Hoan Kiem lake, Hanoi is the habitat of many rare species of microalgae with endemicity and especially diversity of microalgae. Microcystis aeruginosa in Hoan Kiem lake may fonn hannful blooms and secrete the microcystin toxin that can damage animals in the same aquarium. Microcystin is cyclic heptapedtides that affect liver cells and lead to hyperphosphorylation of proteins and breakdown of cytoskeletal filaments, liver haemorrhage and death. Biodegradation is the one of the safe and nature treatments for removing microcystins from water. This process involves indigenous bacteria and complex communities of microorganisms in lake water. Biodegradation can reduce both intracellular toxins within cyanobacterial cells and extracellular toxin released into waterbodies. Therefore, the investigation for a biodegradation solution for this toxin by domestic bacteria is of necessity. In this study, effects of culture conditions on the growth of heterotrophic bacteria which biodegrade microcystin were examined. Two strains of heterotrophic bacteria were isolated from the water bodies ofHoan Kiem lake, called S1 and S2 and subjected to a polyphasic taxonomic study. Both strains S1, S2 were Gram-negative bacteria, motile and rod-shaped Sphingomonas-like bacterial strains, the surface was relatively smooth and the size ranges from 0.31 to 0.5 um x 0.92 to 1.2 um viewed with a scanning electron microscope (SEM x 10 000). Phylogenetic analysis based on 16S rDNA sequence showed that strains S1, S2 belonged to the genus Sphingomonas. Levels of 16S rDNA sequence similarity between strain S1, S2 and the type strains of Sphingomonas species were in the range of 97-99 percent. Optimal growth conditions of both strains were pH 7-8 and temperature 28-30°C.
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