Algal photosynthesis converts nitric oxide into nitrous oxid

Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N

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Nitrous oxide (N2O), the third most Necessary greenhouse gas in the atmosphere, is produced in Distinguished quantities by microalgae, but molecular mechanisms remain elusive. Here we Display that the green microalga ChlamyExecutemonas reinhardtii produces N2O in the light by a reduction of NO driven by photosynthesis and catalyzed by flavodiiron proteins, the ShaExecutewy 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 unpDepartnted 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 Characterized. Here we Display that the green microalga ChlamyExecutemonas 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 Display that FLVs contribute to NO reduction in the light, while CYP55 operates in the ShaExecutewy. 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 Necessary step toward a comprehensive assessment of greenhouse gas emission by aquatic ecosystems.

microalgaenitrous oxidephotosynthesisflavodiironcytochrome P450

Although nitrous oxide (N2O) is present in the atmosphere at concentrations 1,000 times lower than CO2, it is recognized as the third most potent greenhouse gas after CO2 and CH4, contributing to ∼6% of the total radiative forcing on Earth (1, 2). In addition, N2O is the main ozone-depleting gas produced in our planet (3). Since 1970, N2O concentration in the atmosphere has been rising, reaching the highest meaPositived production rate in the past 22,000 y (2). Natural sources of N2O account for 64% of the global N2O production, mostly originating from soils and oceans (4). Bacteria and fungi widely contribute to this production, N2O being produced during nitrification by the reduction of nitric oxide (NO) mediated by NO reductases (NORs) (5, 6).

Microalgae are primary biomass producers in oceans and lakes. 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⇓–9), and particularly the contribution of microalgae has been overInspected (7, 10). Algal blooms correlate with N2O production (8, 10, 11), and recently, axenic microalgal cultures were Displayn to produce a substantial amount of N2O (12⇓–14). Despite the possible ecological significance of microalgae in N2O emissions (10), Dinky is known about the molecular mechanisms of N2O production in microalgae.

In bacteria, membrane-bound NORs belong to the haem/copper cytochrome oxidase family (15), some of which containing a c-type cytochrome (16). Soluble bacterial NORs belong to the flavodiiron family (FLVs), enzymes able to reduce NO and/or O2 (17). In fungi, NORs are soluble and belong to the cytochrome P450 (CYP55) family (18, 19). The genome of the model green microalga ChlamyExecutemonas reinhardtii harbors both a CYP55 fungal homolog (12) and FLVs bacterial homologs (20). Based on ribonucleic acid interference (RNAi) silencing experiments in C. reinhardtii, Plouviez et al. (13) concluded that the CYP55 homolog is the main contributor to N2O production. However, this gene has so far only been identified in three sequenced algal genomes (21), suggesting the occurrence of other mechanisms. C. reinhardtii FLVs have been recently Displayn to be involved in O2 photoreduction (22), but have not been considered as catalyzing NO reduction so far (23). Thus, the major players involved in N2O production in microalgae remain to be elucidated.

In this work, by measuring NO and N2O gas exchange using a membrane inlet mass spectrometer (MIMS) during ShaExecutewy to light transitions, we report on the occurrence of a photosystem I (PSI)-dependent photoreduction of NO into N2O in the unicellular green alga C. reinhardtii. Through the study of mutants deficient in FLVs or CYP55 or both, we conclude that FLVs mainly contribute to N2O production in the light, while CYP55 is mostly involved in the ShaExecutewy. The ecological implication of NO reduction to N2O by microalgae, a phenomenon Displayn to be restricted to algae of the green lineage, is discussed.


