(b) How does heterocyst differ from vegetative cell ? Mention the factors controlling its formation and add a note on functions of heterocysts.

Differences between Heterocyst and Vegetative Cell

Heterocysts and vegetative cells are two distinct cell types found in filamentous cyanobacteria, exhibiting significant structural and functional differences to facilitate their specialized roles.

Feature	Heterocyst	Vegetative Cell
Function	Primarily nitrogen fixation (conversion of N₂ to NH₃).	Primarily photosynthesis (oxygenic, producing O₂ and carbohydrates).
Photosynthetic Apparatus	Lacks Photosystem II (PSII), thus no oxygen production. Retains Photosystem I (PSI) for ATP generation.	Possesses both Photosystem I and Photosystem II, performing oxygenic photosynthesis.
Cell Wall	Thicker, multilayered cell envelope with an outer laminated layer (OLL) and an inner heterocyst glycolipid layer (HGL) to restrict oxygen diffusion.	Typical bacterial cell wall, thinner, permeable to oxygen.
Pigmentation	Diminished pigmentation, often appearing pale or yellowish, due to the degradation of phycobiliproteins (light-harvesting pigments for PSII).	Contains abundant photosynthetic pigments (chlorophyll a, phycobiliproteins), giving the characteristic blue-green color.
Shape and Size	Generally larger, rounder, or oval-shaped than vegetative cells.	Smaller, typically cylindrical or barrel-shaped, and actively dividing.
Cytoplasm	Less dense, often appearing transparent or hyaline. Contains polar plugs composed of cyanophycin.	Dense cytoplasm with numerous thylakoid membranes centrally organized.
Enzymes Present	Contains oxygen-sensitive nitrogenase, a higher level of glutamine synthetase. Lower level of glutamine-oxoglutarate aminotransferase (GOGAT).	Contains RuBisCO for carbon fixation, lower level of glutamine synthetase. GOGAT found in vegetative cells for nitrogen assimilation.
Reproduction	Terminally differentiated, cannot divide.	Actively dividing, responsible for filament growth.

Factors Controlling Heterocyst Formation

The differentiation of a vegetative cell into a heterocyst is a tightly regulated process, primarily influenced by environmental conditions and genetic mechanisms:

• Nitrogen Availability: The most critical factor is the depletion or scarcity of fixed nitrogen sources (like ammonium or nitrate) in the environment. This nitrogen starvation triggers a cascade of genetic events leading to heterocyst differentiation.
• Oxygen Levels: While not a direct trigger for differentiation, oxygen levels play a crucial role. The nitrogenase enzyme, responsible for nitrogen fixation, is highly sensitive to oxygen. Therefore, mechanisms to create a micro-anaerobic environment are integral to heterocyst formation and function.
• Genetic Regulation: Several genes and regulatory proteins govern heterocyst development and patterning:
  • NtcA: This transcriptional regulator is activated under low nitrogen conditions, initiating the differentiation process by influencing the expression of other key genes.
  • HetR: A master regulator, HetR is crucial for heterocyst differentiation. It up-regulates genes like hetR itself, patS, and hepA by binding to their promoters.
  • PatS: This gene encodes a small peptide that acts as a diffusible inhibitor, preventing adjacent vegetative cells from differentiating into heterocysts and thus maintaining a regular spacing pattern along the filament (typically every 10-20 vegetative cells).
  • PatA: Involved in patterning heterocysts along the filaments and important for cell division.
  • HetN: An enzyme dependent on heterocyst maintenance.
  • Other genes like devA, hetC, and the hetMNI locus are also implicated in understanding intercellular interactions that influence heterocyst formation.
• Intercellular Communication: The spatial pattern of heterocysts suggests a system of communication between cells. Differentiating cells produce inhibitory signals that diffuse to neighboring cells, preventing their differentiation and establishing the characteristic spacing.

Functions of Heterocysts

Heterocysts perform vital functions for filamentous cyanobacteria, primarily centered around nitrogen fixation:

• Nitrogen Fixation: This is the primary role. Heterocysts house the oxygen-sensitive enzyme nitrogenase, which converts atmospheric nitrogen gas (N₂) into ammonia (NH₃). Ammonia is a biologically usable form of nitrogen that can be incorporated into amino acids, proteins, and nucleic acids, making it available to the entire cyanobacterial filament. This process is highly energy-intensive, requiring significant ATP.
• Creation of Micro-anaerobic Environment: To protect nitrogenase from oxygen inactivation, heterocysts undergo several modifications:
  • Development of a thick, multilayered cell wall that acts as a physical barrier to oxygen diffusion.
  • Degradation of Photosystem II, preventing oxygen production within the cell.
  • High rates of respiration that consume any residual oxygen.
  • Presence of polar plugs (composed of cyanophycin) at the junctions with vegetative cells, which slows down cell-to-cell diffusion, further isolating the internal environment.
• Metabolic Exchange: Heterocysts establish a symbiotic relationship with adjacent vegetative cells. Vegetative cells provide carbohydrates (primarily sucrose) and reductants (e.g., in the form of NADPH) produced during photosynthesis to the heterocysts, fueling the energy-demanding process of nitrogen fixation. In return, heterocysts export fixed nitrogen (mainly as glutamine) to the vegetative cells, supporting their growth and metabolic activities.
• ATP Production: While lacking PSII, heterocysts retain Photosystem I (PSI), enabling them to perform cyclic photophosphorylation to generate ATP, which is essential for the nitrogen fixation process.