Heterocyst – Introduction

Multicellular creatures can have more diversified and efficient structures, activities, and behaviours than unicellular ones. Heterocyst-forming cyanobacteria are a good model for studying cellular differentiation and multicellular pattern creation among multicellular prokaryotes. Cyanobacteria are gram-negative prokaryotes that can undergo oxygenic photosynthesis.

Nitrogen-fixing heterocysts, spore-like akinetes, and the cells of motile hormogonia filaments have all evolved. The generation of heterocysts in the filamentous cyanobacterium Anabaena (also Nostoc) sp. strain PCC 7120 has been the most well-investigated of these.

In a very basic form, heterocyst formation provides a fascinating example of cell differentiation and developmental biology. Filaments are made up of only two types of cells, which are arranged in a one-dimensional arrangement like beads on a string.

Table of Contents

• Heterocyst – Definition
• Formation of Heterocysts
• Symbiotic Relationships of Heterocysts
• Frequently Asked Questions (FAQs)

Heterocyst – Definition

Blue-green algae or cyanobacteria including Nostoc and Anabaena acquire specialised cells known as heterocysts. These are cells that have been changed from vegetative cells. These are the N₂ fixation sites. The heterocyst contains an enzyme called nitrogenase, which aids in nitrogen fixation. Oxygenic photosynthesis is also carried out by these cells.

Because oxygen inhibits nitrogenase, the heterocyst must produce a micro anaerobic environment. Because of the heterocysts’ distinctive form and behaviour, a global shift in gene expression is required. Heterocysts, for example:

• develop three additional cell walls, one of which is glycolipid-based and acts as a hydrophobic barrier to oxygen.
• generate nitrogenase, as well as other proteins involving nitrogen fixation, degrade photosystem II, which produces oxygen.
• up-regulate glycolytic enzymes.
• create proteins that scavenge any oxygen that is still available.
• contain cyanophycin-based polar plugs that inhibit cell-to-cell diffusion.

Many cyanobacterial species can fix nitrogen. Because oxygen inactivates nitrogenase, oxygenic photosynthesis and nitrogen fixation are conflicting activities. Cyanobacteria primarily use two ways to distinguish these activities: a biological circadian clock for temporal separation and multicellularity and cellular differentiation for geographical separation. Photosystem I is maintained by heterocysts, which allows them to create ATP through cyclic photophosphorylation.

Heterocysts differ from vegetative cells by having a somewhat bigger and rounder form, thicker cell envelopes, less pigmentation, and frequently noticeable cyanophycin granules at poles close to vegetative cells. The extra envelope layers that surround heterocysts assist to protect nitrogenase from oxygen.

vegetative cells and heterocysts are mutually interdependent. Heterocysts rely on vegetative cells for a resource of reductant and carbon, which is likely given in part as sucrose, as they lack photosystem II and carbon fixation.

Formation of Heterocysts

In the production of heterocysts from a vegetative cell, the following sequences occur:

• The cell grows in size.
• The number of granular inclusions decreases.
• The photosynthetic lammel shifts its orientation.
• Finally, the wall is triple-layered. These three layers grow outside the cell’s outer layer.

The middle layer is homogenous and the inner layer is laminated in nature.

The senescent heterocyst vacuolates and eventually breaks away from the filament, fragmenting it. Hormogonia (singular hormogonium) is the name for these fragments, which reproduce asexually.

The orders Nostocales and Stigonematales, which form simple and branched filaments respectively, divide the cyanobacteria that generate heterocysts. They form a monophyletic group with very minimal genetic variability when grouped together.

Symbiotic Relationships of Heterocysts

Certain plants may form a symbiotic connection with the bacteria. In this type of connection, the bacteria respond to the plant’s signals for heterocyst differentiation rather than the availability of nitrogen. Up to 60% of the cells could become a heterocyst, supplying the plant with fixed nitrogen in exchange for fixed carbon.

Anabaena cyanobacteria and Azolla plants have a notable symbiotic relationship. Azolla plants have Anabaena on their stems and within their leaves. Photosynthesis occurs in the Azolla plant, which gives fixed carbon to the Anabaena for use as a source of energy for dinitrogenases in heterocyst cells. In exchange, the heterocysts can deliver fixed nitrogen in the form of ammonia to the vegetative cells and the Azolla plant, allowing both species to flourish.

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