Effects of titanium dioxide nanoparticles on algal and bacterial communities in periphytic biofilms

The widespread application of commercial TiO₂ NPs inevitably leads to their release into environmental waters through various ways. TiO₂ NPs released into water might be absorbed by and react with periphytic biofilms, which are a kind of aquatic environmental media of important ecological significance, and influence the physiological activity and ecological function of periphytic biofilms. This study investigated the effects of exposure to 1 mg/L and 5 mg/L of TiO₂ NPs on periphytic biofilms cultured indoors. After a 10-day exposure to TiO₂ NPs, the growth (measured by chlorophyll-a content) of microalgal community was inhibited greatly (more than 60%); however, the primary production (indicated by quantum yield) of periphytic biofilms maintained changeless. As for bacteria, TiO₂ NP-exposure increased the bacterial diversity and altered the composition structure. Significant changes were observed in the bacterial communities at the class level, mainly including Alphaproteobacteria, Gammaproteobacteria, Cytophagia, Flavobacteriia, Sphingobacteriia, Synechococcophycideae and Oscillatoriophycideae. The enhancement of metabolic activities (the production of extracellular polymeric substances, especially proteins content increased by 48.51%) of periphytic biofilms was a resistance mechanism to toxicity of NPs. As for extracellular enzyme activities of periphytic biofilms, alkaline phosphatase activity was inhibited (22.43%) after exposed to 5 mg/L of TiO₂ NPs, which posed a threat to phosphorus metabolism of periphytic biofilms. Overall, this study demonstrated that 1 mg/L and 5 mg/L of TiO₂ NPs negatively influenced physiological activities and ecological functions of periphytic biofilms, highlighting that the ecological risks of TiO₂ NPs should be paid attention to.