Is the atmosphere Earth’s largest microbial ecosystem?

“We live submerged at the bottom of an ocean of air.” – Evangelista Torricelli, 1644

In 1644, the physicist Evangelista Torricelli wrote a letter which stated “Noi viviamo sommersi nel fondo d’un pelago d’aria” or “We live submerged at the bottom of an ocean of air.”

“Torricelli was addressing the significance of the barometric pressure, but this ‘ocean of air’ is an apt concept for considering the atmosphere as a microbial habitat,” Dr Rachael Lappan explains.

Dr Lappan is a SAEF Associate Investigator and ARC DECRA Fellow who researches microbes in the atmosphere. She is interested in the question of whether the atmosphere should be considered our planet’s largest microbial ecosystem. 

“The atmosphere is connected to all of Earth’s surface ecosystems and plays an important role in microbial dispersal on local to global scales. Despite this grand scale, surprisingly little is understood about the atmosphere as a habitat,” says Dr Lappan. 

Is the atmosphere like a road or an entire city?

It’s clear to scientists that the atmosphere is a transport medium, enabling microbes to travel from one location to another. For example, SARS-CoV-2 and the fungal spores responsible for wheat rust are both dispersed through the air. However, what Dr Lappan wants to know is whether the atmosphere is actually an ecosystem that plays home to microbial communities specially adapted to live there. In other words, is it just a road or an entire city? 

Recent advances in microbiology have increased our understanding of life’s limitations, revealing the extremes under which microbes can survive. These include extreme temperatures, such as those found in hot springs and the Antarctic dry valleys and extreme pressures, such as deep below the ocean’s surface. In some cases, they can survive in these extreme environments by metabolising atmospheric concentrations of trace gases such as hydrogen, carbon monoxide and methane. 

Dr Lappan and her colleagues think that just as hot springs and Antarctica are challenging but surmountable environments for microbes to call home, so may the Earth’s atmosphere. To solve this mystery, they’re looking into a range of different questions: Is the atmosphere a permanent home to microbial communities? How do these microbes survive? And do they play an active role in this ecosystem?

Everything is everywhere, but the environment selects, or does it?

Lourens Baas-Becking was a Dutch botanist and microbiologist who is often quoted for his hypothesis, “Everything is everywhere, but the environment selects.” This theory posits that microbes are uniformly distributed throughout the atmosphere and that if they arrive in a suitable environment, they will establish and survive. 

However, this theory has been challenged by the realisation that microorganisms can’t be evenly distributed throughout the atmosphere. Evidence suggests that they are limited in their ability to disperse, indicating that microbial communities in the atmosphere are influenced by their immediate environment, lifespan, and other factors. 

“Airborne microbial assemblages can be highly variable, fluctuating daily and seasonally, differing with biogeography, and have been observed to “wash out” of the atmosphere with precipitation events,” Dr Lappan explains.  

“These findings imply that the atmospheric microbiome is a “moving imprint” of surface ecosystems, rather than a community of organisms endemic only to the atmosphere.”

A microbe’s garden

“If the atmosphere were considered to be a “true” ecosystem, by definition this would comprise metabolically active organisms interacting with their environment and each other while suspended in air,” Dr Lappan explains. 

These microbes would need to be able to survive and thrive in extreme conditions and be specially adapted to withstand low temperatures, a lack of organic matter for food, high UV, and other harsh conditions. 

“The atmospheric ecosystem could have multiple distinct habitats like dust or clouds, with resident microorganisms that could profoundly influence global biology, chemistry and climate beyond what we currently understand,” Dr Lappan says.

For example, bacteria are known to help clouds form because they promote the freezing of water molecules. They are now widely accepted as a major driver of cloud formation. However, other particles, such as dust, are also known to support cloud formation, so the extent to which these microbes contribute to cloud formation processes across Earth is one of the many unresolved questions. 

Getting airborne

These knowledge gaps open up a plethora of research avenues which come with challenges. Recent advances in metagenomics enable scientists to use DNA sequencing techniques to isolate and analyse the genetics of entire microbial communities directly from samples. These analyses provide insights into their diversity and potential functions within ecosystems. 

However, when collecting samples from the air, studies can be severely impacted by contaminating DNA from the scientist who collected the sample, their equipment, the DNA extraction reagents or the laboratory air where they conducted their analysis. Even if these challenges are overcome, it’s difficult to reproduce the environment and potential interactions of atmospheric microbes in the laboratory. Dr Lappan says further work needs to be done to standardise sampling methodology to help overcome some of these issues. 

While there are many unknowns about life in our ocean of air, ongoing research promises to uncover new understanding about the unseen life that floats above us and its impact on our world. This could reshape our understanding of how these tiny organisms influence Earth’s global processes and change our perception of the atmosphere.

Read more

Lappan, R., Thakar, J., Molares Moncayo L., Besser, A., Bradley, J.A., Goordial, J., Trembath-Reichert, E. & Greening, C. (2004) The atmosphere: a transport medium or an active microbial ecosystem?, The ISME Journal, 18, 1, wrae092. https://doi.org/10.1093/ismejo/wrae092