الفهرس 
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Abstract
The current global discussion about the imminent depletion of fossil fuels and the urgent need for alternative, renewable, and sustainable fuel sources served as the inspiration for the current work. As dependable sources of renewable biofuels, microalgae are now the subject of intense research interest. The main goal of this thesis was to examine the possibilities of various Monoraphidium species as a practical and sustainable feedstock for the production of biodiesel. The following succinctly describes the main findings and research activities of the current study.Different Monoraphidium and Chlorella isolates namely; Monoraphidium griffithii (Berkeley) Komárková - Legnerová, Monoraphidium contortum (Thuret) Komárková - Legnerová, Monoraphidium sp, Monoraphidium minutum (Nägeli) Komárková-Legnerová, Monoraphidium convolutum (Corda) Komárková – Legnerová, and Chlorella saccarophilia from the Nile River in Delta region, Egypt.The microalgal isolates were cultivated on BBM to select the most productive algae. Three Monoraphidium isolates namely; M. griffithii, M. contortum and M. convolutum SSDM23 were selected to evaluate the biomass production (0.26 g L-1, 0.16 g L-1 and 0.34 g L-1), respectively. Also, the lipid content (% DW) was calculated (13.3%, 16%, and 22.6%).The transesterified lipids, of three M. isolates, were analyzed by GC/MS to determine FAMEs (wt.%). Significant variations of FAMEs were observed between the various microalgal isolates where, M. convolutum SSDM23 exhibited the highest FAMEs (wt.%), (82.4%). Therefore, M. convolutum SSDM23 was selected to be evaluated as a promising applicant for biodiesel production as it maintained the highest growth characteristics, dry weight, lipid content and FAMEs (wt.%).M. convolutum SSDM23 was cultivated on three different media namely; modified Navicula, BBM, and BG11 media in 10L capacity transparent plastic carboy to estimate dry weight (g L-1) and lipid content (% DW). It was reported that modified Navicula maintained a significant increase in dry weight and lipid content (0.32 g L-1 and 26%) respectively. Afterward, M. convolutum SSDM23 was cultivated on four different nitrogen sources namely; calcium nitrate, ammonium sulphate, ammonium acetate, and urea. It was observed that ammonium acetate exhibited a significant increase in lipid content (36%). Consequently, ammonium acetate was selected as an alternative nitrogen source. Therefore, different nitrogen concentrations of ammonium acetate (15, 12, 9, 6, and 3 mg L-1 N) were estimated. The concentration of ammonium acetate equal to 12 mg L-1 maintained a significant increase in lipid productivity (8 mg L-1 d-1). To determine the efficiency of the produced lipids of M. convolutum SSDM23, the transesterified lipids of such alga were analyzed by GC/MS analysis. It was revealed that the SFAMEs increased significantly (55.49%) compared to a control culture (23.16%).In order to trigger the lipid accumulation, it was decided to test low concentrations of phosphorus (19, 1.425, 0.95, 0.475, 0.237 mg L-1 P) on the growth performance and lipid production of M. convoltum SSDM23. M. convolutum SSDM23 cultures were cultivated on a modified Navicula medium with ammonium acetate as an alternative nitrogen source. The concentration of phosphorus equal to 0.475 exhibited a significant increase in lipid content (38.44%) including MUFAMEs (52.4%).Similarly, low different concentrations of iron (1, 0.75, 0.5, 0.25, 0.125 mg L-1 Fe3+) were examined on dry weight and lipid content of M. convoltum SSDM23. The concentration of 0.75 mg L-1 Fe3+ enhanced the lipid production (39.1%) containing SFAMEs (60%).After that, the lipid production of M. convolutum SSDM23 was optimized through a statistical Central Composite Design (CCD) with previous nutritional elements (nitrogen, phosphorus, and iron) salts in response to dry weight and lipid content. Twenty trials with varying salt concentrations were carried out to cultivate M. convolutum SSDM23 on a modified Navicula medium. The trial ranges were as follows: phosphorus (0.237-1.829 mg L-1), iron (0.125-1.298 mg L-1), and nitrogen (7-17 mg L-1). The optimized trial included 12 mg L-1 ammonium acetate, phosphorus at 1.829 mg L-1, and iron at 0.562 mg L-1 maintained the highest lipid content (50.2% DW). Accordingly, it was suggested for the cultivation of M. convolutum SSDM23 in a RWP system for ten days.The chemical and physical characteristics of lipids produced from both control and optimized M. convolutum cultures were determined using the European standard (EDIN 51605). The chemical characteristics of optimized lipids, including free fatty acid (% mg KOH/g oil), acid value (% mg KOH/g oil), and saponification value (mg KOH/g oil), registered a noticeable minimization of their values compared to control culture, whereas the iodine value (mg I2/g oil) of optimized lipids recorded an increment compared to control culture. Additionally, the physical characteristics of optimized lipids containing specific gravity, flash point, fire point, and boiling point complied perfectly with European standard (EDIN 51605). Therefore, the optimized lipids of M. convolutum were esterified through an esterification process into their corresponding FAMEs (biodiesel) with a biodiesel yield of 95%.The lipids of M. convolutum GC/MS analysis revealed a significant increase in methyl hexadecanoate (C17:0) and methyl cis-9-octadecenoate (C19) by three folds compared with the control culture. Also, the lipid productivity of optimized M. convoltum SSDM23 cultures increased by two folds compared with the control culture.