How trees in your local park protect you from disease

On your next visit to the park, try to count all the different species you can see. Away from the closely mowed grass, you might spot wildflowers frequented by pollinating insects, such as bees, wasps, and hoverflies. Above are the gnarled branches of mature trees, some of which will have lived for hundreds of years, providing food and refuge for generations of fungi and insects.

You may find yourself immersed in the chorus of songbirds fervently arguing over companions. There will no doubt be floating-footed mammals scurrying through the bushes and amphibians hiding under logs.

But there is also another world of wild animals floating all around you. It is the biodiversity that we cannot see with the naked eye – the secret life of the air we breathe.

Invisible nature

The air is teeming with microscopic life forms: dense clouds of bacteria, tiny fungi and algae that surround us. There are single-celled organisms called protozoa and large amounts of viruses, moss spores, and plant pollen. There may even be a few microscopic moss-living animals called tardigrades, also known as water bears or moss piglets because of their mammalian appearance (under a microscope, at least).

The retardigrades are eight-legged micro-animals found throughout the Earth’s biosphere.
Schokraie et al. (2012), CC BY

Humans are bombarded daily by all these little organisms. Studies have shown that up to a million microbial cells can be found in a single cubic meter of air and that people can inhale 100 million bacteria every day.

But where does all this invisible life come from? And what does our exposure to this mean for our health? Together with colleagues, we set out to find out the types of germs people are likely to encounter when walking in city parks.

Our recent study in Scientific Reports suggests that many of the life forms floating in the air actually originate from the ground beneath our feet. It makes a lot of sense. Soil is arguably the most biodiverse habitat on Earth, and a single gram of it can contain more microbes than there are humans on the planet.

Germs are incredibly light, so they become very easy to fly and are carried very far by the wind. They can be lifted off the ground in air bubbles that form in raindrops and clump together on dust particles that fall from the atmosphere.

A petri dish with pink and orange bacteria growing on nutrient agar.
Soil microbes grown in a petri dish.
Sarawut Chainawarat / Shutterstock

Our study showed that distinct layers of bacteria form in the air, with different species and amounts of microbes occurring at different heights. At the average height of the head of a standing adult, there were less but also different types of bacteria compared to those present in the air lower at the height of the head of a child or adult sitting down.

This means that we can be exposed to different types of germs – some good for us, some bad – depending on our height and posture. Exposure to many types of microbial life, especially in childhood, is generally considered a good thing, as it allows our immune system to build a strong army of cells that protect us from pathogens. The greater number of microbial species we have detected closer to the ground could be vital in ensuring that children develop robust immunity later in life.

But the environments in which we spend time also matter. After collecting 135 samples, we found that the air in the wooded areas of an urban park near Adelaide in Australia contained more bacterial species but fewer potential human pathogens than nearby athletic fields. Trees appear to filter out microbial communities in a given airspace, reducing the risk of exposure to disease-causing microbes. Because trees also appear to increase microbial diversity in the air, allowing more of them to grow in urban areas could provide a significant health benefit by boosting our immune system.

A perfectly mowed football field in a public park.
Fewer trees meant more pathogens in the recent study.
CLS Digital Arts / Shutterstock

It would not only benefit human health. While we can’t see microbes and other members of the microscopic world around us, they are fundamental to healthy ecosystems functioning, plant health and communication (yes, plants talk to each other), and even regulation. of the climate.

We still know relatively little about the unseen life in the air we breathe, but our preliminary findings reveal some of their secrets. It would be wise to learn them and encourage them in the important roles they play.


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About Lucille Thompson

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