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Archipelago skyI’ve already written several times about the bacteria in the clouds and what they do up there; now, a new study from the Georgia Institue of Technology in Atlanta has described the communities formed by the bugs in the sky.  In a paper published in the journal PNAS, the researchers detailed the communities of skybugs and how their composition is affected by storms, giving us a better understanding of life in the sky. Not only might this help us better understand atmospheric chemistry, but it may also shed further light on how microbes spread, which could impact the dynamics of everything from ecosystems to diseases.

One of the things that makes this study different from earlier ones is where they collected their samples from.  In most of the work I’ve written about previously, the researchers took samples from air over land, usually just a few kilometers up.  In this case, though, Natasha DeLeon-Rodriguez and her colleagues took advantage of a NASA project researching tropical hurricanes over the Caribbean.  By hitching a ride, the team was able to sample a habitat that hasn’t been sampled before — cloud water 10km above the open ocean — and to collect samples before, after and during two hurricanes in 2010.  They also collected cloud water off the coast of California and on a flight from California to Florida, as well as from several flights over the Gulf of Mexico, the Caribbean Sea and the midwestern Atlantic.

To identify the kinds of bacteria and fungi in their samples, the team extracted DNA, sequenced a fragment of a gene, and compared the sequences against known sequences.  They also estimated the number of cells in their samples by dividing the number of copies of the gene they found by the average number in a bacterial or fungal cell.  This gave them an estimate of about 5000 bacteria per cubic meter and 10-100 times fewer fungal cells — probably because fungal cells are larger and so can’t stay in the air for as long — which were divided into around 300 species.  That’s not much when compared with communities on the surface, where a single gram of soil can have thousands of species of bacteria and millions (or even billions) of individual cells.

Even though the cloud-borne communities might not be as rich or complex as those down here, the researchers noticed a few interesting things about their make-up.  Samples from higher altitudes had a different composition from those at lower altitudes; the researchers suggest that this might be because species that are better at nucleating ice get washed out and don’t make it to higher altitudes.  The hurricanes also had a big impact on the community composition — different kinds of bacteria were found in samples before, after and during the hurricanes, which seemed to bring up many new microbial cells into the atmosphere.  By tracing back the path of the clouds and comparing their genetic data against the greengenes database, the researchers hoped to figure out where the bacteria in their samples came from.  They found bacteria originating from nearly all the habitats on Earth, but most of the bacteria came from aquatic habitats.  This highlighted another unique feature of the samples from hurricanes, which were the only ones with significant quantities of bacteria associated with human feces — probably because they passed over populated areas.

Perhaps the most striking finding was that, despite all of the variability in community structure, 17 species were present in all of the samples.  The researchers think that these might be the core members of the microbiome of the clouds, capable of surviving and persisting for long periods at the high altitudes of the clouds, where harsh conditions like UV light, dessication, and oxidation present daunting challenges to most life.  Some of these organisms can break down oxalic acid, a common chemical in clouds, and use it as an energy source.  “For these organisms, perhaps, the conditions may not be that harsh,” said Konstantinos Konstantinidis, an assistant professor at the Georgia Institute of Technology who led this study. “I wouldn’t be surprised if there is active life and growth in clouds, but this is something we cannot say for sure now.”

Part of my reason for writing about bacteria in the clouds again is to show how science proceeds.  Despite the popular image of dramatic breakthroughs, most science doesn’t happen that way.  Instead, we accumulate knowledge bit by bit and try to piece it together, asking questions and gleaning nuggets of insight as we go.  It’s only afterwards, looking back, that we can see how far we’ve come.  Every study of these microbes uncovers a bit more about them, slowly forming a coherent picture.  This study showed that bacteria aren’t just floating around in the clouds or using them to get from one place to another — some bacteria actually make their living up there.  That’s the other reason I wrote about this — I just love that image!  It’s such a wonderful example of how strange this rich and complex world can be.  The surface of the planet is teeming with life and we know that microbes play an extremely important role in every habitat down here.  With growing evidence that they also persist in aerial habitats and make cloud-borne ecosystems, it’s important to consider what impact they might be having.

Deleon-Rodriguez N, Lathem TL, Rodriguez-R LM, Barazesh JM, Anderson BE, Beyersdorf AJ, Ziemba LD, Bergin M, Nenes A, & Konstantinidis KT (2013). Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications. Proceedings of the National Academy of Sciences of the United States of America, 110 (7), 2575-80 PMID: 23359712

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