الفهرس 
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Abstract
This study aims to ameliorate the accumulation of polyunsaturated fatty acids from two isolated marine microalgal strains Auxenochlorella pyrenoidosa and Synechocystis salina, molecular identification of the two isolates, studying the behavior of these microalgal species and their response on lipid percentage under different stressors such as nitrate depletion and different salinities, confirming the study by relative gene expression of two genes acetyl Co-A carboxylase A (accA) and Δ9-fatty acid desaturase (Δ9-FAD) involved in the metabolic pathway of PUFAs, therefore, investigating fatty acids profile of the two isolates cultivated under nitrate depletion and 25 ppt salt concentration using GC-MS. Polyunsaturated fatty acids play critical role in multiple disciplines specifically human health, PUFAs reduce cardiovascular diseases, minimize the bad cholesterol levels in the blood, assist in the treatment of inflammations, improve the brain functions, and enhance the regulation of neurotransmitters, therefore, the maximization of PUFAs productivity from biological sustainable sources such microalgae become an urgent demand. The Polyunsaturated fatty acids play critical role in multiple disciplines specifically human health, PUFAs reduce cardiovascular diseases, minimize the bad cholesterol levels in the blood, assist in the treatment of inflammations, improve the brain functions, and enhance the regulation of neurotransmitters, therefore, the maximization of PUFAs productivity from biological sustainable sources such microalgae become an urgent demand. This experiment aimed to combine two different stressors with each other’s, the best salt concentration which was 25 ppt with nitrate stress for photosynthetic activity determination, RNA extraction and fatty acid profile analysis (GC-MS). RNA was extracted from algal samples using TRIZOL, definite volumes of the extracted RNA converted to cDNA using hexamer primers and oligo dt, NCBI used to retrieve the primers for Δ9-FAD and accA, qRT-PCR was performed using syber green master mix, The 2-ΔΔCt method was used to determine the fold change. The expression level of A. pyrenoidosa Δ9-FAD was upregulated 4.03 under nitrate depletion and 3.82 folds under 25 ppt salt stress while, in S. salina, the salt stress had shown a superiority in Δ9-FAD expression level compared with nitrate stress. The Δ9-FAD expression level was 5.5 and 4.6 folds up-regulated under salt and nitrate stress, respectively. The transcription level of accA gene in S. salina under and nitrate depletion and 25 ppt salt stress were upregulated respectively by 8.38 and 35.41 folds.The percentage of unsaturated fatty acids in A. pyrenoidosa under nitrate depletion was elevated to 68.22% while, the 25 ppt salt concentration elevated unsaturated fatty acids to 65.13%. It could be concluded as nitrate depletion and 25 ppt salt can elevate the percentage of unsaturated fatty acids. According to the results, salt stress can accumulate high amount of lipids mostly are polyunsaturated fatty acids in S. salina, nitrate depletion showed a superiority in unsaturated fatty acids by 92.55%. The most abundant chain length under nitrate depletion were C16:1, C18:1n9 and C20:3n6 with C16:1 dominating. Increasing the percentage of C18:1 under the two different stressors (nitrate depletion and 25 ppt) could be explained as the stress induced lipid biosynthesis as a result of the action of Δ9-FAD, therefore these results could be confirmed by the up regulation of Δ9-FAD that is responsible to covert the saturated C18 to monounsaturated C18:1.