This study investigated the accumulation and effects of metal nanoparticles in two seaweed species, Ulva lactuca and Agardhiella subulata. Both seaweeds were exposed to silver nitrate (AgNO 3), silver nanoparticles, and copper oxide (CuO) nanoparticles for 48 h. Metal accumulation occurred in both seaweed species in a concentration-dependent manner after 48 h exposure to each form of metal. In several cases, seaweeds exposed to AgNO 3 (the dissolved form) accumulated comparatively higher tissue Ag concentration than seaweed exposed to Ag nanoparticles; and A. subulata had higher tissue Ag concentrations than U. lactuca after exposure to AgNO 3 for 48 h. Additionally, clear differences were observed in the regulation of Ag between the two seaweed species. Photosynthetic toxicity (primarily due to decreased maximum electron transport rate) was observed in U. lactuca after exposure to AgNO 3, Ag nanoparticles, and CuO nanoparticles. These results increase current knowledge about the differences in dissolved metal versus nanoparticle exposure in marine seaweeds and have implications in marine food webs.