Biology subjects

Burlacot, A.

Publications and source records attributed to Burlacot, A..

18 recordsLinked to original sources

bioRxiv2026

Dual-targeted NADK2 Links Mitochondrial Redox Homeostasis to Carbon Partitioning and Heterotrophic Growth in Chlamydomonas reinhardtii

NAD kinases (NADKs) can modulate NADP(H) levels across shifting environmental conditions. In Chlamydomonas reinhardtii, we localized CreNADK1 to the chloroplast, CreNADK3 to the cytoplasm, and CreNADK2 to both the chloroplast and mitochondria. Importantly, during mixotrophic (light + acetate) and photoautotrophic (light) growth, most CreNADK2 is chloroplast localized whereas an increased proportion resides in mitochondria when the cells are grown heterotrophically (dark + acetate). In Crenadk2 mutants, a diminished ability to synthesize mitochondrial NADP(H) inhibits the TCA cycle and markedly retards heterotrophic growth. In the light, photosynthesis becomes the primary energy source, and the presence of chloroplastic CreNADK1 enables near-normal growth in mutant cells, with inhibition of TCA cycle activity partially ameliorated via rerouting of acetate metabolism through the glyoxylate shunt, bypassing the TCA cycle and diminishing mitochondrial ROS production. These findings demonstrate spatial plasticity of NAD(H)Ks and the implementation of bypass metabolism to enable growth of Crenadk2 mutants.

cell biology↗

bioRxiv2026

CO2 fixation mediated by the carbon concentrating mechanism enables a rapid response to nitrogen deprivation in cyanobacteria

Cyanobacteria are leading biomass producers of the ocean whose ecological success relies on their ability to respond to dynamic availability of nutrients like CO2 and nitrogen, which require distinct adaptive mechanisms. To survive nitrogen deprivation, cyanobacteria undergo a reversible transition to a dormant mode. Under low CO2 levels, a CO2 concentrating mechanism (CCM) supports their CO2 fixation. While the CCM and nitrogen assimilation have been shown to share some regulatory pathways, how the CCM impacts the response to nitrogen deprivation remains underexplored. In this study, by using mutants of the coastal cyanobacteria Synechococcus sp. PCC 7002 lacking a CCM component, we show that the high rate of carbon fixation mediated by the CCM tunes the speed of the nitrogen deprivation response in {beta}-cyanobacteria. We first show that CCM mutants are deficient in inducing their typical nitrogen deprivation response under atmospheric CO2. However, at higher CO2 concentrations, the CCM mutants induce the nitrogen deprivation response. By combining Rubisco kinetics modeling with measurement of the response speed to nitrogen in various CO2 concentrations, we show that the speed of the nitrogen deprivation response increases linearly with Rubiscos carboxylation rate. We further reveal that the regulation of nitrogen response by the CCM is also present in the distantly related freshwater cyanobacteria Synechococcus elongatus PCC 7942, suggesting a widespread role of this regulation across {beta}-cyanobacteria. This study demonstrates that CO2 fixation by the cyanobacterial CCM is a key regulator of the nitrogen deprivation response, favoring a rapid response to dynamic environments.

plant biology↗

bioRxiv2025

A genome-wide CRISPR screen reveals how diatoms thrive in dynamic light

Diatoms are a highly diverse group of phytoplankton that have a large impact on global primary production and carbon sequestration in the ocean1,2. However, they are evolutionarily divergent from model phototrophs of the green lineage, and limited screening tools have hampered discovery of unique diatom biology. To address this challenge, we developed a genome-wide CRISPR/Cas9 screen in the model marine diatom, Phaeodactylum tricornutum. The screen was applied to identify genes required for survival in different light regimes, including both high light and fluctuating light. We identified a broad set of uncharacterized genes, providing the foundation for mechanistic studies of diatom adaptation to dynamic light. Among these genes, we demonstrated that the red lineage-exclusive gene STROBE1 is a new potentiator of cyclic electron flow (CEF) required for CEF to generate a trans-thylakoid proton gradient3,4. As dynamic light conditions are common in marine environments, STROBE1 and other genes identified in this screen may contribute to the broad ecological success of diatoms5,6. This genome-wide genetic screen in P. tricornutum will accelerate the unbiased discovery of novel gene functions in these ecologically important organisms.

