Industry has already commenced the large-scale production of some nanomaterials. Evidence for toxic effects of engineered nanoparticles (ENP) on model organisms is increasing. However, in order to assess the consequences of environmental hazards, a better understanding is required of the behavior of ENP in aquatic ecosystems and their impact on complex communities. In this research, through experimenting with different TiO(2) nanoparticles in stream microcosms, we have shown that microbial membranes were significantly compromised, even under ambient ultraviolet radiation and nano-TiO(2) concentrations predicted for surface waters. Our results suggest adverse effects are not necessarily only attributable to individual particles smaller than 100 nm but also to low concentrations of larger, naturally agglomerating TiO(2) nanoparticles. Cell membrane damage was more pronounced in free-living cells than in biofilm cells, indicating the protective role of cell encapsulation against TiO(2) nanoparticles. The generation of intracellular reactive oxygen species (ROS) further suggests nano-TiO(2)-induced effects inside the microbial cells. Our findings indicate a high sensitivity of microbial communities to levels of ENP concentration that are to be expected in the environment, with as yet unknown implications for the functioning and health of ecosystems.