Unit 4: Environmental and Agricultural Microbiology

Environmental Microbiology

This field studies the roles microbes play in the environment, particularly in recycling essential elements.

Nutrient Cycling

Microbes are the primary drivers of biogeochemical cycles, converting elements between their organic and inorganic forms.

• Carbon Cycle:
  • Fixation: Photoautotrophs (cyanobacteria, algae) and chemoautotrophs fix atmospheric CO2 into organic compounds (sugars).
  • Release: Heterotrophs (fungi, bacteria) perform respiration and decomposition, breaking down organic compounds and releasing CO2.
• Nitrogen Cycle:
  • Nitrogen Fixation: The most critical step. Bacteria (e.g., *Rhizobium*, *Azotobacter*) convert N2 gas (unusable by plants) into ammonia (NH3).
  • Nitrification: Bacteria (e.g., *Nitrosomonas*) convert ammonia (NH3) to nitrite (NO2-), and then others (*Nitrobacter*) convert nitrite to nitrate (NO3-). Nitrate is the main form of nitrogen used by plants.
  • Denitrification: Bacteria (e.g., *Pseudomonas*) convert nitrate (NO3-) back to N2 gas, returning it to the atmosphere.
• Sulphur Cycle: Microbes convert organic sulfur (in proteins) into hydrogen sulfide gas (H2S), which is then oxidized by other bacteria into sulfate (SO42-), a form plants can use.

Role in Decomposition

• Decomposers (Saprophytes): Microbes (especially fungi and bacteria) are the planet's primary decomposers.
• Mechanism: They secrete extracellular enzymes (like cellulase and protease) to break down complex dead organic matter (leaves, animals) into simple molecules, which they absorb.
• Importance: This process recycles all nutrients, making them available for new plant growth. Without decomposers, life would cease.

Microbial Products

• Antibiotics:
  • Definition: Chemical substances produced by a microbe that kill or inhibit the growth of other microbes.
  • Source: Microbes produce them in nature to compete for resources.
  • Example: Penicillin is produced by the fungus Penicillium. Streptomycin is produced by the bacterium Streptomyces.
• Enzymes:
  • Microbes are "factories" for useful enzymes used in industry.
  • Examples:
    • Proteases and Lipases (from bacteria) are added to laundry detergents to break down protein and fat stains.
    • Amylases (from fungi) are used to break down starch into glucose for making biofuels.

Agricultural Microbiology

This field focuses on using microbes to improve crop production and sustainability, reducing the need for chemical fertilizers and pesticides.

Plant Growth Promoting Bacteria (PGPB)

A broad group of soil bacteria that live in the rhizosphere (the area around plant roots) and promote plant growth through various mechanisms.

Plant-Microbe Interaction (e.g., *Rhizobium*)

• Organism: Rhizobium bacteria.
• Plant: Legumes (e.g., peas, beans, soybeans, clover).
• Interaction: A classic symbiosis (mutualism).
  1. The plant's roots release chemicals that attract *Rhizobium*.
  2. The bacteria invade the root hairs, causing the plant to form a root nodule.
  3. Inside the nodule, the bacteria are safe and receive food (sugars) from the plant.
  4. In return, the bacteria perform nitrogen fixation, converting N2 gas from the air into ammonia (NH3) that the plant uses as a direct fertilizer. This is why legumes are "self-fertilizing" for nitrogen.

Siderophores

• Problem: Iron (Fe) is essential for life, but in most soils, it is in an insoluble form (Fe3+) that plants and microbes cannot use.
• Solution: Many PGPB produce siderophores.
• Mechanism: Siderophores are compounds that are "iron-grabbers" (chelators). The bacteria secrete them to bind tightly to the insoluble Fe3+. This complex is then reabsorbed by the bacterium.
• Benefit to Plant: This process not only helps the plant get iron (indirectly) but also acts as a biocontrol by "starving" pathogenic fungi of iron.

Biocontrol Agents

• Definition: Using one microbe (a biocontrol agent) to control a plant pest or pathogen.
• Example:
  • Bacterium: Bacillus thuringiensis (Bt). This bacterium produces a protein crystal (Cry toxin) that is highly toxic to specific insects (like caterpillars) but harmless to humans and other animals.
  • Application: Bt is sold as an organic pesticide spray. The gene for the Bt toxin is also used to create genetically modified (GM) crops like "Bt cotton."