Types of Microorganisms in Soil
Living organisms both plants and
animals, constitute an important component of soil. The pioneering
investigations of a number of early microbiologists showed for the first time
that the soil was not an inert static material but a medium pulsating with
life. The soil is now believed to be a dynamic or rather a living system,
containing a dynamic population of organisms/microorganisms. Cultivated soil
has relatively more population of microorganisms than the fallow land, and the
soils rich in organic matter contain much more population than sandy and eroded
soils. Microbes in the soil are important to us in maintaining soil fertility /
productivity, cycling of nutrient elements in the biosphere and sources of
industrial products such as enzymes, antibiotics, vitamins, hormones, organic
acids etc. At the same time certain soil microbes are the causal agents of
human and plant diseases.
The soil organisms are broadly classified in to two groups viz soil flora and soil fauna, the detailed classification of which is as follows.
The soil organisms are broadly classified in to two groups viz soil flora and soil fauna, the detailed classification of which is as follows.
Soil Organisms
A. Soil Flora
a) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces 4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.
a) Microflora: 1. Bacteria 2. Fungi, Molds, Yeast, Mushroom 3. Actinomycetes, Stretomyces 4. Algae eg. BGA, Yellow Green Algae, Golden Brown Algae.
1. Bacteria is again classified in I)
Heterotrophic eg. symbiotic & non - symbiotic N2 fixers,
Ammonifier, Cellulose Decomposers, Denitrifiers II) Autrotrophic
eg. Nitrosomonas, Nitrobacter, Sulphur oxidizers, etc.
b) Macroflora:
Roots of higher plants
B. Soil Fauna
a) Microfauna:
Protozoa, Nematodes
b) Macrofauna:
Earthworms. moles, ants & others.
As soil inhabit several diverse
groups of microorganisms, but the most important amongst them are: bacteria,
actinomycetes, fungi, algae and protozoa. The characteristics and their
functions / role in the soil are described in the next topics.
Soil Microorganism: Bacteria
Amongst
the different microorganisms inhabiting in the soil, bacteria are the most
abundant and predominant organisms. These are primitive, prokaryotic,
microscopic and unicellular microorganisms without chlorophyll.
Morphologically, soil bacteria are divided into three groups viz Cocci (round/spherical),
(rod-shaped) and Spirilla I Spirllum (cells with long wavy chains). Bacilli
are most numerous followed by Cocci and Spirilla in soil.
The most common method used for isolation of soil bacteria is the "dilution plate count" method which allows the enumeration of only viable/living cells in the soil. The size of soil bacteria varies from 0.5 to 1.0 micron in diameter and 1.0 to 10.0 microns in length. They are motile with locomotory organs flagella.
Bacterial population is one-half of the total microbial biomass in the soil ranging from 1,00000 to several hundred millions per gram of soil, depending upon the physical, chemical and biological conditions of the soil.
Winogradsky (1925), on the basis of ecological characteristics classified soil microorganisms in general and bacteria in particular into two broad categories i.e. Autochnotus (Indigenous species) and the Zymogenous (fermentative). Autochnotus bacterial population is uniform and constant in soil, since their nutrition is derived from native soil organic matter (eg. Arthrobacter and Nocardia whereas Zymogenous bacterial population in soil is low, as they require an external source of energy, eg. Pseudomonas & Bacillus. The population of Zymogenous bacteria increases gradually when a specific substrate is added to the soil. To this category belong the cellulose decomposers, nitrogen utilizing bacteria and ammonifiers.
As per the system proposed in the Bergey's Manual of Systematic Bacteriology, most of the bacteria which are predominantly encountered in soil are taxonomically included in the three orders, Pseudomonadales, Eubacteriales and Actinomycetales of the class Schizomycetes. The most common soil bacteria belong to the genera Pseudomonas, Arthrobacter, Clostridium Achromobacter, Sarcina, Enterobacter etc. The another group of bacteria common in soils is the Myxobacteria belonging to the genera Micrococcus, Chondrococcus, Archangium, Polyangium, Cyptophaga.
