Toxic Effects Of Tetracycline And Metronidazole On Chlorella Pyrenoidosa

In recent years,antibiotics have become an emerging organic pollutant that has received widespread attention,but the degree of pollution in China’s water environment has also intensified due to the existence of abuse.Tetracycline antibiotics are broad-spectrum antibiotics represented by Tetracycline（TC）and produced or semi-synthesized by actinomycetes.Nitroimidazole antibiotics are a class of inexpensive antibiotics with strong anti-anaerobic bacteria,and Metronidazole（MTZ）is one of the first nitroimidazoles discovered.TC and MTZ,as commonly used drugs in the clinical treatment of periodontal disease,are often frequently detected in medical wastewater.As primary producers of aquatic ecosystems,algae are sensitive to pollutants,so when large amounts of antibiotics are released into the water,they can lead to a mixture of multiple antibiotics,which may pose a greater risk to algae growth.There has been much research on the effects of a single antibiotic on aquatic organisms,but limited research on the effects of two antibiotics on aquatic communities.Therefore,it is important to study the combined toxic effects of multiple antibiotics on algae.In this paper,TC and MTZ were taken as the research objects,and Chlorella pyrenoidosa（C.pyrenoidosa）was used as the indicator organism to determine the toxicity mechanism by studying the effects of TC and MTZ on the growth,cell morphology,extracellular polymer and oxidative stress of chlorella under single and combined action.Then,the Toxic Unit（TU）and the Addition Index（AI）were used to jointly analyze the types of toxic interactions between different components of the TC and MTZ mixtures.The results of the thesis study are as follows:（1）When TC was single-acting,each treatment group showed a"time-dose-effect"relationship on the amount of algal cells of C.pyrenoidosa,that is,with the increase of TC concentration,the greater its inhibition rate on C.pyrenoidosa biomass.At high concentrations（15 mg/L）,it seriously destroys the cell morphology of C.pyrenoidosa,affects the functional groups on the cell surface,and even inhibits the synthesis of humic acids,but 1-2 mg/L TC will promote the production of soluble protein and soluble sugars.The content of chlorophyll a increased under 1 mg/L and 2 mg/L conditions,but decreased under other conditions.Reactive oxygen species（ROS）,malondialdehyde content（MDA）and catalase activity（CAT）increased with the increase of concentration,and all reached the peak at 15 mg/L.Superoxide dismutase activity（SOD）first increased and then decreased with the increase of concentration,reaching the maximum of 17.3 U/mgprot at10 mg/L.Its effect on the permeability of algal cell membrane also increased with the increase of concentration,indicating that the higher the TC concentration,the more severe the damage to the cell membrane.（2）When MTZ has a single effect,10 mg/L has a promoting effect on the growth of algal cells.The MTZ of 50 mg/L agglomerates algal cells,and at the same time leads to redshift of some peaks of algal cells,and changes in C=O bonds,thereby inhibit algal cell protein synthesis.At 10-30 mg/L,soluble protein and soluble sugar contents increased.At50 mg/L,it induces the formation of humic acids.The content of chlorophyll a decreases with increasing concentration;ROS,MDA content,SOD and CAT activities were elevated and peaked at 50 mg/L.The cell membrane permeability of C.pyrenoidosa also increased with the increase of concentration.At 50 mg/L,the cell membrane permeability was 2.5times that of the control group,and the damage was the most severe.（3）The 96 h-EC₅₀ of TC and MTZ were 8.720 mg/L and 45.125 mg/L,respectively,and TC was much more toxic to C.pyrenoidosa than MTZ.When TC and MTZ combined with a toxicity ratio of 1:1 for 7 days,TU and AI were jointly evaluated for toxicity,and the combined toxicity effect was presented as a synergistic effect.（4）When TC and MTZ combined action,the growth of algal cells in the early stage of the experiment conformed to the law of"low promotion,high inhibition";When the mixed system was 3T（TC7.5 mg/L,MTZ39 mg/L）,the surface of algal cells dried up and the functional groups were seriously affected.When the 3T system is mixed,the synthesis of soluble proteins and soluble sugars is blocked,and algal cells begin to synthesize fulvicic acid and humic acids.When the mixed system was 0.04T（TC0.1 mg/L,MTZ0.52 mg/L）and 0.4T（TC1 mg/L,MTZ5.2 mg/L）,the synthesis of chlorophyll a was promoted.ROS,MDA content and CAT activity increased significantly with the increase of mixing concentration,and SOD activity first increased and then decreased,reaching a maximum when the mixed system was 1T（TC2.5 mg/L,MTZ13 mg/L）,which was 2.2 times that of the control group.When the mixed system was 1-3T,the permeability of algal cell membrane increased with the increase of the mixing concentration,and 3T was 5.8 times that of the control group.