Abstract. Pure cultures of the freshwater cyanobacterium Microcystis aeruginosa synthesized nitrous oxide (N2O) when supplied with nitrite (NO2-) in darkness (198.9 nmol g-DW−1 h−1 after 24 h) and illumination (163.1 nmol g-DW−1 h−1 after 24 h), whereas N2O production was negligible in abiotic controls supplied with NO2- and in cultures deprived of exogenous nitrogen. N2O production was also positively correlated to the initial NO2- and M. aeruginosa concentrations but was low to negligible when nitrate (NO3-) and ammonium (NH4+) were supplied as the sole exogenous N source instead of NO2-. A protein database search revealed that M. aeruginosa possesses protein homologous to eukaryotic microalgae enzymes known to catalyze the successive reduction of NO2- into nitric oxide (NO) and N2O. Our laboratory study is the first demonstration that M. aeruginosa possesses the ability to synthesize N2O. As M. aeruginosa is a bloom-forming cyanobacterium found globally, further research (including field monitoring) is now needed to establish the significance of N2O synthesis by M. aeruginosa under relevant conditions (especially in terms of N supply). Further work is also needed to confirm the biochemical pathway and potential function of this synthesis.

Abstract. Pure cultures of Microcystis aeruginosa synthesized nitrous oxide (N2O) when supplied with nitrite (NO2-) in darkness (198.9 nmol·g-DW-1·h-1 after 24 hours) and illumination (163.1 nmol∙g-DW-1∙h-1 after 24 hours) whereas N2O production was negligible in abiotic controls supplied with NO2- and in cultures deprived of exogenous nitrogen. N2O production was also positively correlated to the initial NO2- and M. aeruginosa concentrations, but low to negligible when nitrate (NO3-) and ammonium (NH4+) were supplied as the sole exogenous N source instead of NO2-. A protein database search revealed M. aeruginosa possesses protein homologues to eukaryotic microalgae enzymes known to catalyse the successive reduction of NO2- into nitric oxide (NO) and N2O. Our laboratory study is the first demonstration that M. aeruginosa possesses the ability to synthesize N2O. As M. aeruginosa is a bloom-forming cyanobacterium found globally, further research (including field monitoring) is now needed to establish the significance of N2O synthesis by M. aeruginosa under relevant conditions (especially in terms of N supply). Further work is also needed to confirm the biochemical pathway and potential function of this synthesis.

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