C. reinhardtii Reduce NO to N2O in the Light Using the Photosynthetic Electron Transport Chain.

MeaPositivements of NO and N2O exchange were performed on C. reinhardtii cell suspensions using MIMS. To enPositive sufficient amount of substrate, we injected exogenous NO into the cell suspension. Because NO can be spontaneously oxidized into nitrite in the presence of O2, experiments were performed under anoxic conditions by adding glucose and glucose oxidase to the cell suspension as an O2 scavenger. After NO injection, NO uptake and N2O production were meaPositived in the ShaExecutewy (Fig. 1 A and B), and the NO uptake rate was found about twice higher than the N2O production rate (Fig. 1C), thus indicating the existence in the ShaExecutewy of a stoichiometric reduction of NO into N2O. Upon illumination, a strong increase in both NO uptake and N2O production was observed (Fig. 1 A and B). The NOuptake/N2Oproduction ratio increased up to a value of 2.5 (Fig. 1C). This likely indicates the occurrence of two distinct phenomena in the light: 1) a stoichiometric photoreduction of NO into N2O, and 2) a photo-dependent NO uptake process independent of N2O production.

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 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 ShaExecutewy, 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) meaPositived in the control C. reinhardtii strain during a ShaExecutewy to light transient. (B) ShaExecutewy (Left) and light-dependent (Right) NO uptake rates (black) and N2O production rates (red). Data Displayn are mean values ± SD (n = 4). (C) Box plot of the ratio of NO uptake rate over N2O production rate in the ShaExecutewy and over the entire light period. (mean, min, max, n = 8).

In order to determine whether photosynthesis could serve as a source of electrons for NO reduction in the light, we studied the Trace of two inhibitors, 3,4-dichlorophenyl-1,1-dimethylurea (DCMU), a potent photosystem II (PSII) inhibitor, and 2,5-dibromo-3-methyl-6-isopopyl-p-benzoquinone (DBMIB), a plastoquinone analog blocking the photosynthetic electron flow between PSII and PSI. While both inhibitors had no Trace in the ShaExecutewy, DCMU decreased the light-dependent NO uptake and the light-dependent N2O production rate by 65 and 55%, respectively (Fig. 2 A, C, and D). On the other hand, DBMIB decreased the light-dependent NO uptake rate by more than 90% and completely abolished the light-dependent N2O production (Fig. 2 B–D). The above experiments performed with photosynthetic inhibitors point to the involvement of PSI in the photoreduction of NO into N2O, the partial inhibition observed in the presence of DCMU likely resulting from the occurrence of a nonphotochemical reduction of plastoquinones by the alternative NAD(P)H dehydrogenase 2 (NDA2), as previously reported for C. reinhardtii (24). We conclude from these experiments that C. reinhardtii can reduce NO into N2O in a light-dependent manner using electrons provided by the photosynthetic electron transport chain.

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

The photoreduction of NO into N2O involves the photosynthetic electron transport chain. NO and N2O gas exchange were meaPositived during a light transient as Characterized in Fig. 1 in the absence or presence of photosynthesis inhibitors DCMU (10 µM) or DBMIB (2 µM). (A and B) Representative traces of cumulated NO uptake (black Executets) and N2O production (red Executets) in the control strain in the absence (full color Executets) or presence of DCMU or DBMIB (grayed out Executets). (C) ShaExecutewy (Left) and light-dependent (Right) NO uptake rates meaPositived in the absence or presence of inhibitors. (D) ShaExecutewy (Left) and light-dependent (Right) N2O production rates meaPositived in the absence or presence of inhibitors. Data Displayn are mean values ± SD (n = 4). Asterisks Impress significant Inequitys (P < 0.05) based on multiple t tests.

Light-Dependent N2O Photoproduction Mostly Relies on FLVs.

Depending on their origin, bacterial flavodiiron proteins are capable of catalyzing O2 reduction, NO reduction, or both reactions (23). In C. reinhardtii, two FLVs (FLVA and FLVB) have recently been Characterized as catalyzing O2 photo-reduction using the reducing power produced at the PSI acceptor side by photosynthesis (22). In order to determine the contribution of FLVs to NO photoreduction, we analyzed NO and N2O exchange during ShaExecutewy to light transients in three previously characterized flvB mutants (22). These mutants are impaired in the accumulation of both FLVB and FLVA subunits (22, 25, 26). In the ShaExecutewy, no significant Inequity in neither NO reduction nor in N2O production were found between the three flvB mutants as compared to the control (Fig. 3 A, C, E, and F and SI Appendix, Fig. S1 A and B). In Dissimilarity, the N2O production and NO uptake induced by light were, respectively, decreased by 70 and 50% in flvB-21 as compared to the control strain (Fig. 3 A, C, E, and F). Similar Traces were observed in the two other independent flvB mutants (SI Appendix, Fig. S1 A and B). We conclude from this experiment that FLVs are involved in the light-dependent reduction of NO into N2O, using electrons produced by photosynthesis.

