Hunting the main player enabling Chlamydomonas reinhardtii growth under fluctuating light

Modulating cultivation regimes of Messastrum gracile SVMIICT7 for biomass productivity integrated with resource recovery via hydrothermal liquefaction

The present study was designed to assess Messastrum gracile SVMIICT7 potential in treating dairy wastewater (autoclaved (ADWW) and raw (DWW)) with relation to nutrient removal, in-vivo Chl-a-based biomass, and bio-oil synthesis. Chlorophyll a fluorescence kinetics revealed improved photochemical efficiency (0.639, Fv/Fm) in M. gracile when grown with DWW. This may be owing to enhanced electron transport being mediated by an effective water-splitting complex at photosystem (PSII) of thylakoids. The increase in ABS/RC observed in DWW can be attributed to the elevated chlorophyll content and reduced light dissipation, as evident by higher values of ETo/RC and a decrease in non-photochemical quenching (NPQ). M. gracile inoculated in DWW had the highest Chl-a-biomass yield (1.8 g L-1) and biomolecules while maximum nutrient removal efficiency was observed in ADWW (83.7% TN and 60.07% TP). M. gracile exhibited substantial bio-oil yield of 29.6% and high calorific value of 37.19 MJ kg-1, predominantly composed of hydrocarbons along with nitrogen and oxygen cyclic compounds. This research offers a thorough investigation into wastewater treatment, illustrating the conversion of algal biomass into valuable energy sources and chemical intermediates within the framework of a biorefinery.

Roles of ApcD and orange carotenoid protein in photoinduction of electron transport upon dark-light transition in the Synechocystis PCC 6803 mutant deficient in flavodiiron protein Flv1

Flavodiiron proteins Flv1/Flv3 accept electrons from photosystem (PS) I. In this work we investigated light adaptation mechanisms of Flv1-deficient mutant of Synechocystis PCC 6803, incapable to form the Flv1/Flv3 heterodimer. First seconds of dark-light transition were studied by parallel measurements of light-induced changes in chlorophyll fluorescence, P700 redox transformations, fluorescence emission at 77 K, and OCP-dependent fluorescence quenching. During the period of Calvin cycle activation upon dark-light transition, the linear electron transport (LET) in wild type is supported by the Flv1/Flv3 heterodimer, whereas in Δflv1 mutant activation of LET upon illumination is preceded by cyclic electron flow that maintains State 2. The State 2-State 1 transition and Orange Carotenoid Protein (OCP)-dependent non-photochemical quenching occur independently of each other, begin in about 10 s after the illumination of the cells and are accompanied by a short-term re-reduction of the PSI reaction center (P700+). ApcD is important for the State 2-State 1 transition in the Δflv1 mutant, but S-M rise in chlorophyll fluorescence was not completely inhibited in Δflv1/ΔapcD mutant. LET in Δflv1 mutant starts earlier than the S-M rise in chlorophyll fluorescence, and the oxidation of plastoquinol (PQH2) pool promotes the activation of PSII, transient re-reduction of P700+ and transition to State 1. An attempt to induce state transition in the wild type under high intensity light using methyl viologen, highly oxidizing P700 and PQH2, was unsuccessful, showing that oxidation of intersystem electron-transport carriers might be insufficient for the induction of State 2-State 1 transition in wild type of Synechocystis under high light.

Dysfunction of Chloroplast Protease Activity Mitigates pgr5 Phenotype in the Green Algae Chlamydomonas reinhardtii

Researchers have described protection mechanisms against the photoinhibition of photosystems under strong-light stress. Cyclic Electron Flow (CEF) mitigates electron acceptor-side limitation, and thus contributes to Photosystem I (PSI) protection. Chloroplast protease removes damaged protein to assist with protein turn over, which contributes to the quality control of Photosystem II (PSII). The PGR5 protein is involved in PGR5-dependent CEF. The FTSH protein is a chloroplast protease which effectively degrades the damaged PSII reaction center subunit, D1 protein. To investigate how the PSI photoinhibition phenotype in pgr5 would be affected by adding the ftsh mutation, we generated double-mutant pgr5ftsh via crossing, and its phenotype was characterized in the green algae Chlamydomonas reinhardtii. The cells underwent high-light incubation as well as low-light incubation after high-light incubation. The time course of Fv/Fm values in pgr5ftsh showed the same phenotype with ftsh1-1. The amplitude of light-induced P700 photo-oxidation absorbance change was measured. The amplitude was maintained at a low value in the control and pgr5ftsh during high-light incubation, but was continuously decreased in pgr5. During the low-light incubation after high-light incubation, amplitude was more rapidly recovered in pgr5ftsh than pgr5. We concluded that the PSI photoinhibition by the pgr5 mutation is mitigated by an additional ftsh1-1 mutation, in which plastoquinone pool would be less reduced due to damaged PSII accumulation.

Deciphering the structure, function, and mechanism of lysine acetyltransferase cGNAT2 in cyanobacteria

Lysine acetylation is a conserved regulatory posttranslational protein modification that is performed by lysine acetyltransferases (KATs). By catalyzing the transfer of acetyl groups to substrate proteins, KATs play critical regulatory roles in all domains of life; however, no KATs have yet been identified in cyanobacteria. Here, we tested all predicted KATs in the cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) and demonstrated that A1596, which we named cyanobacterial Gcn5-related N-acetyltransferase (cGNAT2), can catalyze lysine acetylation in vivo and in vitro. Eight amino acid residues were identified as the key residues in the putative active site of cGNAT2, as indicated by structural simulation and site-directed mutagenesis. The loss of cGNAT2 altered both growth and photosynthetic electron transport in Syn7002. In addition, quantitative analysis of the lysine acetylome identified 548 endogenous substrates of cGNAT2 in Syn7002. We further demonstrated that cGNAT2 can acetylate NAD(P)H dehydrogenase J (NdhJ) in vivo and in vitro, with the inability to acetylate K89 residues, thus decreasing NdhJ activity and affecting both growth and electron transport in Syn7002. In summary, this study identified a KAT in cyanobacteria and revealed that cGNAT2 regulates growth and photosynthesis in Syn7002 through an acetylation-mediated mechanism.

Quantifying the roles of algal photosynthetic electron pathways: a milestone towards photosynthetic robustness

During photosynthesis, electron transport reactions generate and shuttle reductant to allow CO2 reduction by the Calvin-Benson-Bassham cycle and the formation of biomass building block in the so-called linear electron flow (LEF). However, in nature, environmental parameters like light intensity or CO2 availability can vary and quickly change photosynthesis rates, creating an imbalance between photosynthetic energy production and metabolic needs. In addition to LEF, alternative photosynthetic electron flows are central to allow photosynthetic energy to match metabolic demand in response to environmental variations. Microalgae arguably harbour one of the most diverse set of alternative electron flows (AEFs), including cyclic (CEF), pseudocyclic (PCEF) and chloroplast-to-mitochondria (CMEF) electron flow. While CEF, PCEF and CMEF have large functional overlaps, they differ in the conditions they are active and in their role for photosynthetic energy balance. Here, I review the molecular mechanisms of CEF, PCEF and CMEF in microalgae. I further propose a quantitative framework to compare their key physiological roles and quantify how the photosynthetic energy is partitioned to maintain a balanced energetic status of the cell. Key differences in AEF within the green lineage and the potential of rewiring photosynthetic electrons to enhance plant robustness will be discussed.

In situ self-induced electrical stimulation to plants: Modulates morphogenesis, photosynthesis and gene expression in Vigna radiata and Cicer arietinum

Specific stimuli to plants influence intracellular and intercellular communications, activation of ion channels, gene expression, growth and development. The functional role of self-induced in situ electrical stimuli at the rhizosphere of the plant by placing electrode assembly in a defined circuit mode was studied on the growth and development of Vigna radiata and Cicer arietinum plants. Experiments were designed with three-circuit mode configurational variations (CC-P, OC-P and SC-P) and compared with the relative performance of control system (non-potential). The plants cultivated under the in situ electrical stimuli (low-current) showed a marked influence on growth and photosynthetic performance of the plants. CC-P operation showed improved vegetative growth, characterized by increased roots, shoots and biomass along with accelerated plant growth from seed germination to vegetation, flowering and pod formation leading towards earlier and more robust flowering compared to control system. Plants also showed higher aquaporin gene expression levels in CC-P operation. The control operation showed 10 days additional maturation time compared to CC-P operation. The strategy can be beneficially applied to augment the bioremediation capacity of complex pollutants with reference to phytoremediation or constructed wetland systems where the plant and its roots are the main enabler apart from agriculture applications specific to nursery-raised or transplanted plants.

