Effect Of Humic Acid On The Toxicity Of Polystyrene Microplastics To Scenedesmus Obliquus

Seven microplastics of different sizes,charges and groups（n-plain-0.1μm,n-plain-0.5μm,n-plain-1μm,n-plain-2μm,n-NH₂-0.1μm,n-COOH-0.3μm and p-NH₂-0.1μm,"n"and"p"represent negative and positive charges,respectively）were selected to study the toxicity effect on scenedesmus obliquus.The growth inhibition rate,total chlorophyll content and maximum photochemical efficiency（Fv/Fm）of microalgae were monitored after exposure to different types of microplastic.In addition,the effect of humic acid（HA）which is widely presented in the water environment was also considered during the toxicity test.The main findings are as follows:（1）All kinds of microplastics showed inhibition on microalgae cell density and total chlorophyll content.As microplastics concentration increased,the growth rate and total chlorophyll content of microalgae cells decreased.Among them,n-plain-1μm had minimal toxicity,while p-NH₂-0.1μm had maximum toxicity.Besides,n-plain-2μm showed the greatest effect on the Fv/Fm value of microalgae.The EC50 of n-plain-1μm and p-NH₂-0.1μm after exposure to microalgae for 96 hours were 26.8 mg/L and 0.580mg/L,respectively.（2）The addition of HA significantly alleviated the growth inhibition of microalgae when microalgae was exposed to n-plain-0.1μm through the whole experimental period（1-144 hours）.However,the addition of HA had little effect on the growth rate of microalgae when microalgae was exposed to n-plain-2μm.（3）The addition of HA could reduce the effect of n-plain-0.1μm on photosynthesis of microalgae,but showed little effect on the other two microplastics.Additionally,n-plain-2μm had a significant effect on the photosynthesis chlorophyll fluorescence parameters of microalgae.After microalgae was exposed to n-plain-2μm for 24 hours,it had reduced the Fv/Fm value of microalgae by 39.5%,which indicated that the toxicity of2μm microplastics to microalgae was mainly by blocking light to affect the photosynthesis of microalgae.（4）Further research speculates that the detoxification effect of HA on microplastics was to form a protective layer on the surface of micro-plastics,thereby reducing the adsorption of microplastics on the surface of microalgae and alleviating the toxic effects.