**6. Emission sources**

*Extremophilic Microbes and Metabolites - Diversity, Bioprospecting and Biotechnological...*

Scanning electron microscopy (SEM) also provides much information of the aerobiology [7]. Specifically, it allows the characterization of eukaryotic cells (e.g., diatoms) and, above all, pollens and fungal spores, from which we can obtain great information with good images alone. **Figure 6A** shows pine tree pollen observed via SEM in a

*SEM images of different airborne samples. (A) Pinus pollen. Ground sample after 2 hours sampling. (B) Air sample collected from C-212-200 aircraft during a Saharan dust intrusion (February 24, 2017). Filter appear completely cover of mineral particles. (B and C) Biological particles sampled using C-212-200 aircraft. (E) Diatomea sampled by C-212-200 aircraft in a fligth along the northern coast of Spain (9 March 2017). (F) Cell* 

As mentioned above, factors, such as the shortage of nutrients and substrates, high UV radiation, drying, changes in temperature and pH, or the presence of

sample obtained after a 30 minutes flight of the C-212 aircraft.

*attached to mineral particles and organic matter.*

**5. Mechanisms of microbial survival of airborne bacteria**

**50**

**Figure 6.**

Global and regional models have been used to explain bioaerosol emission, transport, and atmospheric impact [17, 18, 79–84]. Even so, it is not an easy phenomenon to explain, since it depends on a large number of factors. On the one hand, there are numerous sources of tropospheric aerosols, which include sea salt, volcanic dust, cosmic dust, industrial pollutants, and desert and semidesert areas [6, 85]. We must also consider the factors that make the transfer of particles possible, for example, meteorological phenomena, solar radiation, temperature, tides, erosion, etc. [85]. On the other hand, anthropogenic activities can also affect dust emissions indirectly, by changing the climate and the hydrological cycle. In these aerosols, microorganisms will be included in a greater or lesser number. The degree of richness in cells of tropospheric aerosols will depend largely on the source of emission. Thus, the large wooded masses or fields of crops provide the atmosphere with a good number of microorganisms due to the effect of air or the aerosols produced by rain. Similarly, anthropogenic activity contributes large amounts of bacteria to the environment, treatment plants, and composting areas being sources of airborne microorganisms [85].

Desert dust storms play a major role in particle emissions and with them that of microorganisms. In this way, most of the material reaching the atmosphere from the surface comes from desert and semidesert areas, which is known as desert dust. The Sahara-Sahel desert, the Middle East, central and eastern Asia, and Australia are the major sources of desert dust, although all the arid zones of the world are emission sources [9, 86]. Dust storms are atmospheric events typically associated with dry lands due to the preponderance of dried and unconsolidated substrates with

little vegetation cover. The strong and turbulent winds that blow on these surfaces raise fine-grained material, a large part of which consists of particles the size of silt (4–62.5 μm) and clay (<4 μm), reducing visibility to less than 1 km. The atmospheric concentrations of PM10 dust exceed 15,000 μg/m3 in severe events [87], although the concentrations naturally decrease with the distance from the areas of origin, extending hundreds of kilometers. The dust particles and cells associated with them are transported in this manner and will be deposited finally, by the effect of rain, snow, or other meteorological phenomena. Therefore, there is a continuous transfer of mineral and biological matter through the atmosphere that moves from the air to the terrestrial environment and changes its geographical area [7, 24].

## **7. Saharan dust**

The Sahara-Sahel desert located in northwestern Africa is one of the major sources of windblown dust in the world [9]. This phenomenon has an impact on the Mediterranean coastline, but Saharan dust has been transported toward the north of Europe and has been found on numerous occasions in the Alps [88, 89] or blown toward the Atlantic and Caribbean [8, 90]. It has been estimated that 80–120 tons of dust are transported annually through the Mediterranean toward Europe [23, 91, 92]. In particular, dust transported by the winds can reach an elevation of up to 8 km in the atmosphere over the Mediterranean basin [93]. Because of its geographic position, the Iberian Peninsula is often affected by these dust events. Specifically, the Sahara-Bodele depression, located at the southern edge of the Sahara desert, has been described as the richest dust source reaching the Iberian Peninsula. Southern Spain is the main area affected, but dust can reach the Pyrenees and even France [43]. Different researchers have studied the mineralogical and chemical composition of Saharan dust, which has been observed to contain calcite, dolomite, quartz, different clay minerals, and feldspars as the main mineral components [94]. The intrusion of big amounts of these components is an important influence on nutrient dynamics and biogeochemical cycling in the atmosphere of the Iberian Peninsula.

Despite the large number of studies on dispersion, geochemistry, and mineralogy of African dust, few are focused on microbiology. All these studies conclude that there are microbes associated with dust because there are higher concentrations of aerosolized microorganisms during dust events [43, 90, 93–96]. However, the magnitude of the concentrations and the specific microbes associated with dust events remain the subject of debate. On the other hand, the viability of these microorganisms is another big question. The United States Geological Survey (USGS) develops the Global Dust Program to investigate the viability of microorganisms transported in dust masses. USGS authors using DNA sequencing of the ribosomal gene were able to isolate and identify more than 200 viable bacteria and fungi in St. John's samples in the USA [8, 36, 90]. Fungi and bacteria associated with atmospheric dust can be recovered and cultivated, but they must be gram-positive bacteria and many spore formers, which makes them resistant to the extreme conditions of the atmosphere.