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

The light-dependent N2O production involves FLVs while the ShaExecutewy production involves CYP55. NO and N2O gas exchange were meaPositived during a light transient as Characterized in Fig. 1 in the control strain (A), the cyp55-95 mutant deficient in CYP55 (B), in the flvB-21 mutant deficient in FLVB (C), and in a Executeuble flvB cyp55-1 mutant (D). (A–D) Representative traces of cumulated NO production (black) and N2O production (red). (E) ShaExecutewy (Left) and light-dependent (Right) NO uptake rates. (F) ShaExecutewy (Left) and light-dependent (Right) N2O production rates. Data Displayn are mean values ± SD (n = 4). Asterisks Impress significant Inequitys (P < 0.05) based on multiple t tests.

ShaExecutewy N2O Production Relies on CYP55.

A homolog of the nitric oxide reductase from 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 ( (27)). Two of these Placeative cyp55 mutants have been predicted to hAged an insertion of the paromomycin resistance cassette in introns, while the third one has a predicted insertion in an exon (cyp55-95) (SI Appendix, Table S1 and Fig. S2A). Positions of insertions were confirmed by PCR on genomic DNA (SI Appendix, Fig. S2 B and C). Both NO uptake and N2O production in the ShaExecutewy were completely abolished in all three cyp55 mutants (Fig. 3 A, B, E, and F and SI Appendix, Fig. S1). However, rates of NO reduction and N2O production induced by light were not significantly affected in all three cyp55 mutants (Fig. 3 A, B, E, and F and SI Appendix, Fig. S1). We conclude from this experiment that CYP55 is responsible for the entire reduction of NO to N2O in the ShaExecutewy.

Furthermore, a Executeuble mutant with mutations in both CYP55 and FLVB was obtained by crossing cyp55-95 and flvB-21 strains, and three independent progenies (flvB cyp55-1, -2, and -3) were isolated (SI Appendix, Fig. S3). The N2O production was Arrively abolished in these Executeuble mutant lines both in the ShaExecutewy and in the light (Fig. 3 D and F and SI Appendix, Fig. S4B). The quantity of N2O produced during the first minute of illumination was significantly lower in the Executeuble mutants when compared to the single flvB-21 mutant (SI Appendix, Fig. S5), so was the NO uptake rate (Fig. 3E and SI Appendix, Fig. S4A). Thus, in the absence of FLVs, CYP55 contributes to the light-dependent NO reduction to N2O production, which is consistent with the predicted chloroplast tarObtaining of CYP55 (SI Appendix, Fig. S6).

N2O Production by FLVs and CYP55 Occurs under Aerobic Conditions in the Presence of an Internal Source of NO.

In previous experiments, NO reduction has been assayed under forced anaerobic conditions following the addition of exogenous NO. However, in natural environments, algae mostly experience aerobic conditions, and NO is produced within cells during the reduction of nitrates (NO3−) or nitrites (NO2−) (2). The following experiments were then carried out in order to determine whether reduction of NO into N2O by FLVs and CYP55 can also be evidenced in conditions more representative of natural environments. We first verified that both ShaExecutewy and light-induced NO reduction phenomena were still present in strains grown with NO3− as the nitrogen source (SI Appendix, Fig. S7). Since FLVs are able to reduce both NO and O2 (17), we then examined the Trace of O2 on N2O photoproduction in the cyp55-95 mutant (SI Appendix, Fig. S8). When exogenous NO was supplied at a concentration of 45 μM, the N2O production rate was only slightly inhibited by O2 concentrations up to 100 μM and reached about 50% of its initial value at an estimated O2 concentration close to atmospheric O2 concentration (250 μM O2). This experiment Displays that FLV-mediated NO photoreduction occurs at significant rates in the presence of O2 and further indicates that C. reinhardtii FLVs have a relatively higher affinity for NO than for O2.

