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Microbiomes, microorganisms that populate specific environments, are known to include both beneficial and harmful bacterial species. Understanding how destructive microbiomes originate in changing environments and their effects on both the environment and human health could help to tackle global well-being challenges more effectively.

Researchers at the Hong Kong Polytechnic University and institutions and universities in China and the U.S. have recently carried out a study investigating the compositions and origins of airborne (i.e., transported in the air) microbiomes on Earth. Their findings, published in the Proceedings of the National Academy of Sciences, shows that humans and animals are among the primary sources of global airborne bacteria.

"We spent a total of about nine years on this global study, including drafting the initial proposal, conducting sampling across the world, collecting and processing data, and drafting and revising the manuscript," Xiangdong Li, one of the leading researchers who carried out the study, told Â鶹ÒùÔº. "We established a comprehensive atlas of global airborne bacteria with implications for microbiology, ecology, air pollution, and public health, and we believe that airborne bacteria will attract more and more attention from all sectors of society."

To compile their atlas of global airborne bacteria, Li and his colleagues attained and organized a dataset containing 76 newly collected air samples (compounds of 803 weekly samples). They then combined this dataset with another 294 air samples gathered as part of reputable scientific studies, covering 63 places around the world.

The samples used by the researchers were collected at a vast variety of sites, ranging from to rooftops in urban areas to high mountain tops and the Artic Circle. Their hope was that this would allow them to identify the biogeographic distribution of airborne bacteria, such as the ecosystems and environments associated with different types of bacteria.

"One of our most interesting findings is the maximum microbial diversity of microorganisms in intermediate latitudinal regions, which is radically different from the typical latitudinal decreasing gradient of diversity pattern of macroscopic organisms," Li said.

When analyzing the airborne bacteria library they compiled, Li and his colleagues found that places on Earth with the broadest range of existing airborne microbiomes are in the so-called intermediate (or mid-) latitudinal regions. This would make sense, as these regions are known to be associated with richer microbiome ecosystems and greater biodiversity of bacteria communities.

"We have verified in the study that human activities have had a greater impact on the microbial community in comparison with background and relatively pristine air environments, such as 4000 m asl in Tibet," Li said. "Human activities have certainly changed the structure of microbiomes in the ambient air, particularly with a higher abundance of pathogenic bacteria in urban air. Thus, we still need to advocate for efforts to protect natural ecosystems."

Li and his colleagues estimated that the richness of bacteria in the atmosphere (~109) is comparable to the richness of bacteria in the so-called hydrosphere (i.e., the region containing all waters on the Earth's surface).

"With the rapid development of biotechnology, we can now see more of the global microbial world and gradually break through the limitations of traditional biological observations," Li explained. "Thus, we highly recommend that more microbiologic knowledge be covered in future textbooks, including a discussion of the new biogeographic pattern of microbiomes, to help the public understand more about the invisible microbial world on the Earth."

After the COVID-19 pandemic, many people worldwide have become more aware of how the air on Earth can promote the rapid transmission of harmful microbiomes, such as viruses and pathogens. The recent work by this team of researchers offers interesting insight about the distribution of these invisible and yet widely impactful microorganisms.

As part of their recent study, Li and his colleagues primarily focused on the mechanisms behind the geographical distribution of different microbiomes on Earth. As the samples they collected were obtained from only one year cycle, they were unable to reliably explore the processes underpinning the evolution of these microbiomes over time.

"Evolutionary processes also drive microbial biogeography and even take place at ecological scales due to changing environmental conditions," Li explained. "Therefore, future systematic sampling work with a longer period (around 10 years) should be conducted to achieve a comprehensive understanding of mechanisms shaping various macroecological patterns and bacterial community structures."

In the future, the findings gathered by this team of researchers could inspire the development of strategies aimed at increasing biodiversity, to promote the proliferation of a broader range of ecologically beneficial microbiomes. In addition, the microbiome library they compiled could be used by other environmental scientists to assess the effects of urbanization and other human activities on microbiome distribution.

"We now plan to probe the metabolically active bacterial community with respect to their function in the atmosphere (e.g., ice-nucleating, pollutant transformation, interactions with other biological components, etc.), and impacts on human health (e.g., benefits arising from healthful bacterial communities/secondary metabolites, and/or hazardous effects, such as allergens, pathogens, antimicrobial resistance, etc.)," Li added.

"We also plan to expand the scope of airborne microbiomes (e.g., fungi, viruses, allergens, etc.) to enrich the research field of bioaerosols and atmospheric science in relation to human health, as well as contributing to expanding the frontiers of understanding of the Earth's microbiome."

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