, revealing that hurricanes
aerosolize
a large amount of new cells. Nonetheless,
17 bacterial taxa, including taxa that are known to use C1–C4 carbon
compounds present
in the atmosphere, were found in all
samples, indicating that these organisms possess traits that allow
survival in the troposphere.
The findings presented here suggest that
the microbiome is a dynamic and underappreciated aspect of the upper
troposphere
with potentially important impacts on the
hydrological cycle, clouds, and climate.
