Inorganic Nitrogen And Phosphorus Alleviates Chromium Toxicity In The Freshwater Alga Chlorella Vulgaris

Heavy metal pollution has been a global problem.Different from other organic pollutants,Heavy metal compounds with biodegradable,easy enrichment to other organisms through the food chain and endanger human health.Chromium(Cr)is one of the most toxic metal compounds to aquatic organism.Algae are the primary producers of aquatic ecosystems and the cornerstone of aquatic ecosystem structure stability which is commonly used in the assessment of water pollution.Nitrogen(N)and phosphorus(P)affect the external stress tolerability of the algae through influence its growth status.In this theory,We selected a freshwater alga Chlorella vulgaris as experimental objects to study the effects of chromium at different concentrations of N and P on the growth of Chlorella,through the ion absorption,photosynthesis and the cell structure,thus clarify the mechanism of N and P changing C.vulgaris tolerability to chromium.The results showed that Cr affected the equilibrium of ion absorption,and inhibited C.vulgaris growth.The growth inhibitory rate was 50.5%after 96h of exposure to 100 μM Cr in low-P(10 μM)medium.The toxicity of Cr can be reduced by improving P concentration.The growth inhibitory rate reduced to 23.5%in high-P(1000 μM)medium.The effect of N availability on Cr toxicity was similar with P.The concentration of N and P can affect the ion absorption of C.vulgaris after Cr exposure.Low N and P could induce C.vulgaris to absorb more Cr ion to cause more severe cellular injure.Cr can destroy the photosynthesis system of C.vulgaris by affecting the transcript abundance of psaB、psbA、rbcL and decrease photosynthetic pigment content.The transcription of rbcL was only 42%and 30%after 96h of exposure to 100μM Cr in the low N and P(0.5 mM)of the control.High concentration of N and P could alleviate Cr toxicity on the transcripts of photosynthetic gene.Cr also affected the morphology and substructure of C.vulgaris.Cr treatment increased the algal cell volume.From electron microscopy results,we found that the lamellar structure in the Cr-treated cells in at low P became smeared,and the pyrenoids were destroyed,the number of starch granules were increased in the Cr-treated cells in at low N.improving the concentration of N and P can alleviate chromium to chlorella C.vulgaris.These results suggest that Cr inhibited C.vulgaris growth in a concentration-and time-dependent manner,and its inhibitory effect was related to the P and N concentration.This modulating effect of N and P on intracellular Cr toxicity could be due to several factors,such as decrease Cr absorption,increase the absorption of beneficial ions,increase transcription of a photosynthetic-related gene and protection of subcellular structure.