microbiology↗

bioRxiv2025

Sustaining the cell energy in dynamic environments requires photosynthetic electron flows with diverse bandwidths

All living cells depend on the dynamic balance between their energy supply and demand to survive and thrive in dynamic environments. In extreme cases, like for photosynthetic organisms, their energy source, light, can fluctuate dramatically in intensity over timescales of seconds to hours. While various photosynthetic electron flows (EF) are crucial for maintaining bioenergetic homeostasis, how EFs are modulated to respond to dynamic energy intake remains unclear. Here, we show in the model green alga Chlamydomonas reinhardtii that each EF is best suited to a specific domain of energetic fluctuation periodicity for which it can support the cells energetic needs, which we term bandwidth. By systematically exposing cells to a range of light periodicities, we show that while cyclic EF has a large bandwidth, pseudo-cyclic EF (PCEF) can only sustain the cells energetic needs for fast light fluctuations, and that the interplay between the chloroplast and the mitochondria (CMEF) has a limited bandwidth. We further show that the bandwidths of PCEF and CMEF, specialized for dynamic lights, are related to their capacity to generate ATP and protect the photosynthetic apparatus. Finally, we show that in wild-type cells, the activity level of PCEF matches its bandwidth, and we propose that cells tune the relative activity of AEFs depending on the light fluctuation frequency. Our work opens an avenue of research to characterize the molecular mechanisms that can sustain phototrophic growth in complex and dynamic energetic landscapes. It further provides a generalizable framework for understanding the physiological importance of molecular mechanisms in a dynamic environment.

plant biology↗

bioRxiv2025

N-terminal region of PetD is essential for cytochrome b6f function and controls STT7 kinase activity via STT7-dependent feedback loop phosphorylation

The cytochrome b6f complex (b6f) links photosystem I (PSI) and photosystem II (PSII) in the photosynthetic electron transport chain and is distributed across appressed and non-appressed thylakoid membranes. The b6f also activates the state-transition 7 kinase (STT7), which phosphorylates light-harvesting complex proteins, triggering their migration to enable energy redistribution between PSII and PSI. STT7-dependent phosphorylation has also been observed at T4 in the N-terminal domain of the b6f subunit-IV (PetD), though its functional significance has remained unclear. To investigate its role, we generated several chloroplast mutants. The phosphomimic mutation PetD T4E - but not T4A - inhibited STT7 kinase activity, as indicated by the absence of STT7-dependent phosphorylation and a State 1-locked phenotype. This reveals a novel feedback mechanism regulating STT7-dependent phosphorylation. Deletion of the first five N-terminal amino acids similarly inhibited STT7 activity and additionally disrupted electron transfer, underscoring a crucial role of the PetD N-terminus in b6f function.

plant biology↗

bioRxiv2024

The cell biology and genome of Stentor pyriformis, a giant cell that embeds symbiotic algae in a microtubule meshwork

