Scientists have now quantified the number of viruses that are transported in the clouds and the rate at which they fall back to the surface. The team included researchers from Universidad de Granada, San Diego University and the University of British Columbia. There is a wealth of evidence suggesting that bacteria and viruses are carried aloft attached to small particles and transported around the globe. Previous studies have investigated this process for bacteria and near-surface viruses. The experiment began in 2007 and this is the first time anyone has tried to quantify high altitude virus transport and compare the sources and deposition rates of viruses and bacteria. One of the senior authors, Curtis Scuttle, released a statement saying ‘Every day, more than 800 million viruses are deposited per square metre above the planetary boundary layer’. Very few of these viruses are likely to cause any illness in humans. Not only would they need to infect a suitable host wherever they happen to land in order to replicate, but the majority are bacteriophages which drive natural selection in many of Earths bacterial species.
This study focused on particles in the free troposphere, the layer of Earth’s atmosphere between 2.5-3.0km above the surface. This is above the effect of most weather systems but just below the fast flowing jet stream. At this altitude, there are fewer contaminants from local sources and the dispersion of microbes, both viruses and bacteria, is more uniform.
Viruses and bacteria are more often attached to organic particles than free floating. Dust storms in deserts can throw particles high enough to introduce a lot of microbes into the atmosphere. The Saharan desert generates a global dust belt that covers most of the northern hemisphere. Seaspray and cyclones over the Atlantic ocean can also aerosolise particles carrying microbes. Despite not being able to establish the exact origin of the particles, the scientists used NASA weather reports in the days leading up to each collection to determine the source of each sample as either Atlantic or Saharan.
The scientists set up collectors 3km above sea level in the Sierra Nevada mountains (Spain), chosen because it is under the influence of both the global dust belt and winds blowing off the Atlantic ocean. The particle collectors used could also distinguish whether the particles fell during wet and dry conditions and separated the samples accordingly. A proportion of the samples were left untreated to count the free-floating viruses and bacteria while the rest were chemically and mechanically treated to separate the microbes from any organic particles they were attached to.
The study deposition rate of bacteria was significantly higher in wet conditions than on days when the weather was dry. This supports a long-held theory that aerosolised particles act as condensation nuclei for droplets to form around, promoting rain which washes bacteria out of the atmosphere. As viruses are the most abundant microbes on earth, far outnumbering bacteria, the overall number of viruses falling on the collectors was greater than bacteria. The rate at which viruses fell did not seem to be affected by the weather but was affected by the source of the particles. Viruses were deposited at a rate 52-fold higher than bacteria when the Atlantic was the source but only 28-fold higher when coming from the Sahara.
The scientists filtered the particles by size and found there were proportionally many more viruses on particles <0.7mm in diameter. This allows them to stay in the atmosphere for longer and be carried further over the ocean. They also found that the particles carried up by Saharan dust intrusions were generally larger, accounting for the discrepancy in virus to bacteria ratios depending on the source.
While bacterial viability and virus-host availability are not guaranteed, evidence suggests many bacteria and viruses can survive atmospheric transport. This research illuminates a mechanism for long-range virus dispersal, explaining why we have been observing genetically similar viruses in disparate ecosystems for 20 years. This model allows for local genetic variation but global similarity, or the seed bank model, which allows ecosystems to adapt quickly to changes by frequently seeding the microbial population with new genetic material.