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Microbial Ecology (GATE)

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Endophytes are organisms living within the tissue of a plant as endosymbionts, without causing symptoms of a disease. Which of the following fungi resides as an endophyte in only grasses?

  1. Colletotrichum

  2. Curvularia

  3. Epichloë

  4. Fusarium


Correct Option: C
Explanation:

Epichloë resides in only grasses. It colonises the shoot portion of grasses. It provides several benefits to the host, including the production of different herbivore-deterring alkaloids, increased stress resistance and growth promotion.

Plant-derived compounds are responsible for providing the additional carbon that allows the rhizosphere to host such a large variety of microorganisms. Which of the following plant products are the result of abrasive forces of the root against soil particulate matter that remove cells from the root cap?

  1. Mucilages

  2. Mucigels

  3. Exudates

  4. Secretions


Correct Option: A
Explanation:

Mucilages are the result of abrasive forces of the root against soil particulate matter that remove cells from the root cap. Mucilages are polar glycoproteins and exopolysaccharides.

Hyphomycete is a type of fungus. Which of the following hyphomycetes either live their life-cycles in the bodies of dead nematodes or trap and kill nematodes in order to supplement their nitrogen requirements?

  1. Arthrobotrys

  2. Gibberella

  3. Angulimaya

  4. Akanthomyces


Correct Option: A
Explanation:

Species of the hyphomycete genus (Arthrobotrys) produce constricting loops that quickly shut to grab nematodes or non-constricting loops or hyphal networks that entangle nematodes or sticky knobs that adhere to nematodes as they swim by.

Epibiont is an organism that lives on the surface of another living organism without causing any harm. Which of the following organisms live as an epibiont?

  1. Liverworts

  2. Algae

  3. Rhizobia

  4. Rhodococci


Correct Option: B
Explanation:

Algae live as epibionts on many larger marine animals.

Archaea were initially viewed as extremophiles living in harsh environments but they have also been found in habitats like soil, oceans etc. Which of the following is their ecological role?

  1. Nutrient absorption

  2. Sugar metabolism

  3. Ammonification

  4. Infection


Correct Option: C
Explanation:

Archaea are very important for ammonification in rhizospheres. It is a process that includes transforming organically bound nitrogen from microbial or plant biomass to ammonia.

Pseudomonas syringae is a Gram-negative rod-shaped bacterium. Which of the following spheres does it colonise?

  1. Caulosphere

  2. Rhizosphere

  3. Laimosphere

  4. Phyllosphere


Correct Option: D
Explanation:

Pseudomonas syringae colonises phyllosphere (above-ground portions of plants). It lives in the leaf ecosystem as an epiphyte and a pathogen.

Pseudomonas putida is a Gram-negative, rod-shaped, saprotrophic soil bacterium. Which of the following roles does it play in the ecosystem?

  1. Phosphate fixation

  2. Phosphate metabolism

  3. Phosphate solubilisation

  4. Phosphate generation


Correct Option: C
Explanation:

Pseudomonas putida is a beneficial bacterium that solubilises inorganic phosphorus from insoluble compounds.

Which of the following kinds of nutrient transfer takes place in seed stage of orchid and mycorrhiza?

  1. Fungus absorbs water for seed.

  2. Fungus provides essential nutrients to seed.

  3. Fungus inhibits the abscisic acid in seeds to help break their dormancy.

  4. Fungus provides sugars to seed.


Correct Option: D
Explanation:

Orchid seeds cannot germinate unless they have fungus that is attached. They depend on fungus to provide simple sugars.

Recently, genome of Laccaria Bicolor (an ectomycorrhizal fungi) was sequenced. Which of the following kinds of relationship does it form with plants?

  1. Neutralism

  2. Mutualism

  3. Commensalism

  4. Parasitism


Correct Option: B
Explanation:

The relationship between Laccaria Bicolor and plants can be described as mutualism. Laccaria Bicolor shows an excess of enzymes used in ammonia uptake and a lack of enzymes that would be needed to degrade plant wall material. Thus, the fungus provides nitrogen and does not cause any harm to the trees in return for glucose, which it gets from photosynthetic plants. 

In a mutualistic relationship, bacteria live in the roots of plants by forming nodules and provide fixed nitrogen to the plant. Which of the following kinds of nodules are found in alfalfa and clover?

  1. Legume nodules

  2. Perennial legume nodules

  3. Angular legume nodules

  4. Frankia


Correct Option: B
Explanation:

Perennial legume nodules are long finger-like nodules, half an inch in diameter. They can be found on alfalfa and clover. These nodules grow mostly around the tap root and live for quite a long time.

In a hypothetical situation, if all the heterotrophic bacteria disappear from the earth, then which of the following steps of carbon cycle would suffer?

  1. Photosynthesis

  2. Methanogenesis

  3. Biodegradation

  4. CO2 fixation


Correct Option: C
Explanation:

Biodegradation step of carbon cycle would suffer in the absence of heterotrophs. They are important in the carbon chain for the processes of biodegradation and decomposition under aerobic and anaerobic conditions.

Phyllosphere refers to the habitat of microorganisms on above-ground portions of plants. Which of the following about phyllosphere is false?

  1. Phyllosphere microbiota is adapted to tolerate high irradiation and low humidity stresses.

  2. Phyllosphere microorganisms antagonise airborne pathogens, thereby protecting the plant.

  3. An understanding of phyllosphere is of great importance to agricultural industry.

  4. Phyllosphere provides the most benign environment for microorganisms.


Correct Option: D
Explanation:

This is false. Phyllosphere can be considered a very hostile community because there is a limited availability of nutrients, strong sun irradiation and variation in water availability.

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia. Which of the following organisms is supposed to perform almost half of the nitrogen fixation in marine systems on a global scale?

  1. Gloeobacter

  2. Johannesbaptistia

  3. Nostoc

  4. Trichodesmium


Correct Option: D
Explanation:

Trichodesmium belongs to the genus of filamentous cyanobacteria. It performs almost half of the nitrogen fixation in marine systems on a global scale.

Sulphur cycle is a biogeochemical cycle by which sulphur moves to and from minerals and living systems. Which of the following bacteria reduces sulphate ions to hydrogen sulphide in the process of sulphur cycle?

  1. Desulfotomaculum

  2. Thiobacillus

  3. Thiomicrospora

  4. Sulpholobus


Correct Option: A
Explanation:

The direct reduction of sulphate ions to hydrogen sulphide is effected in nature by specialised, strictly anaerobic bacterium of the genus Desulfotomaculum. These sulphate reducing bacteria (SRB) are heterotrophic organisms that utilise sulphate, thiosulphate, S2O3--, sulphite, SO3-- or other reducible sulphur-containing ions as terminal electron acceptors in their respiratory metabolism. In the process, these sulphur-containing ions are reduced to hydrogen sulphide.

Acid rock drainage (ARD) is the weathering of rocks caused by acids. Which of the following bacteria is involved in acid rock drainage?

  1. Chromtiacceae

  2. Chlorobiaceae

  3. Thiomicrospira

  4. Desulfovibrio


Correct Option: C
Explanation:

Thiomicrospira produces sulphuric acid (hydrogen ions, H+ and sulphate ions, SO4--) as a metabolic product. This bacterium is involved in acid rock damage from pyritic and pyrrhotitic rocks under appropriate conditions.

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