Endosymbiotic events in which an endosymbiont is retained within a cell that remains capable of phagocytosis, a situation known as mixotrophy, provide potentially important clues about the evolutionary origins of eukaryotes, particularly regarding the relative evolutionary sequence of phagocytosis and endosymbiosis. Mixotrophy in ciliates is commonplace, but has been investigated at a cellular and molecular level almost entirely in one organism, Paramecium bursaria. Reliance on just one model system makes it difficult to know which cell biological aspects of this system represent general features of ciliate mixotrophy versus accidental features of the specific organism. Here we describe the cell biology and genome of the giant heterotrichous ciliate Stentor pyriformis. We show that this giant unicellular organism contains Chlorella variabilis as its endosymbiont, that the Chlorella can live freely outside the host, that within the host the Chlorella cells are docked near the cell surface, surrounded by microtubule "baskets", and that the photosynthetic efficiency of the Chlorella is reduced inside the Stentor cell compared to when it is free-living outside the host, with photon energy instead being shunted to non-photochemical quenching. Compared to the non-mixotrophic Stentor coeruleus, S. pyriformis has several distinct cellular features that may be related to endosymbiosis: the presence of microtubule baskets, which are absent in S. coeruleus; positive rather than negative phototaxis, which is likely an adaptation to allow the photosynthetic symbionts access to sufficient light; and a lack of pigment in the host cell, which may be an adaptation to tolerate high light levels. Compared to P. bursaria, S. pyriformis has several similar cellular features: in both organisms, the symbiont is a strain of Chlorella variabilis; the Chlorella endosymbiont retains the ability to live freely when separated from the host; and the algal symbionts contained in perialgal vesicles are docked at the cell surface. One potentially informative difference between P. bursaria and S. pyriformis is that S. pyriformis employs a standard genetic code, similar to other Stentor species but different from most other ciliates, including P. bursaria, which use a non-standard code in which one or more stop-codons are respecified to encode amino acids. This difference in genetic code could serve as a barrier to impede gene transfer from symbiont to host in other ciliates, but this would not be a factor in S. pyriformis. A second cell biological difference is that whereas P. bursaria performs phototaxis by a kinetic accumulation mechanism, in which swimming is non-directional but cells slow down in regions of higher light intensity, S. pyriformis performs directed swimming towards the direction of high light intensity. However, as in P. bursaria, phototaxis in S. pyriformis requires the presence of the Chlorella, implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We propose that S. pyriformis will serve as a useful model system for studying the evolution of mixotrophy and endosymbiosis, with unique advantages in terms of size and regenerative ability as well as distinct cellular and genomic features compared with other mixotrophic ciliate models.

cell biology↗

bioRxiv2024

Joint operation of the CO2 concentrating mechanism and photorespiration in green algae during acclimation to limiting CO2

Due to low availability of CO2 in aquatic environment, microalgae have evolved a CO2 concentrating mechanism (CCM). It has long been thought that operation of CCM would suppress photorespiration by increasing the CO2 concentration at the Rubisco active site, but experimental evidence is scarce. To better explore the function of photorespiration in algae, we first characterized a Chlamydomonas reinhardtii mutant defected in low-CO2 inducible 20 (LCI20) and show that LCI20 is a chloroplast-envelope glutamate/malate transporter playing a role in photorespiration. By monitoring growth and glycolate excretion in mutants deficient in either CCM or photorespiration, we conclude that: i. CCM induction does not depend on photorespiration, ii. glycolate excretion protects algal cells from the toxicity of unmetabolized photorespiratory intermediates, iii. photorespiration is active at low CO2 when the CCM is operational. This work provides a foundation for a better understanding of the carbon cycle in the ocean where significant glycolate concentrations have been found.

plant biology↗

bioRxiv2024

Dramatic Changes in Mitochondrial Subcellular Location and Morphology Accompany Activation of the CO2 Concentrating Mechanism

Dynamic changes in intracellular ultrastructure can be critical for the ability of organisms to acclimate to environmental conditions. Microalgae, which are responsible for [~]50% of global photosynthesis, compartmentalize their Rubisco into a specialized structure known as the pyrenoid when the cells experience limiting CO2 conditions; this compartmentalization appears to be a component of the CO2 Concentrating Mechanism (CCM), which facilitates photosynthetic CO2 fixation as environmental levels of inorganic carbon (Ci) decline. Changes in the spatial distribution of mitochondria in green algae have also been observed under CO2 limiting conditions, although a role for this reorganization in CCM function remains unclear. We used the green microalgae Chlamydomonas reinhardtii to monitor changes in the position and ultrastructure of mitochondrial membranes as cells transition between high CO2 (HC) and Low/Very Low CO2 (LC/VLC). Upon transferring cells to VLC, the mitochondria move from a central to a peripheral location, become wedged between the plasma membrane and chloroplast envelope, and mitochondrial membranes orient in parallel tubular arrays that extend from the cells apex to its base. We show that these ultrastructural changes require protein and RNA synthesis, occur within 90 min of shifting cells to VLC conditions, correlate with CCM induction and are regulated by the CCM master regulator CIA5. The apico-basal orientation of the mitochondrial membrane, but not the movement of the mitochondrion to the cell periphery, is dependent on microtubules and the MIRO1 protein, which is involved in membrane-microtubule interactions. Furthermore, blocking mitochondrial electron transport in VLC acclimated cells reduces the cells affinity for inorganic carbon. Overall, our results suggest that CIA5-dependent mitochondrial repositioning/reorientation functions in integrating cellular architecture and energetics with CCM activities and invite further exploration of how intracellular architecture can impact fitness under dynamic environmental conditions.

