Reply on RC1

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.