The Earth’s surface is teeming with microorganisms but we know little about the microbial population in the air that surrounds us. After nearly a decade of effort, a PolyU-led team has compiled a comprehensive map of the world’s airborne microbes, providing fresh insights into how these species interact with the surface environment – as well as their likely future changes.
In collaboration with researchers from the mainland and the US, the research team led by Prof. Li Xiang-dong, Director of Research Institute for Sustainable Urban Development (RISUD), spent around one year sampling airborne microbes across the world, from ground level to mountaintops. Combining their own results with the most accurate global data collected in past studies, they compiled the first-ever atlas of the global airborne microbiome, providing an important new perspective for public health research.
The atlas provides a wealth of insights into the microbial communities floating above the ground. Air is a unique harbour of bacterial life. Genetic analysis by the team showed that the core communities in air – the handful of keystone species that form an outsized proportion of the microbe population – were not the same in the air as those in marine or soil ecosystems. In fact, even though the air is a free-flowing medium with seemingly no internal boundaries, these core bacterial communities are distinctly localised and stable.
The research team analysed the bacterial communities of 370 individual air particulate samples collected from 63 sites around the world, ranging from those at ground level (1.5 – 2 m high) to rooftops (5 – 25 m high) and high mountains (5,238 m asl), as well as from densely populated urban centres to the uninhabited Arctic Circle, for a more diverse coverage in terms of altitudes and geographic regions.
Prof. Li Xiang-dong, Chair Professor of Environmental Science and Technology, Director of Research Institute for Sustainable Urban Development and Dean of Faculty of Construction and Environment of PolyU, leader of the research team, said, “We have verified that human activities have certainly changed the structure of microbiomes in the natural ambient air, particularly with a higher abundance of pathogenic bacteria in urban air. After three years of the pandemic, people are now paying more attention to this invisible but influential microbial community. The research outcomes could serve as a critical reference for predicting the response of planetary microbiome to future environmental changes, as well as the health impacts of inhalable microbiomes.”
The researchers estimated that the total number of microbes in the sea or soil is thousands of times larger than those in the air. Nonetheless, the diversity of microbes – known as “richness” – is just as high, suggesting that surface habitats directly contribute to microbes in the air. Overturning previous assumptions, vegetation is not the main terrestrial source of airborne bacteria, and the Earth’s vast tracts of soil provide very small fraction. The crashing of ocean waves, the shaking of leaves, and even frequent activities and constant respiration of animals and humans are bigger drivers of bacterial exchange between the surface and the air.
From a macroscopic perspective, living things, especially animals and plants, are most diverse in equatorial regions such as warm and humid rainforests, and its diversity decreases in regions closer to the poles. For microbes, the situation is more interesting – microbial diversity is highest in at mid-latitudes and decreases away from the equator. This pattern was well established for terrestrial and aquatic microorganisms, but the new atlas confirms that the same pattern applies to the airborne microbiome. The authors speculated that the concentration of diversity at the mid-latitudes is caused by stronger sources of microbial input.
Overall, the researchers estimated that half of airborne bacteria originate from ground sources. Urban air has especially high levels of human-associated bacteria – some harmless, some pathogenic. Direct transmission of germs from human breaths to air is not the only impact that humans have on the airborne microbial world. Large-scale activities such as industrialisation and urbanisation have disrupted natural environments and affected air quality. This impairs the environmental filtering of microbial structures, making the composition of airborne bacteria more susceptible to random processes – although weather still plays an important role.
The close relationship between modern human activities and the microbes around us underscores the need to predict future changes accurately. The inhalable infectious bacteria that proliferate in cities are of particular concern considering the rapid urbanisation and our growing understanding of airborne contagion from COVID-19 research. Climate change is another driver of the growth of airborne microbes, given that the atlas revealed the marked effect of temperature on microbial abundance. Hence, the study provides an invaluable resource and an important new perspective for future public health research.
The PolyU team collaborated with Prof. James M. Tiedje, University Distinguished Professor at Michigan State University and scientists from mainland China in the study. The findings were published in the Proceedings of the National Academy of Sciences (https://www.pnas.org/doi/full/10.1073/pnas.2204465119), a peer-reviewed journal of the USA National Academy of Sciences.
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