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==Introduction==
==Introduction==
By Robert Gallo
By Robert Gallo (June 12, 2020)


The atmosphere, despite lacking a constant medium for organisms to live on, contains many microbes that live in it. Microbes enter the atmosphere as soil is lifted from the surface by winds, bringing the microbes that live in that soil into the atmosphere as well.<ref name = Aguilera>[https://www.intechopen.com/online-first/microbial-ecology-in-the-atmosphere-the-last-extreme-environment]</ref> While previously these microbes were thought to be exclusively dormant, there is a growing body of work demonstrating atmospheric microbes that have an active metabolism.<ref name = Behzad>[https://academic.oup.com/gbe/article/7/5/1216/603778]</ref> While studying these organisms remains difficult due to the lack of a standardized method of capturing and culturing them,<ref name = Aguilera/> these organisms are researched for their interactions with the terrestrial biosphere, their potential role as condensation nuclei, and as a possible model for extraterrestrial life.
[[Image: atmlayers.png |thumb|300px|right| Diagram showing the layers of the atmosphere and the conditions present at each height, as well as sources of atmospheric cells. Figure from [https://www.intechopen.com/online-first/microbial-ecology-in-the-atmosphere-the-last-extreme-environment/ Aguilera et al.]]]
 
The atmosphere, despite lacking a constant medium for organisms to live on, contains many microbes that live in it. Microbes enter the atmosphere as soil is lifted from the surface by winds, bringing the microbes that live in that soil into the atmosphere as well.<ref name = Aguilera>[https://www.intechopen.com/online-first/microbial-ecology-in-the-atmosphere-the-last-extreme-environment Aguilera et. al., "Microbial Ecology in the Atmosphere: The Last Extreme Environment," Intecophen 2018.]</ref> While previously these microbes were thought to be exclusively dormant, there is a growing body of work demonstrating atmospheric microbes that have an active metabolism.<ref name = Behzad>[https://academic.oup.com/gbe/article/7/5/1216/603778 Behzad et al., "Challenges and Opportunities of Airborne Metagenomics," Genome Biology and Evolution 7, no. 5 (2015):1216-1226]</ref> While studying these organisms remains difficult due to the lack of a standardized method of capturing and culturing them,<ref name = Aguilera/> these organisms are researched for their interactions with the terrestrial biosphere, their potential role as condensation nuclei, and as a possible model for extraterrestrial life.


==The Environment of the Atmosphere==
==The Environment of the Atmosphere==


Unlike other habitats, the atmosphere is not the sole adobe of any organism, with all of the microbes in the atmosphere also existing in a terrestrial or oceanic environment. Either natural forces such as wind or human pollution can send these microorganisms into the atmosphere.<ref name = Lighthart>[https://academic.oup.com/femsec/article/23/4/263/539429]</ref> The atmosphere itself is vast, with several physical layers that extend up to space that present extremes in temperature, pressure, and UV radiation.<ref>[https://aem.asm.org/content/84/23/e01942-18]</ref>
[[Image: atmsizedistro.jpeg |thumb|300px|left| Chart showing distribution of bacteria-bearing particles by size on the left and a comparison between the size of a single bacteria and the size of bacteria-bearing particles on the right.[https://academic.oup.com/femsec/article/23/4/263/539429 from Lighthart.]]]
[[Image: atmlayers.png |thumb|300px|right| Diagram showing the layers of the atmosphere and the conditions present at each height, as well as sources of atmospheric cells. Figure from [https://www.intechopen.com/online-first/microbial-ecology-in-the-atmosphere-the-last-extreme-environment/ Aguilera et al.]]]
 