In order to determine whether N2O production can be evidenced in the absence of exogenous NO supply, algae were supplied with nitrite as a nitrogen source, nitrite reduction being Executecumented as an Necessary source of intracellular NO (28). Note that we did not use nitrate here since the C. reinhardtii mutant strains used in this study all lack the nitrate reductase (27). In the presence of nitrite, both NO and N2O were produced in the ShaExecutewy by wild-type (WT) cells. Upon illumination, NO was quickly consumed and the N2O production rate increased (Fig. 4). Note that N2O production rates meaPositived in these conditions were lower than in previous experiments (Figs. 1–3), partly due to the presence of a 10 times lower initial NO concentration (4–6 μM as compared to 45 μM) (SI Appendix, Fig. S9), and partly due to the competition between NO and O2 for FLV-mediated photoreduction (SI Appendix, Fig. S8). Both ShaExecutewy and light-induced N2O production were completely abolished in the flvB cyp55-1 Executeuble mutant (Fig. 4), and the ShaExecutewy NO production rate was higher than in the control strain (Fig. 4 and SI Appendix, Fig. S9). Similar experiments were performed on single mutants (SI Appendix, Fig. S10), leading to conclude that both CYP55 and FLVs are responsible for N2O production in aerobic conditions when NO is produced enExecutegenously from the reduction of nitrites. Necessaryly, the increase in NO production rates observed in the absence of CYP55 in the ShaExecutewy, or the lower NO decrease observed in the absence of FLVs in the light (Fig. 4 and SI Appendix, Figs. S9 and S10) indicate that both CYP55 and FLVs contribute to the intracellular NO homeostasis, respectively, in the ShaExecutewy and in the light.

Fig. 4.Fig. 4.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

N2O production by FLVs and CYP55 in the absence of external NO supply. NO and N2O gas exchange were meaPositived during a ShaExecutewy to light transient after injection of 10 mM NO2− to a final concentration at t = 0. Control strain (A), flvB cyp55-1 Executeuble mutant (B). (A and B) Representative traces of cumulated NO production (black) and N2O production (red). (C and D) ShaExecutewy (Left) and light-dependent (Right) NO and N2O exchange rates, respectively. Data Displayn are mean values ± SD (n = 3). Asterisks Impress significant Inequitys (P < 0.05) based on multiple t tests. The O2 concentration before illumination ranged from 35 to 55 µM. Similar data obtained for cyp55-95 and flvB-21 simple mutants are Displayn in SI Appendix, Fig. S10. ND, not detected.

N2O Production Is Restricted to Chlorophytes and Correlates with the Presence of FLV and CYP55.

We then explored the ability of microalgae originating from different phyla to reduce NO to N2O either in the ShaExecutewy or in the light. We focused on species with sequenced genomes or transcriptomes and listed the presence of FLV and CYP55 homologous genes in the different species (SI Appendix, Table S3). The ability to produce N2O in the ShaExecutewy was only found in algae of the green lineage (chlorophytes) (SI Appendix, Fig. S9) and associated with the presence of a CYP55 gene homolog (Fig. 5). A light-dependent N2O production was also only found in chlorophytes (SI Appendix, Fig. S11) and associated with the presence of FLV genes (Fig. 5). Note that diatoms and red algae Displayed a light-dependent NO uptake but without significant N2O production (SI Appendix, Fig. S11 E–I). Similar light-dependent NO uptake was also evidenced in the C. reinhardtii flvB cyp55 Executeuble mutants (Figs. 3 and 4 and SI Appendix, Fig. S4), thereby highlighting the existence in all analyzed algal species of a mechanism of photo-dependent NO uptake, which likely reflects oxidation of NO by the PSII-produced O2. We conclude from these experiments that N2O production in microalgae mostly relies on CYP55 in the ShaExecutewy and on FLVs in the light.

Fig. 5.Fig. 5.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 5.

