The Heaven is Houseful: High-Flying Microbes of Our Sky

Imagine you are a kid again. Do you remember how you dreamed about flying? Haven’t you ever wondered even if for a moment, ‘what if I can fly?’ Were not the brilliant white clouds floating against the bright blue sky were mesmerizing? Well, till now we haven't flown; not at least on our own. However, microbes can! We have found microbes from as high as 77 Km above the earth, alive. Just to put things into perspective; the Mount Everest is less than 9 Km high, and a passenger plane usually flies at a maximum height of 12 Km. Every year around 1 Billion Trillion (1021) microbial cells fly away into the atmosphere!

Microbes do not have wings. So, how are they flying? They are so small that they can literally ride the winds. Microorganisms are everywhere, on (and in) every surface you see, including soil, water, plants, and even yourself. They can enter the layer of air (gases) surrounding our planet (atmosphere) as a suspension of tiny particles or droplets (aerosol particles). However, usually, they just sit on these surfaces. They need a little motivation to make the jump from sitting on a surface to being airborne. You give them a little energy, and you will have a successful lift-off. This lift-off energy comes from a diverse source from gentle breeze on the land to bursting of small water bubbles on wave crests on the ocean.

Once the microbes are airborne, they can ride the air currents to sky-high. These microbes in the sky can keep flying in the atmosphere, from several days to years, before the touchdown. During their flight, they can travel vast distances. These distances can span almost the whole hemisphere that they took off from. But, how high can they fly? We have found bacteria like Bacillus, Micrococcus, Mycobacterium, and fungi like Aspergillus, Circinella, Papulaspora, Penicillium from as high as 77 Km. However, they had to overcome a barrier to reach such heights. The barrier is the tropopause. It is the border between the troposphere (0-17 Km) and the stratosphere (17-50 Km) located at nearly 17 Km from the earth. The tropopause is an aberration from the typical change of an atmospheric property with height (inversion layer). It does not allow much mixing between the layers of the atmosphere that it separates. Fortunately for the airborne microbes, there are other ways to cross the tropopause than air current.

Aerosols including microbial cells can be pushed from the troposphere to the stratosphere by a few violent, and gentle ways. The volcanic eruptions can eject the microbes through the tropopause into the stratosphere. It is also possible that microorganisms are shot to the stratosphere by the updraft of conical blue jets that travel upwards from the upside of the clouds (blue lightning strikes). The forest fires feed energy into thunderstorms. So, if thunderstorms and forest fires occur together, they can push plumes of smoke, soot, and microorganisms into the stratosphere. Alternatively, the microbes can be transported slowly but steadily, instead of eruptions, thunders, and fires. It is called the gravito-photophoresis. The gravito-photophoresis is a phenomenon where very tiny particles (aerosols) travel towards the light when intense light beam shines on the particles. When the sun shines brightly on the flying microbes, they start to float towards the sun, upwards. Now, the microbes are in the troposphere, stratosphere and even higher. Who are these high-flyers?

We have captured microbes from altitudes of a couple of kilometers to almost 80 Km (upper region of the mesosphere). The Bacteria are mostly Actinobacteria, Bacillus, several Proteobacteria, Brevibacterium, Deinococcus, Micrococcus, Mycobacterium, Paenibacillus, Proteus, and Staphylococcus. The flying fungi are Aspergillus, Circinella, Engyodontium, Eurotiomycetes, Macrosporium, Papulaspora, Penicillium, and Rhizopus. Spending time in the sky is not as much fun as you would like to think. It is tough. The problems are many, starting from almost no nutrients (oligotrophic) to extreme dryness (desiccation), very low atmospheric pressure (hypobaric), low temperatures, and intense solar radiation (UV and ionizing radiations). Ideally, all the flying microbes should be polyextremophiles. However, most of them are not. So, how are these highflyers surviving such unforgiving flights?

One of the fatal challenges for these airborne microorganisms is radiation (cumulative UV exposure). Microbes can find shelter in the shadowed areas of the dust particles (microniches). Alternatively, if they are in clumps or biofilms, the sacrifice of the microbes on the exterior can protect the ones in the interior. They can also form spores to withstand desiccation, low pressure, low temperature as well as radiation. Spores are highly resistant, metabolically inactive (dormant) structures formed in response to adverse conditions. For the microbes that do not form spores, their hardy DNA (G+C-rich genomes) can improve UV tolerance and survival. Sometimes the toughness is inherent like new genes for repairing DNA in Bacillus pumilus. Having photoprotective pigments like carotenoids and melanins can also help, almost like the sunscreen lotions that protect you. They also produce cold shock proteins that help the microbes at temperatures well below zero degrees centigrade. They are surviving the flight all right but what does it mean to us?

Survival of the flying microbes means that infectious diseases are airborne as well. They can take off from one area and attack another separated by vast distances. In fact, they have been doing precisely that. In 2003, legionnaires disease outbreak in Pas-de-Calais, France the microbes (Legionella pneumophila) attacked us from as far as 6 Km away; launching their attack from some cooling towers. In 2001, Foot-and-mouth disease (FMD) outbreak in the United Kingdom caused such panic that the general election was delayed. This time the airstrike came from North Africa and reached as far as northern Britain flying more than 4000 Km! Not all of the flying microbes are after us. Some are doing something completely different. For example, promoting ice nucleation in the stratosphere. The increased presence and actions of microbes, in the sky, can alter the rainfall.

Next time, when you look up to the sky, would not you think, even if for a moment, about the flying microbes? Next time, when you feel the breeze in your face, see the lightning, or hear the waves try to imagine tiny microbes taking off. Next time, when someone sneezes around you, instead of saying ‘bless you,’ say ‘lift-off.’