Every day you move through an ecology without noticing it. The air above us and, regrettably, the air we breathe in, contains creatures and potential creatures that form their own particular system of life. You may be surprised to discover all the living creatures you inhale with every breath, and how they're controlling your world.
How long can the stuff you sneeze out stay in the air? A few minutes? A few hours? Try a week. For simplicity's sake, we think of transmission of infectious diseases as happening when one person sneezes on a doorknob and the next person to go through the door doesn't wash their hands, but many viruses and bacteria are as well-suited to air travel as birds are. Some use far more powerful mechanisms than birds do.
Some, it's true, are more ostriches than albatrosses. The larger and more delicate microorganisms are suited for indoor deployment and can only survive if a new host comes along quickly. But some are champions of the air. The two main qualities a bacterium or virus needs to fly are small size and the ability to survive without water. Roughly speaking, anything under five micrometers (five millionths of a meter) can fly well. As for lack of water, desiccation happens the moment a virus or bacterium leaves its host (or leaves its surrounding droplet). Some microorganisms develop outer layers that can protect them from the inevitable drying, but others have an even better strategy. Droplets keep them moist, but can pull them down out of the air. Once a droplet dries up, the microorganism does as well. As it dries, it shrivels, shrinking down until it becomes adept at staying airborne. So instead of taking one shot at life and landing wherever the breeze takes them, some viruses and bacteria take off and land and take off and land again, traveling until they find a host.
They're more in control of that take-off and landing than we think. Certain bacteria have an outer membrane lined with special proteins, which manage to make ice form at higher temperatures than it usually would. In a cloud, these ice-forming bacteria can serve as seeds which acquire water, build up drops, and bring on the rain. So these bacteria could disperse by getting whisked up into the clouds, traveling to somewhere with favorable weather conditions and then tipping the conditions in their favor, making it rain and giving themselves a way to land. If that's their strategy, it's worked fell for them. They are found in rain everywhere, including Antarctica.
Pollen is kind of picturesque, until you realize that it's just air-borne sperm. Every year, flowering plants wave their genitals in the air and try to con us into finishing the job for them. Since they can't always count on animals, they count on the air.
When it comes to airborne sex, plants releasing pollen face the virus's challenge— overcoming size limits and desiccation, and they navigate this challenge different ways. Corn pollen, for example, is tiny and dries up only a few minutes after being released, but can travel half a mile before it stops being viable. And corn is a relative homebody — some pollen can travel for thousands of miles.
When it comes to fungi, they release spores, not sperm; each spore is able to develop into its own mushroom. To help send it on its way, some mushrooms will literally change the weather. When a mushroom opens up and lets its spores go, it will also release water vapor, which cools the air in the immediate vicinity of the mushroom. Enough of this can generate a breeze, and many mushrooms do coordinate their spore release. In fact, most plants coordinate the release of their pollen and spores.
This sky-orgy isn't as indiscriminate as it seems. This is important, because whether we know it or not, we are walking through a shower of the pollen and spores put out by the flora around us. We are breathing them in, and when we do, they can be a part of us longer than we think they will be. Pollen contains long-lasting components. A grain of pollen has a cellulose shell, stuck through with various kinds of protein. Some pollen grains are also covered in a component called sporopollenin. The oldest plant matter ever found is over two billion years old and is made of sporopollenin. These grains are durable.
The combination of toughness and time-specificity has made pollen the most informative thing about us. Pollen has been used to figure out what people ate thousands of years ago, and how neandertals buried their dead. It has also played a large part in determining who is guilty of appalling political crimes. There have been multiple occasions when pollen has helped identify the governments, if not the actual people, responsible for massacres.
Decades-old bodies were found in mass graves. Determining who killed them depended on determining exactly when they died, so investigators extracted pollen from the victims' nasal cavities. By analyzing the pollen, and figuring out which plants were blooming when the people died, authorities were able to determine what month they died, and therefore determine who killed them — even when they couldn't identify the victims themselves.
The pollen and spores around us aren't just irritants. They settle in our clothes, our hair, under our fingernails, and in our mouths and noses. They literally coat our bodies, inside and out, and anyone who can read them knows exactly where we've been, and when we went there.
Our food comes from the land, right? Wrong. It comes from the air. We're walking through our own food, because the food chain goes a little farther than we can see. Carnivores eat herbivores, and herbivores eat plants. Plants eat something else.
In the 1600s, Jan Baptiste Van Helmont decided to find out what exactly plants ate. He did so by planting a tree in a pot and watching it grow. He carefully monitored the amount of soil in the pot and the amount that the tree grew, and discovered they were wildly out of proportion. The mass gained by the tree was over 10 times the mass lost by the soil. People did know that soil had to be rich in certain minerals to make plants grow, and so the soil was providing trees with something, but the tree had to be getting its bulk from somewhere else.
A tree is made up of our own exhalations. Carbon dioxide isn't a gas that fizzes into and out of the leaf that takes it in. The carbon in carbon dioxide makes up about 95% of the plant. So the circle of life isn't confined to our bodies going back to the soil — we walk through plant food every day. We pause occasionally to eat plants as food, absorbing their carbon, attaching it to the oxygen we've just inhaled, and breathing it out. So we're not just sharing the air with living creatures. We're feeding them, and feeding ourselves. We're breathing out, and then walking around in, the source of our own nutrition.
[Sources: Aerobiology and Its Role in the Transmission of Infectious Diseases, Antibacterial Populations, The Rain-Making Bacteria, Pollen: Nature's Tiny Clues, BBC, Tassel Emergence and Pollen Shed, NPR]
Top Image: USGS Native Bee Inventory and Monitoring Laboratory Bacteria and Pollen Images: Dartmouth College Electron Microscope Facility.