Zooplankton Toxigenic Microcystis And Microcystin Toxin Production

It is well known that one of the most harmful effects of cyanobacterial blooms is the production of cyanotoxins. Recently the interactions between zooplankton and Microcystis species have been extensively concened. Some studies have revealed that zooplankton can cause the change of toxin production in Microcystis. In this paper the abundance of the potential microcystin-producing Microcystis (mcyB-containing Microcystis) in a small pool was monitored using the Real-time PCR, and the relationships between the potential microcystin-producing Microcystis and zooplankton or nutrients were studied. Meanwhile, targeting on the transcriptions of genes responsible for microcystin synthesis under the stress of Daphnia, the response of Microcystis aeruginosa PCC7806 via the regulation of microcystin production toward the different stages of Daphnia were studied using the RT-qPCR.The results showed that the mcyB-containing Microcystis appeared in the studied area all the year and the concentrations of the mcyB-containing Microcystis ranged from 2.41×102 to 3.20×105 cells/ml. The percentage of mcyB-containing Microcystis cells number in total Microcystis was lower in winter and highest up to 39.73% in spring but went down to the lowest as 1.53% after the Microsystis bloom. It was shown that the percentage of mcyB-containing Microcystis in total Microcystis had a positive linear relationship with the number of cladocerans, implying that the stress of cladocerans can induce the presence of toxic Microcystis in the bloom.The RT-qPCR was firstly used to study the change of transcription regulation of genes responsible for microcystin synthesis in Microcystis aeruginosa PCC7806 under the stress of Daphnia. The result showed that the grazing stress from D. manga could increase transcript levels of mcyB, mcyD and mcyH as the inducible defence of M. aeruginosa PCC7806. The change of microcystin content measured with ELISA agreed with the change of transcripts levels of mcy, therefore the RT-qPCR approach can suitably represent the change of microcystin content from genetic view.D. manga with five days old could cause the increased transcripts levels of mcyB, mcyD and mcyH of M. aeruginosa PCC7806 with OD680 as 0.032, but could not cause the similar change of M. aeruginosa PCC7806 at 0.094 of OD680. The change of microcystin content measured with ELISA also agreed with that of transcript levels of mcy. The increased transcripts levels of mcy of M. aeruginosa PCC7806 could not be observed by using the neonate D. magna with one day old, but could be caused by the five days old juvenile and ten days old adult D. magna grazing, and the change of microcystin content measured with ELISA also agreed with the change of transcripts levels of mcy. The above results suggested that the increased transcripts levels of mcy of M. aeruginosa PCC7806 caused by the grazing from D. magna were related to both Microcystis concentration and the age of D. magna.This result implied that microcystin produced by Microcystis is a very important induced defense of Microcystis against the grazing stress from zooplankton, but this induced defense has some threshold trait. The induced defense happens only when the grazing stress from zooplankton reaches the threshold, and the stronger grazing stress can cause the stronger induced defense.