Bacteria are also classified on the basis of physiological activity or mode of nutrition, especially the manner in which they obtain their carbon, nitrogen, energy and other nutrient requirements. They are broadly divided into two groups i.e. a) Autotrophs and b) Heterotrophs
The most common method used for isolation of soil bacteria is the "dilution plate count" method which allows the enumeration of only viable/living cells in the soil. The size of soil bacteria varies from 0.5 to 1.0 micron in diameter and 1.0 to 10.0 microns in length. They are motile with locomotory organs flagella.
Bacterial population is one-half of the total microbial biomass in the soil ranging from 1,00000 to several hundred millions per gram of soil, depending upon the physical, chemical and biological conditions of the soil.
Winogradsky (1925), on the basis of ecological characteristics classified soil microorganisms in general and bacteria in particular into two broad categories i.e. Autochnotus (Indigenous species) and the Zymogenous (fermentative). Autochnotus bacterial population is uniform and constant in soil, since their nutrition is derived from native soil organic matter (eg. Arthrobacter and Nocardia whereas Zymogenous bacterial population in soil is low, as they require an external source of energy, eg. Pseudomonas & Bacillus. The population of Zymogenous bacteria increases gradually when a specific substrate is added to the soil. To this category belong the cellulose decomposers, nitrogen utilizing bacteria and ammonifiers.
As per the system proposed in the Bergey's Manual of Systematic Bacteriology, most of the bacteria which are predominantly encountered in soil are taxonomically included in the three orders, Pseudomonadales, Eubacteriales and Actinomycetales of the class Schizomycetes. The most common soil bacteria belong to the genera Pseudomonas, Arthrobacter, Clostridium Achromobacter, Sarcina, Enterobacter etc. The another group of bacteria common in soils is the Myxobacteria belonging to the genera Micrococcus, Chondrococcus, Archangium, Polyangium, Cyptophaga.
Bacteria are also classified on the basis of physiological activity or mode of nutrition, especially the manner in which they obtain their carbon, nitrogen, energy and other nutrient requirements. They are broadly divided into two groups i.e. a) Autotrophs and b) Heterotrophs
- Autotrophic bacteria are capable synthesizing their food from simple inorganic nutrients, while heterotrophic bacteria depend on pre-formed food for nutrition. All autotrophic bacteria utilize Co2 (from atmosphere) as carbon source and derive energy either from sunlight (photoautotrophs, eg. Chromatrum. Chlorobium. Rhadopseudomonas or from the oxidation of simple inorganic substances present in soil (chemoautotrophs eg. Nitrobacter, Nitrosomonas, Thiaobacillus).
- Majority of soil bacteria are heterotrophic in nature and derive their carbon and energy from complex organic substances/organic matter, decaying roots and plant residues. They obtain their nitrogen from nitrates and ammonia compounds (proteins) present in soil and other nutrients from soil or from the decomposing organic matter. Certain bacteria also require amino acids, B- Vitamins, and other growth promoting substances also.
Functions
/ Role of Bacteria:
Bacteria bring about a number of changes and biochemical transformations in the soil and thereby directly or indirectly help in the nutrition of higher plants growing in the soil. The important transformations and processes in which soil bacteria play vital role are: decomposition of cellulose and other carbohydrates, ammonification (proteins ammonia), nitrification (ammonia-nitrites-nitrates), denitrification (release of free elemental nitrogen), biological fixation of atmospheric nitrogen (symbiotic and non-symbiotic) oxidation and reduction of sulphur and iron compounds. All these processes play a significant role in plant nutrition,
Bacteria bring about a number of changes and biochemical transformations in the soil and thereby directly or indirectly help in the nutrition of higher plants growing in the soil. The important transformations and processes in which soil bacteria play vital role are: decomposition of cellulose and other carbohydrates, ammonification (proteins ammonia), nitrification (ammonia-nitrites-nitrates), denitrification (release of free elemental nitrogen), biological fixation of atmospheric nitrogen (symbiotic and non-symbiotic) oxidation and reduction of sulphur and iron compounds. All these processes play a significant role in plant nutrition,
|
Process/reaction
|
Bacterial
genera
|
|
Cellulose
decomposition (celluloytic bacteria ) most cellulose
decomposers are mesophilic
|
a.
