Study On The Effect Of Nitrogen Forms And Concentration On The Growth Of Cylindrospermopsis Raciborskii

Cylindrospermopsis raciborskii is tropical species,and it has been dominating and forming blooms in several reservoirs in South China.This species has become one of major threat to the water supply in the area.Cylindrospermopsis raciborskii shows a widely flexibility on phosphorus utilization,hence,the species may get competitive advantages from its absorption and utilization of nitrogen.C.raciborskii N8 was isolated from Zhenhai Reservoir and cultured under different nitrogen forms,i.e.ammonium nitrogen,nitrate and urea.We also explored its adaptation strategies under low-nitrogen condition and attempted to find the threshold of nitrogen fixation.Further,we studied the response of cellular N:P ratio of C.raciborskii N8 to different N:P ratios of culture solution.The main results are as follows:1.In continuous culture experiments,high and low dilution rates were not suitable for the growth of C.raciborskii N8.The optimal dilution rates ranged from 0.05 d-1 to 0.08 d-1.2.Under cultured with different nitrogen forms,the specific growth rates of C.raciborskii N8 significantly varied in batch cultures(P<0.05),in which the specific growth rate was highest in ammonium nitrogen treatment,followed by nitrate and urea.The trend were similar to the groups of continuous cultures,but the variation was not significant(P>0.05).In batch cultures groups,trichome length of C.raciborskii N8 was longest in ammonium nitrogen treatment,followed by nitrate and urea treatment,while long trichomes were detected in continuous cultures.Under continuous nitrogen supply conditions,Cylindrospermopsis raciborskii showed a weak selectivity on the forms of nitrogen.3.Under treated with low concentration of nitrogen,the growth of C.raciborskii N8 did not reach a steady state in organic nitrogen groups and the biomass decreased drastically and the specific growth rates were significantly lower than those in inorganic nitrogen groups(P<0.05).The proportions of heterocyst ranged from 2.0%to 3.0%when the concentration of nitrogen was below 0.25 mg/L in continuous cultures,while no heterocysts were found in groups when the concentration of nitrogen was above 0.361 mg/L.In batch cultures,heterocysts were observed in most of treatment groups(nitrogen concentration:0.125mg/L?0.903mg/L).We speculated that the threshold concentration of nitrogen for C.raciborskii N8 fixing N2 might range from 0.250 mg/L to 0.361 mg/L.In high-phosphorus concentration treatment groups,the proportion of heterocyst ranged from 12%to 24%and increased significantly compared with the low-phosphorus concentration treatment groups(P<0.05).It was indicated that phosphorus could promote N2 fixation of C.raciborskii N8.4.The specific growth rates of C.raciborskii N8 were significantly increased with rise of N:P ratios(P<0.05).The N:P ratios of C.raciborskii N8 ranged from 17.0 to 29.2 and were higher than the Redfield ratio(16:1),and the highest N:P ratio was observed in ammonium treatment and lowest in urea treatment,but the variation is not significant.The cellular N:P ratios of C.raciborskii N8 increased with rise of N:P ratios in culture solution.It was indicated high concentration of nitrogen would promote the growth of C.raciborskii N8,while C.raciborskii N8 could maintain the cellular N:P ratio at a relative stable condition,and this flexible stoichiometric character might provide the species more competitively advantage in natural water body..