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

An increasing production and use of titanium dioxide nanoparticles (TiO 2 NPs) pose a huge threat to phytoplankton since they are largely released into aquatic environments, which represent a sink for TiO 2 NPs. However, toxicity and protective mechanisms of cyanobacteria in response to TiO 2 NPs remain elusive. Here we investigated toxic effects of two sizes of TiO 2 NPs (50 and 10 nm) and one bulk TiO 2 (200 nm) on a cyanobacterium, Synechocystis sp. and their possible protective mechanisms. We found that 10 nm TiO 2 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 2 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 2 nanoparticle exposure. This study provides mechanistic insights into our understanding of Synechocystis sp. responses to TiO 2 NPs. This is essential for more accurate environmental risk assessment approaches of nanoparticles in aquatic ecosystems by governmental environmental agencies worldwide.