Aerobic : Angiococcus, Cytophaga, Polyangium, Sporocytophyga, Bacillus,
Achromobacter, Cellulomonas
b. anaerobic: Clostridium Methanosarcina, Methanococcus |
|
Ammonification
(Ammonifiers)
|
Bacillus,
Pseudomonas
|
|
Nitrification
(Nitrifying bacteria)
|
Nitrosomonas,
Nilrobacter Nitrosococcus
|
|
Denitrification
(Denitrifies)
|
Achromobacter,
Pseudomonas, Bacillus, Micrococcus
|
|
Nitrogen
fixing bacteria
|
a
Symbiotic- Rhizobium, Bradyrrhizobium
b Non-symbiotic: aerobic – Azotobacter Beijerinckia (acidic soils), anaerobic-Clostridium |
Bacteria
capable of degrading various plant residues in soil are :
|
Cellulose |
Hemicelluloses
|
Lignin
|
Pectin
|
Proteins
|
|
Pseudomonas
|
Bacillus
|
Pseudomonas
|
Erwinia
|
Clostridium
|
|
Cytophaya
|
Vibrio
|
Micrococcus
|
|
Proteus
|
|
Spirillum
|
Pseudomonas
|
Flavobacteriumm
|
|
Pseudomonas
|
|
Actinomycetes
|
Erwinia
|
Xanthomonas
|
|
Bacillus
|
|
Cellulomonas
|
|
Streptomyces
|
|
|
Soil Microorganism – Fungi
Fungi
in soil are present as mycelial bits, rhizomorph or as different spores. Their
number varies from a few thousand to a few -million per gram of soil. Soil
fungi possess filamentous mycelium composed of individual hyphae. The fungal
hyphae may be aseptate /coenocytic (Mastigomycotina and Zygomycotina) or
septate (Ascomycotina, Basidiomycotina & Deuteromycotina).
As observed by C.K. Jackson (1975), most commonly encountered genera of fungi in soil are; Alternaria, Aspergillus, Cladosporium, Cephalosporium Botrytis, Chaetomium, Fusarium, Mucor, Penicillium, Verticillium, Trichoderma, Rhizopus, Gliocladium, Monilia, Pythium, etc. Most of these fungal genera belong to the subdivision Deuteromycotina / Fungi imperfeacta which lacks sexual mode of reproduction.
As these soil fungi are aerobic and heterotrophic, they require abundant supply of oxygen and organic matter in soil. Fungi are dominant in acid soils, because acidic environment is not conducive / suitable for the existence of either bacteria or actinomycetes. The optimum PH range for fungi lies-between 4.5 to 6.5. They are also present in neutral and alkaline soils and some can even tolerate PH beyond 9.0
Functions / Role of Fungi
1. Fungi plays significant role in soils and plant nutrition.
2. They plays important role in the degradation / decomposition of cellulose, hemi cellulose, starch, pectin, lignin in the organic matter added to the soil.
3. Lignin which is resistant to decomposition by bacteria is mainly decomposed by fungi.
4. They also serve as food for bacteria.
5. Certain fungi belonging to sub-division Zygomycotina and Deuteromycotina are predaceous in nature and attack on protozoa & nematodes in soil and thus, maintain biological equilibrium in soil.
6. They also plays important role in soil aggregation and in the formation of humus.
7. Some soil fungi are parasitic and causes number of plant diseases such as wilts, root rots, damping-off and seedling blights eg. Pythium, Phyiophlhora, Fusarium, Verticillium etc.
8. Number of soil fungi forms mycorrhizal association with the roots of higher plants (symbiotic association of a fungus with the roots of a higher plant) and helps in mobilization of soil phosphorus and nitrogen eg. Glomus, Gigaspora, Aculospora, (Endomycorrhiza) and Amanita, Boletus, Entoloma, Lactarius (Ectomycorrhiza).
