Methanogens are the bacteria which grow anaerobically on cellulosic material and produce large amount of methane along with CO₂ and water.
Present in cattle (rumen) a part of stomach
They help in
(a) Breaking down of cellulose present in food of cattle.
(b) Nutrition of animal for digestion of cellulose. The excreta of cattle is rich in these bacteria (methanogens) and therefore can be used for generation of biogas.

The evolutionary success of ruminants is based on the efficiency of their digestive system in extracting nutrition from low quality food or say cellulose, which is undigestible matter in other mammals. the key to the ruminant digestive system is the presence of symbiotic microorganisms in their stomach. These microorganisms produce cellulase enzyme, which breaks cell walls of plant material to release fatty acid, which constitute a significant portion of overall energy budget input of a ruminant.

Certain bacteria and cyanobacteria are nitrogen fixers.They convert atmospheric free nitrogen into ammonium form which is easily assimilated by higher plants.Thus, these microorganisms increase the fertility of soil without causing any environmental harzards and therefore, called biofertilizers.Example - Rhizobium and Anabaena.   

Biogas plant is installed basically to get the biogas which is used as fuel for heating and cooking, lighting, etc.It also yields nitrogen rich slurry that is an excellent source of manure.

(a) Single Cell Protein (SCP) are basically produced from the microbes, for example Candida utilis, Chlorella, Spirulina, Saccharomyces. SCP may be used directly as human food supplement or else it can be used in animal feed as protein-rich supplement.
(b) Soil It is a unique habitat which harbours a variety of microflora and fauna, and gives mechanical and nutritional support to higher plants on which human civilization is based.
Examples The rhizosphere micro-organisms have beneficial effects on the development of plant. The microorganisms catalyze the reactions in the rhizosphere and produce \({ CO }_{ 2 }\) and form organic acids that in turn solubilize the inorganic nutrients of plants. Some of the rhizosphere micro-organisms also produce growth stimulating substances.

(i) Acetic acid - Acetobacter aceti
(ii) Butyric acid - Clostridium butylicum
(iii) Lacteric acid - Lactobacillus delbruekii

1. Biofertilisers fix atmospheric nitrogen and increase nitrogen content of the soil.
2. Cyanobacteria increase the organic matter of the soil through their photosynthetic acivity.
3. Fungi (of mycorrhizae) help in absorption and storage of phosphorus and other mineral nutrients.

Biological control of pests and pathogens must be preferred to chemical pesticides because
(i) The chemicals are toxic and extremely harmful to human beings and other animals.
(ii) The chemicals also .fause pollution of soil, groundwater and our agricultural products. ~
(a) Bacillus thuringiensis
1. The spores of this bacterium are insecticidal and used to control the larvae of many insect pests.
2. They are available in sachets as dried spores, which have to be mixed with water and sprayed onto vulnerable plants.
3. When they are eaten  the larvae, the toxin becomes active in the gut of the larvae and kills the larvae.
4. Through genetic engineering, the gene coding for the toxic protein is introduced into crop plants which makes them resistant to the insect pest.
(b) Nucleopolyhedrovirus
1. These are baculoviruses, which are pathogens that attack insects and other arthropods.
2. These viruses are species-specific narrow-spectrum insecticidal agents, that have no negative impacts on plants and other animals including beneficial.

(a) When air is pumped into the depletiontanks and constantly agiated the aerobic microbes grow vigorously and from flocs.They consume a major part of the organic reduce the BOD of the sewage.

1. Organic farming is a holistic approach that seeks to develop an understanding of the webs of interaction among the myriads of organisms that form the flora and fauna of the field.
2. The organic farmer works to create a system where the insects (called pests) are not eradicated, but kept at manageable levels by a complex system of checks and balance within a living and vibrant ecosystem.
3. According to the organic farmer, the eradication of the creatures, called pests, is not only possible but also undesirable, because many beneficial predatory and parasitic insects cannot survive without them.
4. Such a use of biocontrol measures reduces the use of chemical pesticides and thereby the pollution.

The treatment of sewage Involves two steps:
(i) Primary treatment and
(ii) Secondary treatment.
(i) Primary treatment
1. Primary treatment is a physical process of removal of small and large particles through filtration and sedimentation.
2. The first step is to remove the floating objects (like polythene bags) by letting the sewage to pass through wire mesh screens of sequentially smaller pore sizes.
3. Then the sewage is passed into the grit chamber, where grit is sedimented.
4. The sewage is allowed to go into the primary settling tank, where the suspended materials settle down to form primary sludge.
The effluent is taken for secondary treatment.
(ii) Secondary Treatment
1. It is a biological process, that employs the heterotrophic bacteria naturally present in the sewage.
2. The effluent from the primary treatment is passed into large aeration tanks, where it is constantly agitated and air is pumped into it.
3. This allows the rapid growth of aerobic bacteria into flocs, which consume the organic matter of the sewage and reduce the BOD.
4. The effluent is passed into a settling tank, where the flocs are allowed to sediment forming the activated sludge.
5. A small part of the activated sludge is pumped back into aeration tank as inoculum.
6. The remaining major part of the sludge is pumped into anaerobic sludge digesters, where the anaerobic bacteria digest the organic matter and produce methane, hydrogen sulphide and carbon dioxide.
7. The effluent from this is passed into the waterbody.

Baculoviruses belong to the genus Nucleopolyhedrovirus.
They are the pathogens which attack insects and other arthropods.
They are species-specific, narrow spectrum insecticides, which have no negative impacts on plants, birds, mammals, fish, and even non-target insects.
This is desirable because beneficial insects are conserved to aid in integrated pest management (IPM) programmes.

(a) Leguminous plants harbour Rhizobium in their root nodules; the bacterium fixes atmospheric nitrogen and enriches soil fertility.
(b) Cyanobacteria enrich soil fertility by
(i) fixing atmospheric nitrogen in the soil as nitrogen compounds.
(ii) adding organic matter through their photosynthetic activity.
(c). Mycorrhizae absorb phosphorus from the soil and pass it on to the plant; they provide resistance to root-borne pathogens and tolerance to salinity and drought.