The Spatial-temporal Distribution And Infection Characteristics Of Virioplankton In Xiangxi Bay Of The Three Gorges Reservoir

The spatial-temporal distribution and the infection characteristics of virioplankton in Xiangxi Bay of the Three Gorges Reservoir were studied,and results are as follows:1.The method modification for the estimation of viral induced hosts mortalityThe widely accepted method to determine viral induced host mortality is modified dilution approach method,and which is time-consuming and laborious.In this study,we tried to modify this modified dilution approach method to obtain a more convenient method.Two treatments were carried out.Lake water samples treated with method combining the virus reduction and oscillation approach were cultured to obtain the host growth rate without viral infection.The raw water samples(filtered to remove zooplankton)were cultured to obtain the host growth rate with viral infection.The difference between the two host growth rates is the viral induced host mortality.This method was tested with samples form 5 different freshwater lakes,comparing with the modified dilution approach method.Results showed that there was no significant difference in the host mortality measured by the two methods.Hence virus reduction and oscillation combining approach is a simple and feasible way to determine the viral-induced host mortality.2.The method modification for estimation of the lysogenic host inductivityThe viral re-infection was ignored in most on lysogenic inductivity tests.In order to explore the effects of viral re-infection and acquire a more suitable methods for lysogenic inductivity tests,4 host pretreatment methods were tried to eliminate the effects viral re-infection.The tests were carried out in six freshwater lake water samples.The results showed that most of the direct methods showed significant difference in the paired t test between the results of oscillating and non-oscillating culture(P<0.05).In the filtration membrane filtration method,only the phytoplankton of Xiaonan lake and the phytoplankton of Nanhu lake showed significant difference in their oscillation and unoscillating culture results(P<0.05).There was no significant difference between the direct method and the filter membrane filtration method under the oscillating culture.Filter membrane filtration and direct oscillation method can be used to determine the frequency of lysogenic cell in fresh water.3.The spatial-temporal distribution and infectivity of planktonic viruses in the Xiangxi River in the Three Gorges Reservoir.From March 2017 to January 2018,viral and host abundance were monitored monthly at 11 sampling sites were sampled at the end of each month in the Xiangxi and Yangtze Bay.The viral abundance in mainstream of Yangtze river was significantly lower than that of tributary(P<0.05).Virus abundance at each sampling site is mainly affected by host abundance.The viral lysis rate showed no significant difference between Xiangxi river and other known freshwater ecosystem.The viral induced bacteria lysis rate showed significant positive correlation with virus abundance and bacterial abundance(P<0.05).The frequency of lysogenic cell is lower than that of other known freshwater ecosystem.There viral induced bacteria lysis ratewas negativly correlated to the frequency of lysogenic bacterioplankton(P<0.05).These results suggested the viral infection showed an obvious regulation effect on bacterioplankton community in the reservoir.The dominant species in summer bloom succession are diatom-cyanobacteria,cyanobacteria-dinoflagellates and cyanobacteria-diatoms.Virus abundance,viral induced host lysis rate and the frequency of lysogenic cell were different in each bloom period.The viral abundance is mainly affected by phytoplankton species,dominant species of algae and the density of algal cells.The mutual transformation mechanism of virus to host lytic infection and lysogenic infection is the main reason for the difference of lysis rate and lysogenic rate in different water bloom period.There was a significant positive correlation between viral lysis rate of phytoplankton and the concentration of orthophosphate.The increasing concentration of orthophosphate may be the reason for the continuous bloom explosion.