Microbes face constant osmotic pressure challenges, adapting to survive in diverse environments. They use clever strategies like accumulating compatible solutes and adjusting cell structures to maintain balance in both high and low osmotic pressure conditions.

Water activity is crucial for microbial growth, with different organisms thriving at various levels. Understanding this helps us preserve food and control microbial growth. Light and chemical energy sources fuel microbial metabolism, shaping their diverse lifestyles and ecological roles.

Osmotic Pressure and Water Activity

Microbial adaptation to osmotic pressure

• Osmotic pressure is the pressure needed to stop net water movement across a semipermeable membrane
  • High osmotic pressure environments have low water concentration (hyperosmotic)
  • Low osmotic pressure environments have high water concentration (hypoosmotic)
• Microbes adapt to high osmotic pressure (hyperosmotic) environments by:
  • Accumulating compatible solutes (osmoprotectants) in their cytoplasm
    • Amino acids (proline), sugars (trehalose), and potassium ions help maintain cell turgor pressure and prevent water loss
  • Synthesizing cell wall components that increase rigidity and prevent cell lysis (peptidoglycan)
• Microbes adapt to low osmotic pressure (hypoosmotic) environments by:
  • Possessing mechanosensitive channels in their cell membrane
    • Allows for rapid efflux of solutes (ions) to prevent cell lysis due to water influx
  • Having aquaporins in their cell membrane to facilitate controlled water movement
• Tonicity refers to the relative concentration of solutes inside and outside the cell, affecting osmosis
  • In hypertonic environments, cells may undergo plasmolysis, where the cell membrane shrinks away from the cell wall due to water loss

Water activity in microbial growth

• Water activity ($a_w$) is the ratio of water vapor pressure in a substance to pure water vapor pressure at the same temperature
  • Ranges from 0 to 1, with pure water having an $a_w$ of 1
• Microbes require a minimum water activity for growth
  • Most bacteria require $a_w$ > 0.90
  • Filamentous fungi (Aspergillus) and yeasts (Saccharomyces) can grow at lower $a_w$ (0.70-0.85)
  • Halophiles (Halobacterium) can grow at even lower $a_w$ due to their adaptations to high osmotic pressure
• Food preservation methods that reduce water activity:
  1. Drying or dehydration (raisins)
  2. Adding solutes (salt, sugar) to create a hypertonic environment (jams)
  3. Freezing, which converts water to ice and reduces available liquid water (frozen vegetables)

Light Utilization and Energy Generation in Microorganisms

Light utilization by microorganisms

• Phototrophs use light as an energy source for growth
  • Photoautotrophs use light energy to fix carbon dioxide into organic compounds
    • Cyanobacteria (Synechococcus), purple and green sulfur bacteria (Chromatium, Chlorobium) use photosynthetic pigments (chlorophyll, bacteriochlorophyll) to capture light energy
  • Photoheterotrophs use light energy to generate ATP but require organic compounds as a carbon source
    • Purple and green non-sulfur bacteria (Rhodospirillum, Chloroflexus) use bacteriochlorophyll and carotenoids to capture light energy
• Chemotrophs use chemical compounds as an energy source for growth
  • Chemoautotrophs oxidize inorganic compounds to generate energy and fix carbon dioxide into organic compounds
    • Nitrifying bacteria (Nitrosomonas) and sulfur-oxidizing bacteria (Beggiatoa) obtain energy by oxidizing ammonia and hydrogen sulfide, respectively
  • Chemoheterotrophs oxidize organic compounds to generate energy and require organic compounds as a carbon source
    • Most bacteria (Escherichia coli) and fungi (Saccharomyces cerevisiae) are chemoheterotrophs that obtain energy and carbon from organic compounds like sugars and amino acids

Energy generation mechanisms

• Photosynthesis is used by photoautotrophs to convert light energy into chemical energy
• Chemiosmosis is a process used by many microorganisms to generate ATP through the creation of a proton gradient across a membrane
• Oxidation-reduction reactions are fundamental to energy generation in both phototrophs and chemotrophs, involving the transfer of electrons between molecules