**Abstract**

Cyanobacterial exopolysaccharides (EPS) are heteropolysaccharides with significant biological importance in various industries. Investigating nanoparticles is gaining interest due to their great potential in improving cyanobacterial growth and co-products accumulation. Nevertheless, green synthesis of nanoparticles offers an alternative, eco-friendly and cost-effective approach to available chemical methods of nanoparticle synthesis. Thus, this study illustrates a novel approach to green synthesizing Ag nanoparticles (AgNPs) from marine cyanobacterium *Phormidium tenue* and investigates their effect on the enhancement of biomass and exopolysaccharide accumulation in the same cyanobacterium by incorporating previously synthesized AgNPs*.* Firstly, the AgNPs were synthesized from *P. teneue* by adding 1 mM silver sulfate into the culture medium, and the obtained AgNPs were characterized by using UV-VIS spectroscopy, XRD, SEM, and FTIR. In order to increase the biomass yield and EPS accumulation, *P. tenue* culture was subjected to different concentrations of AgNPs. Under different concentrations of AgNPs, the biomass yield and exopolysaccharides increased compared to the control condition on the 28th and 35th day of incubation, respectively. The characterization of the obtained EPS was studied by using FTIR which showed a specific absorbance of OH, weak aliphatic C-H stretching, sulfur-containing functional groups, and carboxylic acids, revealing the characteristic feature of EPS.

**Keywords:** cyanobacteria, exopolysaccharides, FTIR, HPLC, XRD, SEM, UV-VIS spectroscopy, silver nanoparticle, *Phormidium tenue*
