Study On The Response Mechanism Of Typical Harmful Microalgae To Different Nitrogen Sources

In this study,Phaeocystis globosa and Karenia mikimotoi were studied using indoor culture methods.The nutrient utilization and physiological and ecological responses of the two nitrogen starved algal in different nitrogen forms?NaNO3,NH4Cl,and urea?were investigated.Both microalgae could use the three kinds of nitrogen sources to maintain their growth.Under the same initial nitrogen source concentration,the P.globosa grown with urea has a significantly higher cell density and chlorophyll-a than grown with NH₄Cl or NaNO₃?P<0.01?,indicating that urea is a more suitable nitrogen source for the growth of P.globosa.The results of Karenia mikimotoi grown with different nitrogen sources showed that there was no significant difference between the growth curves of the experimental groups?P>0.05?,indicating that K.mikimotoi has no significant difference in the growth under these three nitrogen sources.The results of nitrogen utilization showed that the uptake rate of each nitrogen source by the nitrogen-starved microalgae was the highest at the initial stage,and the uptake rate decreased with the decrease of the nitrogen concentration in the culture fluid.There was a positive correlation between the source concentrations,indicating that the rate of nitrogen uptake by the starved algae cells was regulated by the nitrogen source concentration in their growth ambient;in addition,the two microalgae cultured in different nitrogen sources had a lower specific growth rates at the initial stage,this may be due to the fact that the algal cells were starved at the initial,after the addition of the nitrogen source,most of the absorbed nitrogen sources were used to fill the deficits of the nitrogen pool in the cells.Therefore,the two kinds of microalgae has a higher nitrogen absorption rate but lower specific growth rate at the initial stage.The results of physio-ecological responses of two microalgae to different nitrogen sources showed that the content of soluble protein in the cells of P.globosa and K.mikinotoi was extremely negatively correlated with the concentration of nitrogen in the culture medium?P<0.01?,the content of soluble protein in the algal cells increased at the initial,and decreased when the ambient nitrogen almost exhausted,indicating that part of the nitrogen source absorbed by the algae cells was assimilated into protein and stored in the cells.Internally,it can be used to maintain cell growth when the outside nitrogen source was depleted.The nitrate reductase and urease activities of P.globosa were regulated by nitrogen sources and expressed maximum activity when nitrate and urea were used as nitrogen sources respectively.Moreover,the urease activity of the cells was significantly higher than that of nitrate reductase.?P<0.01?.This may be the main reason for the higher algal density of P.globosa grown with urea.The urease activity of K.mikimotoi expressed the maximum activity when urea was used as the nitrogen source,and the urease activity of the alga cells in the high concentration urea supplement group was stronger than that in the low concentration group;further analysis revealed that the glutamine synthesis of the algae and algae cells.The activity of enzymes?GS?was significantly positively correlated with the concentration of nitrogen sources in each culture medium?P<0.01?.The GS activity decreased with the decrease of the nitrogen concentration in the culture medium,and the GS activity in the cells of each group were almost similar.This result indicated that the rate of conversion of nitrate and urea to ammonium at a similar rate in K.mikimotoi.This may be the main reason for the insignificant difference in the growth of K.mikimotoi under three different nitrogen sources.The results of this study showed that the P.globose,which in nitrogen starvation state,could better use urea to maintain its growth.However,there is no significant difference in the algae density in the nitrogen starved state of K.mikimotoi under three nitrogen sources.This is mainly related to the enzymatic activity of nitrogen-assisted absorption of related enzymes in the algae cells of the two microalgae and the differences in the use of different nitrogen sources by the algae cells.