Characterizing compensatory mechanisms in the absence of photoprotective qE in Chlamydomonas reinhardtii

Rapid fluctuations in the quantity and quality of natural light expose photosynthetic organisms to conditions when the capacity to utilize absorbed quanta is insufficient. These conditions can result in the production of reactive oxygen species and photooxidative damage. Non-photochemical quenching (NPQ) and alternative electron transport are the two most prominent mechanisms which synergistically function to minimize the overreduction of photosystems. In the green alga Chlamydomonas reinhardtii, the stress-related light-harvesting complex (LHCSR) is a required component for the rapid induction and relaxation of NPQ in the light-harvesting antenna. Here, we use simultaneous chlorophyll fluorescence and oxygen exchange measurements to characterize the acclimation of the Chlamydomonas LHCSR-less mutant (npq4lhcsr1) to saturating light conditions. We demonstrate that, in the absence of NPQ, Chlamydomonas does not acclimate to sinusoidal light through increased light-dependent oxygen consumption. We also show that the npq4lhcsr1 mutant has an increased sink capacity downstream of PSI and this energy flow is likely facilitated by cyclic electron transport. Furthermore, we show that the timing of additions of mitochondrial inhibitors has a major influence on plastid/mitochondrial coupling experiments.

Algal PETC-Pro171-Leu suppresses electron transfer in cytochrome b6f under acidic lumenal conditions

Linear photosynthetic electron flow (LEF) produces NADPH and generates a proton electrochemical potential gradient across the thylakoid membrane to synthesize ATP, both of which are required for CO2 fixation. As cellular demand for ATP and NADPH varies, cyclic electron flow (CEF) between Photosystem I and the cytochrome b6f complex (b6f) produces extra ATP. b6f regulates LEF and CEF via photosynthetic control, which is a pH-dependent b6f slowdown of plastoquinol oxidation at the lumenal site. This protection mechanism is triggered at more alkaline lumen pH in the pgr1 (proton gradient regulation 1) mutant of the vascular plant Arabidopsis (Arabidopsis thaliana), which contains a Pro194Leu substitution in the b6f Rieske Iron-sulfur protein Photosynthetic Electron Transfer C (PETC) subunit. In this work, we introduced the equivalent pgr1 mutation in the green alga Chlamydomonas reinhardtii to generate PETC-P171L. Consistent with the pgr1 phenotype, PETC-P171L displayed impaired NPQ induction along with slower photoautotrophic growth under high light conditions. Our data provide evidence that the ΔpH component in PETC-P171L depends on oxygen availability. Only under low oxygen conditions was the ΔpH component sufficient to trigger a phenotype in algal PETC-P171L where the mutant b6f was more restricted to oxidize the plastoquinol pool and showed diminished electron flow through the b6f complex. These results demonstrate that photosynthetic control of different stringency are established in C. reinhardtii depending on the cellular metabolism, and the lumen pH-sensitive PETC-P171L was generated to read out various associated effects.

Alternative electron transport pathways contribute to tolerance to high light stress in lichenized algae

The photosynthetic apparatus of lichen photobionts has been well-characterized by chlorophyll fluorescence analysis (e.g., by pulse amplitude modulation [PAM]), which provides a proxy of the activity of photosystem II (PSII) and its antenna. However, such kinetics are unable to directly characterize photosystem I (PSI) activity and the associated alternative electron pathways that may be involved in photoprotection. Instead, PSI can be probed in vivo by near-infrared absorption, measured at the same time as standard chlorophyll fluorescence (e.g., using the WALZ Dual PAM). Here, we used the Dual PAM to investigate cyclic electron flow and photoprotection in a range of mostly temperate lichens sampled from shaded to more open microhabitats. Sun species displayed lower acceptor side limitation of PSI (Y[NA]) early in illumination when compared to shade species, indicative of higher flavodiiron-mediated pseudocyclic electron flow. In response to high irradiance, some lichens accumulate melanin, and Y[NA] was lower and NAD(P)H dehydrogenase (NDH-2)-type cyclic flow was higher in melanised than pale forms. Furthermore, non-photochemical quenching (NPQ) was higher and faster relaxing in shade than sun species, while all lichens displayed high rates of photosynthetic cyclic electron flow. In conclusion, our data suggest that (1) low acceptor side limitation of PSI is important for sun-exposed lichens; (2) NPQ helps shade species tolerate brief exposure to high irradiance; and (3) cyclic electron flow is a prominent feature of lichens regardless of habitat, although NDH-2-type flow is associated with high light acclimation.

Flv3A facilitates O2 photoreduction and affects H2 photoproduction independently of Flv1A in diazotrophic Anabaena filaments

The model heterocyst-forming filamentous cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a typical example of a multicellular organism capable of simultaneously performing oxygenic photosynthesis in vegetative cells and O₂ -sensitive N₂ -fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O₂ (a Mehler-like reaction). Here, we performed a phenotypic and biophysical characterization of Anabaena mutants impaired in vegetative-specific Flv1A and Flv3A in order to address their physiological relevance in the bioenergetic processes occurring in diazotrophic Anabaena under variable CO₂ conditions. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler-like reaction upon a sudden increase in light intensity, which is likely important for the activation of carbon-concentrating mechanisms and CO₂ fixation. Under ambient CO₂ diazotrophic conditions, Flv3A is responsible for moderate O₂ photoreduction, independently of Flv1A, but only in the presence of Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst-specific uptake hydrogenase, which led to enhanced H₂ photoproduction under both oxic and micro-oxic conditions. These results reveal a novel regulatory network between the Mehler-like reaction and the diazotrophic metabolism, which is of great interest for future biotechnological applications.

Transcriptomic and metabolic signatures of diatom plasticity to light fluctuations

Unlike in terrestrial and freshwater ecosystems, light fields in oceans fluctuate due to both horizontal current and vertical mixing. Diatoms thrive and dominate the phytoplankton community in these fluctuating light fields. However, the molecular mechanisms that regulate diatom acclimation and adaptation to light fluctuations are poorly understood. Here, we performed transcriptome sequencing, metabolome profiling, and 13C-tracer labeling on the model diatom Phaeodactylum tricornutum. The diatom acclimated to constant light conditions was transferred to six different light conditions, including constant light (CL5d), short-term (1 h) high light (sHL1h), and short-term (1 h) and long-term (5 days) mild or severe light fluctuation conditions (mFL1h, sFL1h, mFL5d, and sFL5d) that mimicked land and ocean light levels. We identified 2,673 transcripts (25% of the total expressed genes) expressed differentially under different fluctuating light regimes. We also identified 497 transcription factors, 228 not reported previously, which exhibited higher expression under light fluctuations, including 7 with a light-sensitive PAS domain (Per-period circadian protein, Arnt-aryl hydrocarbon receptor nuclear translocator protein, Sim-single-minded protein) and 10 predicted to regulate genes related to light-harvesting complex proteins. Our data showed that prolonged preconditioning in severe light fluctuation enhanced photosynthesis in P. tricornutum under this condition, as evidenced by increased oxygen evolution accompanied by the upregulation of Rubisco and light-harvesting proteins. Furthermore, severe light fluctuation diverted the metabolic flux of assimilated carbon preferentially toward fatty acid storage over sugar and protein. Our results suggest that P. tricornutum use a series of complex and different responsive schemes in photosynthesis and carbon metabolism to optimize their growth under mild and severe light fluctuations. These insights underscore the importance of using more intense conditions when investigating the resilience of phytoplankton to light fluctuations.

Photoinhibition of Photosystem I Induced by Different Intensities of Fluctuating Light Is Determined by the Kinetics of ∆pH Formation Rather Than Linear Electron Flow

Fluctuating light (FL) can cause the selective photoinhibition of photosystem I (PSI) in angiosperms. In nature, leaves usually experience FL conditions with the same low light and different high light intensities, but the effects of different FL conditions on PSI redox state and PSI photoinhibition are not well known. In this study, we found that PSI was highly reduced within the first 10 s after transition from 59 to 1809 μmol photons m^-2 s^-1 in tomato (Solanum lycopersicum). However, such transient PSI over-reduction was not observed by transitioning from 59 to 501 or 923 μmol photons m^-2 s^-1. Consequently, FL (59-1809) induced a significantly stronger PSI photoinhibition than FL (59-501) and FL (59-923). Compared with the proton gradient (∆pH) level after transition to high light for 60 s, tomato leaves almost formed a sufficient ∆pH after light transition for 10 s in FL (59-501) but did not in FL (59-923) or FL (59-1809). The difference in ∆pH between 10 s and 60 s was tightly correlated to the extent of PSI over-reduction and PSI photoinhibition induced by FL. Furthermore, the difference in PSI photoinhibition between (59-923) and FL (59-1809) was accompanied by the same level of linear electron flow. Therefore, PSI photoinhibition induced by different intensities of FL is more related to the kinetics of ∆pH formation rather than linear electron flow.

