Investigation on cyanobacterial production of the proposed neurotoxin β-N-methylamino-L-alanine (BMAA)

Abstract

β-N-methylamino-L-alanine (BMAA) is an environmental neurotoxin thought to be produced by cyanobacteria. However, the cyanobacterial origin of BMAA remains controversial. The detection method and culture conditions of cyanobacteria are often cited as factors behind the discrepancy of published results. We showed previously that BMAA was highly toxic to the cyanobacterium Nostoc PCC 7120, and it is taken up via an amino acid transport system. Using a mutant ΔnatAΔbgtA deficient in amino acid transport as a genetic control, we show here that BMAA taken up from the medium can be detected quantitatively at a threshold similar to, or below those reported, but was undetectable in the mutant. The BMAA isomer, 2,4-diaminobutanoic acids (DAB), but not BMAA, could be detected in cell free extracts of Nostoc PCC 7120. Long-term (20 ​days) diazotrophic growth or non-limiting supply of phosphate, conditions reported to enhance BMAA synthesis, did not lead to the detection of BMAA. An UPLC-MS/MS signal with a similar retention time to BMAA was found after prolonged diazotrophic incubation, but did not have fragment ions of BMAA after further analysis. When extended to 29 different cyanobacterial strains and 6 natural cyanobacterial bloom samples, none of them was found to produce BMAA. The cytotoxicity of BMAA to cyanobacteria, and the lack of a cellular protective mechanism against such toxicity, contradict the presence of a BMAA synthesis pathway in these organisms. More specific methods for BMAA detection in vivo need to be developed to clarify the cyanobacterial origin of BMAA.