MICROBIOLOGY: EXERCISE 7 SOIL MICROBIOLOGY

• Soil

  is a complex mixture of organic matter consisting of mineral particles, water, air, and organic matter from decaying biomass of plants, animals, and microorganisms
• Soil

  It contains extensive microbial populations of bacteria, yeast, molds, algae, and protozoa
• Topsoil

  has the highest number of microorganisms
• Rhizosphere


  soil region adjacent to plant roots where nitrogen-fixing bacteria is found; high microbial activity is observed
• Factors affecting population of microorganisms in soil

  • physical structure

  • nutrient composition

  • temperature

  • water potential

  • availability of oxygen

  • pH
• Effect of Enrichment on Soil Microorganisms

  Urea and Starch
• Urea


  • Nitrogenous organic compound (mainly as nitrogen source for

  microbes); used as solid nitrogenous fertilizer

  • Hydrolyzed by urease (enzyme that breaks down urea)
• Starch


  • Polysaccharide (carbohydrate or carbon source)

  • Hydrolyzed by amylases (breaks down the amylose in starch)
• Importance of microorganisms in the soil

  • weathering of rocks and in soil formation

  • responsible for the decomposition of complex organic materials into utilizable form

  • they are involved in the geo-chemical cycling of nitrogen, sulfur, carbon, and phosphorus

  • reservoir of pathogenic microorganisms responsible for some plant and animal diseases

  • reservoir of microorganisms with unique metabolic pathways or enzymes
• Nitrogen Fixation


  Conversion of atmospheric N2 into ammonia by some species of bacteria; these bacteria are either free-living or form symbiotic associations with plants or other organisms.
• Nitrogen-Fixing Bacteria


  Free-living and Symbiotic
• Free living

  ability to fix nitrogen without the association of other microorganism (e.g. cyanobacteria and bacteria classified

  under the genus Azotobacter, and other Clostridia)
• Symbiotic

  a bacterium capable of fixing nitrogen with the aid of a symbiont which is usually a plant (e.g. legumes) or other organisms (e.g. termites, protozoa).
• Plant-symbiotic nitrogen-fixing bacteria

  are classified under the genus Rhizobium (Gram negative, small rods) and are capable of infecting the roots of legumes (peas, soybeans, clover, alfalfa, etc.)
• Root nodules


  red-brown in color due to leghemoglobin
• Leghemoglobin


  binds oxygen and supplies it to the bacteria and at the same time, protects the oxygen-sensitive nitrogenous enzyme system.
• nitrogen-fixing bacteria look under the microscope


  They appear as short rods and are stained red, as a result of their gram stain reaction, since they are gram negative bacteria.