Cellular Responses Of Dunaliella Salina To The Osmotic Stress And Changes Of Salinity Besides Accumulation Of Lipid

Some species in genus Dunaliella are unique in their remarkable abilities to accumulate large numbers of ?-carotene and thrive in media containing a wide range of NaCl concentrations ranging from about 1.5%to saturation(around 15%).The algae contain no rigid polysaccharide cell wall and thus have been found to be able to rapidly change their volume and shape in response to changes in the extracellular hypo-or hyper-osmotic pressure.In this study,we investigate the osmotic response process of Dunaliella under salinity stress,including the changes of cell volume and neutral lipids accumulation.The results are as the following:(1)The salt-tolerant green microalga Dunaliella salina can survive both hyper-and hypo-osmotic shock.Upon osmotic shock,the cells transiently and rapidly decreased or increased in size within minutes and slowly over hours acquired their original cell size and volume.(2)The effect of salinity also influences the cell growth and the time required to reach maximal cell density in different salinity is also different.The higher the salinity is,the slower of the cell growth and lower of the cell density are,and vice versa.(3)Only osmotic upshift from 9%NaCl to higher NaCl concentration induced transient neutral lipid appearance in Dunaliella salina,while those transferred to 15%NaCl growth medium stimulated the most neutral lipid accumulation.It is evident that even though D.salina grew well in 9%salt,it does not have the capacity to recover from a shift to 15%NaCl and cell division is accordingly slowed down.However,this salt shock is not lethal.In contrast,cells shifted from 3 to 9%quickly recovered and resumed growth.Shifting cells from 3 to 9%NaCl did not result in neutral lipid emergence.Therefore,the transient appearance of neutral lipid could be dependent on the inhibition of cell division experienced during the NaCl shift from 9 to 15%.(4)The effect of nutrient limitation slows down cell division and photosynthesis as a secondary result,which triggers the cells to accumulate neutral storage lipids when they entered the stationary phase,which is seen in all the batch cultures of D.salina grown in the salinity range of 3-15%.Overall,we have analyzed the short-and long-term effects of salinity on cell growth and lipid productivity in D.salina by changing NaCl concentration in the growth media.For short-term effect,a sudden change in salinity resulted in a transiently increased neutral lipid production in D.salina only by hyper-saline shock until the cell growth slowed down and cell division was inhibited as a result of the stress.For long-term effect,the changes in salt concentration did not significantly influence the overall fatty acid composition in the adapted culture of D.salina cells grown in different salinity.Although there showed increased amounts of total lipids and neutral lipids in the cells grown in higher salinity,lipid productivity is however compromised by the slower growth rate of the cells.Therefore,the cell growth rates,maximum cell density as well as the lipid productivity are the important parameters for evaluating the potential of a microalga used for lipid production.