Here we show that the green microalga Chlamydomonas reinhardtii produces N2O in the light by a reduction of NO driven by photosynthesis and catalyzed by flavodiiron proteins, the dark N2O production being catalyzed by a cytochrome p450. Both mechanisms of N2O production are present in chlorophytes, but absent from diatoms. Our study provides an unprecedented mechanistic understanding of N2O production by microalgae, allowing a better assessment of N2O-producing hot spots in aquatic environments.
Nitrous oxide (N2O), a potent greenhouse gas in the atmosphere, is produced mostly from aquatic ecosystems, to which algae substantially contribute. However, mechanisms of N2O production by photosynthetic organisms are poorly described. Here we show that the green microalga Chlamydomonas reinhardtii reduces NO into N2O using the photosynthetic electron transport. Through the study of C. reinhardtii mutants deficient in flavodiiron proteins (FLVs) or in a cytochrome p450 (CYP55), we show that FLVs contribute to NO reduction in the light, while CYP55 operates in the dark. Both pathways are active when NO is produced in vivo during the reduction of nitrites and participate in NO homeostasis. Furthermore, NO reduction by both pathways is restricted to chlorophytes, organisms particularly abundant in ocean N2O-producing hot spots. Our results provide a mechanistic understanding of N2O production in eukaryotic phototrophs and represent an important step toward a comprehensive assessment of greenhouse gas emission by aquatic ecosystems.
Figure 1: Reduction of NO into N2O in the green alga C. reinhardtii. After 1 min anaerobic acclimation, NO was injected in the cell suspension to a final concentration of 45 µM. After 3 min in the dark, cells were illuminated with green light (3,000 µmol photon m−2 s−1). (A) Representative traces of cumulated amounts of NO uptake (black circles) and N2O production (red circles) measured in the control C. reinhardtii strain during a dark to light transient. (B) Dark (Left) and light-dependent (Right) NO uptake rates (black) and N2O production rates (red). Data shown are mean values ± SD (n = 4). (C) Box plot of the ratio of NO uptake rate over N2O production rate in the dark and over the entire light period. (mean, min, max, n = 8).