As observed by C.K. Jackson (1975), most commonly encountered genera of fungi in soil are; Alternaria, Aspergillus, Cladosporium, Cephalosporium Botrytis, Chaetomium, Fusarium, Mucor, Penicillium, Verticillium, Trichoderma, Rhizopus, Gliocladium, Monilia, Pythium, etc. Most of these fungal genera belong to the subdivision Deuteromycotina / Fungi imperfeacta which lacks sexual mode of reproduction.
As these soil fungi are aerobic and heterotrophic, they require abundant supply of oxygen and organic matter in soil. Fungi are dominant in acid soils, because acidic environment is not conducive / suitable for the existence of either bacteria or actinomycetes. The optimum PH range for fungi lies-between 4.5 to 6.5. They are also present in neutral and alkaline soils and some can even tolerate PH beyond 9.0
Functions / Role of Fungi
1. Fungi plays significant role in soils and plant nutrition.
2. They plays important role in the degradation / decomposition of cellulose, hemi cellulose, starch, pectin, lignin in the organic matter added to the soil.
3. Lignin which is resistant to decomposition by bacteria is mainly decomposed by fungi.
4. They also serve as food for bacteria.
5. Certain fungi belonging to sub-division Zygomycotina and Deuteromycotina are predaceous in nature and attack on protozoa & nematodes in soil and thus, maintain biological equilibrium in soil.
6. They also plays important role in soil aggregation and in the formation of humus.
7. Some soil fungi are parasitic and causes number of plant diseases such as wilts, root rots, damping-off and seedling blights eg. Pythium, Phyiophlhora, Fusarium, Verticillium etc.
8. Number of soil fungi forms mycorrhizal association with the roots of higher plants (symbiotic association of a fungus with the roots of a higher plant) and helps in mobilization of soil phosphorus and nitrogen eg. Glomus, Gigaspora, Aculospora, (Endomycorrhiza) and Amanita, Boletus, Entoloma, Lactarius (Ectomycorrhiza).
Soil Microorganism – Algae
Algae
are present in most of the soils where moisture and sunlight are available.
Their number in soil usually ranges from 100 to 10,000 per gram of soil. They
are photoautotrophic, aerobic organisms and obtain CO2 from atmosphere and
energy from sunlight and synthesize their own food. They are unicellular,
filamentous or colonial. Soil algae are divided in to four main classes or
phyla as follows:
1.
Cyanophyta (Blue-green algae)
2. Chlorophyta (Grass-green algae)
3. Xanthophyta (Yellow-green algae)
4. Bacillariophyta (diatoms or golden-brown algae)
2. Chlorophyta (Grass-green algae)
3. Xanthophyta (Yellow-green algae)
4. Bacillariophyta (diatoms or golden-brown algae)
Out
of these four classes / phyla, blue-green algae and grass-green algae are more
abundant in soil. The green-grass algae and diatoms are dominant in the soils
of temperate region while blue-green algae predominate in tropical soils.
Green-algae prefer acid soils while blue green algae are commonly found in
neutral and alkaline soils. The most common genera of green algae found in soil
are: Chlorella, Chlamydomonas, Chlorococcum, Protosiphon etc. and that
of diatoms are Navicula, Pinnularia. Synedra, Frangilaria.
Blue
green algae are unicellular, photoautotrophic prokaryotes containing
Phycocyanin pigment in addition to chlorophyll. They do not posses flagella and
do not reproduce sexually. They are common in neutral to alkaline soils. The
dominant genera of BGA in soil are: Chrococcus, Phormidium, Anabaena,
Aphanocapra, Oscillatoria etc. Some BGA posses specialized cells know as "Heterocyst"
which is the sites of nitrogen fixation. BGA fixes nitrogen
(non-symbiotically) in puddle paddy/water logged paddy fields (20-30
kg/ha/season). There are certain BGA which possess the character of symbiotic
nitrogen fixation in association with other organisms like fungi, mosses,
liverworts and aquatic ferns Azolla, eg Anabaena-Azolla association fix
nitrogen symbiotically in rice fields.
Functions
/ role of algae or BGA:
1.
Plays important role in the maintenance of soil fertility especially in
tropical soils.
2. Add organic matter to soil when die and thus increase the amount of organic carbon in soil.
3. Most of soil algae (especially BGA) act as cementing agent in binding soil particles and thereby reduce/prevent soil erosion.