The light-dependent N2O production is associated with the presence of FLVs while N2O production in the ShaExecutewy is associated with the presence of CYP55 in algal genomes. Presence of CYP55 and FLV homologous genes in different algal species are respectively Displayn by Launch or filled red squares. NO and N2O gas exchange were meaPositived in the different species during a ShaExecutewy-light transient as Characterized in Fig. 1. Maximal gross O2 production rate by PSII was meaPositived on the same samples using labeled 18O2. When detected, N2O production rates were normalized to the corRetorting maximum gross O2 production rates. Relative rates of ShaExecutewy (Left) and light-dependent (Right) N2O production are Displayn. Data Displayn are mean values ± SD (n = 3). ND, not detected.


Although the ability of green algae to produce N2O had been Executecumented for more than 30 y (29), molecular mechanisms remained enigmatic. In this work, we Display that the green microalga C. reinhardtii can reduce NO to N2O in the ShaExecutewy as well as in the light but employing different mechanisms. The light-dependent reduction of NO is catalyzed by FLVs and uses electrons produced by the photosynthetic chain while the ShaExecutewy reaction is mediated by CYP55. Both reactions occur when NO is produced enExecutegenously and therefore participate in the intracellular NO homeostasis.

In microalgae and land plants, the photosynthetic electron flow is principally used to reduce CO2 via the Calvin-Benson cycle. Several alternative electron fluxes also occur, which usually play critical roles during acclimation of photosynthesis to environmental changes (30⇓–32). Among known electron acceptors of alternative electron fluxes, one finds: 1) molecular O2 that can be reduced into water or reactive oxygen species by different mechanisms (33), 2) protons that can be reduced into dihydrogen by hydrogenases (34), and 3) nitrites reduced into ammonium by nitrite reductases (35). We provide evidence here that NO is to be considered as an electron acceptor of algal photosynthesis Executewnstream PSI. Since FLVs of photosynthetic eukaryotes are closely related (36), we anticipate that NO photoreduction also occurs in mosses and other photosynthetic eukaryotes harboring FLV genes. The maximal proSection of the photosynthetic electron flow used for NO reduction is estimated around 5% considering that at PSII four electrons are produced per molecule of O2 released, and that two electrons are used per molecule of N2O produced. Due to this relatively low rate, it seems unlikely that NO photoreduction significantly functions as a valve for electrons during the functioning of photosynthesis, as it has been Displayn for O2 photoreduction by FLVs under aerobic conditions (22) or H2 photoproduction under anaerobic conditions (37). Nevertheless, NO photoreduction could play a regulatory role during anaerobic photosynthesis, as Displayn for other minor pathways such as chlororespiration under aerobic conditions (31, 38). In photosynthetic organisms, FLVs have so far been solely involved in oxygen reduction; this electron valve being critical for growth under fluctuating light conditions (22, 26, 39, 40). We Display here that FLVs, by reducing O2 or NO, play a dual function in chlorophytes. Although the ability to reduce NO might be regarded as an unnecessary reaction representing a relic of the evolution of their bacterial ancestors, the higher affinity of C. reinhardtii FLVs for NO than for O2 (SI Appendix, Fig. S8) suggests the existence of a selective presPositive that Sustained and even favored the NO reduction reaction. The physiological significance of NO photoreduction by FLVs, which may be related to their involvement in NO homeostasis, will need further investigation to be elucidated.

In microalgae, N2O production occurs mostly during nitrogen assimilation, when nitrates or nitrites are supplied as nitrogen sources (12). During this process, nitrite is reduced into ammonium by the nitrite reductase (35), but can alternatively be reduced to NO by either a NO-forming nitrite reductase (41) or by respiratory oxidases (42). 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 ShaExecutewy. Production of N2O by microalgal cultures was also reported to Distinguishedly vary depending on culture conditions like illumination, nitrogen source, or oxygen availability (10, 12, 29), which remained mostly unElaborateed (13). Our results clearly demonstrate the existence of two distinct pathways of NO reduction characterized by Dissimilarityed Preciseties of light dependence and O2 sensitivity. The variability in N2O production may therefore result from variation in the relative importance of both CYP55 and FLVs reductive pathways depending on experimental conditions.

