MCB 150 Atmoecosphere

• Atmosphere


  Comprised of concentric layers of gases that are vertically delineated by differences in thermal properties

  
• The atmosphere is composed mainly of 78.08% nitrogen, 20.95% oxygen, and 0.93% argon by volume. The remaining roughly 0.04% includes trace gases like carbon dioxide, methane, nitrous oxide, and ozone
• The atmosphere also contains water vapor (with fluctuating saturation levels), liquid water droplets, ice crystals, and dust particles
• Exosphere


  • Outermost atmospheric layer, stretching from 375 miles (600 km) to 6,200 miles (10,000 km) above Earth
  • A molecule traveling upward will fall back to Earth by virtue of gravity unless it is traveling at 11 km/s —the escape velocity
• Thermosphere


  • Upper atmosphere, spans from about 53 miles (85 km) to 375 miles (600 km) above Earth
  • Gas density increases closer to Earth within this very thin layer
  • Sun's ultraviolet and x-ray radiation is absorbed here, significantly raising temperatures
  • Temperatures range from -184°F (-120°C) at the lower edge to up to 3,600°F (2,000°C) at the upper boundary
  • Despite high temperatures, this layer will be cold to microbes as a result of the scarcity of gas molecules for heat transfer
• Mesosphere


  • Spans from 31 miles (50 km) to 53 miles (85 km) above Earth
  • Gases densify with descent
  • Temperature increases with decreasing altitude, reaching around 5°F (-15°C) near its base

  
• Stratosphere


  • Stretches from 4-12 miles (6-20 km) to around 31 miles (50 km) above Earth
  • Contains 19% of atmospheric gases but little water vapor
  • Temperature rises with altitude due to ozone formation, heating the layer from -60°F (-51°C) at the tropopause to about 5°F (-15°C) at its top
  • Temperature gradient, with warmer air above the cooler, inhibits convection, preventing gases from moving upward
• Troposphere


  • Where nearly all weather occurs
  • Height varies: around 11-12 miles (18-20 km) at the equator, 5½ miles (9 km) at 50°N and 50°S, and just under 4 miles (6 km) at the poles
  • Gas density and air thickness decrease with altitude
  • Temperature decreases with height, dropping from an average of 62°F (17°C) at ground level to -60°F (-51°C) at the tropopause
• The troposphere contains the majority of microbial cells found in the atmosphere
• Vertical mixing of gases between the troposphere and the stratosphere is limited due to thermal inversion

  
• Gases carrying microbes still reach the stratosphere, hence the isolation of viable microorganisms from the layer
• The atmospheric ozone in the lower region of the stratosphere likely sets the altitudinal limit to the survival of the majority of microbes due to reduced UV attenuation beyond the ozone layer
• Relative humidity


  The relative water content in the atmosphere, one of the most important properties that affects the survival of airborne microorganisms
• Inactivation due to very low relative humidity


  Occurs by virtue of water loss from the cells, changing the crystalline structure of the lipid bilayers of the cell membrane to a gel phase, affecting cell surface protein configurations and leading to the loss of viability of the cells
• Microorganisms that are not adapted to xeric environments are still supported by regions of high relative humidity and clouds
• Inactivation due to high temperatures


  Commonly associated with desiccation (at low relative humidity) and protein denaturation in microbial cells
• Inactivation due to low temperatures


  Loss of viability is due to the formation of ice crystals on cell structures
• Temperature shock


  Causes freeze-thaw cycles that causes mechanical stress that damages the cell membrane
• Ionizing radiation


  Causes single-strand and double-strand breaks, as well as alterations in nucleic acid base structures
• Non-ionizing radiation


  Causes intrastrand dimerization of thymines, distorting the DNA helix and disrupting normal replication, transcription, and translation
• Radiation serves as a strong filter to microbial survival, but it is unlikely to be a limiting factor to radiation-tolerant microbial taxa
• Limited nutrients in the atmosphere


  However, clouds and rainwater can harbor sulphate and nitrate reaching levels of oligotrophic lakes, and carbon sources such as carboxylic acids, alcohols, and hydrocarbons are also present in the atmosphere and clouds, albeit in lower concentrations
• Oxygen, open air factor and ions


  Combine to inactivate airborne microorganisms
• There is a lack of information regarding microbial communities in the atmosphere as compared to host-associated, terrestrial, and aquatic environments
• The atmosphere is still able to support the transport of microorganisms between habitats and of biological particles such as bacterial and fungal spores, cell fragments, microbial cells, and viruses that account for the primary biological aerosol particles or bioaerosols

  
• Most cells remain metabolically inactive during transport in the atmosphere
• Groups of microorganisms in the atmosphere


  • Not metabolically active
  • Metabolically active but rarely reproduce
  • Metabolically active and can actively reproduce (considered "residents" of the atmosphere)
• Despite the assumptions that the atmosphere cannot support the life of microorganisms, the atmosphere can act as a habitat for microbial life
• Biological particles such as bacterial and fungal spores, cell fragments, microbial cells, and viruses account for the primary biological aerosol particles or bioaerosols
• Most cells remain metabolically inactive during transport
• Groups of microorganisms in the atmosphere


  • Not metabolically active
  • Metabolically active but rarely reproduce
  • Metabolically active and can actively reproduce
• The atmosphere can act as a habitat for microbial life
• Troposphere


  • Lowermost layer that contains 75% of the atmosphere's molecules, gas, water vapor, atmospheric particles, and the majority of microbial cells
  • Conditions in this region do not favor microbial growth, hence the lack of known microbial communities
• Bacterial spores and fungal spores are especially common and taxa-adapted to the conditions of the atmosphere such as desiccation and oxidative stress
• Microbial dispersal may be limited in the stratosphere due to more extreme environmental stress and extended residence times
• Microorganisms were found in the mesosphere at an altitude of around 48-77 km
• Five out of the six species detected in the mesosphere can synthesize pigments, suggesting that natural selection is happening in the mesosphere since cells with chromogenic pigments are more resistant to UV radiation
• Airborne microbes


  • Diseases caused by viruses, bacteria, and fungi can spread through the atmosphere, often following prevailing wind patterns
  • Temporary locations like clouds could potentially harbor microorganisms, benefitting from concentrated water and sufficient light intensities and carbon dioxide for growth, particularly for photoautotrophic microorganisms
  • Condensation nuclei within clouds may provide mineral nutrients to support microbial growth
  • In industrial areas, organic chemicals in the atmosphere might even sustain growth for certain heterotrophs

  
• Spores


  Less metabolically active than vegetative cells, are better adapted for survival in the atmosphere
• Characteristics of atmospheric spores


  • Metabolically dormant
  • Extremely thick walls and a relatively low density
  • Pigmented, light, and small
  • Produced in very high numbers