“Role of Cyanobacteria in Nitrogen Fixation”
- What is the primary role of cyanobacteria in nitrogen fixation? a) To break down nitrogen gas into ammonia
b) To convert ammonia into nitrogen gas
c) To fix atmospheric nitrogen into a usable form
d) To produce oxygen for plantsAnswer: c) To fix atmospheric nitrogen into a usable form
Explanation: Cyanobacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can then be used by plants for growth.
- Which enzyme is essential for the nitrogen fixation process in cyanobacteria? a) Amylase
b) Nitrogenase
c) Lactase
d) ProteaseAnswer: b) Nitrogenase
Explanation: Nitrogenase is the enzyme complex responsible for catalyzing the conversion of nitrogen gas (N₂) to ammonia (NH₃) in nitrogen fixation.
- Where do nitrogen-fixing cyanobacteria typically live? a) In the roots of plants
b) In aquatic environments
c) In soil only
d) Inside animal cellsAnswer: b) In aquatic environments
Explanation: Cyanobacteria are commonly found in aquatic environments, including oceans, lakes, and ponds, where they fix nitrogen.
- Which of the following is a symbiotic relationship between cyanobacteria and plants? a) Mycorrhiza
b) Rhizobium and legumes
c) Lichens
d) Cyanobacteria in rice paddiesAnswer: b) Rhizobium and legumes
Explanation: Cyanobacteria can also form symbiotic relationships, such as with legumes, to help fix nitrogen for plants, though this is often more associated with Rhizobium.
- Which gas is fixed by cyanobacteria during the nitrogen fixation process? a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) MethaneAnswer: c) Nitrogen
Explanation: Cyanobacteria fix nitrogen gas (N₂) from the atmosphere into ammonia, a process essential for replenishing soil nutrients.
- What is the main product of nitrogen fixation by cyanobacteria? a) Nitric acid
b) Ammonia
c) Nitrate
d) UreaAnswer: b) Ammonia
Explanation: The main product of nitrogen fixation in cyanobacteria is ammonia (NH₃), which is a usable form of nitrogen for plants.
- What type of nitrogen fixation is primarily carried out by cyanobacteria? a) Symbiotic fixation
b) Free-living fixation
c) Denitrification
d) AmmonificationAnswer: b) Free-living fixation
Explanation: Cyanobacteria are mostly free-living nitrogen-fixers, although some can form symbiotic relationships with plants.
- Which structure in cyanobacteria is involved in nitrogen fixation? a) Thylakoid membrane
b) Heterocyst
c) Ribosome
d) FlagellumAnswer: b) Heterocyst
Explanation: Heterocysts are specialized cells in cyanobacteria that carry out nitrogen fixation under anaerobic conditions, essential for nitrogenase activity.
- What is the role of the heterocyst in cyanobacteria? a) Photosynthesis
b) Nitrogen fixation
c) Storage of carbon
d) RespirationAnswer: b) Nitrogen fixation
Explanation: Heterocysts are specialized cells that provide an anaerobic environment, crucial for the enzyme nitrogenase to fix nitrogen gas.
- Why do cyanobacteria need to protect the nitrogenase enzyme from oxygen? a) Oxygen enhances nitrogen fixation
b) Oxygen does not affect nitrogen fixation
c) Oxygen inactivates nitrogenase
d) Oxygen provides the energy for nitrogen fixation
Answer: c) Oxygen inactivates nitrogenase
Explanation: Nitrogenase is sensitive to oxygen and is inactivated in its presence, which is why cyanobacteria employ heterocysts to protect it.
- Which of the following is an example of a nitrogen-fixing cyanobacterium? a) Oscillatoria
b) Spirulina
c) Anabaena
d) Chlamydomonas
Answer: c) Anabaena
Explanation: Anabaena is a well-known nitrogen-fixing cyanobacterium that forms heterocysts for nitrogen fixation.
