الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
SUMMARY
The present study was designed to investigate the effect of different conditions on the growth, total lipid yield, and the composition of extracted FAMEs of three green microalgal species; Chlamydomonas sp., Scenedesmus ecornis, and S. communis, in order to determine the best conditions for subsequent biodiesel production.
To accomplish this goal, various techniques of cell disruption and some organic solvents were tested for their effect on extracting the highest yield of total lipids as well as the best proportions of palmitic and oleic fatty acids from each microalga. Box˗Behnken model was then applied to identify the optimum level of each variable; involved both nutritional and environmental factors, which suit every microalga. After that, each microalga was exposed to stress conditions (gamma irradiation and macronutrient depletion) were explored for their impact on its growth and lipids productivity. The microalgal co˗culture was applied to enhance the productivity of biomass and total lipids and to track its impact on the extracted FAMEs composition from mixed biomass.
The most significant findings could be summarized as below:
1. The osmotic shock was shown as the best disruption procedure that boosted the efficiency of lipid extraction from wet microalgal biomass, by releasing the highest yields of lipids and total FAMEs; involving SFAs and USFAs ratios. Results indicated the following:
The total lipids yield and total FAMEs extracted from Chlamydomonas sp. reached a maximum of 25.1 and 17.7 % (w/w), respectively. The highest percentages of SFAs and USFAs were observed at 24.1 and 39.3 % of total FAMEs (w/w), to reach 1.5˗ and 2.0˗fold of control levels, respectively.
For S. ecornis, the extracted total lipids and total FAME yields reached up to 28.3 and 23.7 % (w/w), respectively. Furthermore, the increased proportions of total SFAs and USFAs were recorded to reach 25.5 and 37.8 % of total FAMEs (w/w), which equal 1.4˗ and 1.6˗fold of control values, respectively.
The highest levels of total lipids and total FAMEs were obtained at 26.6 and 17.8 % (w/w), respectively from S. communis biomass. A significant increase in the total levels of SFAs and USFAs was noticed, giving 24.1 and 32.7 % of total FAMEs (w/w) which between 1.3˗ to 1.5˗fold of the control ratios, respectively.
The rates of palmitic and oleic acids represented the majority of total FAMEs concentrations for the three microalgae.
2. The influence of organic solvents on the proportions of total FAMEs; that extracted from wet cells disrupted by osmotic shock method. Findings were as follows:
The best yield of total FAMEs extracted from Chlamydomonas sp. was achieved (at 17.7 % w/w) using chloroform: methanol (2:1 v/v) mixture, followed by hexane at 17.0 % (w/w). The highest ratios of both palmitic and oleic acids were also extracted by chloroform: methanol (2:1 v/v) mixture to reach 20.3 and 22.1 % of total FAMEs (w/w), respectively.
A mixture of chloroform and methanol (2:1 v/v) has extracted the maximum total FAMEs level (23.7 % w/w) from S. ecornis. The best concentrations of palmitic and oleic acids up to 20.8 and 25.6 % of total FAMEs (w/w) were attained by chloroform: methanol co˗solvent.
The co˗solvent of chloroform and methanol (2:1 v/v) released the highest proportion of total FAMEs up to 18.2 % (w/w). Meanwhile, the co˗solvent has extracted significant levels of palmitic and oleic acids to reach 19.7 and 21.0 % of total FAMEs (w/w), respectively.
3. Utilizing the Box–Behnken model resulted in a marked increase in the biomass and lipid production from microalgae through the two groups of factors.
a) Based on the statistical analysis of the 1st group’s variables (nutritional factors), run 5 was selected to be used in microalgal cultivation afterward, since it offered the most appropriate concentrations of NaNO3, KH2PO4, and C6H12O6 for the three microalgae. Results were illustrated by the following:
from Chlamydomonas sp., the optimum biomass concentration and yield of lipids were reached by run 5, giving 3.5 g L˗1 and 30.6 % w/w, respectively. The highest productivity of biomass and lipids were obtained also through run 5, reaching 250.0 and 76.5 mg L˗1 day˗1, respectively.
The highest biomass concentration and total lipids yield for S. ecornis were 3.6 g L˗1 and 36.7 % (w/w), which were resulted from run 5. The best productivity of biomass and lipids were up to 260.7 and 95.7 mg L˗1 day˗1, respectively, and obtained by run 5.
The maximum biomass concentration of S. communis (3.6 g L˗1) with the highest yield of total lipids (33.7 % w/w), which were achieved by run 5. The optimum productivity of biomass and lipids (255.1 and 85.9 mg L˗1 day˗1, respectively) were obtained from run 5.
b) According to statistical analysis of the 2nd group’s variables, run 7 was considered the most suitable set for the growth of the three microalgae, which increased the lipids accumulation inside cells. Results were explained as below:
For Chlamydomonas sp., the highest biomass concentration reaching 3.7 g L˗1 was obtained by runs 2 and 4. Otherwise, the maximum yield of lipids was attained by run 4, giving 36.4 % (w/w). The highest productivity of biomass and lipids (264.3 and 96.2 mg L˗1 day˗1, respectively) were recorded by run 4.
