L'osmo-Resistenza di Dunaliella Salina Ricerche Ultrastrutturali

Abstract

The resistance of DUNALIELLA SALINA TO OSMOTIC STRESSES. ULTRASTRUCTURAL RESEARCHES. — The mechanisms which confer to Dunaliella salina its high resistance to osmotic stresses have been studied from a cytological point of view. For this purpose the Authors have examined the ultrastructural changes following a great rise or fall in the osmolarity of the normal culture medium.

When the tonicity of the nutrient medium is suddenly lowered the response of the alga is given by an instantaneous and general swelling. The whole cell and the single cytoplasmic structures appear equally swollen and the state of dispersion of the stroma of cellular organelles is highly increased. This shows clearly that a conspicuous amount of water has penetrated in every part of cell and suggests that the plasmatic cell membrane and the inner cytoplasmic boundaries may be considered as semipermeable membranes. The response of the different cellular organelles to a hypertonic shock is not quite identical; the mitochondria show for example a quite uniform type of swelling which does not alter deeply their morphological aspect, whilst in the inner space of the nuclear envelope wide sacks appear suggesting that the outer layer of the double membrane of the nucleus may be more permeable to water than the inner one. In the chloroplasts the flattened sacks formed by paired lamellae having blind endings in the peristromium swell up giving rise to conspicuous vesicles. This proves that the permeability to water of the lamellae is greater than that of the external plastidial boundary.

A sudden and strong hypertonic stress causes a great loss of water from every part of the cell. The alga appears indeed shrunken and shrinking phenomena can be see in all cytoplasmic organelles together with a greater electron density of their stroma. The two layers of the nuclear membrane and the lamellar pairs forming the sacculi in the chloroplasts stick closely together causing a nearly total disappearance of the inner space. This shows that the formation of large vesicles after a sudden hypotonic shock is really due to a different permeability of the two layers of the membrane and not to localised osmotic gradients.

In both cases the quite conspicuous structural variations appear reversible and the alga returns to the normal aspect in a comparatively short time.

From these observations it is concluded that the resistance of Dunaliella salina to severe osmotic shocks (which in our experimental conditions were probably more sudded than in the natural environment of the cells) is not due to a mechanism isolating the cells from the external medium or maïntaining homeostatic conditions but rather to a ready permeability of the plasmatic membranes to water and ions. As a consequence an equilibrium between internal and external osmotic conditions is established within the cell in a sufficiently short time allowing normal life conditions.

It has to be observed that during the short time span necessary for the restablishment of a new osmotic equilibrium and of a normal aspect of the plasmatic structures the cells show some clear symptome of suffering consisting in a loss of the motility and in an altered metabolism. Although this time is rather short, it appears clear that the hardness of Dunaliella to osmotic shocks involves a specific resistance of plasma proteins to denaturation.