Photosynthetic regulation in fluctuating light under combined stresses of high temperature and dehydration in three contrasting mosses

Photosynthesis regulation is fundamental for the response to environmental dynamics, especially for bryophytes during their adaptation to terrestrial life. Alternative electron flow mediated by flavodiiron proteins (FLV) and cyclic electron flow (CEF) around photosystem I (PSI) play seminal roles in the response to abiotic stresses in mosses; nevertheless, their correlation and relative contribution to photoprotection of mosses exposed to combined stresses remain unclear. In the present study, the photosynthetic performance and recovery capacity of three moss species from different growth habitats were examined during heat and dehydration with fluctuating light. Our results showed that dehydration at 22 °C for 24 h caused little photodamage, and most of the parameters recovered to their original values after rehydration. In contrast, dehydration at 38 °C caused drastic injuries, especially to PSII, which was mainly caused by the inactivation of non-photochemical quenching (NPQ). Dehydration also induced a high accumulation of O₂^- and H₂O₂. A consistently higher CEF as well as a positive correlation between CEF and FLV was observed in resistant R. japonicum, implying CEF played a more important protective role for R. japonicum. In H. plumaeforme and P. cuspidatum, the positive relationship under mild stress switched to negative when stress became severe. Therefore, FLV pathway was sensitive to environmental fluctuations and maybe less efficient than CEF thus, readily to be lost during land colonization and evolution in angiosperms. Our work provides insights into the coordination of various pathways to fine-tune photosynthetic protection and can be used as a basis for species screening and development of breeding strategies for degraded ecosystem restoration with pioneering mosses.

Comparative appraisal of nutrient recovery, bio-crude, and bio-hydrogen production using Coelestrella sp. in a closed-loop biorefinery

A closed loop algal-biorefinery was designed based on a three-stage integration of dairy wastewater (DWW) treatment, hydrothermal liquefaction (HTL) of defatted algal biomass, and acidogenic process in a semi-synthetic framework. Initially, Coelestrella sp SVMIICT5 was grown in a 5 L photo-bioreactor and scaled up to a 50 L flat-panel photo-bioreactor using DWW. The microalgal growth showed higher photosynthetic efficiency, resulting in a biomass growth of 3.2 g/L of DCW with 87% treatment efficiency. The biomolecular composition showed 26% lipids with a good fatty acid profile (C12-C21) as well as carbohydrate (24.9%) and protein (31.8%) content. In the second stage, the de-oiled algal biomass was valorized via HTL at various temperatures (150°C, 200°, and 250°C) and reaction atmospheres (N2 and H2). Among these, the 250°C (H2) condition showed a 52% bio-crude fraction and an HHV of ∼29.47 MJ/kg (bio-oil) with a saturated hydrocarbon content of 64.3% that could be further upgraded to jet fuels. The energy recovery (73.01%) and elemental enrichment (carbon; 65.67%) were relatively greater in H2 compared to N2 conditions. Finally, dark fermentation of the complex-structured HTL-AF stream resulted in a total bio-H2 production of 231 ml/g of TOC with a 63% treatment efficiency. Life cycle analysis (LCA) was also performed for the mid-point and damage categories to assess the sustainability of the integrated process. Thus, the results of this study demonstrated comprehensive wastewater treatment and valorization of de-oiled algal biomass for chemical/fuel intermediates in the biorefinery context by low-carbon processes.

The Dynamic Changes of Alternative Electron Flows upon Transition from Low to High Light in the Fern Cyrtomium fortune and the Gymnosperm Nageia nagi

In photosynthetic organisms except angiosperms, an alternative electron sink that is mediated by flavodiiron proteins (FLVs) plays the major role in preventing PSI photoinhibition while cyclic electron flow (CEF) is also essential for normal growth under fluctuating light. However, the dynamic changes of FLVs and CEF has not yet been well clarified. In this study, we measured the P700 signal, chlorophyll fluorescence, and electrochromic shift spectra in the fern Cyrtomium fortune and the gymnosperm Nageia nagi. We found that both species could not build up a sufficient proton gradient (∆pH) within the first 30 s after light abruptly increased. During this period, FLVs-dependent alternative electron flow was functional to avoid PSI over-reduction. This functional time of FLVs was much longer than previously thought. By comparison, CEF was highly activated within the first 10 s after transition from low to high light, which favored energy balancing rather than the regulation of a PSI redox state. When FLVs were inactivated during steady-state photosynthesis, CEF was re-activated to favor photoprotection and to sustain photosynthesis. These results provide new insight into how FLVs and CEF interact to regulate photosynthesis in non-angiosperms.

Exogenous melatonin strongly affects dynamic photosynthesis and enhances water-water cycle in tobacco

Melatonin (MT), an important phytohormone synthesized naturally, was recently used to improve plant resistance against abiotic and biotic stresses. However, the effects of exogenous melatonin on photosynthetic performances have not yet been well clarified. We found that spraying of exogenous melatonin (100 μM) to leaves slightly affected the steady state values of CO₂ assimilation rate (A _N ), stomatal conductance (g _s ) and mesophyll conductance (g _m ) under high light in tobacco leaves. However, this exogenous melatonin strongly delayed the induction kinetics of g _s and g _m , leading to the slower induction speed of A _N . During photosynthetic induction, A _N is mainly limited by biochemistry in the absence of exogenous melatonin, but by CO₂ diffusion conductance in the presence of exogenous melatonin. Therefore, exogenous melatonin can aggravate photosynthetic carbon loss during photosynthetic induction and should be used with care for crop plants grown under natural fluctuating light. Within the first 10 min after transition from low to high light, photosynthetic electron transport rates (ETR) for A _N and photorespiration were suppressed in the presence of exogenous melatonin. Meanwhile, an important alternative electron sink, namely water-water cycle, was enhanced to dissipate excess light energy. These results indicate that exogenous melatonin upregulates water-water cycle to facilitate photoprotection. Taking together, this study is the first to demonstrate that exogenous melatonin inhibits dynamic photosynthesis and improves photoprotection in higher plants.

Polyhydroxybutyrate production by Chlorella sorokiniana SVMIICT8 under Nutrient-deprived mixotrophy

Polyhydroxybutyrates (PHBs) are naturally occurring biopolymeric compounds that accumulate in a variety of microorganisms, including microalgae as energy and carbon storage sources. The present study was designed to evaluate nature-based PHB production using microalgae (Chlorella sorokiniana SVMIICT8) in biphasic (growth (GP) and stress phase (SP)) nutritional mode of cultivation. Microalgal PHB accumulation was driven by nutrient constraint, with a maximal production of 29.5% of PHB from 0.94 gm L^-1 of biomass. Fluorescence microscopy revealed PHB granules in the cell cytoplasm, while NMR (¹H and ¹³C), XRD and TGA analysis confirmed the structure. The biopolymer obtained was homopolymer of PHB with carbonyl (C=O) stretch of the aliphatic ester moiety. In GC-MS analysis, major peak representing butyric acid methyl ester also confirmed the PHB. Chlorophyll a fluorescence transients inferred through OJIP, exhibited significant variation in photosynthetic process during growth and nutrient limiting conditions. Mining of bio-based products from microalgae cultivation embrace nature-based approach addressing climate change and sustainability inclusively.

Flavodiiron proteins enhance the rate of CO2 assimilation in Arabidopsis under fluctuating light intensity

The proton concentration gradient (ΔpH) and membrane potential (Δψ) formed across the thylakoid membrane contribute to ATP synthesis in chloroplasts. Additionally, ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen. K+ exchange antiporter 3 (KEA3) relaxes this downregulation by substituting ΔpH with Δψ in response to fluctuation of light intensity. In the Arabidopsis (Arabidopsis thaliana) line overexpressing KEA3 (KEA3ox), the rate of electron transport is elevated by accelerating the relaxation of ΔpH after a shift from high light (HL) to low light. However, the plant cannot control electron transport toward photosystem I (PSI), resulting in PSI photodamage. In this study, we crossed the KEA3ox line with the line (Flavodiiron [Flv]) expressing the Flv proteins of Physcomitrium patens. In the double transgenic line (Flv-KEA3ox), electrons overloading toward PSI were pumped out by Flv proteins. Consequently, photodamage of PSI was alleviated to the wild-type level. The rate of CO2 fixation was enhanced in Flv and Flv-KEA3ox lines during HL periods of fluctuating light, although CO2 fixation was unaffected in any transgenic lines in constant HL. Upregulation of CO2 fixation was accompanied by elevated stomatal conductance in fluctuating light. Consistent with the results of gas exchange experiments, the growth of Flv and Flv-KEA3ox plants was better than that of WT and KEA3ox plants under fluctuating light.

