In the treatment of synthetic dye wastewater by photosynthetic bacteria under optical irradiation, excessive algal growth and adhesion on the walls of the reactor are serious problems. The adverse effects of excessive algal growth on photosynthetic bacterial activity are significantly greater than those of the decreased optical irradiation of the solution. In this report, we investigated the effects of photocatalysis on the growth of algae (Chroococcus sp.) and photosynthetic bacteria. The different sensitivities of Chroococcus sp. and photosynthetic bacteria to photocatalysis were observed by an ATP assay. Moreover, from microscopy findings, some algae were damaged by TiO2 with UV. We suggested that some algae suffered from membrane damage and consequently cell substances were released, resulting in the increase of dissolved material following treatment using TiO2 with UV.