Aeromicrobiology- Atmospheric Layers & Microorganisms in air

Composition of air

The atmosphere consists of 78 % Nitrogen,  21% Oxygen, 0.9% Argon, 0.03 % Carbon dioxide, 0.01% Hydrogen, traces of other gases, dust and condensed vapour.  In addition to all these, air also contains microorganisms. The study of airborne microorganisms is called Aero microbiology. 

Atmospheric layers

Earth is surrounded by the blanket of gases. It has various layers ranging upto the height of thousand kilometres. These layers include Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere. Troposphere is the atmospheric layer nearest to the earth surface and extends up to a height of about 11 to 18 kilometres. Troposphere contains a heavy load of microbes.  However, air is not a natural environment for microorganisms. Physical & chemical parameters of air do not support the growth and multiplication of microorganisms. Microbes present in the troposphere are actually liberated into air from other sources like soil, water, plant & animal surfaces and human beings. Air acts  mainly as a medium for dispersion and transmission of microorganisms. Several infectious diseases are transmitted through air. 

As the height increases, the amount of microorganisms decreases in atmospheric layers. Temperature decreases with increasing height of the troposphere. Temperatures at the top layers of the troposphere range between -43 to - 83 degree Celsius which are below the minimal growth temperatures for microorganisms. As height increases, there is a reduction in atmospheric pressure, concentration of oxygen and water availability. Also at high altitudes, microorganisms are also exposed to high intensity of light radiation and there will be more exposure to UV radiation that may cause lethal mutation and death of microorganisms. Also low concentration of organic carbon is found which will affect the growth of heterotrophs. The physical conditions in high altitude limit even the existence of autotrophs.

 

Atmospheric layer above the troposphere is called the stratosphere. The stratosphere is characterized by low mixing of gases and contains a layer of high ozone concentration. Ozone layer absorbs the ultraviolet rays of sun and protects the Earth surface from excessive UV radiation. But exposure to high concentrations of ozone and UV radiation are detrimental to microbes. Thus, the stratosphere acts as a barrier to the transport of living microorganisms to or from the troposphere. 

Microorganisms in air

Microflora in air is less compared to soil and water. Air is mainly acting as the transport or dispersal media for microorganisms. Microorganisms are mainly confined to the  troposphere.  The microbes in the troposphere include vegetative cells and spores of bacteria, fungi, algae, viruses and cysts of protozoa. As air is exposed to sunlight, it will have higher temperature and less moisture content. Hence microbes cannot survive in the air for longer periods. 

Outdoor Microflora

Microflora of air can be divided into outdoor microflora and indoor microflora. Air  found outside the buildings is called outside air and the microorganisms in outside air are called outdoor microflora. Outside air is of larger volume than the indoor air. Dominant microflora in the outdoor air  are fungi. Two most common fungi include the deuteromycete Cladosporium and the Basidiomycete yeast Sporobolomyces. Other  common fungal forms include Basidiospores,  Ascospores, conidiospores and mycelial fragments. Bacterial forms are also found in outside air.  The common forms include spore forming species like Bacillus, Clostridium, non spore formers like Sarcina lutea, Micrococcus luteus, non pathogenic species of Corynebacterium and coliforms. 

Air microflora over the sea surfaces are quite different from that of landscapes and includes organisms like Achromobacter, Bacillus, Corynebacterium, Flavobacterium, Micrococcus, Sarcina and Staphylococcus.

Microorganisms are detected up to very high altitudes of the troposphere. However, spores  are dominant than vegetative forms at high altitudes and can be seen up to a height of 30 kilometres, but in low concentration. 

Microflora of outdoor air is not constant. Several factors affect the distribution of outdoor microflora. 

1. As height increases microbes are subjected to low temperature, UV radiation and desiccation. 
2. Number of microbes varies from place to place due to differences in human population densities. More microbes are found in  crowded areas and also in air over terrestrial surfaces then over the sea.  
3. Variations in the spore densities in outdoor air occur on an annual and daily basis. Annual variations are due to climatic changes, mainly because of the changes in temperature. Very high temperature and lower winter temperature are less favourable for microorganisms. Also  during the rainy season  microbial spore density in outdoor air is less as rain washes out spores in the air.
4. Diurnal fluctuation in spore liberation also affects outdoor microflora.

Eg: Colorless basidiospores are liberated usually during night, Sporangia of Phytophthora are liberated in the morning when relative humidity is low, spores of Erysiphe are released during midday.

Indoor Microflora

Air inside the buildings is called indoor air and the microorganisms in it are called indoor microflora. Indoor microflora is not constant and affected by air circulation through doors, windows and due to turbulence caused by moving people.  In indoor air also, fungi are the dominant forms, saprophytic fungi like Penicillium and Aspergillus being the most common ones. Bacteria in indoor air  is determined by factors like ventilation and the number of occupants in the room.  Common bacterial forms include Bacillus, Staphylococci and Clostridium perfringens. Indoor bacteria is mainly from human skin and respiratory tracts. Pathogenic bacteria or viruses can be seen in the indoor air if the occupants are infected. 

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