Unlike other habitats, the atmosphere is not the sole abode of any organism, with all of the microbes in the atmosphere also existing in a terrestrial or oceanic environment. Either natural forces such as wind or human pollution can send these microorganisms into the atmosphere.<ref name = Lighthart>[https://academic.oup.com/femsec/article/23/4/263/539429 Lighthart, Bruce. The Ecology of Bacteria in the Alfresco Atmosphere." FEMS Microbial Ecology 23, no. 4 (1997): 263-274.]</ref> The atmosphere itself is vast, with several physical layers that extend up to space that present extremes in temperature, pressure, and UV radiation.<ref name = Pulschen>[https://aem.asm.org/content/84/23/e01942-18 Pulschen et. al. "Survival of Extremophilic Yeasts in the Stratospheric Environment during Balloon Flights and in Laboratory Simulations." Environmental Microbiology 84, no. 23 (2018).]</ref>
Bacteria generally arrive in the atmosphere on relatively large particles, since they cling better to these larger particles that form roundish shapes that evaporate easier.<ref name = Lighthart/>
Bacteria generally arrive in the atmosphere on relatively large particles, since they cling better to these larger particles that form roundish shapes that evaporate easier.<ref name = Lighthart/>
[[Image: atmsizedistro.jpeg |thumb|300px|right| Chart showing distribution of bacteria-bearing particles by size on the left and a comparison between the size of a single bacteria and the size of bacteria-bearing particles on the right.[https://academic.oup.com/femsec/article/23/4/263/539429 from Lighthart.]]]
This is a rapidly emerging field of research, with much still to be learned. In the past, researchers assumed that the only microbial life in the atmosphere consisted of dormant spores. As studies demonstrate that they can metabolize the organic matter that is trapped in evaporated water droplet ,<ref name = Behzad/> the need for research into these effects grows.
This is a rapidly emerging field of research, with much still to be learned. In the past, researchers assumed that the only microbial life in the atmosphere consisted of dormant spores. As studies demonstrate that they can metabolize the organic matter that is trapped in evaporated water droplet ,<ref name = Behzad/> the need for research into these effects grows.


==Microbial Ecology of the Atmosphere==
==Microbial Ecology of the Atmosphere==
The atmosphere, despite having a volume larger than any other habitat, contains very few microbes. The troposphere, the layer of the atmosphere closest to the surface, has fewer than a billionth of the number of cells in terrestrial habitats.<ref name = Aguilera/> Simulations project the distribution of airborne microbes across the world based on wind patterns, and show that the density varies greatly. (Embed this image once I figure out how: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982008/bin/rstb20100283-g1.jpg)Because of the low density and the difficulty in culturing airborne microbes, the diversity of these organisms is difficult to grasp. There is a clear variation in species by altitude (insert this chart https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982008/bin/rstb20100283-g2.jpg). Additionally, airborne microbe transport is a major component in spreading diversity of microbes, but the microbes carried each dust storm varies in such an immense manner that there are no clear trends in diversity in a particular storm, but the overall diversity is quite high in a dust storm compared to the ambient composition. <ref name = Kellogg>[https://reader.elsevier.com/reader/sd/pii/S0169534706002138?token=9CDA186CA76AE9A6DF92203A26C0DA59D3099C6B5B792DC9024E2FC64F95E00993123AC57B463BC3270307965E265006]</ref> While the factors behind the diversity of microbes in the air, they are thought to include the land-use patterns of the source area and wind speeds, with higher wind creating more diversity.<ref name = Dueker>[https://www.frontiersin.org/articles/10.3389/fmicb.2018.02868/full]</ref>
The atmosphere, despite having a volume larger than any other habitat, contains very few microbes. The troposphere, the layer of the atmosphere closest to the surface, has fewer than a billionth of the number of cells in terrestrial habitats.<ref name = Aguilera/> Simulations project the distribution of airborne microbes across the world based on wind patterns, and show that the density varies greatly. Because of the low density and the difficulty in culturing airborne microbes, the diversity of these organisms is difficult to grasp. There is a clear variation in species by altitude. [[Image: atmdustsize.jpeg |thumb|300px|right| Chart showing the microbes isolated at different altitudes. The shaded portion indicates the altitude in which they have been cultured, while the white shows the altitude they were found at in dust. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2982008/ From Womack et al.]]]
Additionally, airborne microbe transport is a major component in spreading diversity of microbes, but the microbes carried each dust storm varies in such an immense manner that there are no clear trends in diversity in a particular storm, but the overall diversity is quite high in a dust storm compared to the ambient composition. <ref name = Kellogg>[https://reader.elsevier.com/reader/sd/pii/S0169534706002138?token=9CDA186CA76AE9A6DF92203A26C0DA59D3099C6B5B792DC9024E2FC64F95E00993123AC57B463BC3270307965E265006 Christina, Kellogg and Griffin, Dale. "Aerobiology and the Global Transport of Desert Dust." Trends in Ecology and Evolution 21, no. 11 (2006): 638-644.]</ref> While the factors behind the diversity of microbes in the air, they are thought to include the land-use patterns of the source area and wind speeds, with higher wind creating more diversity.<ref name = Dueker>[https://www.frontiersin.org/articles/10.3389/fmicb.2018.02868/full Dueker et al. "Comparison of Bacterial Diversity in Air and Water of a Major Urban Center." Frontiers in Microbiology 29 (2018).]</ref> Most of the organisms in the atmosphere are of the domains bacteria, with some eukaryotes (mainly fungal spores and yeast) and an unknown amount of archaea.<ref name = Aguilera/> The fungal spores include Aspergillus, Nigrospora, Arthrinium, and Curvularia, while the bacteria Firmicutes, Proteobacteria, Actinobacteria, and Bacteriodetes. Other Bacteria release spores into the atmosphere, such as Bacillus and Streptococcus.<ref name = Aguilera/>


