biological approaches to the synthesis of nanoparticles are better than chemical and physical procedures because of low energy and time expenditure. This method requires neither toxic solvents nor any dangerous material for the environment. Various microorganisms have the ability to synthesize nanoparticles but the usage of metal reducing bacteria is very noticeable. In this research, biosynthesis of silver nanoparticles has been explored by Shewanella sp., a sulfate reducing bacterium. Methods:Samples from sediments of the Caspian Sea were collected and a sulfate reducing bacterium was isolated. The isolate was evaluated based on its ability to synthesis silver nanoparticles. The sulfate reducing isolate was inoculated in LB medium and incubated at 28 ºC for 24 hours. The culture supernatant was examined for its ability to produce silver nanoparticles. About one mL of the supernatant was added to 100 mL of silver nitrate solution at a concentration of 50 mM then incubated at 28 °C for 24 hours. Results:The appearance of brown color in solution indicated the formation of silver nanoparticles. The UV-Vis spectrum exhibited an absorption band at around 420nm, suggesting the formation of silver nanoparticles. SEM studies showed that the silver nanoparticles were fairly uniform in size with a spherical shape and an average diameter of 44 nm. Conclusion:The results of this study showed that the sulfate reducing isolate has the ability to produce silver nanoparticles.
