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Magazine Chapter 26: Problem 2
Which statement about nitrogen metabolism is not true? a. Certain prokaryotes reduce atmospheric \(\mathrm{N}_{2}\) to ammonia. b. Some nitrifiers are soil bacteria. c. Denitrifiers are obligate anaerobes. d. Nitrifiers obtain energy by oxidizing ammonia and nitrite. e. Without nitrifiers, terrestrial organisms would lack a nitrogen supply.
Short Answer
Expert verified
Statement C is not true.
Step by step solution
01
Understand the Statements
Let's first consider each statement to determine its validity in the context of nitrogen metabolism. Each statement makes a specific claim about a different aspect of the nitrogen cycle, so we need to evaluate the truthfulness of each one.
02
Analyze Statement A
Statement A claims that certain prokaryotes reduce atmospheric \( \mathrm{N}_{2} \) to ammonia. This is true, as certain prokaryotes, like those in the genus Rhizobium, have the ability to fix nitrogen from the atmosphere into ammonia.
03
Verify Statement B
Statement B asserts that some nitrifiers are soil bacteria. This is accurate as nitrifying bacteria, such as Nitrosomonas and Nitrobacter, live in soil and participate in the nitrogen cycle.
04
Check Statement C
Statement C states that denitrifiers are obligate anaerobes. This is false since denitrifiers can be facultative anaerobes, meaning they can thrive in both aerobic and anaerobic environments, although they use anaerobic conditions to denitrify.
05
Assess Statement D
Statement D indicates that nitrifiers obtain energy by oxidizing ammonia and nitrite. This is correct because nitrifying bacteria gain energy through the oxidation of ammonia to nitrite and nitrite to nitrate.
06
Evaluate Statement E
Statement E suggests that without nitrifiers, terrestrial organisms would lack a nitrogen supply. This is not entirely true, as there are other processes and organisms (like nitrogen-fixing bacteria) that also contribute to the nitrogen cycle, ensuring a nitrogen supply to terrestrial organisms.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Nitrogen Cycle
The nitrogen cycle is a crucial process for life on Earth, recycling nitrogen through different forms and environments. This cycle ensures that nitrogen is available in a form that can be used by living organisms. It involves several key steps that transform nitrogen from one chemical form to another, mainly driven by microbial activities.
There are six main stages in the nitrogen cycle:
There are six main stages in the nitrogen cycle:
- Fixation: Atmospheric nitrogen (\(\mathrm{N}_2\)) is converted into ammonia (\(\mathrm{NH}_3\)) usable by plants, primarily performed by nitrogen-fixing bacteria.
- Nitrification: Ammonia is oxidized to nitrites (\(\mathrm{NO}_2^-\)) and then nitrates (\(\mathrm{NO}_3^-\)) by nitrifying bacteria.
- Assimilation: Plants absorb nitrates through their roots and convert them into organic molecules.
- Ammonification: Organic nitrogen is converted back into ammonia by decomposing bacteria when plants and animals die.
- Denitrification: Nitrates are reduced back to nitrogen gas (\(\mathrm{N}_2\)) or nitrous oxide (\(\mathrm{N}_2\mathrm{O}\)), returning to the atmosphere via bacterial processes.
- Leaching: Occasionally, nitrates can be lost to the atmosphere through percolation in water bodies.
Nitrifying Bacteria
Nitrifying bacteria play a pivotal role in the nitrogen cycle, especially in the nitrification process. These bacteria transform ammonia into nitrites and then nitrates, making nitrogen available to plants in a form they can assimilate.
Here are key aspects of nitrifying bacteria:
Here are key aspects of nitrifying bacteria:
- Two-step Oxidation Process: Ammonia-oxidizing bacteria, such as Nitrosomonas, convert ammonia into nitrite. Following this, Nitrobacter bacteria further oxidize nitrites into nitrates.
- Energy Source: These bacteria are chemolithoautotrophs, meaning they derive energy from the chemical oxidation of inorganic compounds like ammonia and nitrite.
- Importance in Soils: Nitrifying bacteria are abundant in soil and critical for soil health, supporting the growth of terrestrial plants by supplying nitrates.
Denitrification
Denitrification is an essential process in the nitrogen cycle, enabling the conversion of nitrate back into nitrogen gas, releasing it into the atmosphere. This process is crucial for maintaining the balance of nitrogen in ecosystems, preventing excess nitrogen buildup.
Important points about denitrification include:
Important points about denitrification include:
- Bacterial Role: Various bacteria, including species of Pseudomonas and Clostridium, perform denitrification.
- Environmental Conditions: Denitrification primarily occurs in anaerobic conditions, such as waterlogged soils or sediments.
- Flexibility of Bacteria: Contrary to being obligate anaerobes, many denitrifiers can act as facultative anaerobes. This means they can switch between aerobic and anaerobic respiration depending on environmental oxygen levels.
- Environmental Impact: Denitrification not only removes excess nitrogen from ecosystems but also influences greenhouse gas emissions through nitrous oxide release.
Prokaryotes
Prokaryotes are a diverse group of microorganisms that play a vital role in the nitrogen cycle. Unlike eukaryotes, prokaryotes lack a nucleus, presenting a simpler cell structure that allows them to thrive in various environments.
In the context of nitrogen metabolism, here is how prokaryotes contribute:
In the context of nitrogen metabolism, here is how prokaryotes contribute:
- Nitrogen Fixation: Some prokaryotes, such as Rhizobium, have symbiotic relationships with plant roots, converting atmospheric nitrogen into ammonia which plants can utilize.
- Diverse Metabolism: Prokaryotes can perform various reactions, including nitrification and denitrification, thanks to their flexible metabolism.
- Habitats: These microorganisms inhabit extreme environments, from deep ocean vents to acidic soils, and are crucial to the global nitrogen economy.