- How does cyanobacterial nitrogen fixation benefit agriculture? a) By producing soil pests
b) By fixing nitrogen into the soil, enriching soil fertility
c) By absorbing pesticides
d) By converting nitrogen into methane
Answer: b) By fixing nitrogen into the soil, enriching soil fertility
Explanation: Cyanobacteria enrich soil fertility by converting atmospheric nitrogen into ammonia, which is accessible to plants.
- Which factor inhibits nitrogen fixation in cyanobacteria? a) Low temperatures
b) High levels of oxygen
c) Low moisture
d) High carbon dioxide levels
Answer: b) High levels of oxygen
Explanation: High levels of oxygen inhibit nitrogen fixation as nitrogenase is oxygen-sensitive.
- How do cyanobacteria contribute to the nitrogen cycle in ecosystems? a) By absorbing nitrogen directly from the air
b) By producing nitrogen oxides
c) By fixing atmospheric nitrogen into ammonia
d) By denitrifying ammonia to nitrogen gas
Answer: c) By fixing atmospheric nitrogen into ammonia
Explanation: Cyanobacteria play a critical role in the nitrogen cycle by converting nitrogen gas into ammonia, making it accessible for plants and other organisms.
- Which process is essential for the sustainable use of nitrogen in agricultural ecosystems? a) Nitrogen fixation by plants
b) Nitrogen fixation by cyanobacteria
c) Ammonification
d) Nitrification
Answer: b) Nitrogen fixation by cyanobacteria
Explanation: Cyanobacteria’s ability to fix nitrogen is essential for the sustainable use of nitrogen in ecosystems, especially in agricultural fields.
- What happens to nitrogen fixed by cyanobacteria in aquatic environments? a) It is used by aquatic plants for growth
b) It is released as a gas back into the atmosphere
c) It is absorbed by algae only
d) It gets converted into oxygen
Answer: a) It is used by aquatic plants for growth
Explanation: The fixed nitrogen is utilized by aquatic plants and algae for growth and development, contributing to the aquatic food web.
- How does the nitrogen fixation process in cyanobacteria influence global nitrogen availability? a) By increasing the amount of oxygen in the atmosphere
b) By reducing the amount of available nitrogen
c) By adding biologically available nitrogen to the ecosystem
d) By preventing nitrogen loss to the atmosphere
Answer: c) By adding biologically available nitrogen to the ecosystem
Explanation: Cyanobacteria contribute to the nitrogen cycle by fixing nitrogen gas and making it available for biological processes, increasing nitrogen availability.
- What is the main environmental condition necessary for cyanobacterial nitrogen fixation? a) High light intensity
b) Anaerobic conditions in heterocysts
c) High temperatures
d) High salinity
Answer: b) Anaerobic conditions in heterocysts
Explanation: Cyanobacteria need anaerobic conditions in heterocysts to fix nitrogen, as nitrogenase is sensitive to oxygen.
- Which of the following can enhance nitrogen fixation by cyanobacteria? a) Adding excess oxygen
b) Providing sufficient carbon dioxide
c) Adding nitrogen-rich fertilizers
d) Increasing salinity levels
Answer: b) Providing sufficient carbon dioxide
Explanation: Cyanobacteria require carbon dioxide for photosynthesis, which powers the energy needed for nitrogen fixation.
- What is a common method used in agricultural practices to enhance nitrogen fixation by cyanobacteria? a) Using synthetic nitrogen fertilizers
b) Applying organic matter to soil
c) Growing cyanobacterial cultures in paddy fields
d) Adding high levels of oxygen to soil
Answer: c) Growing cyanobacterial cultures in paddy fields
Explanation: In paddy fields, cyanobacterial cultures are grown to naturally fix nitrogen and reduce the need for synthetic fertilizers.
- How does cyanobacterial nitrogen fixation affect soil health? a) It decreases soil organic matter
b) It prevents nutrient leaching
c) It increases the nitrogen content, improving soil fertility
d) It acidifies the soil
Answer: c) It increases the nitrogen content, improving soil fertility
Explanation: The nitrogen fixed by cyanobacteria contributes to improving soil fertility, making it more suitable for plant growth.
