Genene Tefera, DVM, PhD
Microbial Genetic Resources Department, Institute of Biodiversity Conservation
Addis Ababa, Ethiopia, January, 2009
Transmission Electron Microscope observations show for Spirulina prokaryotic organization, capsule, pluri-stratified cell wall, photosynthetic or thylakoid lamella system, ribosomes and fibrils of DNA region and numerous inclusions. The capsule has fibrillar structure and covers each filament protecting it. The irregular presence of capsule around the filaments in S. platensis is a differentiating morphological characteristic to compare with S. maxima. Trichome width varies from 6 to 12 μm, and is composed of cylindrical cells. The helix diameter varies from 30 to 70 μm; the trichome length is about 500 μm, although in some cases when stirring of culture is deficient the length of filament reaches approximately 1 mm. It is very important to explain that the helical shape of Spirulina in liquid culture is changed to spiral shape in solid media. These changes are due to hydratation or dehydratation of oligopeptides in the peptidoglycan layer.
Spirulina cell wall is formed by four numbered layers, from the inner most outward as: LI, LII, LIII and LIV. All these layers are very weak, except layer LII made up of peptidoglycan, substance that gives the wall its rigidity. The LI layer contains b-1, 2-glucan, a polysaccharide not very digestible by human beings. However, the low concentration (<1%) of this layer, thickness its (12 nm), and the protein and lipopolysaccharide nature of the LII layer are favorite reasons for the easy human digestion of Spirulina.
In this microorganism chlorophyll a, carotenes and phycobilisomes, which contain phycocyanin (blue pigment), are located in the thylakoid system or photosynthetic lamellas. The inter-thylakoid space is limited by the presence of electronically transparent protein gas vesicles, with the cylindrical form that give Spirulina its floating capacity. Ribosomes and fibrils of DNA region are generally of central localization.
Spirulina contains numerous characteristic peripheral inclusions associated to thylakoids. Those are: cyanophycin granules, polyhedral bodies, polyglucan granules, lipid granules, and polyphosphate granules. The cyanophycin granules, or reserve granules, are important due to their chemical nature and a series of pigments. The polyhedral bodies or carboxysomes mainly contain the enzyme ribulose 1, 5-diphosphate carboxylase that allows the fixation of CO2 in photosynthetic organisms and probably carry out a reserve function. The polyglucan granules or glycogen granules or a-granules are glucose polymers, small, circular and widely diffused in the interthylacoidal space. The lipid granules, b-granules or osmophile granules form the reservation deposit, constituted by poly-b-hydroxybutyrate (PHB), found only in prokaryotes. PHB acts as a carbon and energy reserve.
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