What is the role of heterocysts in a cyanobacterial filament?

• A. They carry out photosynthesis and nitrogen fixation.
• B. They carry out only photosynthesis.
• C. They oxidize inorganic substances to obtain energy.
• D. They store endospores.
• E. They carry out only nitrogen fixation.

Answer: (E)

Verified step by step guidance

1. Understand that cyanobacteria are photosynthetic bacteria that can also fix nitrogen, an essential process for converting atmospheric nitrogen into a form usable by living organisms.
2. Recognize that heterocysts are specialized cells found in some filamentous cyanobacteria, which are crucial for nitrogen fixation.
3. Note that heterocysts have a thickened cell wall to protect the nitrogenase enzyme from oxygen, as nitrogenase is sensitive to oxygen and can be inactivated by it.
4. Acknowledge that while most cells in a cyanobacterial filament perform photosynthesis, heterocysts are specialized to perform only nitrogen fixation, not photosynthesis.
5. Conclude that the role of heterocysts is to provide an anaerobic environment necessary for the nitrogenase enzyme to function, thus enabling the cyanobacteria to fix nitrogen efficiently.