Nano-biotechnology is recognized as offering revolutionary changes in various fields of medicine. Biologically synthesized silver nanoparticles have a wide range of applications. The biosynthesis of silver nanoparticles is an eco-friendly method in the field of nanotechnology. Seaweed extracts of Caulerpa racemosa (Forsskål) J. Agardh and Ulva lactuca Linnaeus was used as a reducing agent in the eco-friendly extracellular synthesis of silver nanoparticles from an aqueous solution of silver nitrate (AgNO₃). High conversion of silver ions to silver nanoparticles was achieved with Ulva lactuca at reaction temperature of 100 ℃ and a seaweed extract concentration of 10% with a residential time of 1 h using reflux extractor when compared with the other methods. Formation of silver nanoparticles was characterized by spectrophotometry and the electron microscopic technique. The average particles size was ranging from 35 to 75 nm. Antimicrobial activities indicate the minimum inhibitory concentration of biologically synthesized nanoparticles tested against the pathogen Staphylococcus aureus (1 mg/ml). High inhibitions over the growth of Pseudomonas aeruginosa, Vibrio cholerae and Escherichia coli were witnessed against the concentrations of 1 mg/ml. Enzyme assay of the collected seaweeds performed using standard protocol to assess the potency level. Further, seed germination test proved that synthesized nanoparticles were environmentally safe, for which the same can be used for effluent treatment process.