Cyanobacterial Heterocysts

Abstract

Cyanobacteria are phototrophic bacteria carrying out oxygen‐producing photosynthesis. Besides showing the capability of building their cellular carbon from carbon dioxide (CO 2 ), available in the atmosphere, several strains of cyanobacteria have also acquired the ability to fix molecular dinitrogen (N 2 ), a ubiquitous source of nitrogen. As the enzyme responsible for nitrogen fixation (nitrogenase) is highly sensitive to oxygen, nitrogen fixation and oxygenic photosynthesis cannot take place simultaneously in cyanobacterial cells. To solve this problem, some filamentous strains are able to restrict N 2 fixation to a specialised cell type, the heterocyst. Heterocysts are morphologically distinct, terminally differentiated cells that develop, in the absence of alternative sources of combined nitrogen, mostly in a semiregular pattern along the filaments. Thus, a filament containing heterocysts provides division of labour between vegetative cells (photosynthetic CO 2 fixation) and heterocysts (anaerobic N 2 fixation). These cyanobacteria represent true multicellular organisms with profound morphological cell differentiation and sophisticated intercellular communication systems, ultimately orchestrated by complex gene expression patterns. Key Concepts Although bacteria, some filamentous cyanobacteria are true multicellular organisms, showing different cell types performing specialised tasks. When lacking a source of combined nitrogen, these cyanobacteria start a program of cell differentiation resulting in a pattern of semiregularly spaced heterocysts along the filament. Heterocysts differentiate from vegetative cells, which carry out oxygenic photosynthesis, and provide a microoxic environment for the activity of oxygen‐labile nitrogenase. Nitrogenase is the enzyme responsible for N 2 fixation and allows the organism to live from sunlight, air (CO 2 and N 2 ) and some minerals. The two cell types rely on each other and exchange metabolites and signalling molecules, via septal junctions mediating cell‐to‐cell contact. Two transcription factors, HetR and NtcA, regulate the changes in gene expression during heterocyst differentiation. The pattern of spaced heterocysts is regulated by inhibitor gradients that promote the decay of HetR, confirming mathematical models of two‐dimensional pattern formation in heterocystous cyanobacteria.

Changes in Envelope Structure and Cell–Cell Communication during Akinete Differentiation and Germination in Filamentous Cyanobacterium Trichormus variabilis ATCC 29413

“…The filaments of the multicellular cyanobacterium Trichormus variabilis ATCC 29413 consist of hundreds of vegetative cells that have the potential to differentiate into specialized cells, heterocysts and akinetes. The semi-regularly spaced heterocysts enable the fixation of atmospheric nitrogen (N 2 ), if no other combined nitrogen sources are available, and provide organic nitrogen to the vegetative cells [ 1 , 2 ]. Akinetes are spore-like nonmotile single cells that develop from the vegetative cells in response to diverse environmental factors, including light intensity, temperature and nutrient deficiency [ 3 , 4 , 5 , 6 ].…”

The Dual Role of the Glycolipid Envelope in Different Cell Types of the Multicellular Cyanobacterium Anabaena variabilis ATCC 29413

“…In A. variabilis, the nitrogen (N 2 ) fixing heterocysts appear in a semi-regular pattern along the filament in response to insufficient supply of combined nitrogen under aerobic conditions (Fay, 1992;Muro-Pastor and Maldener, 2019). Nitrogen fixation is mediated by the enzyme nitrogenase, which is highly sensitive to oxygen.…”

“…To protect the nitrogenase, a microoxic environment is maintained in the heterocysts by inactivation of oxygen-evolving photosynthesis, by increased respiration, and by formation of an additional multilayered cell envelope outside the cell wall (Murry et al, 1984;Walsby, 1985;Murry and Wolk, 1989;Valladares et al, 2003Valladares et al, , 2007Magnuson, 2019). The heterocyst envelope is composed of two layers: an inner heterocyst-specific glycolipid (HGL) layer, which acts as a barrier to oxygen and limits its diffusion into the heterocyst, and an outermost heterocyst envelope polysaccharide (HEP) layer, which protects the HGL layer from physical damage (Wolk et al, 1994;Adams, 2000;Maldener et al, 2014;Muro-Pastor and Maldener, 2019).…”