Problem 28 - Biology E/M

Question

Choose from the terms below about the nitrogen cycle. (A) Nitrogen-fixing bacteria (B) Denitrifying bacteria (C) Nitrifying bacteria Convert nitrates into free nitrogen in the atmosphere

Step-by-Step Solution

Answer

(B) Denitrifying bacteria.

Step 1: Understanding the Nitrogen Cycle

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can involve nitrogen fixation, nitrification, denitrification, and decay. In this exercise, we need to identify the process or organisms responsible for converting nitrates back into atmospheric nitrogen.

Step 2: Identify the Role of Denitrifying Bacteria

Denitrifying bacteria are responsible for the conversion of nitrates (NO₃⁻) into nitrogen gas (N₂), which is then released into the atmosphere. This process completes the nitrogen cycle by returning nitrogen to the atmosphere.

Step 3: Choosing the Correct Term

Given the choices (A) Nitrogen-fixing bacteria, (B) Denitrifying bacteria, (C) Nitrifying bacteria, the correct choice is (B) Denitrifying bacteria, since they convert nitrates into free nitrogen in the atmosphere.

Key Concepts

Denitrifying Bacteria

Denitrifying bacteria play a crucial role in the nitrogen cycle by converting nitrates (NO₃⁻) back into nitrogen gas (N₂), which is released into the atmosphere. This process is known as denitrification and is essential for maintaining the balance of nitrogen in the ecosystem. Without these bacteria, nitrogen could accumulate in the form of nitrates, leading to environmental issues like eutrophication.

Denitrification occurs primarily in anaerobic conditions, meaning areas void of oxygen. These conditions are often found in water-logged soils and deep aquatic sediments.

• Denitrifying bacteria include species like Paracoccus denitrificans and Pseudomonas.
• They perform a step-by-step process, changing nitrate into nitrogen gas through different forms like nitrite (NO₂⁻) and nitrous oxide (N₂O).

This ability to reduce nitrates alleviates excess nitrogen accumulation, ensuring that ecosystems do not become unbalanced.

Nitrogen-fixing Bacteria

Nitrogen-fixing bacteria are vital organisms that help convert atmospheric nitrogen (N₂) into a form that can be used by plants, such as ammonia (NH₃). This process is known as nitrogen fixation and is essential because atmospheric nitrogen is inert and not directly usable by most living organisms.

These bacteria can be found in the soil or in symbiotic relationships with plant roots, particularly in legumes like peas, beans, and clover.

• Free-living nitrogen-fixing bacteria include Azotobacter and Clostridium.
• Symbiotic nitrogen fixers are found in root nodules of legumes and belong to the genus Rhizobium.

Through nitrogen fixation, these bacteria convert atmospheric nitrogen to ammonia, which then can be further transformed into nitrates (NO₃⁻), a form readily taken up by plants for their growth and development.

Nitrifying Bacteria

Nitrifying bacteria are responsible for the process known as nitrification, which involves converting ammonia (NH₃) into nitrates (NO₃⁻) through a two-step oxidation process. This conversion is crucial for soil fertility and the availability of nitrogen to plants.

The first step is carried out by bacteria such as Nitrosomonas, which oxidize ammonia into nitrites (NO₂⁻). Then, other bacteria like Nitrobacter further oxidize nitrites into nitrates.

• Nitrifying bacteria thrive in well-aerated soils, which provide the oxygen needed for these reactions.
• This process negatively correlates with denitrification, as both occur in different environmental settings and conditions.

By converting ammonia, nitrifying bacteria make nitrogen readily available to plants in the form of nitrates, thus playing a vital role in plant nutrition and ecosystem productivity.