Question

Deprived bacteria. Blue-green algae (cyanobacteria) form heterocysts when deprived of ammonia and nitrate. In this form, the cyanobacteria lack nuclei and are attached to adjacent vegetative cells. Heterocysts have photosystem I activity but are entirely devoid of photosystem II activity. What is their role?

Short answer

Heterocysts fix atmospheric nitrogen in an anaerobic environment to provide ammonia to vegetative cells.

Step-by-step solution

Understanding Heterocysts

Heterocysts are specialized cells that form in certain cyanobacteria, such as blue-green algae, when they are deprived of fixed nitrogen sources like ammonia and nitrate.

Photosystem Activity

Heterocysts possess photosystem I activity but lack photosystem II. This means that heterocysts can participate in light-dependent reactions but do not produce oxygen as photosystem II does.

Nitrogen Fixation Role

The lack of photosystem II in heterocysts prevents oxygen production, creating an anaerobic environment necessary for nitrogen fixation. Heterocysts fix atmospheric nitrogen into ammonia, which can be utilized by the vegetative cells of the cyanobacteria colony.

Interdependence with Vegetative Cells

The fixed nitrogen (ammonia) from heterocysts is shared with adjacent vegetative cells. In return, vegetative cells supply heterocysts with carbohydrates and ATP needed for fixation.

Key concepts

Heterocysts

Heterocysts are fascinating structures found in some cyanobacteria, such as blue-green algae. When these microorganisms cannot access fixed nitrogen sources like ammonia and nitrate, they form heterocysts. These specialized cells are different from normal vegetative cells because they lose their nuclei and have a thickened cell wall to help maintain an anaerobic environment.
Heterocysts play a crucial role in nitrogen fixation. They convert atmospheric nitrogen (\( ext{N}_2 \)) into ammonia (\( ext{NH}_3 \)), a form of nitrogen that the organism can use for growth and metabolism. This process requires an environment devoid of oxygen, as oxygen can inhibit the nitrogenase enzyme responsible for fixing nitrogen.

• Heterocysts form when cyanobacteria are deprived of external nitrogen sources.
• They have thick walls to create an anaerobic environment.
• This anaerobic condition is optimal for nitrogen fixation by nitrogenase.

Cyanobacteria

Cyanobacteria, also known as blue-green algae, are aquatic and photosynthetic organisms. They are important for their ability to conduct photosynthesis, similar to plants, and play a vital role in the nitrogen cycle by fixing atmospheric nitrogen.
Cyanobacteria possess both photosystems I and II under normal conditions. However, when heterocysts are formed, they lack photosystem II activity. This is a crucial adaptation because it helps eliminate oxygen production, which is necessary for effective nitrogen fixation.
Despite lacking a nucleus in heterocysts, cyanobacteria form these specialized cells to maintain mutualistic relationships with their vegetative cells. Heterocysts convert atmospheric nitrogen into ammonia, which is then shared with vegetative cells. In exchange, vegetative cells provide heterocysts with carbohydrates and energy in the form of ATP, showcasing a fine-tuned collaboration.

• Cyanobacteria are crucial for both photosynthesis and nitrogen fixation.
• They form heterocysts to enable efficient nitrogen fixation without oxygen interference.

Photosystem I Activity

Photosystem I is part of the light-dependent reactions of photosynthesis. It plays a role in converting light energy into chemical energy, crucial for various cellular processes.
In heterocysts, photosystem I activity remains even though photosystem II is absent. This retention allows heterocysts to generate ATP and NADPH, which are critical energy carriers used in the nitrogen fixation process. The lack of photosystem II ensures no oxygen is produced, protecting the sensitive nitrogenase enzyme needed for converting nitrogen into ammonia.
Photosystem I captures light and uses this energy to facilitate electron transfers, ultimately contributing to the formation of energy-rich molecules. This energy is vital for the heterocysts as they work to fix nitrogen and provide nutrients to the vegetative cells.

• Photosystem I is essential for light-induced electron transfer during photosynthesis.
• In heterocysts, it produces the energy required for nitrogen fixation.
• The absence of photosystem II prevents oxygen, safeguarding nitrogenase activity.