Abstract: Blooms of cyanobacteria have occurred in many regions all over the world and produced a number of toxins,including dermatotoxins,neurotoxins and hepatotoxins.Among hepatotoxins,microcystins are the most widespread and found to have more than 75 kinds of structural analogues.The toxic effects of microcystins on animals have been carried out widely.Microcystins could result in the death of invertebrate,fish,birds and other wild and domestic animals.Moreover,the bioaccumulation of microcystins by aquatic organisms including fish,shellfish and zooplankton determines the risk of health problems among animals and people that eat food contaminated by cyanotoxins.It was reported that mcirocystins could inhibit the growth and induce the antioxidant system reaction of plant.However,few studies have been tried to elucidate the possible effect of microcystins on microbes,which are equally important in aquatic ecosystem.Escherichia coli(E.coli) and Bacillus subtilis(B.subtilis) are procaryotic microorganisms and often used to be representative of Gram-negative and Gram-positive bacteria respectively.Moreover,Microcystin-LR,RR and YR are the most common isomers of microcystin.We investigated the effects of microcystin-RR(MC-RR) on the growth and some physio-biochemical characteristics of representative microbial species E.coli and B.subtilis.The results in the present study showed that the growth of E.coli and B.subtilis were significantly prolonged when exposed to microcystin-RR of 10mg/L compared with that of the control group.But the OD600 only showed a slight decrease when E.coli and B.subtilis exposed to microcystin-RR of 5mg/L or below for 1h.The OD600 of the high concentration(10mg/L) toxin-treated group was lower than that of the control group during the experiment.Since toxin-treated E.coli and B.subtilis cells had to consume a lot of endogenous compounds to conjugate or transform this xenobiotic,the growth and reproduction of E.coli and B.subtilis were unavoidable to be prolonged.But the dose of microcystin-RR did not have a lethal effect.E.coli and B.subtilis only showed growth inhibition at the initial growth phase when cells were treated with microcystin-RR.Indeed,the normal rate of growth was gradually re-established and the growth curves of the toxin-treated and untreated bacteria became parallel.The cell viability of E.coli and B.subtilis were also inhibited for a short period compared with that of the control group,which was similar to the growth of bacteria.The contents of protein and soluble sugar in cells increased compared with that of the control group at the beginning of MC-RR exposure,and then gradually decreased.The results showed the stress of MC-RR on E.coli and B.subtlis.The change of protein and soluble sugar made cells suit for the stress of MC-RR gradually.