The seminar discussed the ultrastructure and reproduction of cyanobacterial cells, with a focus on Microcystis.
The ultrastructure includes a mucilage sheath, cell wall with four layers, plasma membrane, and cytoplasm containing photosynthetic apparatus of thylakoids and phycobilisomes, ribosomes, and storage structures.
Reproduction is mainly through binary fission, but some cyanobacteria also form specialized dormant structures like akinetes or motile fragments called hormogonia. Microcystis can produce the toxin microcystin and is studied for its production of antioxidants.
3. ULTRASTRUCTURE CYANOBACTERIAL CELL:
• The ultra-structural detail of a typical cyanobacterial cell is the following:
• 1. Mucilage Sheath:
The cells and filaments of most cyanobacteria are generally surrounded by a
mucilaginous sheath whose thickness, pigmentation, consistency, and nature is
greatly influenced by environmental factors. It is considered that these
microorganisms secrete the mucilage through pores present in their cell walls.
4. •2. Cell Wall:
The cell wall lies between the plasma membrane and the
mucilage sheath. Like bacteria, peptidoglycan is the main constituent of the
cyanobacterial cell wall. Ultra structurally, the wall consists of four layers (LI,
LII, LIII, LIV) each of which is connected with the other one by a connection
known as plasmodesma .All cyanobacterial cell walls, like that of other
prokaryotes, possess diaminopimelic acid.
5. 3. Plasma Membrane:
A typical cyanobacterial plasma membrane is 70Å thick. It is selectively permeable, lacks
sterol such as cholesterol, and consists of a high proportion of protein to phospholipid (typically
2:1) like other prokaryotes. It generally fuses with the photosynthetic lamellae and gives rise to
inward folding’s in the cytoplasm; the folding’s are called lamellosomes or mesosomas The latter
are mostly similar in functions to mesosomes occurring in other prokaryotes
6. 4. The Cytoplasm:
It lacks eukaryotic organelles such as chloroplasts, mitochondria, endoplasmic reticulum,
Golgi bodies. But, it possesses photosynthetic apparatus, ribosomes, and a large number of
subcellular inclusions such as glycogen or α-granules, polyphosphate bodies, polyhedral
bodies, cyanophycin granules, and the genetic material.
(i) Photosynthetic apparatus:
In place of the chloroplasts of photosynthetic eukaryotes, cyanobacteria have flattened
vesicular structures called thylakoids or lamellae, which resemble the individual thylakoids
of the true chloroplasts of photosynthetic eukaryotes.
The lamellae or thylakoids are both physiologically or structurally complex and possess
photosynthetic pigments. As described earlier, the principal pigment of all cyanobacteria is
chlorophylls.
7. • In addition, there are β-carotene and other accessory pigments, namely, phycobiliproteins.
The phycobiliproteins are phycocyanin (PC), allophycocyanin (AP), allophycocyanin-B
(APB), and phycoerythrin. By possessing phycocyanin and phycoerythrin accessory
pigments, the cyanobacteria resemble with red algae.
• However, the necessary pigments of these organisms are generally organized into
organelles called phycobilisomes and trap light energy of lower wavelengths, which
cannot be absorbed by chlorophyll a, and pass it on to the chlorophyll.
• This is the reason why cyanobacteria, like green algae, can exploit deeper waters where
the quality and quantity of illumination is inappropriate for the photosynthetic plants.
8. • (ii) Ribosomes:
• These are the sites of protein synthesis. Cyanobacteria ribosomes occur freely in the
cytoplasm and are identical to those of bacteria in being 70S ribosomes.
• (iii) Glycogen or α-granules:
• Glycogen or α-granules are the sites for storage of excess photosynthetic products.
The latter is used as energy source in darkness or when CO2 supply is limiting.
• (iv) Polyphosphate bodies:
• These are the spherical structures formed as a result of the aggregation of high
molecular weight linear polyphosphates. These subcellular inclusions are also
called metachromatin granules or volutin granules and serve as phosphate
stores and are consumed during periods of phosphate starvation. These
structures develop mostly in those cyanobacteria that grow in a phosphate-rich
environment.
9. • Reproduction in Cyanobacteria:
• Like bacteria, the cyanobacteria also reproduce asexually and the commonest mode of
reproduction in them is transverse binary fission. In addition, there are certain specialized
structures such as akinetes, hormogonia, harmosts and spores, which are partly involved in the
process of reproduction.
• So far as the sexual reproduction in its true sense is concerned, it is absent in them and the
requirements of sexuality are considered to be met by some alternative pathways referred to as
parasexual-pathways.
10. 1. Akinetes:
• Most filamentous cyanobacteria develop perennating structures (dormant structures) in
adverse condition. These structures are larger than the vegetative cells, are equipped with
thick walls, and are called akinetes . When favorable conditions return, they germinate and
produce new filaments.
11. 2. Hormogonia:
• All filamentous cyanobacteria reproduce by fragmentation of their filaments (trichomes) at
more or less regular intervals to form short pieces each consisting of 5-15 cells. These
short pieces of filaments are called hormogonia. The latter show gliding motility and
develop into new full- fledged filaments.
3. Hormocysts:
• Some cyanobacteria produce hormocysts, which are multicellular structures having a
thick and massive sheath. They may be intercalary or terminal in position and may
germinate from either end or both the ends to give rise to the new filaments.
12. 4. Spores:
Non-filamentous cyanobacteria generally produce spores such as endospores, exospores and
Nano cysts which contribute by germinating and giving rise to new vegetative cells when the
unfavorable condition is over. Endospores are produced endogenously like those in bacteria;
exospores are the result to exogenous budding of cells, and the Nano cysts are produced
endogenously like endospores.
The difference between an endospore and a Nano cyst is that in endospore formation the parent
cell concomitantly enlarges in size, whereas in Nano cyst formation there is no such
enlargement of the cell.
13. SIGNIFICANCE OF MICROCYSTIS
Microcystis aeruginosa produces microcystin toxin under the right
environmental conditions, it can be a source of drinking water pollution.
M. aeruginosa is the subject of research in to the natural production of
butylated hydroxytoluene [BHT] an antioxidant, food additive and
industrial chemical .
