Nitrogen starvation of cyanobacteria results in the production of β-N-methylamino-L-alanine

► First report providing irrefutable evidence for the cyanobacterial origin of neurotoxic β-N-methylamino-L-alanine (BMAA). ► First report of the de novo synthesis of BMAA from provided labelled raw materials by an axenic cyanobacterial culture. ► First report on the environmental modulation of BMAA production by cyanobacteria.