Photosynthesis under fluctuating light in the CAM plant Vanilla planifolia

Photosynthetic induction after a sudden increase in illumination affects carbon gain. Photosynthetic dynamics under fluctuating light (FL) have been widely investigated in C3 and C4 plants but are little known in CAM plants. In our present study, the chlorophyll fluorescence, P700 redox state and electrochromic shift signals were measured to examine photosynthetic characteristics under FL in the CAM orchid Vanilla planifolia. The light use efficiency was maximized in the morning but was restricted in the afternoon, indicating that the pool of malic acid dried down in the afternoon. During photosynthetic induction in the morning, electron flow through photosystem I rapidly reached the 95% of the maximum value in 4-6 min, indicating that V. planifolia showed a fast photosynthetic induction when compared with C3 and C4 plants reported previously. Upon a sudden transition from dark to actinic light, a rapid re-oxidation of P700 was observed in V. planifolia, indicating the fast outflow of electrons from PSI to alternative electron acceptors, which was attributed to the O₂ photo-reduction mediated by water-water cycle. The functioning of water-water cycle prevented photosystem I over-reduction after transitioning from low to high light and thus protected PSI under FL. In the afternoon, cyclic electron flow was stimulated under FL to fine-tune photosynthetic apparatus when photosynthetic CO₂ was restricted. Therefore, water-water cycle cooperates with cyclic electron flow to regulate the photosynthesis under FL in the CAM orchid V. planifolia.

Differential Response of the Photosynthetic Machinery to Fluctuating Light in Mature and Young Leaves of Dendrobium officinale

A key component of photosynthetic electron transport chain, photosystem I (PSI), is susceptible to the fluctuating light (FL) in angiosperms. Cyclic electron flow (CEF) around PSI and water-water cycle (WWC) are both used by the epiphytic orchid Dendrobium officinale to protect PSI under FL. This study examined whether the ontogenetic stage of leaf has an impact on the photoprotective mechanisms dealing with FL. Thus, chlorophyll fluorescence and P700 signals under FL were measured in D. officinale young and mature leaves. Upon transition from dark to actinic light, a rapid re-oxidation of P700 was observed in mature leaves but disappeared in young leaves, indicating that WWC existed in mature leaves but was lacking in young leaves. After shifting from low to high light, PSI over-reduction was clearly missing in mature leaves. By comparison, young leaves showed a transient PSI over-reduction within the first 30 s, which was accompanied with highly activation of CEF. Therefore, the effect of FL on PSI redox state depends on the leaf ontogenetic stage. In mature leaves, WWC is employed to avoid PSI over-reduction. In young leaves, CEF around PSI is enhanced to compensate for the lack of WWC and thus to prevent an uncontrolled PSI over-reduction induced by FL.

Phycoremediation potential ofTetradesmus sp.SVMIICT4in treating dairy wastewaterusingFlat-Panel photobioreactor

Tetradesmus sp. SVMIICT4 was isolated and cultivated mixotrophically in a flat-panel photobioreactor (FP-PBR) for concurrent dairy wastewater treatment, carbon fixation, and biomass production. Integrated wastewater treatment showed good COD and nutrients removal efficiency accounting for biomass with an accumulation of carbohydrate (21.48 mg g^-1) and protein (19.52 mg g^-1). Chlorophyll a fluorescence transients (Fv/Fm, ETo/RC, TRo/RC, and Abs/RC) deduced through OJIP curve fittings, showed consistent improvement in photosynthetic activity throughout the cultivation period. The absorption flux per reaction centre corroborated with increased chlorophyll content (18.94 mg g^-1), resulting in higher electron transport (ET/Rc) and lower non-photochemical quenching (NPQ). The fatty acid profile showed high content of unsaturated, followed by saturated fatty acids, which has multiple applications in food, feed, and fuel industries, enabling a bio-based economy.

Regulation of Leaf Angle Protects Photosystem I under Fluctuating Light in Tobacco Young Leaves

Fluctuating light is a typical light condition in nature and can cause selective photodamage to photosystem I (PSI). The sensitivity of PSI to fluctuating light is influenced by the amplitude of low/high light intensity. Tobacco mature leaves are tended to be horizontal to maximize the light absorption and photosynthesis, but young leaves are usually vertical to diminish the light absorption. Therefore, we tested the hypothesis that such regulation of the leaf angle in young leaves might protect PSI against photoinhibition under fluctuating light. We found that, upon a sudden increase in illumination, PSI was over-reduced in extreme young leaves but was oxidized in mature leaves. After fluctuating light treatment, such PSI over-reduction aggravated PSI photoinhibition in young leaves. Furthermore, the leaf angle was tightly correlated to the extent of PSI photoinhibition induced by fluctuating light. Therefore, vertical young leaves are more susceptible to PSI photoinhibition than horizontal mature leaves when exposed to the same fluctuating light. In young leaves, the vertical leaf angle decreased the light absorption and thus lowered the amplitude of low/high light intensity. Therefore, the regulation of the leaf angle was found for the first time as an important strategy used by young leaves to protect PSI against photoinhibition under fluctuating light. To our knowledge, we show here new insight into the photoprotection for PSI under fluctuating light in nature.

Photorespiration Alleviates Photoinhibition of Photosystem I under Fluctuating Light in Tomato

Fluctuating light (FL) is a typical natural light stress that can cause photodamage to photosystem I (PSI). However, the effect of growth light on FL-induced PSI photoinhibition remains controversial. Plants grown under high light enhance photorespiration to sustain photosynthesis, but the contribution of photorespiration to PSI photoprotection under FL is largely unknown. In this study, we examined the photosynthetic performance under FL in tomato (Lycopersicon esculentum) plants grown under high light (HL-plants) and moderate light (ML-plants). After an abrupt increase in illumination, the over-reduction of PSI was lowered in HL-plants, resulting in a lower FL-induced PSI photoinhibition. HL-plants displayed higher capacities for CO2 fixation and photorespiration than ML-plants. Within the first 60 s after transition from low to high light, PSII electron transport was much higher in HL-plants, but the gross CO2 assimilation rate showed no significant difference between them. Therefore, upon a sudden increase in illumination, the difference in PSII electron transport between HL- and ML-plants was not attributed to the Calvin–Benson cycle but was caused by the change in photorespiration. These results indicated that the higher photorespiration in HL-plants enhanced the PSI electron sink downstream under FL, which mitigated the over-reduction of PSI and thus alleviated PSI photoinhibition under FL. Taking together, we here for the first time propose that photorespiration acts as a safety valve for PSI photoprotection under FL.

Light-harvesting complex stress-related proteins play crucial roles in the acclimation of Physcomitrella patens under fluctuating light conditions

Photosynthetic organisms have evolved photoprotective mechanisms to acclimate to light intensity fluctuations in their natural growth environments. Photosystem (PS) II subunit S (PsbS) and light-harvesting complex (LHC) stress-related proteins (LhcSR) are essential for triggering photoprotection in vascular plants and green algae, respectively. The activity of both proteins is strongly enhanced in the moss Physcomitrella patens under high-light conditions. However, their role in regulating photosynthesis acclimation in P. patens under fluctuating light (FL) conditions is still unknown. Here, we compare the responses of wild-type (WT) P. patens and mutants lacking PsbS (psbs KO) or LhcSR1 and 2 (lhcsr KO) to FL conditions in which the low-light phases were periodically interrupted with high-light pulses. lhcsr KO mutant showed a strong reduction in growth with respect to WT and psbs KO under FL conditions. The lack of LhcSR not only decreased the level of non-photochemical quenching, resulting in an over-reduced plastoquinone pool, but also significantly increased the PSI acceptor limitation values with respect to WT and psbs KO under FL conditions. Moreover, in lhcsr KO mutant, the abundance of PSI core and PSI-LHCI complex decreased greatly under FL conditions compared with the WT and psbs KO. We proposed that LhcSR in P. patens play a crucial role in moss acclimation to dynamic light changes.

Mixoplankton interferences in dilution grazing experiments

It remains unclear as to how mixoplankton (coupled phototrophy and phagotrophy in one cell) affects the estimation of grazing rates obtained from the widely used dilution grazing technique. To address this issue, we prepared laboratory-controlled dilution experiments with known mixtures of phyto-, protozoo-, and mixoplankton, operated under different light regimes and species combinations. Our results evidenced that chlorophyll is an inadequate proxy for phytoplankton when mixoplankton are present. Conversely, species-specific cellular counts could assist (although not fully solve) in the integration of mixoplanktonic activity in a dilution experiment. Moreover, cell counts can expose prey selectivity patterns and intraguild interactions among grazers. Our results also demonstrated that whole community approaches mimic reality better than single-species laboratory experiments. We also confirmed that light is required for protozoo- and mixoplankton to correctly express their feeding activity, and that overall diurnal grazing is higher than nocturnal. Thus, we recommend that a detailed examination of initial and final plankton communities should become routine in dilution experiments, and that incubations should preferably be started at the beginning of both day and night periods. Finally, we hypothesize that in silico approaches may help disentangle the contribution of mixoplankton to the community grazing of a given system.

