The biosynthesis of nitrous oxide in the green alga Chlamydomonas reinhardtii

Abstract

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

Algal photosynthesis converts nitric oxide into nitrous oxide

“…During this process, nitrite is reduced into ammonium by the nitrite reductase (34), but can alternatively be reduced to NO by either a NO-forming nitrite reductase (39) or by respiratory oxidases (40). N2O was observed in conditions where NO is produced and CYP55 was suggested to be involved in its reduction (13). Our work establishes that the CYP55 is indeed involved in this process, but essentially in the dark.…”

“…Although this variability could have been due to differences in the strains used or fine regulatory mechanisms, it remained unexplained (13). Our results clearly demonstrate the existence of two distinct pathways of NO reduction.…”

“…Fusarium oxysporum (CYP55) encoded by the C. reinhardtii genome (Cre01.g007950) was recently proposed to be involved in NO reduction (13). In order to investigate the contribution of the CYP55 homolog to N2O production, we obtained three C. reinhardtii insertion mutants from the CLiP library (https://www.chlamylibrary.org; (27).…”

“…For decades, N2O production has been detected in samples from the ocean, lakes and coastal waters but the respective contribution of prokaryotic and eukaryotic organisms to this phenomenon remains to be investigated (7)(8)(9), and particularly the contribution of microalgae has been overlooked (7,10). Algal blooms correlate with N2O production (8,10,11), and recently, axenic microalgal cultures were shown to produce substantial amount of N2O (12)(13)(14). Despite the possible ecological significance of microalgae in N2O emissions (10), little is known about the molecular mechanisms of N2O production in microalgae.…”

“…The genome of the model green microalga Chlamydomonas reinhardtii harbors both a CYP55 fungal homolog (12) and FLVs bacterial homologs (20). Based on RNAi silencing experiments in C. reinhardtii, Plouviez et al concluded that the CYP55 homolog is the main contributor to N2O production (13). However, this gene has so far only been identified in three sequenced algal genomes (21) suggesting the occurrence of other mechanisms.…”