Therefore, fungi and bacteria associated with dust may have been isolated from dust intrusions, but a percentage of the viable ones already remains an unanswered question. Another big question is the activity of these cells in the atmosphere. It is clear that they are resistant to extremophile conditions, but the question is whether they are developing their life cycle in this particular environment. This question could be answered by molecular ecology methodologies based on the isolation and sequencing of mRNA, but low atmospheric biomass and high variability are, once again, the great problem when developing this type of

**53**

*Microbial Ecology in the Atmosphere: The Last Extreme Environment*

virus-like particles was observed, from 104 to 2105 m<sup>−</sup><sup>3</sup>

the results between the different flows and scenarios.

**after a Saharan dust intrusion**

RNA-based methodologies. On the other hand, clinical records point to many of the viable microorganisms identified in the Saharan dust as the cause of respiratory diseases (asthma and lung infections or allergic reactions), cardiovascular diseases, and skin infections [7, 90, 97, 98]. It is known that other microbes associated with dust in the air are pathogenic to humans, including those that cause anthrax and tuberculosis, or to livestock (such as foot and mouth disease) or plants [7, 90, 97, 98]. Characterization, quantification, and feasibility studies are

It is common to find fungal spores belonging to the genus *Aspergillus*, *Nigrospora*, *Arthrinium*, and *Curvularia* associated with Saharan dust. Bacterial taxa comprised a wide range of phyla, including *Firmicutes*, *Proteobacteria*, *Actinobacteria*, and *Bacteroidetes*. Generators of genus spores such as *Clostridium* and *Bacillus* are very common, along with other gram-positive ones such as *Geodermatophilus* or *Streptococcus*. Also, *Alphaproteobacteria*, a very common bacterium class in soils (e.g., the family *Sphingomonadaceae*), are associated with dust [4, 9]. As regards *Archaea*, there are few studies of the atmosphere, in general, and of dust, in particular, that focus on this domain. Surely, reduced cases of pathogenic archaea have been studied to a lesser extent. *Aeropyrum* is the most detected genus of airborne archaea, but it is related to marine aerosols [11]. On the other hand, studies of pollen associated with dust are widespread. An interesting study investigated pollen transported from North Africa to Spain through Saharan dust and found that pollen from five non-native plant species was detected exclusively during dust events [99]. Lastly, viruses and virus-like particles have a great interest in the emission of dust. One study mentions virus-like particles associated with a transoceanic dust event. This report is based on epifluorescent microscopy of filters stained with a specific nucleic acid stain. An increase in the order of magnitude of

tion and dust conditions in the Caribbean [41]. It is speculated that free airborne viruses show worse resistance to high ultraviolet radiation and dry air associated

**8. Microbial diversity study in the atmosphere of the Iberian Peninsula** 

Four aerobiology sampling flights took place during February and March 2017 using the CASA C-212-200 aircraft from INTA. The study focused on microbial diversity in the atmosphere of the Iberian Peninsula during and after a Saharan dust intrusion. Flights took place under four different conditions: (1) during a strong Sahara dust storm that reached the north of the Iberian Peninsula, from February 22 to 24, 2017 (February 23, 2017) (**Figure 7**); (2) following precipitation (February 28, 2017); (3) following a dry period (March 8, 2017); and (4) along the northern coast of Spain (March 9, 2017). In each flight, samples were collected at different altitudes, and air samples were obtained simultaneously at ground level. A total of 20 samples were collected and are being analyzed. Cell presence was observed by scanning electron microscopy (SEM), and bacterial diversity is being studied by DNA extraction, 16S rRNA gene amplification, and Illumina MiSeq sequencing. Results are being analyzed via bioinformatics and biostatistical software (MOTHUR, SPSS, STAMP, CANOCO, and PAST) which will allow us to compare

Although this study is not yet finished, some data can be advanced in this chapter. **Figure 6** shows SEM microphotographs obtained from samples in different scenarios. In general, the samples obtained during the days of dust intrusion (flight

with long-distance transport in dust events resist worse than others [9].

between the baseline condi-

*DOI: http://dx.doi.org/10.5772/intechopen.81650*

vital to address these problems.

#### *Microbial Ecology in the Atmosphere: The Last Extreme Environment DOI: http://dx.doi.org/10.5772/intechopen.81650*

*Extremophilic Microbes and Metabolites - Diversity, Bioprospecting and Biotechnological...*

spheric concentrations of PM10 dust exceed 15,000 μg/m3

**7. Saharan dust**

little vegetation cover. The strong and turbulent winds that blow on these surfaces raise fine-grained material, a large part of which consists of particles the size of silt (4–62.5 μm) and clay (<4 μm), reducing visibility to less than 1 km. The atmo-

although the concentrations naturally decrease with the distance from the areas of origin, extending hundreds of kilometers. The dust particles and cells associated with them are transported in this manner and will be deposited finally, by the effect of rain, snow, or other meteorological phenomena. Therefore, there is a continuous transfer of mineral and biological matter through the atmosphere that moves from the air to the terrestrial environment and changes its geographical area [7, 24].