Mixotrophic cultivation of isolated Messastrum gracile SVMIICT7: Photosynthetic response and product profiling

The isolated Messastrum gracile SVMIICT7 was mixotrophically cultivated in flat panel photobioreactor (FP-PBR) towards understanding the photosynthetic transient and product profile. Biomass productivity attained a maximum of 45 mg L^-1d^-1, with COD, nitrate and phosphate removal of 83.3%, 84.05%, and 74.98% respectively. Messastrum sp. showed good assimilation of proteins (124 mg g^-1) (w/w), carbohydrates (119 mg g^-1) (w/w) and lipids (26%) (w/w). The myristoleic acid (C14:1-39.1%) and heptadecanoic acid (C17:0-29.1%) are abundant fatty acids with therapeutic, food and feed applications. The cellular ultrastructure studies revealed facile arrangement of chloroplast and starch covered pyrenoids supporting increased carbohydrate accumulation. Photosystem II (PSII) [Y(II), ETR(II), Y(NPQ), and Y(NO)] and photosystem I (PSI) [Y(I), ETR(I), Y(NA), and Y(ND)] transients showed improved photosynthetic efficiency directing microalgae growth and biomass productivity. Higher Fv/Fm values indicates relatively good water splitting and carbon fixation at PSII and PSI facilitating improved photosynthetic electron transport and synthesis of value-added products thereby enabling bioeconomy.

Roles of alternative electron flows in response to excess light in Ginkgo biloba

Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O₂ mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba, FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba, FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light.

Photosynthetic transients in Chlorella sorokiniana during phycoremediation of dairy wastewater under distinct light intensities

The present study is aimed to understand the photosynthetic transients of Chlorella sorokiniana SVMBIOEN2 during treatment of dairy wastewater under different light intensities (100, 150, and 200 µmol m^-2s^-1) in mixotrophic mode. Light intensities showed marked influence on photosystem behavior, lipid profile, and organic pollutant removal. Analysis of Chlorophyll a fluorescence transient including Fv/Fm, ETo/RC, TRo/RC, and Abs/RC showed better photosystem efficiency at 100 µmol m^-2s^-1 operations. OJIP curve fitting depicted a positive L-band at 150 µmol m^-2s^-1 indicating lower kinetic energy of photosystem II (PSII) reaction centres at high light intensities. Better photosynthetic activity at 100 µmol m^-2s^-1 operations resulted in good assimilation of biomass (2.3 g L^-1), carbohydrates (10.2 mg g^-1), and proteins (14 mg g^-1) with a significant reduction in chemical oxygen demand (85%). Phycoremediation of dairy wastewater accumulates predominantly monounsaturated fatty acids followed by polyunsaturated fatty acids showing the application of C. sorokiniana in nutraceutical and food industries.

Chlamydomonas as a model for reactive oxygen species signaling and thiol redox regulation in the green lineage

One-sentence summary: Advances in proteomic and transcriptomic studies have made Chlamydomonas a powerful research model in redox and reactive oxygen species regulation with unique and overlapping mechanisms with plants.

Regulatory electron transport pathways of photosynthesis in cyanobacteria and microalgae: Recent advances and biotechnological prospects

Cyanobacteria and microalgae perform oxygenic photosynthesis where light energy is harnessed to split water into oxygen and protons. This process releases electrons that are used by the photosynthetic electron transport chain to form reducing equivalents that provide energy for the cell metabolism. Constant changes in environmental conditions, such as light availability, temperature, and access to nutrients, create the need to balance the photochemical reactions and the metabolic demands of the cell. Thus, cyanobacteria and microalgae evolved several auxiliary electron transport (AET) pathways to disperse the potentially harmful over-supply of absorbed energy. AET pathways are comprised of electron sinks, e.g. flavodiiron proteins (FDPs) or other terminal oxidases, and pathways that recycle electrons around photosystem I, like NADPH-dehydrogenase-like complexes (NDH) or the ferredoxin-plastoquinone reductase (FQR). Under controlled conditions the need for these AET pathways is decreased and AET can even be energetically wasteful. Therefore, redirecting photosynthetic reducing equivalents to biotechnologically useful reactions, catalyzed by i.e. innate hydrogenases or heterologous enzymes, offers novel possibilities to apply photosynthesis research.

Photosynthetic regulation in response to fluctuating light conditions under temperature stress in three mosses with different light requirements

Under natural field conditions, mosses experience fluctuating light intensities combined with temperature stress. Alternative electron flow mediated by flavodiiron proteins (FLVs) and cyclic electron flow (CEF) around photosystem I (PSI) allow mosses to growth under fluctuating light conditions. However, little is known about the roles of FLVs and CEF in the regulation of photosynthesis under temperature stress combined with fluctuating light. Here, we measured chlorophyll fluorescence and P700 redox state under fluctuating light conditions at 4 °C, 20 °C, and 35 °C in three mosses with different light requirements. Upon a sudden increase in light intensity, electron flow from photosystem II initially increased and then gradually decreased at 20 °C and 35 °C, indicating that the operation of FLV-dependent flow lasted much longer than previously thought. Furthermore, the absolute rates of FLV-dependent flow and CEF were enhanced under fluctuating light at 35 °C, pointing to their important roles in photoprotection when exposed to fluctuating light at moderate high temperature. Furthermore, the downregulation of FLV activity at 4 °C was partially compensated for by enhanced CEF activity. These results suggested the subtle coordination between FLV activity and CEF under fluctuating light and temperature stress. Racomitrium japonicum and Hypnum plumaeforme, which usually grow under relatively high light levels, exhibited higher FLV activity and CEF than the shade-grown moss Plagiomnium ellipticum. Based on our results, we conclude that photosynthetic acclimation to fluctuating light and temperature stress in different mosses is largely linked to the adjustment of FLV activity and CEF.

Improved photosynthetic capacity and photosystem I oxidation via heterologous metabolism engineering in cyanobacteria

Cyanobacteria have been increasingly explored as a biotechnological platform, although their economic feasibility relies in part on the capacity to maximize their photosynthetic, solar-to-biomass energy conversion efficiency. Here we show that cyanobacterial photosynthetic capacity can be increased by diverting cellular resources toward heterologous, energy-storing metabolic pathways and by reducing electron flow to photoprotective, but energy-dissipating, oxygen reduction reactions. We further show that these heterologous sinks can partially contribute to photosystem I (PSI) oxidation, suggesting an engineering strategy to improve both energy storage capacity and robustness by selective diversion of excess photosynthetic capacity to productive processes.

Cyanobacteria must prevent imbalances between absorbed light energy (source) and the metabolic capacity (sink) to utilize it to protect their photosynthetic apparatus against damage. A number of photoprotective mechanisms assist in dissipating excess absorbed energy, including respiratory terminal oxidases and flavodiiron proteins, but inherently reduce photosynthetic efficiency. Recently, it has been hypothesized that some engineered metabolic pathways may improve photosynthetic performance by correcting source/sink imbalances. In the context of this subject, we explored the interconnectivity between endogenous electron valves, and the activation of one or more heterologous metabolic sinks. We coexpressed two heterologous metabolic pathways that have been previously shown to positively impact photosynthetic activity in cyanobacteria, a sucrose production pathway (consuming ATP and reductant) and a reductant-only consuming cytochrome P450. Sucrose export was associated with improved quantum yield of phtotosystem II (PSII) and enhanced electron transport chain flux, especially at lower illumination levels, while cytochrome P450 activity led to photosynthetic enhancements primarily observed under high light. Moreover, coexpression of these two heterologous sinks showed additive impacts on photosynthesis, indicating that neither sink alone was capable of utilizing the full “overcapacity” of the electron transport chain. We find that heterologous sinks may partially compensate for the loss of photosystem I (PSI) oxidizing mechanisms even under rapid illumination changes, although this compensation is incomplete. Our results provide support for the theory that heterologous metabolism can act as a photosynthetic sink and exhibit some overlapping functionality with photoprotective mechanisms, while potentially conserving energy within useful metabolic products that might otherwise be “lost.”