- What is the relationship between cyanobacteria and nitrogen-fixing plants like legumes? a) Cyanobacteria invade plant cells to fix nitrogen
b) They share nitrogen fixation duties
c) Cyanobacteria fix nitrogen independently without involving plants
d) Cyanobacteria provide nitrogen only to non-leguminous plants
Answer: b) They share nitrogen fixation duties
Explanation: Cyanobacteria and nitrogen-fixing plants like legumes can work together to fix nitrogen in the soil.
- Which of the following is true about nitrogenase activity in cyanobacteria? a) It is most active in the presence of oxygen
b) It operates in aerobic environments
c) It is inactivated by oxygen
d) It requires high temperatures for activity
Answer: c) It is inactivated by oxygen
Explanation: Nitrogenase is sensitive to oxygen and is inactivated when exposed to it, which is why it functions best in anaerobic conditions.
- What happens to the ammonia produced by cyanobacterial nitrogen fixation? a) It is stored in vacuoles
b) It is converted into nitrate and released into the soil
c) It is used by cyanobacteria for energy
d) It is immediately released into the atmosphere
Answer: b) It is converted into nitrate and released into the soil
Explanation: Ammonia produced by cyanobacteria is typically converted into nitrates, which are accessible to plants.
- What role do cyanobacteria play in the global nitrogen cycle? a) They convert ammonia into nitrogen gas
b) They release nitrogen gas into the atmosphere
c) They convert nitrogen gas into ammonia, supporting ecosystems
d) They prevent nitrogen from entering the atmosphere
Answer: c) They convert nitrogen gas into ammonia, supporting ecosystems
Explanation: Cyanobacteria convert atmospheric nitrogen into ammonia, which is vital for sustaining ecosystems and agriculture.
- Which characteristic allows cyanobacteria to fix nitrogen in oxygen-rich environments? a) They use a special pigment
b) They contain heterocysts that provide an anaerobic environment
c) They secrete nitrogenase inhibitors
d) They adapt to high oxygen levels
Answer: b) They contain heterocysts that provide an anaerobic environment
Explanation: Heterocysts protect nitrogenase from oxygen by providing an anaerobic environment in which nitrogen fixation can occur.
- What is the main difference between cyanobacterial nitrogen fixation and legume-rhizobium symbiosis? a) Cyanobacteria fix nitrogen more efficiently than rhizobia
b) Legumes and rhizobia fix nitrogen exclusively in the roots
c) Cyanobacteria use sunlight to fix nitrogen, while rhizobia do not
d) Cyanobacteria are free-living, while rhizobia form symbiotic relationships
Answer: d) Cyanobacteria are free-living, while rhizobia form symbiotic relationships
Explanation: Cyanobacteria are often free-living nitrogen fixers, while Rhizobium forms symbiotic relationships with legumes for nitrogen fixation.
- How can cyanobacterial nitrogen fixation benefit aquatic ecosystems? a) By increasing dissolved oxygen levels
b) By contributing to the nitrogen availability for aquatic plants and algae
c) By controlling algal blooms
d) By converting ammonia into phosphate
Answer: b) By contributing to the nitrogen availability for aquatic plants and algae
Explanation: Cyanobacteria provide nitrogen for aquatic plants and algae, promoting biodiversity and supporting the food chain in aquatic ecosystems.
- Which process allows cyanobacteria to synthesize nitrogenase under low-light conditions? a) Photosynthesis
b) Respiration
c) Nitrogen fixation
d) Dark fermentation
Answer: a) Photosynthesis
Explanation: Cyanobacteria use photosynthesis to generate the energy required for nitrogen fixation, even under low-light conditions.
- Why is nitrogen fixation by cyanobacteria important for the biosphere? a) It prevents nitrogen pollution
b) It maintains nitrogen balance in the atmosphere
c) It helps sustain life by making nitrogen available to living organisms
d) It produces toxic gases for ecological balance
Answer: c) It helps sustain life by making nitrogen available to living organisms
Explanation: Nitrogen fixation by cyanobacteria is crucial for providing biologically available nitrogen, supporting life across ecosystems.