The Sahara-Sahel desert located in northwestern Africa is one of the major sources of windblown dust in the world [9]. This phenomenon has an impact on the Mediterranean coastline, but Saharan dust has been transported toward the north of Europe and has been found on numerous occasions in the Alps [88, 89] or blown toward the Atlantic and Caribbean [8, 90]. It has been estimated that 80–120 tons of dust are transported annually through the Mediterranean toward Europe [23, 91, 92]. In particular, dust transported by the winds can reach an elevation of up to 8 km in the atmosphere over the Mediterranean basin [93]. Because of its geographic position, the Iberian Peninsula is often affected by these dust events. Specifically, the Sahara-Bodele depression, located at the southern edge of the Sahara desert, has been described as the richest dust source reaching the Iberian Peninsula. Southern Spain is the main area affected, but dust can reach the Pyrenees and even France [43]. Different researchers have studied the mineralogical and chemical composition of Saharan dust, which has been observed to contain calcite, dolomite, quartz, different clay minerals, and feldspars as the main mineral components [94]. The intrusion of big amounts of these components is an important influence on nutrient dynamics and

biogeochemical cycling in the atmosphere of the Iberian Peninsula.

Despite the large number of studies on dispersion, geochemistry, and mineralogy of African dust, few are focused on microbiology. All these studies conclude that there are microbes associated with dust because there are higher concentrations of aerosolized microorganisms during dust events [43, 90, 93–96]. However, the magnitude of the concentrations and the specific microbes associated with dust events remain the subject of debate. On the other hand, the viability of these microorganisms is another big question. The United States Geological Survey (USGS) develops the Global Dust Program to investigate the viability of microorganisms transported in dust masses. USGS authors using DNA sequencing of the ribosomal gene were able to isolate and identify more than 200 viable bacteria and fungi in St. John's samples in the USA [8, 36, 90]. Fungi and bacteria associated with atmospheric dust can be recovered and cultivated, but they must be gram-positive bacteria and many spore formers, which makes them resistant to the extreme condi-

Therefore, fungi and bacteria associated with dust may have been isolated from dust intrusions, but a percentage of the viable ones already remains an unanswered question. Another big question is the activity of these cells in the atmosphere. It is clear that they are resistant to extremophile conditions, but the question is whether they are developing their life cycle in this particular environment. This question could be answered by molecular ecology methodologies based on the isolation and sequencing of mRNA, but low atmospheric biomass and high variability are, once again, the great problem when developing this type of

in severe events [87],

**52**

tions of the atmosphere.

RNA-based methodologies. On the other hand, clinical records point to many of the viable microorganisms identified in the Saharan dust as the cause of respiratory diseases (asthma and lung infections or allergic reactions), cardiovascular diseases, and skin infections [7, 90, 97, 98]. It is known that other microbes associated with dust in the air are pathogenic to humans, including those that cause anthrax and tuberculosis, or to livestock (such as foot and mouth disease) or plants [7, 90, 97, 98]. Characterization, quantification, and feasibility studies are vital to address these problems.

It is common to find fungal spores belonging to the genus *Aspergillus*, *Nigrospora*, *Arthrinium*, and *Curvularia* associated with Saharan dust. Bacterial taxa comprised a wide range of phyla, including *Firmicutes*, *Proteobacteria*, *Actinobacteria*, and *Bacteroidetes*. Generators of genus spores such as *Clostridium* and *Bacillus* are very common, along with other gram-positive ones such as *Geodermatophilus* or *Streptococcus*. Also, *Alphaproteobacteria*, a very common bacterium class in soils (e.g., the family *Sphingomonadaceae*), are associated with dust [4, 9]. As regards *Archaea*, there are few studies of the atmosphere, in general, and of dust, in particular, that focus on this domain. Surely, reduced cases of pathogenic archaea have been studied to a lesser extent. *Aeropyrum* is the most detected genus of airborne archaea, but it is related to marine aerosols [11]. On the other hand, studies of pollen associated with dust are widespread. An interesting study investigated pollen transported from North Africa to Spain through Saharan dust and found that pollen from five non-native plant species was detected exclusively during dust events [99]. Lastly, viruses and virus-like particles have a great interest in the emission of dust. One study mentions virus-like particles associated with a transoceanic dust event. This report is based on epifluorescent microscopy of filters stained with a specific nucleic acid stain. An increase in the order of magnitude of virus-like particles was observed, from 104 to 2105 m<sup>−</sup><sup>3</sup> between the baseline condition and dust conditions in the Caribbean [41]. It is speculated that free airborne viruses show worse resistance to high ultraviolet radiation and dry air associated with long-distance transport in dust events resist worse than others [9].
