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

An increasing production and use of titanium dioxide nanoparticles (TiO2 NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO2 NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO2 NPs remain elusive. Here we investigated toxic effects of two sizes of TiO2 NPs (50 and 10 nm) and one bulk TiO2 (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO2 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 TiO2 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 TiO2 nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO2 NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.