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Microbes as Biofertilisers, Practice Problems and FAQs

Microbes as Biofertilisers, Practice Problems and FAQs

We know that microbes form an important biological component of the earth along with the plants and animals. These are tiny organisms which cannot be seen through the naked eyes but are found almost everywhere. They include viruses, bacteria, protozoa, fungi etc. Do you know that microbes can survive in the hottest and coldest regions of the earth? Some bacteria can even live in temperatures as high as 100 degrees Celsius. In fact, these microbes may be very small but are one of the most working members of our biological environment.

Did you ever wonder what these microbes really do? Aren’t they harmful to us? We have always known them to be disease causing agents or germs that spoil our food. But an exception to this quality of microbes is their use in our welfare. Interesting, isn’t it?

Now you must be thinking how this happens. Let’s take an example, you know that environmental pollution is increasing day by day which is a major concern for nature. One of the most important causes of pollution is the use of chemical fertilisers in agriculture. Well, what you don’t know is that these microbes can save the environment significantly from pollution. Want to know how? Let’s jump in to study about it in detail.

Table of contents

  • Role of microbes as biofertilizers
  • Bacteria as a biofertiliser
  • Fungi as a biofertiliser
  • Cyanobacteria as a biofertiliser
  • Commercially available biofertilisers
  • Advantages of biofertilisers
  • Practice problems
  • FAQs

Role of microbes as biofertilizers

Fertilisers are the substances that are used to enrich the quality of the soil for agriculture. The nutrients present in the soil are used up by the plants for their growth and development activities. In current age, most of the fertilisers are composed of chemicals. Although these chemical fertilisers help in plant growth, they also contribute to soil and water pollution. The discharge of potassium, nitrate, and phosphorus in the soil washes away and enters the water bodies killing the aquatic organisms and causes water pollution. These substances also cause imbalance in soil pH. The accumulation of these chemicals also occur in the food chain as most organisms on earth consume plants as food.

Due to this alarming condition, organic farming is being advanced as the best approach of agricultural practices. Instead of chemical fertilisers, biofertilisers are used in organic farming. The microorganisms such as bacteria, cyanobacteria, and fungi which are used to improve the nutrient content of the soil are known as biofertilisers.

Bacteria as a biofertiliser

Nitrogen fixation is the process of converting atmospheric nitrogen into compounds of nitrogen that can be directly used by plants. Many symbiotic and free-living bacteria are capable of fixing nitrogen and thus help in making nutrients available to plants. Such bacteria can serve as biofertilisers. The released wastes and dead remains of these bacterial cells helps in increasing the nitrogen content of the soil. 

Some phosphate dissolving bacteria can also serve as biofertilisers.

Symbiotic nitrogen fixing bacteria

An example of biofertiliser is the Rhizobium bacteria which lives in symbiotic association with the nodules on the roots of leguminous plants. Frankia forms root nodules in non-leguminous plants such as Casuarina and Alnus and Xanthomonas lives as a symbiont in the leaf nodules of Ardisia. These symbiotic bacteria fix atmospheric nitrogen into ammonia and make it available to the plant for assimilation into amino acids. In return they obtain food and shelter from the plants.

Fig: Rhizobium

Fig: Rhizobium

Free living nitrogen fixing bacteria

Some free living forms of bacteria in the soil also act as biofertilisers due to their nitrogen fixing ability. These can be - 

  • aerobic saprotrophs such as Azotobacter, Beijernickia, etc. 
  • anaerobic saprotrophs such as Clostridium.
  • facultative aerobic saprotrophic bacteria such as Bacillus, Klebsiella.
  • chemoautotrophic bacteria such as Desulphovibrio.
  • photosynthetic bacteria such as Rhodospirillum, Rhodopseudomonas, etc.

Fig: Azotobacter

Fig: Azotobacter

All of these microorganisms are vital to the nitrogen cycle. They fix nitrogen from the air and produce ammonium or other nitrogen salts that can be used by the plants. 

Loose association nitrogen fixing bacteria

Certain nitrogen fixing bacteria such as Azospirillum live around the roots of the plants but do not form any obligatory or symbiotic relationship with them. The bacteria obtain nourishment from the root exudates. In return a part of the fixed nitrogen compounds exuded by the bacteria are passed into the roots. 

Phosphate bacteria

Thiobacillus, Pseudomonas, Micrococcus, etc. release phosphatases which are capable of dissolving phosphate from various organic and inorganic sources and thus make them available to the plants.

Fungi as a biofertiliser

Plants and fungi have been found to create symbiotic relationships (mycorrhiza). The fungal component obtains nourishment and shelter from the plant roots. In return, the fungal hyphae helps in solubilisation and absorption of nutrients such as phosphorus, nitrogen and potassium from organic matter. Other advantages of plants with such relationships include resistance to root-borne diseases, tolerance to salt and drought, and an overall improvement in plant growth and development by releasing growth promoting substances.

Mycorrhizae are of two types - ectomycorrhiza and endomycorrhiza. Ectomycorrhiza forms a woolly covering over the root and the fungal hyphae remains limited to the intercellular spaces of the root cortex. In endomycorrhiza the fungal hyphae might enter the cortical cells of the root and form vesicles or arbuscules for obtaining nutrients. Endomycorrhizae is hence also known as vesicular arbuscular mycorrhiza or VAM. 

Fungi associated with ectomycorrhiza mostly belong to basidiomycetes and are species specific, e.g, Boletus, Scleroderma, etc. Fungi associated with endomycorrhiza are mostly zygomycetes and a single fungus may form symbiotic associations with the roots of a number of different plants. These include species of Glomus, Sclerocystis, etc.

