Physiological effects of nitrate, ammonium, and urea on the growth and microcystins contamination of Microcystis aeruginosa: Implication for nitrogen mitigation.

The effects of three commonly bioavailable nitrogen (N) sources (nitrate, ammonium, and urea) on regulating the growth and microcystins (MCs) production of Microcystis aeruginosa (M. aeruginosa) at environmentally relevant concentrations were investigated from a physiological perspective. Changes in amino acid quotas as well as the transcripts of target genes associated with N metabolism (ntcA, pipX and glnB) and toxin formation (mcyA and mcyD) were determined. Results indicated that increases in nitrate and urea concentrations enhanced M. aeruginosa growth, but high ammonium concentration (7 mg-N/L) suppressed the growth. The total intracellular MCs (IMCs) content was well correlated (0.65, p < 0.001) to amino acids (the sum of methionine, leucine, serine, alanine, arginine, glutamic acid, and aspartic acid) associated with MCs production. Ammonium favors amino acid synthesis in M. aeruginosa, thus cells grown under high concentrations of ammonium (7 mg-N/L) had sufficient precursors for MCs production, which might lead to higher IMCs. Both high and low ammonium concentration resulted in high total extracellular MCs (EMCs) level in water, despite of their different mechanisms. These results indicated that mitigation of nitrogen in eutrophic waters should be very cautious of unexpected risks, as the reduction of ammonium may have the risk of stimulating M. aeruginosa growth or increasing EMCs levels.