Effect Of Norfloxacin On The Apparent Competition Between Two Different Phytoplankton

Antibiotics are widely used in various fields because of their ability to effectively inhibit or kill pathogenic microorganisms.In addition to human medical treatment,the requirements for antibiotics in livestock and aquaculture are also enormous.With the abuse of antibiotics,however,many hidden troubles gradually revealed,the pollution caused by antibiotics in the natural environment has aroused great concern all over the world.Due to the stable chemical property of antibiotics,they will remain in water for a period of time after entering the water environment,and there is the possibility of continuous enrichment,that means the water environment has become the largest destination for antibiotics.There have been many studies reported that the toxic effect of antibiotics on various aquatic creatures,and the focus is mainly on the response of individual or population to antibiotic contaminants.But there are few studies on the response of relationship among different aquatic organisms to antibiotics,such as interspecific competition,which is essential to predict changes in the structure and function of aquatic ecosystems.Therefore,in this study we select Scenedesmus quadricauda,Scenedesmus obliquus and Chlorella vulgaris to constitute interspecific competition system,and add to grazer Daphnia magna to constitute apparent competition system based on that,explore different concentration of norfloxacin(0,0.5,2 and 8 mg/L)impact on the relationship between two different phytoplankton in these systems.The results showed that:(1)In the absence of D.magna,norfloxacin had no significant effect(P>0.05)on growth rate and cell morphology of the three phytoplankton.In S.quadricauda-C.vulgaris and S.obliquus-C.vulgaris systems,norfloxacin had significant effect(P<0.05)on the competition between two species of phytoplankton.Norfloxacin is beneficial to increase the competitive advantage of Scenedesmus,but the advantage decreases with the change of treatment time,mainly dominated by C.vulgaris in system.(2)When the density of D.magna was constant,both S.quadricauda-C.vulgarisD.magna and S.obliquus-C.vulgaris-D.magna systems,the proportion of three-to fourcelled colonies of Scenedesmus decreased with the increase of norfloxacin concentration in the short-term apparent competition experiment.At the end of the experiment,the proportion of S.obliquus cell colonies in the control group and the 8mg/L treatment group was 82.72% and 64.4% respectively,with significant difference(P < 0.05).The proportion of S.quadricauda cell colonies was 70.32% and 62.02% respectively,with significant difference(P<0.05).It is an important defensive strategy to form cell colonies for Scenedesmus.A lower proportion means that Scenedesmus are more likely to be ingested by D.magna.Compared with C.vulgaris,the competition ability of Scenedesmus is significantly decreased.The algal density ratio of S.obliquus to C.vulgaris in the control group and the 8mg/L treatment group was 2.05 and 0.99 respectively,with significant difference(P < 0.05).The algal density ratio of S.quadricauda to C.vulgaris was 2.13 and 1.14 respectively,with significant difference(P<0.05).Results show that norfloxacin can change the competitive relationship of two species of algae,makes the Scenedesmus gradually lose advantage position in the high concentration of norfloxacin,whereas C.vulgaris transform from disadvantage to slight advantage.(3)When the density of D.magna was non-constant,the morphological changes of S.obliquus was not uniform with the increase of norfloxacin concentration in the shortterm apparent competition experiment.At the same time,the competition between two species of phytoplankton also showed nonuniformly changes.Although there is no obvious pattern of change,it can also demonstrate that norfloxacin can play a role in phytoplankton competition by affecting the morphological changes of phytoplankton in the presence of zooplankton.These results reflect the effect of norfloxacin on the interspecific competition among different phytoplankton.In order to provide some reference for the comprehensive evaluation of the ecological consequences of antibiotic pollutants.And it can also give us some inspirations to pay more attention to the changes of the relationship between aquatic organisms.