The Influence Of Humic Acids On Tetracycline On The Growth Of Coelastrella Sp. And Its Mechanisms

Tetracycline antibiotics have been widely used in humans and animals since their discovery in the 1940 s,and their effective antibacterial effect makes them widely used in livestock production for disease control and sub-therapeutic feeds to improve feed efficiency,growth rate and overall health situation.However,most of the tetracycline administered to animals are partially metabolized,and most of the tetracycline used for animal feeding and disease treatment are excreted through feces and urine in the form of parent compounds or biologically active metabolites.The release of antibiotics into the aquatic environment can cause adverse effects on the organism.Dissolved organic matters(DOM)is a heterogeneous mixture of microorganisms and cells in aquatic plants that autolyze or metabolize.DOM is ubiquitous in the water environment,exists at the level of milligrams per liter,and is rich in organic functional groups with high active sites(for example,carboxyl,phenol,hydroxyl and amide).Therefore,DOM can effectively interact with antibiotics through complexation,cation bridging,adsorption,hydrogen bonding,and cation exchange.Humic acids is an important active component of DOM and can be found in a variety of environments,including sediment,soil,groundwater,and surface water.In this article,we studied the impact of humic acids to the growth and physiological and biochemical properties indicators of microalgae Coelastrella sp.in BG-11 medium under different concentrations of tetracycline(0,0.5,1.0,2.0,5.0 and 10.0 mg/L)and discussed its underlying mechanism.The result shows:(1)Tetracycline obviously affects the growth of Coelastrella sp.and the synthesis of chlorophyll a and protein,which is manifested as dose-response relationship,stimulation at low concentration(<2mg/L),inhibition at high concentration(> 2mg/L).(2)Under tetracycline stress,the contents of SOD,MDA,and GSH in Coelastrella sp.cells increased significantly,indicating that oxidative damage may be the mechanism that causes the toxicity of tetracycline to Coelastrella sp.Cells.(3)The presence of humic acids can reduce the toxicity caused by tetracycline to Coelastrella sp.,and significantly reduce the adverse effects of tetracycline on the growth,physiological and biochemical indicators and oxidative damage of Coelastrella sp..(4)The adsorption of humic acids on the outer membrane of microalgal cells and the formation of the humic acids-tetracycline complex hindered the diffusion of tetracycline into microalgal cells,which resulted in a decrease in the concentration of bioavailable tetracycline forms.This may be the mechanism by which humic acids affects the bioavailability of tetracycline.These findings help to better understand the environmental risks of antibiotics in eutrophic water.