Silver TiO2 nanocomposites were synthesized by two methods: silver UV photodeposition (Ag/P25) and a sol-gel process incorporating silver nitrate during the titanium alkoxide gelling step (TiO2-Ag). The obtained nanocomposites were characterized by means of XRD, N-2 adsorption, XPS, and DRS. All the silver-TiO2 semiconductors have anatase as the principal crystalline TiO2 phase and the average band gap was found in the edge of the visible-ultraviolet region (3.26 eV). According to XPS and HAADF-STEM studies, highly dispersed silver nanoparticles were found on the titania surface as Ag degrees. The photocatalytic effects of the silver TiO2 nanocomposites on the killing of marine algae were examined by using either Amphidinium carterae (red tide) or noxious Terraselmis suecica (green tide) as a probe. After 1 h under UV irradiation, both harmful algae were inactivated by the silver nanocomposites. The fatal damages to these microorganisms induced by the TiO2-Ag so-gel semiconductor occurred faster than those promoted by the Ag/P25 and TiO2 sot-gel isolated supports. In addition to the biocide properties of silver in aqueous medium, the silver nanoparticles acted as electron traps, retarding electron-hole recombination, which enhanced the photocatalytic activity. (C) 2010 Elsevier B.V. All rights reserved.