==Microbes and Cloud Formation==
==Microbes and Cloud Formation==


Microbes in the atmosphere appear to act as condensation nuclei.<ref name = Behzad/> This occurs as certain species of bacteria act as catalysts for the formation of ice, which then follow the normal process for cloud formation. All bacteria that can from ice use the same protein called the Ice+ protein, which is active at temperatures of 2 degrees celsius or less generally.<ref name = Lindow>[https://www.fasebj.org/doi/pdf/10.1096/fasebj.7.14.8224607]</ref> Proteobacteria especially have a role in cloud formation, and bacteria in general make up a higher fraction of particles in the upper atmosphere, indicating that their role in cloud formation may be much more significant than previously believed.<ref name = Deleon>[https://www.pnas.org/content/pnas/110/7/2575.full.pdf]</ref> add chart once I figure out how
Microbes in the atmosphere appear to act as condensation nuclei.<ref name = Behzad/> This occurs as certain species of bacteria act as catalysts for the formation of ice, which then follow the normal process for cloud formation. All bacteria that can form ice use the same protein called the Ice+ protein, which is active at temperatures of 2 degrees celsius or less generally.<ref name = Lindow>[https://www.fasebj.org/doi/pdf/10.1096/fasebj.7.14.8224607 Gurian-Sherman, Douglas and Lindlow, Steven. "Bacterial Ice Nucleation: Significance and Molecular Basis" FASBE Journal 7, no. 14 (1993): 1338-43.]</ref> Proteobacteria especially have a role in cloud formation, and bacteria in general make up a higher fraction of particles in the upper atmosphere, indicating that their role in cloud formation may be much more significant than previously believed.<ref name = Deleon>[https://www.pnas.org/content/pnas/110/7/2575.full.pdf Deleon Rodriguez et al. "Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications." PNAS 110, no. 7 (2013): 2575-2580.]</ref>
[[Image: icenucleus.png |thumb|300px|left| Chart showing the number of ice nuclei formed on a plant when uncolonized and colonized with ice-forming bacteria. Figure from [https://www.fasebj.org/doi/pdf/10.1096/fasebj.7.14.8224607/ Gurian-Sherman and Lindlow]]]


