Seasonal Dynamics And Potential Regulating Factors Of Cylindrospermopsis Raciborskii In A Large Tropical Reservoir:Zhenhai Reservoir,Guangdong Province

It is well known that Cylindrospermopsis raciborskii is increasingly spreading into different aquatic ecostystems and can produce a variety of toxins,but little is known about its bloom forming mechanism.To better understand the factors that may lead to the successful dominance and bloom formation of C.raciborskii,seasonal dynamics of the phytoplankton species composition and chemical-physico factors were monitored monthly during the period from November 2014 to December 2016 in Zhenhai reservoir in Guangdong province.In order to clarify the strategies employed by C.raciborskii to utilise phosphorus,growth experiments were performed under different inorganic phosphorus concentrations（7.13mg/L,0.64mg/L,0.03mg/L）and different phosphorus forms(Dipotassium hydrogen phosphate（K₂HPO₄）,Sodium pyrophosphate（K₄P₂O₇）,Sodium polyphosphate(K₅P₃O₁₀),D-glucose-6-phosphate（G-6-P）,Adenosine triphosphate（ATP）,Cyclic adenosine monophosphate（cAMP）)with C.raciborskii strains from Zhenhai Reservoir,in the meantime alkaline phosphatase activity was monitored.Our results were shown as follows:1.During the sampling period,water temperature was higher and ranged from 14.3℃-32.8℃,with a mean value of 24.9℃;the reservoir had a relatively low Secchi depth and the average value of the Zeu/Zm was 0.36,which indicated phytoplankton population in the reservoir always grown in light-limited conditions;the total nitrogen（TN）and total phosphorus（TP）concentrations varied from 0.76mg/L^-1.8mg/L and 0.013mg/L-0.086mg/L,respectively.Compared with Redfield ratio（N/P=16）,TN/TP molar ratio（37-243）were relatively higher,which indicated that Zhenhai Reservoir can be characterized as phosphorus limitation.2.C.raciborskii dominated during most of sampling period and its biomass showed an obviously seasonal pattern,ranging from 0mg/L^-15.5mg/L（middle of Zhenhai reservoir）and 0mg/L^-19.7mg/L（outlet of Zhenhai reservoir）.C.raciborskii biomass was relatively higher from March to June 2015 and from September to October 2016,with the highest level in June 2015（middle of Zhenhai reservoir for 15.5mg/L;outlet of Zhenhai reservoir for 19.7mg/L）.We found C.raciborskii almost disappeared from February to June 2016,with very low biomass（0mg/L-0.007mg/L）.The trichome length of this cyanobacterium was significantly negatively correlated with water temperature（R²=0.611,n=21,P<0.05（middle of Zhenhai reservoir）;R²=0.733,n=21,P<0.05（outlet of Zhenhai reservoir））,and longer trichomes always occured in the spring and winter and shorter trichomes dominated in the other warm seasons.Multiple continuously factors analysis indicated that the biomass of C.raciborskii was significant-negatively correlated with Zeu/Zm and NO_3--N concentrations,but significant-positively with TP concentrations（R²=0.698,n=26,P<0.01（middle of Zhenhai reservoir）;R²=0.674,n=25,P<0.01（outlet of Zhenhai reservoir））.Further variance decomposition analysis showed that these three factors played an important role in the seasonal variation of C.raciborskii biomass and accounted for 61% of the total variance,but there is no significant difference between their contribution.3.The laboratory experiment showed that the growth rates of C.raciborskii decreased markedly with declined phosphorus concentrations.C.raciborskii can utilize three dissolved inorganic phosphate（DIP）and three dissolved organic phosphorus（DOP）,and the growth rates were significantly higher in the medium containing DIP than DOP.The growth rate was highest in K₂HPO₄-grown culture and lowest in K₅P₃O₁₀-grown culture,which indicated that orthophosphate（K₂HPO₄）may be the optimal DIP for C.raciborskii growth than in poly phosphate(K₄P₂O₇ and K₅P₃O₁₀).Compared with ATP,C.raciborskii grew relatively faster when supplied with G-6-P and cAMP.Higher phosphorus concentration and DIP can promote the growth of C.raciborskii,but lower alkaline phosphatase activity was observed.Alkaline phosphatase activity of C.raciborskii was markedly increased under low phosphorus and only DOP available conditions.These results indicated that C.raciborskii can utilize different phosphorus forms and alkaline phosphatase is necessary for DOP utilization.When exposed to low phosphorus environment,C.raciborskii can use different DOP to support its growth by increasing the activity by alkaline phosphatase.