SOIL MICROBIOLOGY AND SOIL MICROORGANISMS

 SOIL MICROBIOLOGY

·       Soil Microbiology is the scientific discipline that is concerned with the study of all biological aspects of the microorganisms (bacteria, archaea, viruses, fungi, parasites and protozoa) that exist in the soil environment.

·       Soil Microbiology is a sub-discipline of Environmental Microbiology.

·       Soil microorganisms can be classified as Bacteria, Actinobacteria, Fungi, Algae and Protozoa.

·       Sergei N. Winogradsky is considered as the 'Father of Soil Microbiology'.

MAJOR GROUPS OF SOIL MICROORGANISMS

·       Major group of microorganisms which live in soil are

ü  Bacteria

ü  Actinobacteria

ü  Fungi

ü  Algae

ü  Bacteriophages

ü  Protozoa

ü  Nema­todes

·       Basically, Soil microorganisms are classified as

1) Autotrophs

·       Autotrophs are any organisms that are capable of producing their own food. This is achieved by using light energy from sun, water and carbon-di-oxide. Example – Algae and Cyanobacteria.

·       Autotrophs use non-living material (inorganic sources) to make their own food.

·       Autotrophs are “Main producers”.

·       Autotrophs are classified into 2 types. They are: a) Photoautotrophs & b) Chemoautotrophs

a) Photoautotrophs

·       Photoautotrophs are organisms which get the energy to make Organic materials from Sunlight.

·       Photoautotrophs include all Green algae and Bacteria which perform Photosynthesis.

b) Chemoautotrophs

·       Chemoautotrophs are organisms that obtain energy from Inorganic chemical processes.

·       Chemoautotrophs use volatile chemicals such as molecular hydrogen, hydrogen sulfide, elemental sulfur, ferrous iron, and ammonia as their energy sources. 

·       Chemoautotrophs are usually Bacteria or Archaebacteria.

·       Chemoautotrophs include Nitrogen fixing bacteria located in the soil, Iron oxidizing bacteria located in the lava beds (Volcano) and Sulfur oxidizing bacteria located in deep sea thermal vents.

2) Heterotrophs

·       Heterotroph is an organism that cannot manufacture its own food and derives its intake of nutrition from other sources of Organic carbon.

·       Chemoautotrophs are “Consumers and Decomposers”.

·       There are 2 forms of Heterotrophs. They are: a) Photoheterotrops & b) Chemoheterotrophs

a) Photoheterotrophs

·       Photoheterotrophs use light for energy, although are unable to use Carbon-di-oxide as their sole carbon source and, therefore, use organic compounds from their environment. 

·       Heliobacteria and certain Proteobacteria are Photoheterotrophs.

b) Chemoheterotrophs

·       Chemoheterotrophs obtain their energy from ingesting preformed organic energy sources such as Lipids, Carbohydrates and Proteins which have been synthesized by other organisms.

·       Chemoheterotrophs are the most abundant type of organisms and include most Bacteria, Fungi and Protozoa.

QUALITATIVE MICROFLORA OF SOIL

1) Bacteria

·       Bacteria are the smallest unicellular Prokaryotes.

·       In soil, Bacteria are more numerous than others microorganisms.

·       The number of bacteria in soil varies between 10⁸ and 10¹⁰ cells per gram soil. However, in an agriculture field their number goes to about 3 × 10⁹ cells per gram soil.

·       Based on regular presence, Soil bacteria are divided into two groups:

a)    Soil indigenous (True resident) or Autochronous bacteria

ü  Bacterial population is uniform

b)    Soil invader or Allochtonous or Zymogenous bacteria

ü  Bacterial population depends on external source of energy.

ü  Normal population of bacteria is low, but when a specific substrate is added to the soil, they increase in number

·       The number and types of bacteria are influenced by Soil types, Microenvironment, Organic matter and Cultivation practices.

·       The inner region of soil aggregates contained higher level of Gram negative bacteria, while the outer region contained higher level of Gram positive bacteria. This may be due to polymer formation, motility, surface changes, and life cycle of bacteria involved.

·       In soil, bacteria play a ma­jor role in

ü  Organic matter decomposition

ü  Biotransformation

ü  Biogas produc­tion

ü  Nitrogen fixation

·       Example of some of Soil bacteria is Agrobacterium sp., Arthrobacter sp., Bacillus sp., Alcaligenes sp., Clostridium sp., Corynebacterium sp., Erwinia sp., Nitrosomonas sp., Nitrobacter sp., Pseudomonas sp., Rhizobium sp. and Thiobacillus sp.

