Study On Mechanism Of Toxicity Of CuO Nanoparticles To Algae

CuO nanoparticles?CuO NPs?,as a kind of nanomaterial,are widely used in commercial applications.The expansion of CuO NPs'applications has caused release of increasing quantities of CuO NPs into environment,which has resulted in widespread of CuO NPs'biosafety concerns.Many studies have found the adverse biological toxicity of CuO NPs.Algae,as the primary producer and the starting point of bio-concentration,will first be affected by CuO NPs.Therefore,it is of great physiological and ecological significance to study the mechanism of toxicity of CuO NPs to algae.In this study,the effects of CuO NPs on the two most important primary metabolic pathways of algae,photosynthesis and respiration,and the absorption and transformation of CuO NPs by algal cells were studied from two aspects of eukaryotic algae?Chlorella sp.,Scenedesmus sp.?and prokaryotic algae?Arthrospira sp.?.The aim of this study is to understand the mechanism of toxicity of CuO NPs to algae,and to provide a theoretical basis and a new strategy for establishing the toxicity evaluation system of nanomaterials and preventing nanomaterials from polluting the environment.The main results obtained are as follows:?1?CuO NPs were more toxic to algae than the same content of CuO microparticles?CuO MPs?,which indicates that nanoscale particle size could increase the toxicity of the materials.The inhibition degree of CuO NPs to photosynthetic oxygen evolution rate was more serious than that to respiration rate and the inhibition degree continually increased with the increase of content of CuO NPs in both eukaryotic algae and prokaryotic algae.This indicates that photosynthetic apparatus of algae were more sensitive to CuO NPs treatment than respiratory apparatus.It is worth noting that prokaryotic algae were more sensitive to the toxicity of CuO NPs than eukaryotic algae.The content and time that the CuO NPs treatment caused the same damage to prokaryotic algae were significantly reduced than that to eukaryotic algae.?2?In both eukaryotic algae and prokaryotic algae,the thylakoids were damaged by CuO NPs and the structure of thylakoids became looser.Moreover,CuO NPs resulted in the bending deformation of thylakoids in prokaryotic algae.The deformation of thylakoids structure caused by CuO NPs seriously affected the light energy absorption and conversion efficiency to reduce the light energy utilization efficiency of algae,which eventually affected the photosynthetic ability of algal cells.?3?We measured the chlorophyll fluorescence transient curves of algae.The result showed that the maximal photochemical efficiency of PSII?Fv/Fm?in CuO NPs-treated algae decreased,which indicates that CuO NPs damaged the PSII of algae.CuO NPs have specific damage sites to the photosystem of algae,but the damage sites of eukaryotic algae and prokaryotic algae were different.CuO NPs mainly damaged the OEC in the eukaryotic algae but the D1 protein of the PSII reactive center in prokaryotic algae.?4?The destruction of thylakoid structure and the damage to PSII caused by CuO NPs significantly inhibited photosynthetic oxygen evolution rate of algae.CuO NPs release a lot of Cu²⁺once CuO NPs enter the aquatic environment,and algal cells absorbed Cu²⁺released by CuO NPs.When algae was treated with CuSO₄ that released the same content of Cu²⁺as the CuO NPs do into the supernatant,we observed that Cu²⁺inhibited photosynthetic oxygen evolution rate and aggravated photoinhibition and caused the deformation of thylakoids,which was similar to the CuO NPs-treatment.These results indicate that the toxicity of CuO NPs might be associated with Cu²⁺released by CuO NPs.Although Cu²⁺caused a similar toxicity to algae as CuO NPs did,the degree of toxicity to photosynthetic apparatus caused by Cu²⁺was significantly lower than that by CuO NPs,which indicates that to release Cu²⁺by CuO NPs was not the only way to damage algae.Algae absorb CuO NPs by endocytosis and CuO NPs entering the algae cells are mainly enriched around the thylakoid.The CuO NPs around the thylokoids due to its nanoscale particle size reacted with biomolecules such as proteases and nucleic acids in thylokoids,affecting their function to directly damage thylokoids.?5?The size of Cu₂O NPs particles?2-5 nm?were far smaller than that of CuO NPs observed in thylokoids.However,we analyzed CuO NPs by XRD without finding Cu₂O NPs.We proved that Cu₂O NPs within the cell are converted directly from the CuO NPs that is absorbed into the cell whereas non-extracellular or intracellular Cu²⁺is reduced.?6?Previous studies have attributed the damage of plant cells caused by CuO NPs to oxidative stress,but it is not clear to the reason and site of the production of reactive oxygen specie?ROS?in the cells by CuO NPs-treatment.We proved that the accumulation of ROS caused by the CuO NPs was not due to an inhibition of the ROS scavenging capacity but due to aggravation of the production of ROS.In addition,we observed that ROS were produced in thylokoids after CuO NPs treatment,which coincided with our observation that CuO NPs mainly damaged photosynthetic apparatus of algae.The accumulation of ROS affected the normal turnover of PSII and resulted in the peroxidation of membrane lipids and caused the destruction of membrane structure,which exacerbated the destruction of photosynthetic apparatus.In summary,we demonstrated that CuO NPs are highly toxic to algae,and the prokaryotic algae are more sensitive to the toxicity of CuO NPS than eukaryotic algae,and the photosynthetic apparatus of algae are more sensitive to the toxicity of CuO NPs than the respiratory apparatus,and CuO NPs have different damage sites in the photosynthetic apparatus between eukaryotic algae and prokaryotic algae.CuO NPs damage photosynthetic apparatus restulting in over production of ROS to cause oxidative stress on algae.