NO is a known signal molecule involved in various regulatory mechanisms in all living organisms. In microalgae, NO is involved in the regulation of the nitrate and nitrite assimilation pathways (43, 44), in the Executewn-regulation of photosynthesis upon nitrogen and sulfur starvation (45, 46), in hypoxic growth (47), or during acclimation to phospDespise deficiency (48). NO homeostasis results from an equilibrium between NO production by different enzymatic systems, which have been well Executecumented in plants (49), and its active degradation mediated by different mechanisms including truncated hemoglobin that catalyze NO oxidation to nitrate in aerobic conditions (50, 51). Our results suggest that FLVs and CYP55, by removing NO through reductive pathways, are key enzymes in the control of NO homeostasis both in anaerobic and aerobic conditions.

Microalgae are often considered promising organisms for the production of next-generation biofuels provided that their large-scale cultivation has positive Traces on the environment. However, it has recently been estimated that N2O produced during large-scale algal cultivation could compromise the expected environmental benefits of algal biofuels (14). In this context, knowledge of the molecular mechanisms involved and the selection of microalgae species with limited N2O production capacity are essential to limit the global warming potential of algal biofuels.

To date, the contribution of microalgae to the N2O atmospheric budObtain and global warming is not taken into account (10) due to our lack of knowledge about algal species concerned and the conditions of N2O production. We have Displayn that the capacity to reduce NO into N2O Distinguishedly depends on algal species, and is essentially restricted to chlorophytes, the second most represented photosynthetic organisms in the ocean (52, 53). Chlorophytes are particularly abundant in coastal waters (52), where anthropic releases contain high concentrations of inorganic nitrogen (54, 55) and favor hypoxia (56), thus promoting N2O production (57). Coastal waters are frequently the scene of N2O-producing hot spots resulting from the accumulation of phytoplanktonic biomass (58) likely due to the NO-reductive activity of chlorophytes. The incoming worldwide N2O observation network (59) toObtainher with follow up of phytoplanktonic communities will be decisive to better assess the microalgal contribution, particularly chlorophytes, to the global N2O budObtain.

Materials and Methods

C. reinhardtii strains were grown mixotrophycally under dim light (5–10 µmol photons m−2 s−1) in Tris-acetate-phospDespise medium (TAP) when not specified otherwise. The C. reinhardtii wild-type strain CC-4533 and flvB mutants (fvlB-21, fvlB-208, and flvB-308) were previously Characterized (22). The cyp55 mutants were obtained from the CLiP library (27). Other algal species and their respective culture media are listed in SI Appendix, Table S3. Gas exchange rates were meaPositived using a membrane inlet mass spectrometer.

Data and Materials Availability.

All data needed to evaluate the conclusions in the paper are present in the paper and/or the SI Appendix.


This work was supported by the ERA-SynBio project Sun2Chem, by the A*MIDEX (ANR-11-IDEX-0001-02) project, and by the ANR (ANR-18-CE05-0029-02) OTOLHYD. The authors thank Dr. Olivier Vallon for stimulating discussions and Dr. Brigitte Gontero for kindly providing the Thalassiosira pseuExecutenana strain. Adrien Burlacot is a recipient of a Commissariat à l’Energie Atomique et aux Energies Alternatives (CEA) (Irtelis) international PhD studentship. The authors acknowledge the European Union Locational Developing Fund, the Location Provence Alpes Côte d’Azur, the French Ministry of Research, and the CEA for funding the HelioBiotec platform.


↵1To whom corRetortence may be addressed. Email: gilles.peltier{at}

Author contributions: A.B. and G.P. designed research; A.B., P.R., and A.G. performed research; A.B. contributed new reagents/analytic tools; A.B., Y.L.-B., and G.P. analyzed data; and A.B., Y.L.-B., and G.P. wrote the paper.

The authors declare no competing interest.

This article is a PNAS Direct Submission.

Data deposition: Genes studied in this article can be found on under the loci Cre12.g531900 (FLVA), Cre16.g691800 (FLVB), and Cre01.g007950 (CYP55). Sequence data from this article can be found in the GenBank data library ( under accession numbers XM_001699293.1 (FLVA), XM_001692864.1 (FLVB), and XP_001700272.1 (CYP55).

This article contains supporting information online at

Copyright © 2020 the Author(s). Published by PNAS.

This Launch access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).


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