Your gut isn’t the only place that harbors a community of microbes. There are also microbiomes coating your skin and most household, industrial, and commercial surfaces. There's even a community hanging out in the lower atmosphere. Scientists in Spain have monitored this airborne microbiome by taking rain and snow samples every two weeks for seven years at a site in the central Pyrenees. The samples were then run through a DNA sequencer to reveal the airborne microbiome. They found that the bacteria, archaea, protists, and fungi all varied predictably by season.

In the wintertime, microbes frequently had marine origins, coming primarily from the Atlantic, although these were mixed in with bugs from forest and other terrestrial sources. Overall, the winter atmospheric microbiome was the most diverse, and that diversity included the highest levels of pathogens in any season.

In the summer, the microbiome was more regional, coming from the Mediterranean as well as fresh water, cropland, and cities. There was more pollution in the summer; the scientists monitored atmospheric levels of chemicals, including nitrates and sulphates, in addition to microbes. One of the most abundant and recurring taxa over the seven summers was Ramlibacter, related to a bacterium first isolated in 2011 from meteorite fragments buried in the sands near Tatouine, in Tunisia. It is specifically adapted to live in hot, dry, desert climes, so the researchers suggested that it could be used as a forensic signature for “summertime in Europe—African dust in the air.”

There were some constants. Bacteria were more prevalent than eukaryotes and archaea, although this finding could be biased by the relative lack of eukaryotic sequences in public databases. There were about 1,200 bacterial genera represented. 75 percent of the eukaryotes were fungi. Most of the species were already known. Some species were found year round, and some of these core group were found in the upper troposphere above the Caribbean Sea as well.

Water and air currents used to be the only forces moving microbes and their genes across the planet. Now, we are contributing, too. Once microbes get injected into the atmosphere, they can remain suspended for days before being transported by wind and dispersed by precipitation. Their movements may have consequences for ecology, agriculture, and human health.

Now that we know which species are more likely to be airborne when, we might be able to better prepare for their arrival and can look to correlate events on the ground to their presence.

PNAS, 2018. DOI: 10.1073/pnas.1812826115 (About DOIs).