INTRODUCTION

Algal biomass can serve as a feedstock for the production of a variety of different biofuels, e.g.biodiesel, hydrogen, methane and bioethanol. Biodiesel is a non-toxic and biodegradable alternative fuel derived from renewable sources (Hussain et al., 2008). Microalgal biofuel are derived from the lipid content of the algal cells that serve as the feedstock for many high energy density transportation fuels, including biodiesel as well as green diesel, green jet fuel and green gasoline (Pienkos, 2009). The lipid and fatty acid composition of microalgae differs according to the culture conditions. The lipid accumulation in algae usually occurs during environmental stress, including growth under nutrient deficient conditions. The cells adapt themselves to stress by undergoing changes in morphological and developmental pattern as well as physiological and biochemical processes. The increase in salt concentration affects the rate of respiration, distribution of minerals, ion toxicity, photosynthetic rate and permeability of the cell membranes (Sudhir, 2004). Some unicellular green alga likeDunaliellasalinavar. bardawilresponded to salinity stress by regulating carbon fluxes between starch production in the chloroplast and the synthesis of glycerol in the cytoplasm (Cowan,1991). The growth of microalgae is retarded during salinity stress due to the accumulation of compatible solutes like proline and glycine to balance the external salt concentrations (Ahmed et al, 1989). Compatible solutes like proline are formed within cells during osmotic stress and acts as osmoprotectants to stabilize enzymes (Fatma et al, 2007). The percentage of saturated fatty acid in microalgae decreased as the concentration of NaCl increased, while the percentage of highly unsaturated fatty acid increased (Kirrolia et al, 2011).The decrease in photosynthetic activity commonly observed under salt stress may be due to limitations in photosynthetic electron transport and partial stomatal closure (Zhang et al, 2010). In Synechocystis, it has been found that the combination of light and salt stress has a strong synergistic and damaging effect on PSII, which is due to the fact that salt stress inhibits the activity of PSII from light-induced inactivation (Allakhverdiev et al, 2002).