Elevated CO2 Concentration Alters Photosynthetic Performances under Fluctuating Light in Arabidopsis thaliana

In view of the current and expected future rise in atmospheric CO₂ concentrations, we examined the effect of elevated CO₂ on photoinhibition of photosystem I (PSI) under fluctuating light in Arabidopsis thaliana. At 400 ppm CO₂, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO₂ conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO₂, indicating that elevated atmospheric CO₂ facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO₂ than in 400 ppm CO₂, suggesting that elevated atmospheric CO₂ mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO₂ does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO₂ fixation. Therefore, elevated CO₂ mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO₂ can have large effects on thylakoid reactions under fluctuating light.

Different Strategies for Photosynthetic Regulation under Fluctuating Light in Two Sympatric Paphiopedilum Species

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. Cyclic electron flow (CEF) around PSI and electron flux from water via the electron transport chain to oxygen (the water-water cycle) play important roles in coping with fluctuating light in angiosperms. However, it is unclear whether plant species in the same genus employ the same strategy to cope with fluctuating light. To answer this question, we measured P700 redox kinetics and chlorophyll fluorescence under fluctuating light in two Paphiopedilum (P.) Pftzer (Orchidaceae) species, P. dianthum and P. micranthum. After transition from dark to high light, P. dianthum displayed a rapid re-oxidation of P700, while P. micranthum displayed an over-reduction of P700. Furthermore, the rapid re-oxidation of P700 in P. dianthum was not observed when measured under anaerobic conditions. These results indicated that photo-reduction of O₂ mediated by the water-water cycle was functional in P. dianthum but not in P. micranthum. Within the first few seconds after an abrupt transition from low to high light, PSI was highly oxidized in P. dianthum but was highly reduced in P. micranthum, indicating that the different responses of PSI to fluctuating light between P. micranthum and P. dianthum was attributed to the water-water cycle. In P. micranthum, the lack of the water-water cycle was partially compensated for by an enhancement of CEF. Taken together, P. dianthum and P. micranthum employed different strategies to cope with the abrupt change of light intensity, indicating the diversity of strategies for photosynthetic acclimation to fluctuating light in these two closely related orchid species.

Photosynthesis acclimation under severely fluctuating light conditions allows faster growth of diatoms compared with dinoflagellates

BACKGROUND

Diatoms contribute 20% of the global primary production and are adaptable in dynamic environments. Diatoms always bloom earlier in the annual phytoplankton succession instead of dinoflagellates. However, how diatoms acclimate to a dynamic environment, especially under changing light conditions, remains unclear.

RESULTS

We compared the growth and photosynthesis under fluctuating light conditions of red tide diatom Skeletonema costatum, red tide dinoflagellate Amphidinium carterae, Prorocentrum donghaiense, Karenia mikimotoi, model diatom Phaeodactylum tricornutum, Thalassiosira pseudonana and model dinoflagellate Dinophycae Symbiodinium. Diatoms grew faster and maintained a consistently higher level of photosynthesis. Diatoms were sensitive to the specific inhibitor of Proton Gradient Regulation 5 (PGR5) depending photosynthetic electron flow, which is a crucial mechanism to protect their photosynthetic apparatus under fluctuating light. In contrast, the dinoflagellates were not sensitive to this inhibitor. Therefore, we investigate how PGR5 functions under light fluctuations in the model diatom P. tricornutum by knocking down and overexpressing PGR5. Overexpression of PGR5 reduced the photosystem I acceptor side limitation (Y (NA)) and increased growth rate under severely fluctuating light in contrast to the knockdown of PGR5.

CONCLUSION

Diatoms acclimatize to fluctuating light conditions better than dinoflagellates. PGR5 in diatoms can regulate their photosynthetic electron flow and accelerate their growth under severe light fluctuation, supporting fast biomass accumulation under dynamic environments in pioneer blooms.

The water-water cycle is not a major alternative sink in fluctuating light at chilling temperature

The water-water cycle (WWC) has the potential to alleviate photoinhibition of photosystem I (PSI) in fluctuating light (FL) at room temperature and moderate heat stress. However, it is unclear whether WWC can function as a safety valve for PSI in FL at chilling temperature. In this study, we measured P700 redox state and chlorophyll fluorescence in FL at 25 °C and 4 °C in the high WWC activity plant Dendrobium officinale. At 25 °C, the operation of WWC contributed to the rapid re-oxidation of P700 upon dark-to-light transition. However, such rapid re-oxidation of P700 was not observed at 4 °C. Upon a sudden increase in light intensity, WWC rapidly consumed excess electrons in PSI and thus avoided an over-reduction of PSI at 25 °C. On the contrary, PSI was highly reduced within the first seconds after transition from low to high light at 4 °C. Therefore, in opposite to 25 °C, the WWC is not a major alternative sink in FL at chilling temperature. Upon transition from low to high light, cyclic electron transport was highly stimulated at 4 °C when compared with 25 °C. These results indicate that D. officinale enhances cyclic electron transport to partially compensate for the inactivation of WWC in FL at 4 °C.

Coordination of Cyclic Electron Flow and Water-Water Cycle Facilitates Photoprotection under Fluctuating Light and Temperature Stress in the Epiphytic Orchid Dendrobium officinale

Photosystem I (PSI) is the primary target of photoinhibition under fluctuating light (FL). Photosynthetic organisms employ alternative electron flows to protect PSI under FL. However, the understanding of the coordination of alternative electron flows under FL at temperature stresses is limited. To address this question, we measured the chlorophyll fluorescence, P700 redox state, and electrochromic shift signal in leaves of Dendrobium officinale exposed to FL at 42 °C, 25 °C, and 4 °C. Upon a sudden increase in illumination at 42 °C and 25 °C, the water-water cycle (WWC) consumed a significant fraction of the extra reducing power, and thus avoided an over-reduction of PSI. However, WWC was inactivated at 4 °C, leading to an over-reduction of PSI within the first seconds after light increased. Therefore, the role of WWC under FL is largely dependent on temperature conditions. After an abrupt increase in light intensity, cyclic electron flow (CEF) around PSI was stimulated at any temperature. Therefore, CEF and WWC showed different temperature responses under FL. Furthermore, the enhancement of CEF and WWC at 42 °C quickly generated a sufficient trans-thylakoid proton gradient (ΔpH). The inactivation of WWC at 4 °C was partially compensated for by an increased CEF activity. These findings indicate that CEF and WWC coordinate to protect PSI under FL at temperature stresses.

Leaf photosynthetic and anatomical insights into mechanisms of acclimation in rice in response to long-term fluctuating light

Long-term fluctuating light (FL) conditions are very common in natural environments. The physiological and biochemical mechanisms for acclimation to FL differ between species. However, most of the current conclusions regarding acclimation to FL were made based on studies in algae or Arabidopsis thaliana. It is still unclear how rice (Oryza sativa L.) integrate multiple physiological changes to acclimate to long-term FL. In this study, we found that rice growth was repressed under long-term FL. By systematically measuring phenotypes and physiological parameters, we revealed that: (a) under short-term FL, photosystem I (PSI) was inhibited, while after 1-7 days of long-term FL, both PSI and PSII were inhibited. Higher acceptor-side limitation in electron transport and higher overall nonphotochemical quenching (NPQ) explained the lower efficiencies of PSI and PSII, respectively. (b) An increase in pH differences across the thylakoid membrane and a decrease in thylakoid proton conductivity revealed a reduction of ATP synthase activity. (c) Using electron microscopy, we showed a decrease in membrane stacking and stomatal opening after 7 days of FL treatment. Taken together, our results show that electron flow, ATP synthase activity and NPQ regulation are the major processes determining the growth performance of rice under long-term FL conditions.

Photosystem I is tolerant to fluctuating light under moderate heat stress in two orchids Dendrobium officinale and Bletilla striata

Under natural field conditions, plants usually experience fluctuating light (FL) under moderate heat stress in summer. However, responses of photosystems I and II (PSI and PSII) to such combined stresses are not well known. Furthermore, the role of water-water cycle (WWC) in photoprotection in FL under moderate heat stress is poorly understood. In this study, we examined chlorophyll fluorescence and P700 redox state in FL at 42 °C in two orchids, Dendrobium officinale (with high WWC activity) and Bletilla striata (with low WWC activity). After FL treatment at 42 °C, PSI activity maintained stable while PSII activity decreased significantly in these two orchids. In D. officinale, the WWC could rapidly consume the excess excitation energy in PSI and thus avoided an over-reduction of PSI upon any increase in illumination. Therefore, in D. officinale, WWC likely protected PSI in FL at 42 °C. In B. striata, heat-induced PSII photoinhibition down-regulated electron flow from PSII and thus prevented an over-reduction of PSI after transition from low to high light. Consequently, in B. striata moderate PSII photoinhibition could protected PSI in FL at 42 °C. We conclude that, in addition to cyclic electron flow, WWC and PSII photoinhibition-repair cycle are two important strategies for preventing PSI photoinhibition in FL under moderate heat stress.

