Carbon Mechanism On The Growth Of Marine Blooms Microalgae

Recent years,with the rapid development of the marine economy,marine phytoplankton blooms in coastal areas have become increasingly prominent,and pose numbers of adverse effects to marine environment.Therefore,based on the status quo,in order to prevent and manage microalgae blooms effectively.This study was the first attempt to investigate the effect of inorganic carbon restriction on the growth of microalgae during marine phytoplankton blooms.The study simulated the environment of blooms and further explored the mechanism of microalgae blooms.Nitzschia closterium,Chaetoceros gracilis,Skeletonema costatum,Alexandrium tamareme and Heterosigma akashiwo were selected as the target microalgae in this work,including 3 species of diatoms,1 species of dinoflagellate and 1 species of raphidophyceae.Experiments were implemented in the CO2-unrestricted and CO2-restricted systems(both include three nutritional groups),respectively.Therefore,CO2 was the only inorganic carbon used in this study.Meanwhile,this study conducted with nitrogen and phosphorus co-restricted on 'the growth 'of microalgae.The three nutritional groups were normal group,nitrogen deficiency group and phosphorus deficiency group.The microalgae density,chlorophyll concentration,stable isotope signatures(?13C and ?15N),fatty acids(FAs)profiles and compound-specific stable carbon' isotopic composition of FAs(?13CFAs)were measured.Additionally,five nutrients concentrations,carbonate system concentrations,pH and salinity of culture medium were also measured.In the same nutritional conditions,the microalgae density of N.closterium,A.tamareme and H.akashiwo under CO2-unrestricted system were higher than those under CO2-restricted system.Furthermore,the microalgae' density in nitrogen deficiency group was the lowest of three nutritional groups.The pH of culture medium under CO2-restricted was significantly higher than that of CO2-unrestricted.Additionally,the range of pH under CO2-restricted could reach to more than 2 pH units,which was significantly greater than that of CO2-unrestricted.This indicated that with the increase of the culture time,the inorganic carbon concentrations gradually decreased,and CO32-concentration gradually,especially CO2-restricted system.As a result,the pH of culture medium sharply increased.The chlorophyll a and c concentrations of N.closterium,A.tamareme and H.akashiwo under CO2-restricted were lower than those of CO2-unrestricted,besides chlorophyll a and c concentrations in nitrogen deficiency group were the lowest of three nutritional groups.This may because that inorganic carbon is the important raw material for photosynthesis,moreover nitrogen is also one of the basic elements of chlorophyll.The decreased of chlorophyll concentrations suggested that inorganic carbon and nitrogen restriction would significantly affect the synthesis of chlorophyll and reduce the pigment.The ?13C of N.closterium,H.akashiwo and A.tamareme under CO2-restricted were significantly greater than those of CO2-unrestricted.This demonstrated that with the decreased of inorganic carbon concentrations,microalgae had to increase the absorption of 13C,thus,made the ?13C greater and greater.The ?15N of N.closterium,C.gracilis,S.costatum and H.akashiwo gradually increased,and ?15N in nitrogen deficiency group was the significantly higher than that of normal and phosphorus deficiency groups.This showed that microalgae had to absorb more 15N when nitrogen deficiency,and led to a greater ?15N.The major FAs of N.closterium,C.gracilis and S.costatum were 16:0,16:1n-7,20:5n-3 and 22:6n-3.The major FAs of A.tamareme were 16:0,18:1n-9 and 22:6n-3.The major FAs of H.akashiwo were 12:0,16:0,16:1n-7 and 22:6n-3.In the same nutritional conditions,the FAs composition of the same microalgae was different under CO2-unrestricted and CO2-restricted systems.The ?13CFAs of N.closterium under CO2-unrestricted system first increased and then decreased,made a peak ?13CFAs around day 4.The ?13CFAs of A.tamareme under CO2-unrestricted first decreased and then increased,made a lowest ?13CFAs around day 6.The ?13CFAs of N.closterium,C.gracilis and H.akashiwo under CO2-restricted all rapidly increased during the exponential phase then to be stable in stationary phase,had peak ?13CFAs.The ?13CFAs of A.tamareme under CO2-restricted first decreasedand then increased,had lowest ?13CFAs.This may because the different types of enzymes that contained in A.tamareme.The ?13CFAs of N.closterium,A.tamareme and H.akashiwo under CO2-restricted were significantly higher than those of CO2-unrestricted.Indicated that more 13C was participate in the synthesis of FAs when inorganic carbon concentration less and less.The innovative results of this research:(1)The bulk ?13C and ?13CFAs of blooms microalgae under CO2-restricted system was more positive than that of CO2-unrestricted.Additionally,the bulk ?13C and ?13CFAs rapidly increased during the exponential phase and had the peak value.(2)The ?13CFAs of blooms microalgae was more obvious and sensitive than the bulk?13C.Thus,the ?13CFAs was more sensitive to the change of inorganic carbon concentrations.The rapidly increase of ?13CFAs indicates that the inorganic carbon concentrations has decreased in seawater with density of blooms microalgae increased.Consequently,the marine phytoplankton blooms may occur.(3)The occurrence time of peak ?13CFAs of blooms microalgae may be earlier than that of microalgae peak density.The time difference could be used to predict phytoplankton blooms.