4. Mucilage secreted by the BGA is hygroscopic in nature and thus helps in increasing water retention capacity of soil for longer time/period.
5. Soil algae through the process of photosynthesis liberate large quantity of oxygen in the soil environment and thus facilitate the aeration in submerged soils or oxygenate the soil environment.
6. They help in checking the loss of nitrates through leaching and drainage especially in un-cropped soils.
7. They help in weathering of rocks and building up of soil structure.
2. Add organic matter to soil when die and thus increase the amount of organic carbon in soil.
3. Most of soil algae (especially BGA) act as cementing agent in binding soil particles and thereby reduce/prevent soil erosion.
4. Mucilage secreted by the BGA is hygroscopic in nature and thus helps in increasing water retention capacity of soil for longer time/period.
5. Soil algae through the process of photosynthesis liberate large quantity of oxygen in the soil environment and thus facilitate the aeration in submerged soils or oxygenate the soil environment.
6. They help in checking the loss of nitrates through leaching and drainage especially in un-cropped soils.
7. They help in weathering of rocks and building up of soil structure.
Soil Microorganism – Protozoa
These
are unicellular, eukaryotic, colourless, and animal like organisms (Animal
kingdom). They are larger than bacteria and size varying from few microns to a
few centimeters. Their population in arable soil ranges from l0,000 to 1,00,000
per gram of soil and are abundant in surface soil. They can withstand adverse
soil conditions as they are characterized by "cyst stage" in their
life cycle. Except few genera which reproduce sexually by fusion of cells, rest
of them reproduces asexually by fission / binary fission. Most of the soil
protozoa are motile by flagella or cilia or pseudopodia as locomotors organs.
Depending upon the type of appendages provided for locomotion, protozoa are
- Rhizopoda (Sarcondia)
- Mastigophora
- Ciliophora (Ciliata)
- Sporophora (not common Inhabitants of soil)
Class-Rhizopoda:
Consists protozoa without appendages usually have naked protoplasm without
cell-wall, pseudopodia as temporary locomotory organs are present some times.
Important genera are Amoeba, Biomyxa, Euglypha, etc.
Class
Mastigophora: Belongs flagellated protozoa, which are predominant in soil.
Important genera are: Allention, Bodo, Cercobodo, Cercomonas, Entosiphon
Spiromonas, Spongomions and Testramitus. Many members are
saprophytic and some posses chlorophyll and are autotrophic in nature. In this
respect, they resemble unicellular algae and hence are known as
"Phytoflagellates".
The
soil protozoa belonging to the class ciliate / ciliophora are characterized by
the presence of cilia (short hair-like appendages) around their body, which
helps in locomotion. The important soil inhabitants of this class are Colpidium,
Colpoda, Balantiophorus, Gastrostyla, Halteria, Uroleptus, Vortiicella,
Pleurotricha etc.
Protozoa
are abundant in the upper layer (15 cm) of soil. Organic manures protozoa. Soil
moisture, aeration, temperature and PH are the important factors affecting soil
protozoa.
Function
/ Role of Protozoa
1.
Most of protozoans derive their nutrition by feeding or ingesting soil bacteria
belonging to the genera Enterobacter, Agrobacterium, Bacillus, Escherichia,
Micrococcus, and Pseudomonas and thus, they play important role in
maintaining microbial / bacterial equilibrium in the soil.
2. Some protozoa have been recently used as biological control agents against phytopathogens.
3. Species of the bacterial genera viz. Enterobacter and Aerobacter are commonly used as the food base for isolation and enumeration of soil protozoans.
4. Several soil protozoa cause diseases in human beings which are carried through water and other vectors, eg. Amoebic dysentery caused by Entomobea histolytica.
2. Some protozoa have been recently used as biological control agents against phytopathogens.
3. Species of the bacterial genera viz. Enterobacter and Aerobacter are commonly used as the food base for isolation and enumeration of soil protozoans.
4. Several soil protozoa cause diseases in human beings which are carried through water and other vectors, eg. Amoebic dysentery caused by Entomobea histolytica.
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