Expression of Flavodiiron Proteins Flv2-Flv4 in Chloroplasts of Arabidopsis and Tobacco Plants Provides Multiple Stress Tolerance

With the notable exception of angiosperms, all phototrophs contain different sets of flavodiiron proteins that help to relieve the excess of excitation energy on the photosynthetic electron transport chain during adverse environmental conditions, presumably by reducing oxygen directly to water. Among them, the Flv2-Flv4 dimer is only found in β-cyanobacteria and induced by high light, supporting a role in stress protection. The possibility of a similar protective function in plants was assayed by expressing Synechocystis Flv2-Flv4 in chloroplasts of tobacco and Arabidopsis. Flv-expressing plants exhibited increased tolerance toward high irradiation, salinity, oxidants, and drought. Stress tolerance was reflected by better growth, preservation of photosynthetic activity, and membrane integrity. Metabolic profiling under drought showed enhanced accumulation of soluble sugars and amino acids in transgenic Arabidopsis and a remarkable shift of sucrose into starch, in line with metabolic responses of drought-tolerant genotypes. Our results indicate that the Flv2-Flv4 complex retains its stress protection activities when expressed in chloroplasts of angiosperm species by acting as an additional electron sink. The flv2-flv4 genes constitute a novel biotechnological tool to generate plants with increased tolerance to agronomically relevant stress conditions that represent a significant productivity constraint.

The Significance of Chloroplast NAD(P)H Dehydrogenase Complex and Its Dependent Cyclic Electron Transport in Photosynthesis

Chloroplast NAD(P)H dehydrogenase (NDH) complex, a multiple-subunit complex in the thylakoid membranes mediating cyclic electron transport, is one of the most important alternative electron transport pathways. It was identified to be essential for plant growth and development during stress periods in recent years. The NDH-mediated cyclic electron transport can restore the over-reduction in stroma, maintaining the balance of the redox system in the electron transfer chain and providing the extra ATP needed for the other biochemical reactions. In this review, we discuss the research history and the subunit composition of NDH. Specifically, the formation and significance of NDH-mediated cyclic electron transport are discussed from the perspective of plant evolution and physiological functionality of NDH facilitating plants' adaptation to environmental stress. A better understanding of the NDH-mediated cyclic electron transport during photosynthesis may offer new approaches to improving crop yield.

Physiological Roles of Flavodiiron Proteins and Photorespiration in the Liverwort Marchantia polymorpha

Against the potential risk in oxygenic photosynthesis, that is, the generation of reactive oxygen species, photosynthetic electron transport needs to be regulated in response to environmental fluctuations. One of the most important regulations is keeping the reaction center chlorophyll (P700) of photosystem I in its oxidized form in excess light conditions. The oxidation of P700 is supported by dissipating excess electrons safely to O₂, and we previously found that the molecular mechanism of the alternative electron sink is changed from flavodiiron proteins (FLV) to photorespiration in the evolutionary history from cyanobacteria to plants. However, the overall picture of the regulation of photosynthetic electron transport is still not clear in bryophytes, the evolutionary intermediates. Here, we investigated the physiological roles of FLV and photorespiration for P700 oxidation in the liverwort Marchantia polymorpha by using the mutants deficient in FLV (flv1) at different O₂ partial pressures. The effective quantum yield of photosystem II significantly decreased at 2kPa O₂ in flv1, indicating that photorespiration functions as the electron sink. Nevertheless, it was clear from the phenotype of flv1 that FLV was dominant for P700 oxidation in M. polymorpha. These data suggested that photorespiration has yet not replaced FLV in functioning for P700 oxidation in the basal land plant probably because of the lower contribution to lumen acidification, compared with FLV, as reflected in the results of electrochromic shift analysis.

PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow

Proton gradient regulation 5 (PGR5) is involved in the control of photosynthetic electron transfer, but its mechanistic role is not yet clear. Several models have been proposed to explain phenotypes such as a diminished steady-state proton motive force (pmf) and increased photodamage of photosystem I (PSI). Playing a regulatory role in cyclic electron flow (CEF) around PSI, PGR5 contributes indirectly to PSI protection by enhancing photosynthetic control, which is a pH-dependent down-regulation of electron transfer at the cytochrome b6f complex (b6f). Here, we re-evaluated the role of PGR5 in the green alga Chlamydomonas reinhardtii and conclude that pgr5 possesses a dysfunctional b6f. Our data indicate that the b6f low-potential chain redox activity likely operated in two distinct modes - via the canonical Q cycle during linear electron flow and via an alternative Q cycle during CEF, which allowed efficient oxidation of the low-potential chain in the WT b6f. A switch between the two Q cycle modes was dependent on PGR5 and relied on unknown stromal electron carrier(s), which were a general requirement for b6f activity. In CEF-favoring conditions, the electron transfer bottleneck in pgr5 was the b6f, in which insufficient low-potential chain redox tuning might account for the mutant pmf phenotype. By attributing a ferredoxin-plastoquinone reductase activity to the b6f and investigating a PGR5 cysteine mutant, a current model of CEF is challenged.

LHCSR3-Type NPQ Prevents Photoinhibition and Slowed Growth under Fluctuating Light in Chlamydomonas reinhardtii

Natural light intensities can rise several orders of magnitude over subsecond time spans, posing a major challenge for photosynthesis. Fluctuating light tolerance in the green alga Chlamydomonas reinhardtii requires alternative electron pathways, but the role of nonphotochemical quenching (NPQ) is not known. Here, fluctuating light (10 min actinic light followed by 10 min darkness) led to significant increase in NPQ/qE-related proteins, LHCSR1 and LHCSR3, relative to constant light of the same subsaturating or saturating intensity. Elevated levels of LHCSR1/3 increased the ability of cells to safely dissipate excess light energy to heat (i.e., qE-type NPQ) during dark to light transition, as measured with chlorophyll fluorescence. The low qE phenotype of the npq4 mutant, which is unable to produce LHCSR3, was abolished under fluctuating light, showing that LHCSR1 alone enables very high levels of qE. Photosystem (PS) levels were also affected by light treatments; constant light led to lower PsbA levels and F_v/F_m values, while fluctuating light led to lower PsaA and maximum P700⁺ levels, indicating that constant and fluctuating light induced PSII and PSI photoinhibition, respectively. Under fluctuating light, npq4 suffered more PSI photoinhibition and significantly slower growth rates than parental wild type, whereas npq1 and npq2 mutants affected in xanthophyll carotenoid compositions had identical growth under fluctuating and constant light. Overall, LHCSR3 rather than total qE capacity or zeaxanthin is shown to be important in C. reinhardtii in tolerating fluctuating light, potentially via preventing PSI photoinhibition.

Water oxidation by photosystem II is the primary source of electrons for sustained H2 photoproduction in nutrient-replete green algae

Photosynthetic H₂ production in the green alga Chlamydomonas reinhardtii is catalyzed by O₂-sensitive [FeFe]-hydrogenases, which accept electrons from photosynthetically reduced ferredoxin and reduce protons to H₂. Since the process occurs downstream of photosystem I, the contribution of photosystem II (PSII) in H₂ photoproduction has long been a subject of debate. Indeed, water oxidation by PSII results in O₂ accumulation in chloroplasts, which inhibits H₂ evolution. Therefore, clear evidence for direct water biophotolysis resulting in simultaneous H₂ and O₂ releases in algae has never been presented. This paper demonstrates that sustained H₂ photoproduction in C. reinhardtii is directly linked to PSII-dependent water oxidation and brings insights into regulation of PSII activity and H₂ production by CO₂/HCO₃^– under microoxic conditions.

The unicellular green alga Chlamydomonas reinhardtii is capable of photosynthetic H₂ production. H₂ evolution occurs under anaerobic conditions and is difficult to sustain due to 1) competition between [FeFe]-hydrogenase (H₂ase), the key enzyme responsible for H₂ metabolism in algae, and the Calvin–Benson–Bassham (CBB) cycle for photosynthetic reductants and 2) inactivation of H₂ase by O₂ coevolved in photosynthesis. Recently, we achieved sustainable H₂ photoproduction by shifting algae from continuous illumination to a train of short (1 s) light pulses, interrupted by longer (9 s) dark periods. This illumination regime prevents activation of the CBB cycle and redirects photosynthetic electrons to H₂ase. Employing membrane-inlet mass spectrometry and H218O, we now present clear evidence that efficient H₂ photoproduction in pulse-illuminated algae depends primarily on direct water biophotolysis, where water oxidation at the donor side of photosystem II (PSII) provides electrons for the reduction of protons by H₂ase downstream of photosystem I. This occurs exclusively in the absence of CO₂ fixation, while with the activation of the CBB cycle by longer (8 s) light pulses the H₂ photoproduction ceases and instead a slow overall H₂ uptake is observed. We also demonstrate that the loss of PSII activity in DCMU-treated algae or in PSII-deficient mutant cells can be partly compensated for by the indirect (PSII-independent) H₂ photoproduction pathway, but only for a short (<1 h) period. Thus, PSII activity is indispensable for a sustained process, where it is responsible for more than 92% of the final H₂ yield.

