Dose response curve and population growth rate alterations of marine Chlorophyte Dunaliella tertiolecta derived from the exposure to ZnO nanoparticles were evaluated. Bulk ZnO and ionic zinc were also investigated for comparison. At the same time, the aggregation state and particle size distribution were monitored. The evaluated 50% effect concentration (EC50 1.94 [0.78-2.31] mg Zn L(-1)) indicates that nano ZnO is more toxic than its bulk counterpart (EC50 3.57 [2.77-4.80] mg Zn L(-1)). Cross-referencing the toxicity parameters calculated for ZnCl(2) (EC50 0.65 [0.36-0.70] mg Zn L(-1)) and the dissolution properties of the ZnO, it can be gathered that the higher toxicity of nano ZnO is most likely related to the peculiar physicochemical properties of the nanostate with respect to the bulk material. Furthermore growth rate of D. tertiolecta was significantly affected by nano ZnO exposure. Our findings suggest that the primary particle size of the dispersed particles affect the overall toxicity.