Influence of Nitrogen Source and Growth Phase on Extracellular Biosynthesis of Silver Nanoparticles Using Cultural Filtrates of Scenedesmus obliquus

In this study, silver nanoparticles (AgNPs) were green-synthesized extracellularly by the action of bioactive compounds in cultural filtrates of green microalga Scenedesmus obliquus (KY621475). The influences of six different nitrogen sources (i.e., NaNO3, CO(NH4)2, (NH4)2CO3, KNO3, NH4NO3, and (NH4)2SO4) on extracellular biosynthesis of AgNPs were observed by UV–Visible spectroscopy (380–425 nm) and confirmed using high-resolution transmission electron microscopy (HRTEM). The highest biomass production was observed in the case of urea and ammonium carbonate treatments, which, surprisingly, showed negative activity for AgNPs biosynthesis. Considering their coupling and compatible presence in cultural filtrates, reductases (especially nitrate reductase) as reduction agents are assumed to play a key role in the extracellular biosynthesis of AgNPs. The cultural filtrates of the potassium and sodium nitrate treatments produce AgNPs of relatively small size (5–10 and 4–10 nm, respectively), smaller than those produced by filtrate of ammonium nitrate treatment. The antimicrobial activity of produced AgNPs was a function mainly of particle size, which was influenced by the nitrogen source of the microalgal culture. The AgNPs produced from the KNO3 and NaNO3 cultural filtrates performed the best as antimicrobial agents.