Toxic and protective mechanisms of cyanobacteria Synechococcus sp. in response to zinc oxide nanoparticles

The rapid growth of zinc oxide nanoparticles (ZnO NPs) production leads to their accumulation in the environment. However, the impact of ZnO NPs on aquatic ecosystems has not been fully studied. The question of assessing the impact of this pollutant on microalgae and cyanobacteria as the primary-productive link of aquatic biocenoses remains relevant. In the presented study, changes in morphology, structural-functional and fluorescent indices of cyanobacterium Synechococcus sp. were established for concentrations of zinc oxide nanoparticles (ZnO NPs) ranging from 0.3-14 mg L^-1. ZnO NPs have mechanical and cytotoxic effects on Synechococcus sp. cells. At high pollutant concentrations (8.4-14 mg L^-1), pronounced deformation of Synechococcus sp. cell membranes was observed, which was caused by the mechanical effect of the pollutant on the cells and heteroaggregation of ZnO particles with cyanobacterial cells. At the same time, no effect of NPs on the cell morphometric indices was revealed. Physiological and biochemical parameters of single cell Synechococcus sp. and cells aggregated with NPs do not differ significantly at ZnO NPs concentrations of 1.4-14 mg L^-1. At concentrations above 1.4 mg L^-1, the production of reactive oxygen species in Synechococcus sp. significantly increased in both groups of cells. At the same time, deterioration of other physiological and biochemical parameters of cells was also observed. Growth inhibition, decrease of intracellular content of chlorophyll and phycoerythrin, dissociation of phycoerythrobilin in antenna complexes, decrease of metabolic activity of cells were observed. High sensitivity of the photosynthetic apparatus of Synechococcus sp. to ZnO NPs was shown. It was found that in Synechococcus sp. Unlike eukaryotic algae, the maximum efficiency of light quantum utilization and the minimum values of non-photochemical quenching of chlorophyll fluorescence are registered under light conditions corresponding to the growth conditions of cyanobacteria. The results of the presented study contribute to the understanding of the mechanisms of toxicity of dispersed ZnO NPs and effective assessment of their probable ecological risk and interaction with phototrophic microorganisms.