Toxicity of nanoparticles of CuO, ZnO and TiO 2 to microalgae Pseudokirchneriella subcapitata

Abstract

Toxicities of ZnO, TiO 2 and CuO nanoparticles to Pseudokirchneriella subcapitata were determined using OECD 201 algal growth inhibition test taking in account potential shading of light. The results showed that the shading effect by nanoparticles was negligible. ZnO nanoparticles were most toxic followed by nano CuO and nano TiO 2. The toxicities of bulk and nano ZnO particles were both similar to that of ZnSO 4 (72 h EC50 ~ 0.04 mg Zn/l). Thus, in this low concentration range the toxicity was attributed solely to solubilized Zn 2+ ions. Bulk TiO 2 (EC50 = 35.9 mg Ti/l) and bulk CuO (EC50 = 11.55 mg Cu/l) were less toxic than their nano formulations (EC50 = 5.83 mg Ti/l and 0.71 mg Cu/l). NOEC (no-observed-effect-concentrations) that may be used for risk assessment purposes for bulk and nano ZnO did not differ (~ 0.02 mg Zn/l). NOEC for nano CuO was 0.42 mg Cu/l and for bulk CuO 8.03 mg Cu/l. For nano TiO 2 the NOEC was 0.98 mg Ti/l and for bulk TiO 2 10.1 mg Ti/l. Nano TiO 2 formed characteristic aggregates entrapping algal cells that may contribute to the toxic effect of nano TiO 2 to algae. At 72 h EC50 values of nano CuO and CuO, 25% of copper from nano CuO was bioavailable and only 0.18% of copper from bulk CuO. Thus, according to recombinant bacterial and yeast Cu-sensors, copper from nano CuO was 141-fold more bioavailable than from bulk CuO. Also, toxic effects of Cu oxides to algae were due to bioavailable copper ions. To our knowledge, this is one of the first systematic studies on effects of metal oxide nanoparticles on algal growth and the first describing toxic effects of nano CuO towards algae.