plant biology↗

bioRxiv2023

Alka(e)nes contribute to membrane lipid homeostasis and resilience of photosynthesis to high light in cyanobacteria

Alka(e)nes are produced by many living organisms and exhibit diverse physiological roles, reflecting a high functional versatility. Alka(e)nes serve as water proof wax in plants, communicating pheromones for insects, and microbial signaling molecules in some bacteria. Although alka(e)nes have been found in cyanobacteria and algal chloroplasts, a possible role in photosynthesis and chloroplast function remains elusive. In this study, we investigated the consequences of the absence of alka(e)nes on membrane lipid remodeling and photosynthesis using the cyanobacteria Synechocystis PCC6803 as a model organism. By following the dynamics of membrane lipids and the photosynthetic performance in strains defected and altered in alka(e)ne biosynthesis, we show that a profound remodeling of the membrane lipidome and carotenoid content occur in the absence of alka(e)nes, including a decrease in the membrane carotenoid content, a decrease in some digalactosyldiacylglycerol (DGDG) species and a parallel increase in monogalactosyldiacylglycerol (MGDG) species. Under high light, this effect is accompanied in alka(e)ne deficient strains by a higher susceptibility of photosynthesis and growth, the effect being reversed by expressing an algal photoenzyme producing alka(e)nes from fatty acids. We conclude that alka(e)nes play a crucial role in maintaining lipid homeostasis of photosynthetic membranes, thereby contributing to the proper functioning of photosynthesis, particularly under elevated light intensities. Significance statementWe used cyanobacteria as a model organism to explore the role of alka(e)nes related to photosynthesis. Our findings reveal that the absence of alka(e)nes induces alterations in the composition of membrane lipids and carotenoid content, resulting in an increased susceptibility of photosynthesis. By introducing a fatty acid photodecarboxylase to produce alkanes, we could reverse these effects, highlighting the critical role of alka(e)nes in maintaining lipid balance in photosynthetic membranes and ensuring efficient photosynthesis. Uncovering the physiological role of alka(e)nes provides insights to a better understanding of the widespread presence of genes encoding alka(e)nes-synthesizing enzymes in cyanobacteria and microalgae, organisms of major ecological and evolutionary importance in the global CO2 assimilation.

microbiology↗

bioRxiv2023

Algal CO2 capture is powered by alternative electron pathways of photosynthesis

On Earth, microalgae contribute to about half of global net photosynthesis. During photosynthesis, sunlight is converted into chemical energy (ATP and NADPH) used by metabolism to convert CO2 into biomass. Alternative electron pathways of photosynthesis have been proposed to generate additional ATP that is required for sustaining CO2 fixation, but the relative importance of each pathway remains elusive. Here, we dissect and quantify the contribution of cyclic, pseudo-cyclic and chloroplast to mitochondria electron flows for their ability to sustain net photosynthesis in the microalga Chlamydomonas reinhardtii. We show that each pathway has the potential to energize substantial CO2 fixation, can compensate each other, and that the additional energy requirement to fix CO2 is more than 3 times higher than previous estimations. We further show that all pathways have very different efficiencies at energizing CO2 fixation, with the chloroplast-mitochondria interaction being the most efficient, thus laying bioenergetic foundations for biotechnological improvement of CO2 capture.

plant biology↗

bioRxiv2023

Coordinated wound responses in a regenerative animal-algal photosymbiotic metaorganism

