Cylindrospermopsin Production At Different Phosphorus Levels And Its Allelopathic Effects On Three Phytoplankton Species

There are reports that cylindrospermopsin(CYN)as allelochemicals induced with sympatric phytoplankton produce extracellular alkaline phosphatase(ALP),promote the increase of inorganic phosphorus in water,which is conducive to cylindrospermopsin-producing cyanobacteria(CYN-producers)to obtain phosphorus in low-phosphorus environment.Thus,the production of CYN may be one of the competitive strategies of CYN-producers to obtain phosphorus nutrition.In order to verify the above hypothesis,this paper observed the growth and CYN production characteristics of toxogenic C.raciborskii at different phosphorus concentrations,and further experiments the effects of CYN on the growth and ALP activity of cyanobacteria Microcystis aeruginosa,green algae Chlorella pyrenoidesa and Scenedesmus bijuba by toxin addition experiments at high/low phosphorus concentrations.The results of this paper are helpful to elucidate the ecological function of CYN.The main results are as follows:1.Ten strains of C.raciborskii were isolated from Qiandanghu Lake,foshan city,guangdong province,and there were significant differences in their growth and morphology(LSD,P < 0.05).PCR detection of CYN toxin synthase gene and toxin LC-MS/MS analysis showed that only QDH7 was the producing strain,and the main toxin isomer produced was deoxy-CYN.2.The growth of toxigenic C.raciborskii QDH7 showed a significant positive correlation with phosphorus concentration(LSD,P<0.05),while the ALP activity and CYN yield showed opposite changes,as shown by low phosphorus>medium phosphorus>and high phosphorus(LSD,P<0.05).RT-q PCR of the toxigenic genes cyr A and cyr J showed that the expression of the two genes in the low phosphorus group was significantly higher than that in the high phosphorus group(LSD,P<0.05).RT-q PCR analysis of phosphorus transportation-related genes pst S,pit and alkaline phosphatase gene pho A showed that compared with the high-phosphorus group,the expression of the above three genes was significantly upregulated in the toxigenic C.raciborskii QDH7 in the medium/low phosphorus group(LSD,P<0.05).Therefore,C.raciborskii could up-regulate the expression of related genes under phosphorus restriction,thus increasing the production of CYN and the activity of ALP.3.Under high phosphorus conditions,the growth and ALP activity of Microcystis aeruginosa were not affected by the presence of CYN.However,the growth and ALP activity of Chlorella pyrenoidesa were not affected by the addition of CYN at low concentration,but were significantly increased under the treatment of CYN at medium/high concentration(LSD,P<0.05).In addition,the growth and ALP activity of Scenedesmus bijuba were significantly increased at three CYN concentrations(LSD,P<0.05).This indicated that under high phosphorus conditions,different CYN concentrations could have allelopathic effect on green algae,but had no significant effect on cyanobacteria.Under low phosphorus conditions,the growth of Microcystis aeruginosa was still unaffected by the presence of CYN,but the ALP activity in the medium/high toxin treatment group was significantly higher than that in the control group.RT-q PCR results showed that the expression of pst S,sph X and alkaline phosphatase gene pho X in Microcystis aeruginosa were significantly up-regulated at 48 h.However,the growth and ALP activity of Chlorella pyrenoidesa under low phosphorus were significantly increased under high CYN treatment(LSD,P<0.05).There was no significant difference in the growth of Scenedesmus bijuba between the CYN-treated group and the control group,but the ALP activity was significantly higher than that of the control group(LSD,P<0.05).This indicated that the addition of CYN under low phosphorus conditions could produce allelopathic effects on all three phytoplankton,among which Scenedesmus bijuba was the most sensitive,and low concentration of CYN could increase the activity of ALP.4.Under high/low phosphorus conditions,QDH7 crude extract had no effect on the growth and ALP activity of Scenedesmus bijuba.Indicating that crude extract dominated by deoxy-CYN from QDH7 had no allelopathic effect on Scenedesmus bijuba.