The biosynthesis of nitrous oxide in the green alga Chlamydomonas reinhardtii

Summary Over the last decades, several studies have reported emissions of nitrous oxide (N 2 O) from microalgal cultures and aquatic ecosystems characterized by a high level of algal activity (e.g. eutrophic lakes). As N 2 O is a potent greenhouse gas and an ozone‐depleting pollutant, these findings suggest that large‐scale cultivation of microalgae (and possibly, natural eutrophic ecosystems) could have a significant environmental impact. Using the model unicellular microalga Chlamydomonas reinhardtii , this study was conducted to investigate the molecular basis of microalgal N 2 O synthesis. We report that C. reinhardtii supplied with nitrite ( NO 2 − ) under aerobic conditions can reduce NO 2 − into nitric oxide ( NO ) using either a mitochondrial cytochrome c oxidase ( COX ) or a dual enzymatic system of nitrate reductase ( NR ) and amidoxime‐reducing component, and that NO is subsequently reduced into N 2 O by the enzyme NO reductase ( NOR ). Based on experimental evidence and published literature, we hypothesize that when nitrate ( NO 3 − ) is the main Nitrogen source and the intracellular concentration of NO 2 − is low (i.e. under physiological conditions), microalgal N 2 O synthesis involves the reduction of NO 3 − to NO 2 − by NR followed by the reduction of NO 2 − to NO by the dual system involving NR . This microalgal N 2 O pathway has broad implications for environmental science and algal biology because the pathway of NO 3 − assimilation is conserved among microalgae, and because its regulation may involve NO .