In filamentous cyanobacteria, what is a heterocyst and what is its primary function?

Introduction:
Heterocysts are specialized cells in some filamentous cyanobacteria that enable nitrogen fixation in the presence of oxygenic photosynthesizers. This question evaluates understanding of their structure, function, and differentiation relative to vegetative cells.

Given Data / Assumptions:

• Nitrogenase is oxygen sensitive and requires a microoxic environment.
• Filamentous cyanobacteria compartmentalize functions along their trichomes.
• Heterocysts differ morphologically and physiologically from vegetative cells.

Concept / Approach:
Heterocysts are terminally differentiated, thick-walled cells that downregulate oxygenic photosystem II activity and upregulate nitrogenase machinery. They provide an internal low-oxygen niche through enhanced respiration and barrier layers, allowing N2 reduction to NH3 while neighboring vegetative cells perform oxygenic photosynthesis and share fixed carbon in exchange for fixed nitrogen.

Step-by-Step Solution:
Recall oxygen sensitivity of nitrogenase and the need for spatial separation from O2 evolution. Recognize heterocyst features: glycolipid layers, reduced PSII, elevated respiration, nitrogenase expression. Differentiate from spores or simple vegetative cells; heterocysts do not divide. Select the option describing a terminally differentiated nitrogen-fixing cell.

Verification / Alternative check:
Microscopy shows heterocysts at regular intervals in filaments, and gene expression profiles confirm nitrogenase and heterocyst-specific proteins.

Why Other Options Are Wrong:

• A type of spore: Akinetes are spores; heterocysts are functional N2-fixing cells, not dormant forms.
• Progenitor of vegetative cells: Heterocysts are terminally differentiated, not progenitors.
• A cell that carries out oxygenic photosynthesis: Heterocysts suppress PSII to limit oxygen.

Common Pitfalls:
Confusing akinetes with heterocysts; akinetes are survival spores, whereas heterocysts are active in nitrogen fixation.

Final Answer:
a terminally differentiated cell that fixes nitrogen is the correct description of a heterocyst.