Fig: Mycorrhiza

Fig: Mycorrhiza

Cyanobacteria as a biofertiliser

Cyanobacteria are autotrophic prokaryotic microorganisms that may be found in both aquatic and terrestrial habitat. Many of them, such as Anabaena, Nostoc, and Oscillatoria, can fix nitrogen from the air. These occur in water as well as moist soils and are important biofertilisers in paddy fields as they enhance nitrogen content of the soil by exuding fixed nitrogen and also add a lot of organic matter to the soil.

Fig: Cyanobacteria

Fig: Cyanobacteria

Anabaena azollae is a cyanobacterium that thrives in a symbiotic relationship with Azolla, a free-floating water fern. Anabaena azollae, like bacteria, fixes atmospheric nitrogen and forms organic molecules. Thus, Azolla is often inoculated into paddy fields to increase soil fertility.

Fig: Cyanobacteria in Paddy field

Fig: Cyanobacteria in Paddy field

Commercially available biofertilisers

Several biofertilizers are now commercially accessible for farmers to purchase and use. These not only assist to replenish soil nutrients, but they also aid to lessen reliance on artificial fertilisers. This helps to preserve the mineral composition of the soil and greatly lowers pollution.

Fig: Commercially available biofertilisers

Fig: Commercially available biofertilisers

Advantages of biofertilisers

Biofertilisers do not pollute the soil, ground water or surface water. Crop yield is increased by 15-35% and the number of soil borne pathogens also decreases. They do not alter the soil pH and maintain the crumb structure of the soil. These are also less expensive compared to chemical fertilisers.

Practice problems

Q1. Which of the following statements is incorrect regarding Rhizobium?

A. Fixes nitrogen in the leaves of flowering plants.
B. Helps in absorption of phosphorus by roots.
C. Fixes nitrogen in the root nodules of legumes.
D. Forms symbiotic relationship with fungi.

Solution: Rhizobium is a bacteria that lives in symbiotic relationship with the nodules present in the roots of leguminous plants. It makes ammonium by fixing nitrogen from the air. This ammonium is then converted to nitrogen molecules, which is consumed by the plants. In this approach, this bacteria increase soil nutrient quality and function as biofertiliser. Hence, option c is correct.

Q2. Which of the following is correctly matched?

A. Anabaena - Azolla 
B. Glomus - Free-living nitrogen fixer
C. Rhizobium - Mycorrhiza
D. Nostoc - Symbiotic nitrogen fixer

Solution: Azolla, a free-floating water fern, lives in a symbiotic association with Anabaena azollae, a cyanobacterium. Anabaena azollae fixes atmospheric nitrogen and creates organic compounds, just like bacteria. In this interaction, Anabaena takes carbon and nitrogen from the plant in exchange for fixed nitrogen. It also adds organic matter to the soil. Hence by inoculating Azolla into rice fields, they are made more fertile. Hence, option a is correct.

Q3. Assertion (A): Fungi is used as a biofertiliser.
Reason (B): It associates with plants to form symbiotic relationships.

a. Both A and R are correct, but the R is the correct explanation of A.
b. Both A and R are correct and R is not the correct explanation of A.
c. A is correct and R is incorrect.
d. Both A and R are incorrect.

Solution: Symbiotic relationships have been discovered between roots of plants and fungus and are called mycorrhiza. The fungal component obtains nourishment and shelter from the plant roots and in return, the fungal hyphae helps in solubilisation and absorption of nutrients such as phosphorus, nitrogen and potassium from organic matter. Other advantages of plants with such relationships include resistance to root-borne diseases, tolerance to salt and drought, and an overall improvement in plant growth and development by releasing growth promoting substances. Thus, fungi belonging to species such as Glomus, Boletus, etc which form mycorrhizae are used as biofertilisers. Hence, option a is correct.

Q4. How are biofertilisers beneficial over chemical fertilisers?
Answer: A biofertiliser boosts plant development by using live microorganisms like bacteria and fungus. Biofertilisers do not harm the environment by causing pollution whereas chemical fertilisers frequently result in release of too much phosphate and nitrogen in the soil. Runoff then carries the surplus into lakes and streams. The condition of the water deteriorates causing algal overgrowth and death of aquatic organisms. Chemical fertilisers also alter soil pH which remains intact when biofertilisers are used.


Question 1. Name a free living nitrogen fixing cyanobacterium which increases the fertility of maize fields.
Answer: Many free living cyanobacteria are capable of fixing nitrogen and act as excellent biofertilisers as they increase the nitrogen content of the soil and also add more organic matter to the soil. Cylindrospermum licheniforme is a cyanobacterium that is active in maize and sugarcane fields.

Question 2. What is a heterocyst?
Answer: Heterocysts are specialised nitrogen fixing cells in cyanobacteria such as Nostoc, Anabaena, etc that help to fix atmospheric nitrogen under aerobic conditions. They are formed when the cyanobacteria are starved of nitrogen. Nitrogen fixation by nitrogenase enzyme requires an oxygen free environment. The heterocysts provide a micro anaerobic environment with the help of a number of different adaptations, such that the nitrogenase enzyme can work efficiently.

Question 3. What is the role of leghemoglobin in nitrogen fixation by Rhizobium?
Answer: The nitrogenase enzyme in Rhizobium is responsible for fixing nitrogen in Rhizobium. The enzyme requires an oxygen free environment to function. Leg haemoglobin is a pigment that develops in the cytoplasm of the root cells infected by Rhizobium. It acts as an oxygen scavenger that binds with oxygen and makes it unavailable to the cell such that nitrogenase can work efficiently.

Question 4. Can mycorrhizae survive without plant roots?
Answer: Mycorrhizal spores are highly resistant and can survive for years even in the absence of plant roots. Upon coming into contact with plant roots, these spores germinate and colonise the roots.



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