Toxic and protective mechanisms of cyanobacterium Synechocystis sp. in response to titanium dioxide nanoparticles.

An increasing production and use of titanium dioxide nanoparticles (TiO NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO NPs remain elusive. Here we investigated toxic effects of two sizes of TiO NPs (50 and 10 nm) and one bulk TiO (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO NPs caused significant growth and photosynthesis inhibition in Synechocystis sp. cells, largely reflected in decreased growth rate (38%), operational PSII quantum yields (40%), phycocyanin (51%) and allophycocyanin (63%), and increased reactive oxygen species content (245%), superoxide dismutase activity (46%). Also, transcriptomic analysis of Synechocystis sp. exposure to 10 nm TiO NPs showed the up-regulation of D1 and D2 protein genes (psbA and psbD), ferredoxin gene (petF) and F-type ATPase genes (e.g., atpB), and the down-regulation of psbM and psb28-2 in PS II. We further proposed a conceptual model to explore possible toxic and protective mechanisms for Synechocystis sp. under TiO nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.