Animal regeneration requires coordinated responses of many cell types throughout the animal body. In animals carrying endosymbionts, cells from the other species may also participate in regeneration, but how cellular responses are integrated across species is yet to be unraveled. Here, we study the acoel Convolutriloba longifissura, which hosts symbiotic Tetraselmis green algae and can regenerate entire bodies from small tissue fragments. We show that animal injury leads to a decline in the photosynthetic efficiency of the symbiotic algae and concurrently induces upregulation of a cohort of photosynthesis-related genes. A deeply conserved animal transcription factor, runt, is induced after injury and required for the acoel regeneration. Knockdown of runt also dampens algal transcriptional responses to the host injury, particularly in photosynthesis related pathways, and results in further reduction of photosynthetic efficiency post-injury. Our results suggest that the runt-dependent animal regeneration program coordinates wound responses across the symbiotic partners and regulates photosynthetic carbon assimilation in this metaorganism.

developmental biology↗

bioRxiv2023

Cyclic and pseudo-cyclic electron pathways play antagonistic roles during nitrogen deficiency in Chlamydomonas reinhardtii

Nitrogen (N) scarcity is a frequently encountered situation that constrains global biomass productivity. In response to N deficiency, cell division stops and photosynthetic electron transfer is downregulated, while carbon storage is enhanced. However, the molecular mechanism downregulating photosynthesis during N deficiency and its relationship with carbon storage are not fully understood. The Proton Gradient Regulator-like 1 (PGRL1) controlling cyclic electron flow (CEF) and Flavodiiron proteins involved in pseudo-(CEF) are major players in the acclimation of photosynthesis. To determine the role of PGRL1 or FLV in photosynthesis under N deficiency, we measured photosynthetic electron transfer, oxygen gas exchange and carbon storage in Chlamydomonas pgrl1 and flvB knockout mutants. Under N deficiency, pgrl1 maintains higher net photosynthesis and O2 photoreduction rates, while flvB shows a similar response compared to control strains. Cytochrome b6f and PSI are maintained at a higher abundance in pgrl1. The photosynthetic activity of flvB and pgrl1 flvB double mutants decreases in response to N deficiency similar to the control strains. Furthermore, the preservation of photosynthetic activity in pgrl1 is accompanied by an increased accumulation of triacylglycerol depending on the genetic background. Taken together, our results suggest that in the absence of PGRL1-controlled CEF, FLV-mediated PCEF maintains net photosynthesis at a high level and that CEF and PCEF play antagonistic roles during N deficiency. It further illustrates how nutrient status and genetic makeup of a strain can affect the regulation of photosynthetic energy conversion in relation to carbon storage and provides new strategies for improving lipid productivity in algae. Significance statementNitrogen (N) deficiency, an often-encountered phenomenon in nature, results in growth arrest, downregulation of photosynthesis and massive carbon storage in microalgae. However, more mechanistic insights involved in tuning photosynthetic electron transfer during N deficiency are required. Here, we provide evidence that a well-conserved protein in chlorophytes, the Proton Gradient Regulator-like 1 (PGRL1), is a key regulator of photosynthesis during N deficiency. In its absence, cells exhibited sustained photosynthesis thanks to the Flavodiiron (FLV) proteins. We propose that both PGRL1 and FLV, by having antagonistic roles in N deficiency, manage the redox landscape, carbon storage and biomass production. Our work revolves around the current paradigm of photosynthesis regulation during N deficiency and provides a new framework for improving biomass production and carbon storage in microalgae for biotechnological purposes.

plant biology↗

bioRxiv2022

Boosting chloroplast ribosome biogenesis by a plastidial DEAD-box RNA helicase is critical for high light acclimation

