The biosynthesis of nitrous oxide in the green algae Chlamydomonas reinhardtii

Abstract

Over the last decades, several studies have reported emissions of nitrous oxide (N2O) from microalgal cultures and aquatic ecosystems characterized by a high level of algal activity (e.g. eutrophic lakes). As N2O 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 N2O synthesis. We report that C. reinhardtii supplied with nitrite (NO2−) under aerobic conditions can reduce NO2− 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 N2O by the enzyme NO reductase (NOR). Based on experimental evidence and published literature, we hypothesize that when nitrate (NO3−) is the main Nitrogen source and the intracellular concentration of NO2− is low (i.e. under physiological conditions), microalgal N2O synthesis involves the reduction of NO3− to NO2− by NR followed by the reduction of NO2− to NO by the dual system involving NR. This microalgal N2O pathway has broad implications for environmental science and algal biology because the pathway of NO3− assimilation is conserved among microalgae, and because its regulation may involve NO.