Viruses and bacteria are everywhere. Coating our bodies, covering our bedspreads, lurking in hospital catheter tubes. You name it, they’re probably there. To some degree, most people are aware of it. However, we tend to think in terrestrial terrestrial terms, as if microorganisms are inherently surface bound. Evidently they’re not. They’re floating in the clouds and even above them. And every day, they are falling out of the sky and back down to earth (Yes, on top of us, as well). Curtis Suttle, a researcher at the University of British Columbia, studies the virosphere and the extreme places viruses can be found. His research led him to turn his gaze from the Earth up to the sky. He discussed some of his research with SCINQ.
SCIENTIFIC INQUIRER: Let’s start at the beginning of the flying virus cycle. How do they manage to go from the Earth’s surface and into the troposphere?
CURTIS SUTTLE: Viruses are extremely small. If an average virus was the size of a pinhead, and I was scaled by the same amount, I would be about 50 km high or 31 miles tall. Because they are so small, they can be swept off the surface of the Earth, high up into the atmosphere. However, most of the viruses (and bacteria) are associated with very small particles that can also be swept up above the planetary boundary layer. This is above where most weather occurs and where there is little friction between the air and the Earth’s surface.
SI: What are the most prevalent viruses you find floating in the sky? Are any of them potentially dangerous?
CS: Almost certainly, the vast majority of viruses circulating in the atmosphere will be bacteriophages, often called phages. These are viruses that infect bacteria, and are unable to infect animals or plants.
SI: What is the general timespan for which these long residence viruses can remain airborne? How do these viruses stay in the air for so long?
CS: The residence time is a function of the particle size. Even though most viruses are particle associated, the particles are extremely small and hence can remain airborne for many days. Dust from the Sahara Desert makes it from Africa to North America; hence, viruses and bacteria would not have trouble making this journey, as well. My co-author, who designed the study (cc’d above) provided the following explanation: “To determine residence times (days) we need to know the concentration of viruses in the atmosphere (i.e. viruses /m2 of air integrating the air column) then divided by the deposition rates (viruses/m2/day) assuming steady state. The problem is to obtain an integration of atmosphere column (an integration similar to layers in the ocean)- Low troposphere layers should have higher concentrations and very variable across space (similar to the epipelagic ocean) than the high troposphere (more homogeneous-similar to the deep ocean), so there is not an easy answer to your question”
SI: Is there an optimal size and structure for viruses that can get swept up into the troposphere? At what size do microorganisms become too large to make the journey up?
CS: This will be a function of wind speed, particle size and particle density; therefore, there is not an easy answer. However, it is safe to say that the smaller and less dense the particle, the higher probability of being swept up into the troposphere.
SI: Most of the viruses remain in the troposphere before descending back to earth. But why don’t they get swept into the stratosphere or even out to space?
CS: Although we didn’t sample the stratosphere, almost certainly there will be viruses and bacteria present, as well, albeit at lower abundances. I don’t know about space, but I have often argued that if we want to look for life on other planets, we should be looking for viruses in the planet’s atmosphere. Viruses are by far the most abundant biological entities, they readily aerosolize, and we have the methods to detect as little as a single virus particle.
SI: How can you tell a virus has been swept into the air? What is its signature? Does being in the earth’s atmosphere — albeit a low part — change the viruses in any way?
CS: There is not a specific signature for an aerosolized virus. In the atmosphere, viruses and bacteria will be exposed to higher levels of UV and other radiation that would potentially reduce their viability by damaging their nucleic acids and proteins.
SI: Are there any other microorganisms in the sky? Bacteria?
CS: Yes, in our paper we also discuss the high levels of bacteria, although the deposition rates of bacteria ranged from about 10 to several hundred fold less than that of viruses.
SI: So when we are outside, are we literally being bombarded with microorganisms?
CS: Yes, with every breath we are inhaling thousands of microbes. If we swim in a lake or in the ocean, on average we would swallow several hundred million viruses, just from the amount of water we take into our mouths. However, these viruses don’t have any negative health consequences for us. In fact, we would not be able to exist without them. They are an essential part of the machinery that allows life to exist on Earth.
SI: Can you briefly describe how you conducted your study thousands of feet in the air?
CS: Essentially, my colleagues placed collectors on mountain peaks in the Sierra Nevada Mountains in Spain, at about 3000m elevation, and measured deposition rates over several months during a two-year period.
SI: How do the long-residence viruses in the sky fit into the larger scheme of the virosphere?
CS: Viruses are found wherever life exists, and are very much a part of the essential biological functions on the planet that allow life to exist. Our study also provides an explanation to a puzzling observation that we made more than 10 years ago that identical viruses can be found in very different environments (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC544240/). The possible explanations were that the organisms that these viruses infected were found in all of these environments (unlikely), or that viruses must be dispersed over vast distances. This study shows that dispersal is the likely explanation.
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