Characterization and Impact of Silver nanoparticles on cell growth, lipid, carbohydrate and fatty acids of Chlorella vulgaris and Dictyochloropsis splendida

Background: Many efforts have been made to increase the productivity of microalgae for biodiesel productions. The use of silver nanoparticles is the novel way to elicit stress responses with enhanced lipid level. Results: : In this study, the biosynthesis of extracellular silver nanoparticles (AgNPs) was reported and their impacts as elicitors on the cell growth and metabolite contents of Chlorella vulgaris and Dictyochloropsis splendida were evaluated. The production of AgNPs was achieved by the reduction of silver nitrate (AgNO 3 ) solution, after incubation at 35°C overnight with Pseudomonas aeruginosa supernatant, and exposed to gamma irradiation at 100 Gy for 1.5 mins. The biosynthesis was confirmed by the maximum absorption peak at 455 nm with the UV–Vis Spectrophotometer. The Atomic Force Microscopy (AFM) recorded the spherical nanoparticles size of 10 nm, while the Dynamic Light Scattering (DLS) recorded the size range of 6.7 to 12.1 nm (84.2%) and the particles were monodispersed. The Gas Chromatography/Mass Spectroscopic analysis of the bacterial filtrate before reaction with AgNO 3 suggested the presence of ethylene glycol derivatives which may act as a reducing agent of silver ions to silver nanoparticles. Lower AgNPs concentrations (1, 3 and 5 mg/L) enhanced the lipid production but at the expense of cell growth. All AgNPs concentrations however displayed a negative impact on carbohydrates content. The lipid profile of the AgNPs-treated algae showed the appearance or disappearance, and increase or decrease of certain fatty acids, as compared to the untreated control. The Saturated Fatty Acids represented the highest composition (61-67%) of the total fatty acids and Palmitic acids (16:0) were dominant (43.06-46.57%). Conclusion: Lipids of this composition could withstand autoxidation during storage and are perfect feedstock for biodiesel and other lipid based applications.