==A Model for Extraterrestrial Life?==
==A Model for Extraterrestrial Life?==
At least two studies have proposed examining eukaryotic microbes in the atmosphere as models for extraterrestrial life. One study exposed extremophilic yeast to high altitudes and conditions as an analog to conditions on Mars, and found that the strains N. Friedmannii and Exophelia were good models for life on Mars.<ref>[https://aem.asm.org/content/84/23/e01942-18#sec-2]</ref> [[Image: extremeyeast.jpeg |thumb|300px|right| Chart showing the survival of different organisms based on exposure to altitude and atmospheric conditions from [https://aem.asm.org/content/84/23/e01942-18/ Pulschen et al.'s study on extreomphilic yeast surival.] The N. Friedmanii yeast survived quite well.]]. Another study of other extremophilic yeasts also highlights the potential of these yeasts as models for extraterrestrial life, by first isolating yeasts in the Atacama desert at altitude that were the first yeast discovered in that desert, and found them to have high survivability at high and low temperature, as well as high UV levels.<ref name = Pulschen>[https://onlinelibrary.wiley.com/doi/full/10.1002/mbo3.262]</ref> These yeasts demonstrate that eukaryotes can survive in the atmosphere and in extreme environments, and signal that further research into these airborne eukaryotes is needed.
At least two studies have proposed examining eukaryotic microbes in the atmosphere as models for extraterrestrial life. By exposing extremophilic yeasts to high altitudes, the low pressure, high UV, and low temperatures, researchers can match the conditions found on Mars. Researchers doing this found that the strains N. Friedmannii and Exophelia were good models for life on Mars.<ref name = Pulschen/> [[Image: extremeyeast.jpeg |thumb|300px|right| Chart showing the survival of different organisms based on exposure to altitude and atmospheric conditions from [https://aem.asm.org/content/84/23/e01942-18/ Pulschen et al.'s study on extreomphilic yeast surival.] The N. Friedmanii yeast survived quite well.]]
Another study of other extremophilic yeasts also highlights the potential of these yeasts as models for extraterrestrial life, by first isolating yeasts in the Atacama desert at altitude that were the first yeast discovered in that desert, and found them to have high survivability at high and low temperature, as well as high UV levels.<ref name = Pulschen2>[https://onlinelibrary.wiley.com/doi/full/10.1002/mbo3.262 Pulschen et al. "UV ‐resistant yeasts isolated from a high‐altitude volcanic area on the Atacama Desert as eukaryotic models for astrobiology." Microbiology Open 4, no. 4 (2015): 574-578.]</ref> These yeasts demonstrate that eukaryotes can survive in the atmosphere and in extreme environments, and signal that further research into these airborne eukaryotes is needed.


==Key Microbes of the Atmosphere==
==Key Microbes of the Atmosphere==


Due to the varied conditions worldwide that contribute to its diversity, there are no clear microbes that dominate in the atmosphere.
Due to the varied conditions worldwide that contribute to its diversity, there are no clear microbes either by species or phyla that dominate in the atmosphere. Bacteria in general dominate, but the field remains open for further research to better answer this question.


==Conclusion==
==Conclusion==

Latest revision as of 18:46, 16 July 2021

This student page has not been curated.

Introduction

By Robert Gallo (June 12, 2020)

Diagram showing the layers of the atmosphere and the conditions present at each height, as well as sources of atmospheric cells. Figure from Aguilera et al.

The atmosphere, despite lacking a constant medium for organisms to live on, contains many microbes that live in it. Microbes enter the atmosphere as soil is lifted from the surface by winds, bringing the microbes that live in that soil into the atmosphere as well.[1] While previously these microbes were thought to be exclusively dormant, there is a growing body of work demonstrating atmospheric microbes that have an active metabolism.[2] While studying these organisms remains difficult due to the lack of a standardized method of capturing and culturing them,[1] these organisms are researched for their interactions with the terrestrial biosphere, their potential role as condensation nuclei, and as a possible model for extraterrestrial life.

The Environment of the Atmosphere

Chart showing distribution of bacteria-bearing particles by size on the left and a comparison between the size of a single bacteria and the size of bacteria-bearing particles on the right.from Lighthart.