Photosynthetic organisms have developed sophisticated strategies to fine-tune light energy conversion to meet the metabolic demand, thereby optimizing growth in fluctuating light environments. Although mechanisms such as energy dissipation, photosynthetic control, or the photosystem II (PSII) damage and repair have been widely studied, little is known about the regulation of protein synthesis capacity during light acclimation. By screening a Chlamydomonas reinhardtii insertional mutant library using chlorophyll fluorescence imaging, we isolated a high chlorophyll fluorescence mutant (hf0) defected in a gene encoding a putative plastid targeted DEAD-box RNA helicase called CreRH22. CreRH22 is rapidly induced upon illumination and belongs to the GreenCut, a set of proteins specific to photosynthetic organisms. While photosynthesis is slightly affected in the mutant under low light (LL), exposure to high light (HL) induces a marked decrease in both PSII and PSI, and a strong alteration of the light-induced gene expression pattern. These effects are explained by the inability of hf0 to increase plastid ribosome amounts under HL. We conclude that CreRH22, by promoting ribosomal RNA precursor maturation in a light-dependent manner, enables the assembly of extra-ribosomes required to synthesize photosystem subunits at a higher rate, a critical step in the acclimation of algae to HL.

plant biology↗

bioRxiv2022

Interplay between LHCSR proteins and state transitions governs the NPQ response in intact cells of Chlamydomonas during light fluctuations.

Photosynthetic organisms use sunlight as the primary energy source to fix CO2. However, in the environment, light energy fluctuates rapidly and often exceeds saturating levels for periods ranging from seconds to hours, which can lead to detrimental effects for cells. Safe dissipation of excess light energy occurs primarily by non-photochemical quenching (NPQ) processes. In the model green microalga Chlamydomonas reinhardtii, photoprotective NPQ is mostly mediated by pH-sensing light-harvesting complex stress-related (LHCSR) proteins and the redistribution of light-harvesting antenna proteins between the photosystems (state transition). Although each component underlying NPQ has been documented, their relative contributions to the dynamic functioning of NPQ under fluctuating light conditions remains unknown. Here, by monitoring NPQ throughout multiple high light-dark cycles with fluctuation periods ranging from 1 to 10 minutes, we show that the dynamics of NPQ depend on the frequency of light fluctuations. Mutants impaired in the accumulation of LHCSRs (npq4, lhcsr1, and npq4lhcsr1) showed significantly less quenching during illumination, demonstrating that LHCSR proteins are responsible for the majority of NPQ during repetitive exposure to high light fluctuations. Activation of NPQ was also observed during the dark phases of light fluctuations, and this was exacerbated in mutants lacking LHCSRs. By analyzing 77K chlorophyll fluorescence spectra and chlorophyll fluorescence lifetimes and yields in a mutant impaired in state transition, we show that this phenomenon arises from state transition. Finally, we quantified the contributions of LHCSRs and state transition to the overall NPQ amplitude and dynamics for all light periods tested and show that both processes interact to facilitate NPQ during light fluctuations. We further assess the role of LHCSRs in cell growth under various periods of fluctuating light. These results highlight the dynamic functioning of photoprotection under light fluctuations and open a new way to systematically characterize the photosynthetic response to an ever-changing light environment. One sentence summaryThe roles of LHCSR and STT7 in NPQ vary with the light fluctuation period and duration of light fluctuation.

plant biology↗

bioRxiv2021

Coral symbionts exhibit a polycistronic flavodiiron gene leading to functional proteins in photosynthesis

Photosynthesis in cyanobacteria, green algae, and basal land plants is protected against excess reducing pressure on the photosynthetic chain by flavodiiron proteins (FLV) that dissipate photosynthetic electrons by reducing O2. In these organisms, the genes encoding FLV are always conserved in the form of a pair of two-type isozymes (FLVA and FLVB) that are believed to function in O2 photo-reduction as a heterodimer. While coral symbionts (dinoflagellates of the family Symbiodiniaceae) are the only algae to harbor FLV in photosynthetic red plastid lineage, only one gene is found in transcriptomes and its role and activity remain unknown. Here, we characterized the FLV genes in Symbiodiniaceae and found that its coding region is composed of tandemly repeated FLV sequences. By measuring the O2-dependent electron flow and P700 oxidation, we suggest that this atypical FLV is active in vivo. Based on the amino-acid sequence alignment and the phylogenetic analysis, we conclude that in coral symbionts, the gene pair for FLVA and FLVB have been fused to construct one coding region for a hybrid enzyme, which presumably occurred when or after both genes were inherited from basal green algae to the dinoflagellate. Immunodetection suggested the FLV polypeptide to be cleaved by a post-translational mechanism, adding it to the rare cases of polycistronic genes in eukaryotes. Our results demonstrate that FLV are active in coral symbionts with genomic arrangement that is unique to these species. The implication of these unique features on their symbiotic living environment is discussed.

