Vesiculation of thylakoids in Myxobaktron salinum Walsby strain PCC 8305

Myxobaktron salinum WALSBY strain PCC 8305 (also known as Dactylococcopsis salina) is a unicellular cyanobacterium isolated from hypersaline waters of Solar Lake (Sinai). This cyanobacterium is characterised by fusiform cells showing a netlike cytoplasm, i.e. keritomy. Transmission electron microscopy studies showed that the keritomy is due to the enlargement of thylakoid lumen in electron-transparent areas, i.e. intrathylakoidal vesicles. In young cells grown photo-autotrophically under standard conditions the content of intrathylakoidal vesicles appears mostly electron-transparent. With the culture ageing the content becomes more electron-dense, the intrathylakoidal vesicles are sometimes filled with small osmiophilic granules, and the vesiculation of thylakoids increases; under dark condition there is a progressive decrease of the electron-dense content up to complete disappearance of the osmiophilic granules. There is also a reduction of the dimension of intrathylakoidal vesicles. During growth under nitrogen depleted conditions there is a marked accumulation of low electron-dense granules inside the intrathylakoidal vesicles. These granules tend to disappear when the culture is transferred to dark conditions, and the intrathylakoidal vesicle content becomes completely electron-transparent. This behaviour is typical of carbohydrates, which are usually consumed under dark conditions. Using a specific staining (THIERY 1967) we confirmed the polysaccharidic nature of the content of the intrathylakoidal vesicles. The dynamic of the formation and role of these vesicles are discussed together with their occurrence among other unicellular cyanobacteria.