Effects of arginine on the growth and microcystin-LR production of Microcystis aeruginosa in culture

Abstract

Although toxic cyanobacterial blooms and their toxins threaten drinking water and ecology and are promoted by nutrient loading, the precise nutrient regime that increases cyanobacterial populations and toxin production is poorly understood. Here, the influences of arginine (Arg), as a common amino acid with high nitrogen content, on the growth and microcystins (MCs) production of Microcystis aeruginosa (M. aeruginosa) were investigated by an isotope method (¹⁵N). The results showed that the biomass and production of microcystin-LR (MC-LR) increased with an increase in initial Arg concentrations in the range of 0.3-1.4 mmol-N L^-1, whereas a higher Arg concentration (3.6 mmol-N L^-1) inhibited the growth. MC-LR on different days (days 0, 6, 12, and 18) was detected by liquid chromatography with tandem mass spectrometry (LC-MS/MS) after incubation with ¹⁵N-Arg. The MC-LR molecular weight increased from 995 to 1004 with 100% relative abundance with 10 ¹⁵N atoms bound by the Adda, Arg (4 ¹⁵N), Glu, Mdha, Ala, Leu, and MeAsp residues on day 18. It seems that there was a sequential order when M. aeruginosa assimilated Arg to synthesize MC-LR. The Arg residue in the molecule of MC-LR was the last one to be labeled by ¹⁵N from ¹⁵N-arginine. This study not only presents a deeper insight into the biosynthesis of free amino acids that are incorporated into MCs but also reminds us of the potential risk caused by Arg, which should arouse concerns.

Keywords: Arginine; Growth; Isotope; Microcystin-LR; Microcystis aeruginosa.