plant biology↗

bioRxiv2021

Alternative electron pathways of photosynthesis drive the algal CO2 concentrating mechanism

Global photosynthesis consumes ten times more CO2 than net anthropogenic emissions, and microalgae account for nearly half of this consumption1. The great efficiency of algal photosynthesis relies on a mechanism concentrating CO2 (CCM) at the catalytic site of the carboxylating enzyme RuBisCO, thus enhancing CO2 fixation2. While many cellular components involved in the transport and sequestration of inorganic carbon (Ci) have been uncovered3,4, the way microalgae supply energy to concentrate CO2 against a thermodynamic gradient remains elusive4-6. Here, by monitoring dissolved CO2 consumption, unidirectional O2 exchange and the chlorophyll fluorescence parameter NPQ in the green alga Chlamydomonas, we show that the complementary effects of cyclic electron flow and O2 photoreduction, respectively mediated by PGRL1 and flavodiiron proteins, generate the proton motive force (pmf) required by Ci transport across thylakoid membranes. We demonstrate that the trans-thylakoid pmf is used by bestrophin-like Ci transporters and further establish that a chloroplast-to-mitochondria electron flow contributes to energize non-thylakoid Ci transporters, most likely by supplying ATP. We propose an integrated view of the CCM energy supply network, describing how algal cells distribute photosynthesis energy to power different Ci transporters, thus paving the way to the transfer of a functional algal CCM in plants towards improving crop productivity. One sentence summaryPhotosynthetic alternative electron flows and mitochondrial respiration drive the algal CO2 concentrating mechanism

plant biology↗

bioRxiv2020

Fatty acid photodecarboxylase is an ancient photoenzyme responsible for hydrocarbon formation in the thylakoid membranes of algae

ABSTRACTFatty acid photodecarboxylase (FAP) is one of the three enzymes that require light for their catalytic cycle (photoenzymes). FAP has been first identified in the green microalga Chlorella variabilis NC64A and belongs an algae-specific subgroup of the glucose-methanol-choline oxidoreductase family. While the FAP from Chlorella and its Chlamydomonas reinhardtii homolog CrFAP have demonstrated in vitro activity, their activity and physiological function have not been studied in vivo. Besides, the conservation of FAP activity beyond green microalgae remains hypothetical. Here, using a Chlamydomonas FAP knockout line (fap), we show that CrFAP is responsible for the formation of 7-heptadecene, the only hydrocarbon present in this alga. We further show that CrFAP is associated to the thylakoids and that 90% of 7-heptadecene is recovered in this cell fraction. In the fap mutant, photosynthesis activity was not affected under standard growth conditions but was reduced after cold acclimation. A phylogenetic analysis including sequences from Tara Ocean identified almost 200 putative FAPs and indicated that FAP was acquired early after primary endosymbiosis. Within Bikonta, FAP was kept in photosynthetic secondary endosymbiosis lineages but absent in those that lost the plastid. Characterization of recombinant FAPs from various algal genera (Nannochloropsis, Ectocarpus, Galdieria, Chondrus) provided experimental evidence that FAP activity is conserved in red and brown algae and is not limited to unicellular species. These results thus indicate that FAP has been conserved during evolution of most algal lineages when photosynthesis was kept and suggest that its function is linked to photosynthetic membranes.One sentence summary FAP is present in thylakoids and conserved beyond green algae.Competing Interest StatementThe authors have declared no competing interest.View Full Text

plant biology↗

bioRxiv2019

Algal photosynthesis converts nitric oxide into nitrous oxide

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. Furthermore, NO reduction by both pathways is restricted to Chlorophytes, organisms particularly abundant in ocean N2O-producing hotspots. 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.\n\nOne sentence summaryGreen microalgae produce N2O using flavodiiron proteins in the light and a cytochrome P450 NO reductase in the dark.

plant biology↗