Effect of urea on growth and microcystins production of Microcystis aeruginosa

•MC-LR production of M. aeruginosa was restricted by high urea concentration.•Growth and MC-LR production was promoted by low urea concentration.•The first urea-N was biosynthesized into the Ala or Leu residue.•Mdha was the last residue to assimilate urea-15N.

The effects of urea on the growth and toxin content of Microcystis aeruginosa isolated from Dianchi Lake in China were investigated. Experiments were carried out in lab using 15N isotopic technique to characterize urea-N biosynthesis to microcystins. High urea concentration (3.6 mmol-N L−1) would restrict the growth of M.aeruginosa and the production of microcystin-LR, while low urea concentration (0.4–1.4 mmol-N L−1) would promote the growth of M.aeruginosa and the production of microcystin-LR. The 15N labeling experiment further demonstrated that there existed selectivity when M.aeruginosa assimilated urea to form its structure. The majority of M.aeruginosa assimilated 1 urea molecule at first which was biosynthesized into the Ala or Leu residue. On day 18, The m/z = 1004 parent ion assimilated 9 15N except that the Mdha residue did not assimilate any urea-15N. The results give deeper insight to the biosynthesis of urea into microcystins.