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Read this article to learn about soil microorganisms. After reading this article you will learn about: 1. Types of Soil Microorganisms 2. Importance of Soil Microorganisms 3. Cultivation.
Types of Soil Microorganisms:
1. Bacteria:
More dominant group of microorganisms in the soil and equal to one half of the microbial biomass in soil. Population 100,000 to several hundred millions for gram of soil. (Common soil bacteria – Arthrobacter, Bacillus, Clostridium, Micrococcus).
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2. Actinomycetes:
Intermediate group between bacteria and fungi. Numerous and widely distributed in soil. Abundance is next to bacteria. 104 – 108/g soil. 70% of soil actinomycetes are Streptomyces. Many of them are known to produce antibiotics. Population increases with depth of soil.
3. Fungi:
More numerous in surface layers of well-aerated and cultivated soils-dominant in acid soils. Common genera in soil are Aspergillus, Mucor, Penicillium Trichoderma, Alternaria and Rhizopus. Algae – found in most of the soils in number ranges from 100 to 10,000 per g.
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4. Protozoa:
Unicellular – population ranges from 10,000 to 100,000 per g of soil. Most of the soil forms are flagellates, amoebae or ciliates. Derive their nutrition by devouring soil bacteria. Abundant in upper larger of the soil. They are regulating the biological equilibrium in soil.
Average Size of Microbes:
(a) Bacteria-0.5-3.0mm
(b) Fungi-1.5-10mm
(c) Protozoa-2-200mm
(d) Viruses-100-600 Nano m(0.002mm)
(e) MLOS-0.1-0.3mm
(f) Algae – 0.1mm (BGA) to several feet (higher algae)
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Virus:
(a) Virus = Nucleic acid + Protein
(b) Lipo-virus = Nucleic acid + Protein + lipid (e.g. Influenza virus)
(c) Animal virus = DNA + Protein (or Bacteriophage)
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(d) Plant Virus = RNA + Protein
Importance of Soil Microorganisms:
(Involved in nutrient transformation process)
1. Decomposition of resistant components of plant and animal tissue
2. Role in microbial antagonism
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3. Participate in humus formation
4. Predator of nematodes
5. Surface blooming reduces erosion losses
6. Improve soil structure
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7. Involved soil structure
8. Maintenance of biological equilibrium
(a) Rhizosphere:
The term ‘rhizosphere’ was introduced in 1904 by the German scientist Hiltner to denote that region of the soil which is modified as a result of the uptake and deposition of substances by a growing root or the root surface (rhizoplane) together with that region of the surrounding soil in which the microbial population is affected, qualitatively and/or quantitatively, by the presence of a root.
The rhizosphere may extend a few millimeters, or centimeters, from the rhizoplane.
(b) Rhizoplane:
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Root surface along with the closely adhering soil particles is termed as rhizoplane.
(c) Antibiotics
A. Important antibiotics:
Penicillin, Cephalosporin, Erythromycin, Carbomycin, Streptomycin, Neomycin, Oxytracycline, Polymixin G, Bulbiformin, aureofurgin Bacitracin etc.
B. Discovery of antibiotics:
(i) Alexander Fleming – discovered penicillin (first antibiotic) in 1929
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(ii) S. A. Wakesman – streptomycin in 1943, used against Mycobacterium tuberculosis (the cause of tuberculosis)
(iii) Vasudeva I et al. – Bulbiformin in 1958
(iv) Thirumalachar et al, – Aureofungin in 1964, the antibiotic against fungi
Cultivation of Soil Microorganisms:
Microbes growing on a nutrient medium are referred to as a culture.
1. Spallanzani (1776) – foundation of sterile culture
2. Robert Koch – Gelatin nutrient medium
3. Frau Hesse (1883) – Agar-agar medium
(a) Sterilization:
Sterilization is the complete removal of all microbes through chemicals, filters and ionization.
A. Direct flaming:
The objects to be sterilized are passed through flame of spirit lamp, e.g. slide sterilization.
B. Hot air sterilization:
Objects like vessels are sterilized in hot air oven at 160-180˚C for two hours.
C. Moist heat with pressure (Autoclaving):
Material is sterilized in an instrument called autoclave at 15-lb/psi (per square inch) pressure (which almost equals to 121.6°C) for 15 minutes.
D. Chemical sterilization:
Chemical agents suitable and used for sterilization is called ‘sanitizer’ are used to reduce microbial population. Sanitizers are commonly used to sterile laboratory equipment’s, e.g. Propylene oxide @ 5 ml/10ml of culture
(b) Gram’s Staining Techniques:
A. A Danish physician, Hans Christian Gram, devised gram staining technique for identification and characterization of bacteria in 1884.
Procedure:
(i) A slide containing heat-fixed smear of bacterial cell, is treated with crystal- violet stain (a basic dye), during which the cells turn purple.
(ii) The slide is then flushed with Gram’s iodine solution.
(iii) Then an organic solvent acetone is used for decolourization the bacteria. At this stage, gram-positive bacteria remain purple because they have a single thick cell wall that is not easily penetrated by the acetone alcohol whereas gram-negative bacteria are decolorized as they have cell walls with much thinner layers that allow easy penetration.
(iv) In a final step, a counterstain (safranin) is used to distinguish bacteria into gram positive (purple coloured) and gram negative (pink coloured).
(c) Mineralization:
A. Mineralization is the process in which organic materials are broken down into inorganic materials.
B. Mineralization is effected mainly by saprotrophic bacteria and fungi.
C. Mineralization in organically polluted rivers and lakes may lead to SELF PURIFICATION.
D. Mineralization plays an essential part in the cycles of matter, e.g. Carbon Cycle, Nitrogen Cycle and Sulphur Cycle.
Immobilization of nitrogen is the reverse of mineralization and occurs when large quantities of low nitrogen crop reduces begin decomposition.
Nitrogen fixing organisms was 1st isolated by M.W. Beijerinck, who called the organism Bacillus radicicola.
(d) Mycorrhiza:
A. Mutualistic association between a fungus and the root (or rhizoid) of a plant.
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B. Mycorrhizas occur in the majority of plants, including vascular and some nonvascular species (e.g. liverworts).
C. The fungi involved (e.g. basidiomycetes, ascomycetes, deuteromycetes) are always associated with the primary cortex of the root, and many appear never to occur as free-living saprotrophs.
D. The formation of mycorrhizas leads to improved uptake of nutrients by the host plant; nutrients are apparently absorbed by hyphae (which may extend some distance from the root) and are transported back to the root to be released into the host tissue.
E. Mycorrhiza formation and efficacy is greatest in nutrient-poor soils, and may be reduced or eliminated by application of soil fertilizers.
(e) Vermicomposting:
A. The use of earthworms in composting process is known as Vermicomposting.
B. One kg of earthworms can consume one kg of organic materials in a day and secrete as Castings that are rich in nitrate, available phosphorus, potassium, calcium and magnesium.
C. Germicide (microbicide):
An agent that kills microorganisms but not necessarily their resistant spores.
D. Sanitizer:
An agent that reduces the microbial population to safe levels. Usually, it is a chemical agent that kills 99.9% of growing bacteria.
E. Symbiosis:
A term used originally for any stable condition in which two different organisms (symbionts) live in more or less close physical association – regardless of the nature of the relationship.
F. Phyllosphere:
The region/zone on leaves inhibited by microorganisms has been termed as ‘phyllosphere’ and the leaf surface as ‘phylloplane’.