The maximum biomass concentration of S. ecornis (3.9 g L˗1) was observed from run 7. In addition, the highest total lipids yield was achieved by runs of 13 and 7, giving 36.6 and 36.3 % (w/w), respectively. The best productivity of both biomass and lipids were achieved by run 7, reaching 280.7 and 102.0 mg L˗1 day˗1, respectively.
For S. communis, the maximum levels of biomass and total lipids were up to 3.9 g L˗1 and 38.6 % (w/w), respectively, which were achieved via run 7. The highest productivity of biomass and total lipids were also attained by run 7 at 278.6 and 107.5 mg L˗1 day˗1, respectively.
4. Exposing the microalgal species to gamma irradiation has different effects
depending on each microalga during the cultivation period (20 days). After a cultivation period of 15 days, results demonstrated the following:
A DROP was observed in the growth of irradiated cultures based on the used irradiation dose. The highest biomass concentration of Chlamydomonas sp. was obtained by non˗irradiated culture at 1.27 g L˗1, followed by 1.16 g L˗1 for the irradiated microalgae at 25 Gy. The highest biomass productivity was 84.5 mg L˗1 day˗1, for non˗irradiated culture with lipid content and productivity of 19.8, and 23.4 % (w/w), respectively. Conversely, the highest yield of lipids and lipid productivity were obtained by irradiated microalga at 25 Gy (27.3 % w/w and 21.3 mg L˗1 day˗1, respectively with biomass productivity of 78.2 mg L˗1 day˗1.
By increasing the exposure dose of γ˗rays, the growth of S. ecornis has significantly increased up to 1.32 g L˗1 when exposed to 300 Gy, compared to 1.10 g L˗1 of the non˗irradiated culture. The lipid yield and productivity of irradiated S. ecornis have significantly increased even at lower doses. The highest yield lipid (28.4 % w/w), biomass productivity (87.8 mg L˗1 day˗1), and lipid productivity (24.9 mg L˗1 day˗1) were all achieved by an irradiated culture at 300 Gy.
As for S. communis, the growths of irradiated cultures have slightly reduced compared to the non˗irradiated culture, which produced the maximum biomass concentration at 1.02 g L˗1. Moreover, lipid yield and productivity lipids have significantly declined with the increase in an irradiation dose, which is different from the non˗irradiated culture.
5. Three different strategies of macronutrients depletion were tested to investigate their effect on the growth and production of total lipids by microalgal species. Results were as below:
For Chlamydomonas sp., using N&P˗reduction strategy led to induce lipids accumulation over the biomass production, giving the highest yield of total lipids and lipid productivity (37.1 % w/w and 26.3 mg L˗1 day˗1, respectively) with reasonable biomass productivity of 71.0 mg L˗1 day˗1.
A noticeable increase was detected in the production of biomass and total
lipids by S. ecornis at relatively high concentrations of NaNO3 (31.9 g L˗1) and K2HPO4 (3.3 g L˗1). The best yield of total lipids (38.7 % w/w) along with lower productivity of biomass and lipids (84.6 and 32.7 mg L˗1 day˗1, respectively) were attained at NaNO3 concentration was of 31.9 g L˗1.
The production of biomass and total lipids by S. communis have significantly enhanced by decreasing both NaNO3 and K2HPO4 concentrations. The best yield of total lipids was detected at 40.6 % (w/w) accompanied by the highest productivity of biomass and lipids at 76.0 and 30.9 mg L˗1 day˗1, under N&P˗depletion conditions.
6. The use of a co˗culture of the three microalgae with three different rates of inoculum; 10, 15, and 20 % (v/v) to batch cultures and resulted in remarkably improved biomass production, lipids accumulation, and FAMEs composition. Results showed the following:
The microalgal co˗cultures exhibited significantly higher yields of biomass and lipids in a comparatively short period. The maximum biomass concentrations reached up to 5.0, 5.1, and 4.9 g L˗1 on day 11 of cultivation period, for the three co˗cultures inoculated at ratios of 10, 15, and 20 % (v/v), respectively.
Within 11 days of cultivation period, the highest yields of total lipids were recorded up to 43.0, 47.2, and 36.5 % (w/w) by co˗cultures with different inoculum rates of 10, 15, 20 % (v/v), respectively.
The maximum proportion of total FAMEs (46.3 % w/w) was achieved when the culture was inoculated at a rate of 15 % (v/v), followed by 45.6 % and 41.8 % (w/w) at rates of 10 and 20 % (v/v). Likewise, the highest levels of SFAs and USFAs were obtained by a co˗culture that was inoculated at a rate of 15 % (v/v).
The best percentages of palmitic and oleic acids were at 22.4 and 39.5 % of total FAMEs (w/w), respectively, which were achieved from a co˗culture inoculated at a rate of 15 % (v/v). Whereas the co˗cultures that inoculated at rates of 10 and 20 % (v/v) yielded less significant ratios of palmitic and oleic acids.