Photosynthetic sea slugs induce protective changes to the light reactions of the chloroplasts they steal from algae

Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own cells, and mechanisms that allow preservation of the chloroplasts are unknown. We found that the slug Elysia timida induces changes to the photosynthetic light reactions of the chloroplasts it steals from the alga Acetabularia acetabulum. Working with a large continuous laboratory culture of both the slugs (>500 individuals) and their prey algae, we show that the plastoquinone pool of slug chloroplasts remains oxidized, which can suppress reactive oxygen species formation. Slug chloroplasts also rapidly build up a strong proton-motive force upon a dark-to-light transition, which helps them to rapidly switch on photoprotective non-photochemical quenching of excitation energy. Finally, our results suggest that chloroplasts inside E. timida rely on oxygen-dependent electron sinks during rapid changes in light intensity. These photoprotective mechanisms are expected to contribute to the long-term functionality of the chloroplasts inside the slugs.

Plants, algae and a few other organisms rely on a process known as photosynthesis to fuel themselves, as they can harness cellular structures called chloroplasts to convert light into usable energy. Animals typically lack chloroplasts, making them unable to use photosynthesis to power themselves. The sea slug Elysia timida, however, can steal whole chloroplasts from the cells of the algae it consumes: the stolen structures then become part of the cells in the gut of the slug, allowing the animal to gain energy from sunlight.

Once they are in the digestive system of the slug, the chloroplasts survive and keep working for longer than expected. Indeed, these structures are often harmed as a side effect of photosynthesis, but the sea slug does not have the right genes to help repair this damage. In addition, conditions inside animal cells are widely different to the ones found inside algae and plants. It is not clear then how the sea slug extends the lifespan of its chloroplasts by preventing damage caused by sunlight.

To investigate this question, Havurinne and Tyystjärvi compared photosynthesis in sea slugs and the algae they eat. A range of methods, including measuring fluorescence from the chloroplasts, was used: this revealed that the slug changes the inside of the stolen chloroplasts, making them more resistant to damage.

First, when exposed to light the stolen chloroplasts can quickly switch on a mechanism that dissipates light energy to heat, which is less damaging. Second, a molecule that serves as an intermediate during photosynthesis is kept in a ‘safe’ state which prevents it from creating harmful compounds. And finally, additional safeguard molecules ‘deactivate’ compounds that could otherwise mediate damaging reactions. Overall, these measures may reduce the efficiency of the chloroplasts but allow them to keep working for much longer.

Early chloroplasts were probably independent bacteria that were captured and ‘domesticated’ by other cells for their ability to extract energy from the sun. Photosynthesizing sea slugs therefore provide an interesting way to understand some of the challenges of early life. The work by Havurinne and Tyystjärvi may also reveal new ways to harness biological processes such as photosynthesis for energy production in other contexts.

Engineering of NADPH Supply Boosts Photosynthesis-Driven Biotransformations

Light-driven biocatalysis in recombinant cyanobacteria provides highly atom-efficient cofactor regeneration via photosynthesis, thereby remediating constraints associated with sacrificial cosubstrates. However, despite the remarkable specific activities of photobiocatalysts, self-shading at moderate-high cell densities limits efficient space-time-yields of heterologous enzymatic reactions. Moreover, efficient integration of an artificial electron sink into the tightly regulated network of cyanobacterial electron pathways can be highly challenging. Here, we used C=C bond reduction of 2-methylmaleimide by the NADPH-dependent ene-reductase YqjM as a model reaction for light-dependent biotransformations. Time-resolved NADPH fluorescence spectroscopy allowed direct monitoring of in-cell YqjM activity and revealed differences in NADPH steady-state levels and oxidation kinetics between different genetic constructs. This effect correlates with specific activities of whole-cells, which demonstrated conversions of >99%. Further channelling of electrons toward heterologous YqjM by inactivation of the flavodiiron proteins (Flv1/Flv3) led to a 2-fold improvement in specific activity at moderate cell densities, thereby elucidating the possibility of accelerating light-driven biotransformations by the removal of natural competing electron sinks. In the best case, an initial product formation rate of 18.3 mmol h^–1 L^–1 was reached, allowing the complete conversion of a 60 mM substrate solution within 4 h.

The chloroplast NADH dehydrogenase-like complex influences the photosynthetic activity of the moss Physcomitrella patens

Alternative electron pathways contribute to regulation of photosynthetic light reactions to adjust to metabolic demands in dynamic environments. The chloroplast NADH dehydrogenase-like (NDH) complex mediates the cyclic electron transport pathway around PSI in different cyanobacteria, algae, and plant species, but it is not fully conserved in all photosynthetic organisms. In order to assess how the physiological role of this complex changed during plant evolution, we isolated Physcomitrella patens lines knocked out for the NDHM gene that encodes a subunit fundamental for the activity of the complex. ndhm knockout mosses indicated high PSI acceptor side limitation upon abrupt changes in illumination. In P. patens, pseudo-cyclic electron transport mediated by flavodiiron proteins (FLVs) was also shown to prevent PSI over-reduction in plants exposed to light fluctuations. flva ndhm double knockout mosses had altered photosynthetic performance and growth defects under fluctuating light compared with the wild type and single knockout mutants. The results showed that while the contribution of NDH to electron transport is minor compared with FLV, NDH still participates in modulating photosynthetic activity, and it is critical to avoid PSI photoinhibition, especially when FLVs are inactive. The functional overlap between NDH- and FLV-dependent electron transport supports PSI activity and prevents its photoinhibition under light variations.

PGRL1 overexpression in Phaeodactylum tricornutum inhibits growth and reduces apparent PSII activity

Proton gradient regulation 5-like photosynthetic phenotype 1 (PGRL1)-dependent cyclic electron transport around photosystem I (PSI) plays important roles in the response to different stresses, including high light. Although the function of PGRL1 in higher plants and green algae has been thoroughly investigated, little information is available on the molecular mechanism of PGRL1 in diatoms. We created PGRL1 overexpression and knockdown transformants of Phaeodactylum tricornutum, the diatom model species, and investigated the impact on growth and photosynthesis under constant and fluctuating light conditions. PGRL1 over-accumulation resulted in significant decreases in growth rate and apparent photosystem II (PSII) activity and led to an opposing change of apparent PSII activity when turning to high light, demonstrating a similar influence on photosynthesis as a PSII inhibitor. Our results suggested that PGRL1 overexpression can reduce the apparent efficiency of PSII and inhibit growth in P. tricornutum. These findings provide physiological evidence that the accumulation of PGRL1 mainly functions around PSII instead of PSI.

Functional redundancy between flavodiiron proteins and NDH-1 in Synechocystis sp. PCC 6803

In oxygenic photosynthetic organisms, excluding angiosperms, flavodiiron proteins (FDPs) catalyze light-dependent reduction of O₂ to H₂ O. This alleviates electron pressure on the photosynthetic apparatus and protects it from photodamage. In Synechocystis sp. PCC 6803, four FDP isoforms function as hetero-oligomers of Flv1 and Flv3 and/or Flv2 and Flv4. An alternative electron transport pathway mediated by the NAD(P)H dehydrogenase-like complex (NDH-1) also contributes to redox hemostasis and the photoprotection of photosynthesis. Four NDH-1 types have been characterized in cyanobacteria: NDH-1₁ and NDH-1₂ , which function in respiration; and NDH-1₃ and NDH-1₄ , which function in CO₂ uptake. All four types are involved in cyclic electron transport. Along with single FDP mutants (∆flv1 and Δflv3) and the double NDH-1 mutants (∆d1d2, which is deficient in NDH-1_1,2 and ∆d3d4, which is deficient in NDH-1_3,4 ), we studied triple mutants lacking one of Flv1 or Flv3, and NDH-1_1,2 or NDH-1_3,4 . We show that the presence of either Flv1/3 or NDH-1_1,2 , but not NDH-1_3,4 , is indispensable for survival during changes in growth conditions from high CO₂ /moderate light to low CO₂ /high light. Our results show functional redundancy between FDPs and NDH-1_1,2 under the studied conditions. We suggest that ferredoxin probably functions as a primary electron donor to both Flv1/3 and NDH-1_1,2 , allowing their functions to be dynamically coordinated for efficient oxidation of photosystem I and for photoprotection under variable CO₂ and light availability.