Unlike other habitats, the atmosphere is not the sole abode of any organism, with all of the microbes in the atmosphere also existing in a terrestrial or oceanic environment. Either natural forces such as wind or human pollution can send these microorganisms into the atmosphere.[3] The atmosphere itself is vast, with several physical layers that extend up to space that present extremes in temperature, pressure, and UV radiation.[4] Bacteria generally arrive in the atmosphere on relatively large particles, since they cling better to these larger particles that form roundish shapes that evaporate easier.[3] This is a rapidly emerging field of research, with much still to be learned. In the past, researchers assumed that the only microbial life in the atmosphere consisted of dormant spores. As studies demonstrate that they can metabolize the organic matter that is trapped in evaporated water droplet ,[2] the need for research into these effects grows.

Microbial Ecology of the Atmosphere

The atmosphere, despite having a volume larger than any other habitat, contains very few microbes. The troposphere, the layer of the atmosphere closest to the surface, has fewer than a billionth of the number of cells in terrestrial habitats.[1] Simulations project the distribution of airborne microbes across the world based on wind patterns, and show that the density varies greatly. Because of the low density and the difficulty in culturing airborne microbes, the diversity of these organisms is difficult to grasp. There is a clear variation in species by altitude.

Chart showing the microbes isolated at different altitudes. The shaded portion indicates the altitude in which they have been cultured, while the white shows the altitude they were found at in dust. From Womack et al.

Additionally, airborne microbe transport is a major component in spreading diversity of microbes, but the microbes carried each dust storm varies in such an immense manner that there are no clear trends in diversity in a particular storm, but the overall diversity is quite high in a dust storm compared to the ambient composition. [5] While the factors behind the diversity of microbes in the air, they are thought to include the land-use patterns of the source area and wind speeds, with higher wind creating more diversity.[6] Most of the organisms in the atmosphere are of the domains bacteria, with some eukaryotes (mainly fungal spores and yeast) and an unknown amount of archaea.[1] The fungal spores include Aspergillus, Nigrospora, Arthrinium, and Curvularia, while the bacteria Firmicutes, Proteobacteria, Actinobacteria, and Bacteriodetes. Other Bacteria release spores into the atmosphere, such as Bacillus and Streptococcus.[1]

Microbes and Cloud Formation

Microbes in the atmosphere appear to act as condensation nuclei.[2] This occurs as certain species of bacteria act as catalysts for the formation of ice, which then follow the normal process for cloud formation. All bacteria that can form ice use the same protein called the Ice+ protein, which is active at temperatures of 2 degrees celsius or less generally.[7] Proteobacteria especially have a role in cloud formation, and bacteria in general make up a higher fraction of particles in the upper atmosphere, indicating that their role in cloud formation may be much more significant than previously believed.[8]

Chart showing the number of ice nuclei formed on a plant when uncolonized and colonized with ice-forming bacteria. Figure from Gurian-Sherman and Lindlow

A Model for Extraterrestrial Life?

At least two studies have proposed examining eukaryotic microbes in the atmosphere as models for extraterrestrial life. By exposing extremophilic yeasts to high altitudes, the low pressure, high UV, and low temperatures, researchers can match the conditions found on Mars. Researchers doing this found that the strains N. Friedmannii and Exophelia were good models for life on Mars.[4]

Chart showing the survival of different organisms based on exposure to altitude and atmospheric conditions from Pulschen et al.'s study on extreomphilic yeast surival. The N. Friedmanii yeast survived quite well.

Another study of other extremophilic yeasts also highlights the potential of these yeasts as models for extraterrestrial life, by first isolating yeasts in the Atacama desert at altitude that were the first yeast discovered in that desert, and found them to have high survivability at high and low temperature, as well as high UV levels.[9] These yeasts demonstrate that eukaryotes can survive in the atmosphere and in extreme environments, and signal that further research into these airborne eukaryotes is needed.

Key Microbes of the Atmosphere

Due to the varied conditions worldwide that contribute to its diversity, there are no clear microbes either by species or phyla that dominate in the atmosphere. Bacteria in general dominate, but the field remains open for further research to better answer this question.

Conclusion

The microbiology of the atmosphere remains deeply understudied and is ripe for further research to understand the role it plays in spreading species and other exchanges with the terrestial biosphere.

References