Effects Of Oil Spill Pollution And Microalgal Allelopathy On Stable Carbon Isotope Fractionation Of Fatty Acids In Ulva Pertusa

In the marine ecological environment,marine algae is the main primary producer.Ulva pertusa is a principal macroalgae in the coastal intertidal zone.It is widely distributed in the area of coast and has abundant resources.Poor living conditions will have a certain impact on the growth of U.pertusa.In this thesis,two kinds of commonly adverse environment on U.pertusa were studied.The first one is oil spill,which is very common in the ocean.Marine oil spill accident occurs frequently,causing tens of thousands of oil to flow into the ocean every year.Petroleum is diverse and complex,and some petroleum hydrocarbons soluble in water can have toxic effects on marine life.After the oil spill occurs,the oil will continue to spread along with the ocean current,flow to the intertidal zone and accumulate,which will affect the algae.The second one is the symbiosis of marine algae.It is inevitable in the marine environment,and different kinds of algae will interact with each other.However,most studies focus on the influence of microalgae,while few studies on the influence of macroalgae.Therefore,this study is the first time to combine fatty acid content and stable carbon isotope composition of fatty acid(δ¹³C_FAs)to explore the mechanism of fatty acid metabolism in U.pertusa under pollution stress,as well as the mechanism of carbon source absorption and utilization.（1）Due to the different types of petroleum have different effects on algae,this study elected three kinds of petroleum that are commonly used in marine transportation to explore the toxicity mechanism of U.pertusa.The toxic effects of water accommodated fraction（WAF）dissolved and dispersed in water with different concentrations of 180#fuel oil,0#diesel oil and Russian crude oil on U.pertusa were analyzed.The results showed that with the increase of WAF concentration,low concentration of WAF promoted the growth of U.pertusa,while high concentration of WAF inhibited the growth of U.pertusa.Although the growth of U.pertusa was inhibited under different concentrations of WAF stress,the content of chlorophyll a would continue to increase.Therefore,the measurement of chlorophyll a could not fully reflect the response of algae to low-concentration oil exposure.In addition,further determination of carbon and nitrogen stable isotope composition of U.pertusa were done.Theδ¹³C value of U.pertusa was found to be significantly higher than that of the control group under low-concentration WAF stress,and theδ¹³C value of U.pertusa under high-concentration WAF stress was significantly lower than that of the control group.There was a high correlation between the growth rate of U.pertusa andδ¹³C,andδ¹³C could reflect the growth state of algae more timely than chlorophyll a.At the same time,the content of eight fatty acids and stable carbon isotopic compositions of fatty acids in U.pertusa were further determined.C16:0 has the highest content as the precursor of other fatty acids.The content of unsaturated fatty acids C16:1,C18:1 and C18:3n-6 increased first and then decreased with the increase of WAF concentration,showing a significant change trend.However,the overall trend ofδ¹³C_FAs in U.pertusa was to increase first and then decrease,and the trend was more obvious than that of the fatty acid content.When the concentration of WAF was low,the growth rate of U.pertusa became faster,and more ¹³C would be absorbed after the consumption of ¹²C,resulting in an increase ofδ¹³C_FAs.When the concentration of WAF increased,the toxicity of petroleum destroyed the normal physiological function of cells and reduced theδ¹³C_FAs.δ¹³C_FAs can better reflect the dynamic metabolic process of algae cells under stress.（2）The effect of two marine microalgae（Alexandrium tamarense and Heterosigma akashiwo）on U.pertusa when they were cultivated separately with U.pertusa were analyzed.The result showed that both A.tamarense and H.akashiwo could have allelopathic effect on U.pertusa,and the growth of U.pertusa was to be inhibited.The longer the co-cultivation time,the more obvious the inhibition effect would be.The MDA content of U.pertusa under the co-cultivation conditiosn was significantly higher than that of the control group,which indicated that the cell membrane of U.pertusa was damaged.The fatty acid and stable carbon isotope compositions in U.pertusa were further determined.After an experimental period of 8 days,the content of fatty acids in U.pertusa under the stress of two kinds of microalgae decreased compared with the control group,especially the content of C16:0,C18:2,C18:3n-3 and C18:3n-6.Among these four kinds of fatty acid,three kinds of unsaturated fatty acids were included.When algae was subjected to external stress,it responded to the poor living environment by regulating the synthesis of unsaturated fatty acids,especially C18 polyunsaturated fatty acids,so unsaturated fatty acid would be affected.With the increase of time(exceptδ¹³C_18:0 andδ¹³C_18:1),δ¹³C_FAsin co-cultivation group were all negative compared to the control group.The co-cultivation group of U.Pertusa in the growth process requires less carbon source than the control group,and the effect of stress on algae leads to a decrease in carbon fixation of photosynthesis.Principal component analysis based on fatty acid can only distinguish algae under different cultivation time,while principal component analysis based on stable carbon isotope of algae fatty acid can effectively distinguish between the control group and co-cultivation group to determine whether algae are threatened by external adverse environment.In conclusion,the stable carbon isotope composition of fatty acids in U.pertusa preferred to the fatty acid content in response to adverse living environment.Firstly,algae absorb different carbon sources to synthesize fatty acid.When algae are stressed,the growth and photosynthesis of them will be affected,resulting in different degrees of carbon fractionation of metabolites.Therefore,δ¹³C_FAs can be used as a new indicator to determine the response of algae to adverse living environment.