| Traditionally, dissolved organic nitrogen (DON) is considered to be composed of insoluble compounds and can not be utilized by phytoplankton.But in recent years,more and more studies showed that small molecular DON may also be the direct nitrogen sources for some harmful algae, stimulating their proliferation and playing important role in the formation of harmful algal blooms. In this study, a representative HAB species Phaeocystis globosa was chosen as material, and a series of different ratios of organic nitrogen were established in laboratory to examine the effects of organic nitrogen concentration on urease activity, photosynthesis and cell toxin production of harmful species. Besides, the regulation of main environmental factors, light intensity and temperature for utilization of P. globosa for organic nitrogen were also discussed, respectively. The main results were as follows:Under different light intensity the organic nitrogen ratio significantly influenced the growth of P.globosa. The cell density and specific growth rate of 20% DON:TDN ratio group were significantly higher than other treatments with high irradiance (120μmol·m2·s-1),whereas specific growth rate increased significantly with the rise of DON:TDN ratio with low irradiance (10μmol·m-2·s-1). Urease activity of P. globosa was not only effluenced by DON:TDN ratio but also regulated by light intensity. At high light intensity, urease activity increased with DON:TDN ratio. However, there was no significant correlation between urease activity and DON:TDN ratio at low irradiance, while urease activity were significant higher.. As to the Fv/Fm value, low light groups were significantly higher than those of high light. And DON:TDN ratio has little effect on Fv/Fm value.Under different temperatures, the experiment result of organic nitrogen ratios on the growth of P.globosa was followed as:the specific growth rate of 20% ratio organic nitrogen group was the highest. With the temperature rised, the gap between 20% ratio organic nitrogen group and the other groups gradually decreased. Under the temperature established artificially, the urease activity of containing organic nitrogen experimental groups were higher than nitrate group, however,In different organic nitrogen ratio treatment groups, the urease activity of 100% urea group is highest In addition,the temperature significantly influenced the growth of P.globosa, specific growth rate and maximal photochemical efficiency were positively correlated with temperatures. Nitrogen sources transfer showed significant effect on urease activity. The urease activity of urea pre-culture group was the highest, followed by ammonia, and alanine was the lowest.which exhibited maximal photochemical efficiency. The nitrogen transfer did not significantly influenced the cell density.Hemolytic toxin was a secondary metabolite during the growth of P. globosa. The cell toxin production showed rising trend with DON:TDN ratio. The cell toxin production was significantly higher at 30℃. Besides, the cell toxin production at highlight intensity was also higher than low light intensity. Nitrogen transfer also had significant effects on toxin production of P.globosa. The urea to urea treatments showed the strongest toxicity, while the ammonium to urea group was the lowest.The nutrient addition experiments showed that organic nitrogen could significantly stimulate the growth of phytoplankton, the biomass of organic nitrogen groups were significantly higher than nitrate group. And DON:TDN ratio also had significant effects on phytoplankton biomass:phytoplankton biomass showed rising trend with DON:TDN ratio.
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