The Hong Kong Polytechnic University draws world's first "atlas" of airborne microorganisms

From soil to oceans, bacteria are abundant
on the Earth's surface.
The number of microbes in the air around us is relatively unknown, but a team led by PolyU scientists is about to change that
.
After nearly a decade of effort, they have compiled a comprehensive map of global airborne microbes, providing new insights
into how these species interact with the surface environment and how they might change in the future.

With 10,000 or more bacteria in one cubic meter of "empty" air since the beginning of the COVID-19 pandemic, interest in the role of air as a habitat for microbes, not just pipes, has increased
dramatically.
In collaboration with researchers in Chinese mainland and the United States, the team, led by Polytechnic University, spent about a year sampling microbes in the air, from the ground to the top of a mountain
, around the world.
Combining their findings with the most accurate global data collected in previous studies, they and their research partners compiled the first-ever global air microbiota atlas.

The atlas provides rich insights
into the microbial communities floating on the ground.
There is no doubt that air is a unique harbor
for bacterial life.
The team's genetic analysis showed that the core communities in the air — the few species that make up a disproportionate proportion of microbial populations — are different
from those found in marine or soil ecosystems.
In fact, even though air is a free-flowing medium that seems to have no internal boundaries, these core bacterial communities are clearly localized and stable
.

The team analyzed bacterial communities of 370 individual airborne particle samples collected from 63 sites around the world, ranging from ground (1.
5 – 2 m high) to rooftops (5 – 25 m high) and mountains (5238 m altitude), as well as from densely populated urban centers to the Arctic Circle, with more diverse
coverage in terms of altitude and geographic area.

Professor Xiang-dong LI, PolyU's Chair Professor of Environmental Science and Technology and Dean of the School of Architecture and Environment, who led the research team, said, "We have confirmed that human activities do alter the microbiota structure in the natural ambient air, especially when
the content of pathogenic bacteria in urban air is high.
After three years of the pandemic, there is now more attention to this invisible but influential microbial community
.
The results can serve as an important reference
for predicting planetary microbiome responses and the health effects of inhalable microbiomes on future environmental changes.
”

Researchers estimate that the total number of microbes that occupy the ocean or soil is thousands of times
the total number of microbes in the air.
Still, the diversity of microbes in the air — known as "richness" — is equally high
.
This suggests that surface habitats directly supply the air with microbes
.
Refuting previous assumptions, vegetation is not the main terrestrial source of airborne bacteria, and that Earth's vast soil provides only a tiny fraction
.
The impact of ocean waves, the shaking of leaves, and even the frequent activity and constant breathing of animals and humans are all greater drivers of bacterial exchange between the surface and the air
.

Macroscopic life, especially animals and plants, is most diverse in equatorial regions (for example, consider warm and humid rainforests), and its diversity decreases
closer to the poles.
For microbes, the situation is more interesting - moving from the equator, maximizing diversity in mid-latitudes, and then disappearing
again.
This pattern is well established in terrestrial and aquatic microbes, but the new atlas confirms that it also applies to airborne microbiota
.
The authors speculate that the "surge" in microbial diversity in the mid-latitudes is caused
by stronger sources of microbial input from these regions.

Overall, the researchers estimated that half of the bacteria in the air came from the ground
.
The proportion of human-related bacteria in urban air is particularly high – some harmless, some
disease-causing — The direct transfer of bacteria from people to the air isn't the only impact we have on
the world of air microbes.
Large-scale activities such as industrialization damage the natural environment and affect air quality
.
This weakens the environment's "filtering" effect on the microbial structure, making the composition of bacteria in the air more influenced by random processes – although weather still plays an important role
.

The close relationship between modern human activity and the microbes around us underscores the need to
accurately predict future changes.
The spread of inhalable infectious bacteria in cities is of particular concern
given the rapid urbanization and COVID-19 research driving our growing understanding of airborne transmission.
Climate change is another driver, as the atlas reveals a significant impact
of temperature on microbial richness.
As such, the study provides valuable resources and important new perspectives
for future public health research.

The PolyU team collaborated with Professor James M.
TIEDJE, Distinguished Professor at Michigan State University, and scientists from Chinese mainland to conduct the study
.
The findings were published in the journal
PNAS.