2) Actinobacteria

·       Actinobacteria (previously called as Actinomycetes) share the characters of both Bacteria and Fungi, and they are commonly known as “Ray-fungi” because of their close affinity with Fungi.

·       Actinobacteria are Gram positive and release antibiotic substances.

·       The earthy odour (due to the compound Geosmin) of newly wetted soils has been found to be a volatile growth product of Actinobacteria.

·       Population of Actinobacteria in soil remains greater in grass land than in the cultivated land.

·       In temperate zones, the number of Actinobacteria ranges from 10⁵ to 10⁸ per gram soil.

·        The luxurient growth is favoured by Neutral or Alkaline pH (6.0 to 8.0).

·       The important members of Actinobacteria are: Actinomyces sp., Actinoplanes sp., Micromonospora sp., Thermomonospora sp., Microbispora sp., Nocardia sp., Frankia sp., Streptomyces sp. and Thermoactinomyces sp.

3) Fungi

·       In most of aer­ated or cultivated soils, fungi share a major part of the total microbial biomass because of their large diameter and extensive net work of mycelium.

·       Popula­tion of soil fungi ranges from 2 × 10⁴ to 1 × 10⁶ per gram dry soil and its number differs according to isolation procedure and composition of media.

·       Fungi derive nutrients for their growth Saprophytically from Organic matters, Living animals (includ­ing Protozoa, Arthropods, Nema­todes, etc.) and Living plants.

·       Fungi also live in the root zone and helps make nutrients available to plants. Mycorrhizae (AM Fungi) facilitate water and nutrient uptake by the roots and plants to provide Sugars, Amino acids and other nutrients.

·       Some of the fungal soil saprophytes are: Alternaria sp., Curvularia sp., Aspergillus sp., Mucor sp., Rhizopus sp., Penicillium sp., Fusarium sp., Cladosporium sp., Dematium sp., Gliocladium sp., Helminthosporium sp., Humicola sp. and Metarrhizium sp.

·       Soil fungi associated with plant disease are: Armillaria sp., Fusarium sp., Helminthosporium sp., Ophiobolus sp., Phytophthora sp., Plasmodiophora sp., Pythium sp., Rhizoctonia sp., Sclerotium sp., Thielaviopsis sp. and Verticillium sp.

4) Algae

·       Soil algae luxuri­antly grow where adequate amount of moisture and light are present. They play a variety of roles in soil.

·       Popula­tion of soil algae ranges from 100 to 10,000 per gram soil.

·       One of the important role of Blue Green Algae (BGA) is that it has revolutionized the field of agriculture microbiology due to use of Cyanobacterial biofertilizer.

·       Soil algae can also be used in reclamation of Sodic soil (alkaline soil) and Sewage treat­ment.

·       Soil algae are divided into four classes

a)     Chlorophyceae – Green algae contains the Green pigment Chlorophyll (Example – Chlorella and Chlamydomonas).

b)    Cyanophyceae – Blue green algae contain Blue pigment Phycocyanin (Example – Nostoc and Anabaena).

c)     Xanthophyceae - Yellow green algae contain Yellow green pigment Xanthophylls (Example – Botrydium).

d)    Bacillaiophyceae – Unicellular algae contains Golden brown pigment (Example - Navicula).  

5) Protozoa

·       In moist soil, most of the members of Protozoa remain in encysted form.

·       The population of each group of Protozoa is 10³ per gram wet soil.

·       The role of soil protozoa is Predatory, as these eat upon bacteria (Enterobacter sp., Agrobacterium sp., Bacillus sp. and Escherichia coli) and thereby regulate their population.

·       The number of Protozoa can be correlated with plant root growth and indirectly with status of soil nutrients.

·       The important members of Protozoa in soil are Cercoboda sp., Euglypha sp. and Colpoda sp.

6) Nematodes

·       Nematodes are microscopic worms that live around or inside the plant.

·       Some Nematodes are Predators while others are beneficial, eating pathogenic nematodes and secreting nutrients to the plant.

·       Example of Soil nematodes are Colpoda sp., Pleurotricha sp., Heteromita sp., Cercomonas sp., Oikomonas sp. and Phalansterium sp.

7) Viruses (Bacteriophages)

·       Soil viruses exist in the form of Bacteriophages.

·        The virus that affect the bacteria are called Bacteriophages.

·       Bacteriophages in soil mainly inhibit the Bacteria, Actinobacteria, Fungi, Algae and Nematodes in soil which are phytopathogenic in nature.