Mehler Reaction Research Articles

Flavodiiron proteins prevent the Mehler reaction in Chlamydomonas reinhardtii

Regulation of photosynthetic electron transfer is fundamental for energetic efficiency and coping with changing environmental factors. All plant groups, except angiosperms, use flavodiiron proteins (FDPs) on the acceptor side of photosystem I (PSI), putatively protecting from PSI photoinhibition under fluctuating light. An alternative electron flow during photosynthesis is the direct reduction of O2 with consequential production of reactive oxygen species (ROS; the Mehler reaction), but to what extent FDPs prevent this is unknown. Here, we quantified O2-dependent electron flow, photosystem activity and the Mehler reaction in Chlamydomonas reinhardtii. Near-infra red absorbance measurement of PSI reaction centre (P700) showed that FDPs remain active long after a dark-to-light transition and their activity increased under hyperoxia. Light-induced hydrogen peroxide (H2O2) production, as a marker of the Mehler reaction, was influenced by O2 concentration, and was up to 67 % higher in an FDP-deficient mutant (flvb) than in the wild-type under saturating constant light. In cultures kept under sub-saturating constant light, flvb produced 315 % more H2O2 and had lower PSII efficiency than wild-type. Inhibiting electron transfer out of photosystem II (PSII) with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) only partially blocked H2O2 production, particularly under hyperoxia, indicating that PSII was an additional ROS source. P700+ reduction in the dark in the presence of DCMU was faster in flvb than wild-type, revealing enhanced cyclic electron flow, which may also have led to PSI mediated Mehler reaction. We conclude that FDPs remain active in constant light and can prevent PSI mediated Mehler reaction, with relevance to PSII photoinhibition.

Photosystems under high light stress: throwing light on mechanism and adaptation

High light stress decreases the photosynthetic rate in plants due to photooxidative damage to photosynthetic apparatus, photoinhibition of PSII, and/or damage to PSI. The dissipation of excess energy by nonphotochemical quenching and degradation of the D1 protein of PSII and its repair cycle help against photooxidative damage. Light stress also activates stress-responsive nuclear genes through the accumulation of phosphonucleotide-3'-phosphoadenosine- 5'-phosphate, methylerythritol cyclodiphosphate, and reactive oxygen species which comprise the chloroplast retrograde signaling pathway. Additionally, hormones, such as abscisic acid, cytokinin, brassinosteroids, and gibberellins, play a role in acclimation to light fluctuations. Several alternate electron flow mechanisms, which offset the excess of electrons, include activation of plastid or plastoquinol terminal oxidase, cytochrome b6/f complex, cyclic electron flow through PSI, Mehler ascorbate peroxidase pathway or water-water cycle, mitochondrial alternative oxidase pathway, and photorespiration. In this review, we provided insights into high light stress-mediated damage to photosynthetic apparatus and strategies to mitigate the damage by decreasing antennae size, enhancing NPQ through the introduction of mutants, expression of algal proteins to improve photosynthetic rates and engineering ATP synthase.

Effect of light fluctuations on photosynthesis and metabolic flux in Synechocystis sp. PCC 6803.

In nature, photosynthetic organisms are exposed to fluctuating light, and their physiological systems must adapt to this fluctuation. To maintain homeostasis, these organisms have a light fluctuation photoprotective mechanism, which functions in both photosystems and metabolism. Although the photoprotective mechanisms functioning in the photosystem have been studied, it is unclear how metabolism responds to light fluctuations within a few seconds. In the present study, we investigated the metabolic response of Synechocystis sp. PCC 6803 to light fluctuations using 13 C-metabolic flux analysis. The light intensity and duty ratio were adjusted such that the total number of photons or the light intensity during the low-light phase was equal. Light fluctuations affected cell growth and photosynthetic activity under the experimental conditions. However, metabolic flux distributions and cofactor production rates were not affected by the light fluctuations. Furthermore, the estimated ATP and NADPH production rates in the photosystems suggest that NADPH-consuming electron dissipation occurs under fluctuating light conditions. Although we focused on the water-water cycle as the electron dissipation path, no growth effect was observed in an flv3-disrupted strain under fluctuating light, suggesting that another path contributes to electron dissipation under these conditions.

The role of alternative electron pathways in the photosynthetic chain in higher plants

Light-dependent reactions of photosynthesis takes place in the thylakoids of chloroplasts where light energy harvested from the sun drives the synthesis of ATP and NADPH. The major pathway of photosynthetic chain is the linear electron transport (LET), in which both photosystems (PSI and PSII) are involved, and ATP and NADPH are produced. However, ratio in production of those components is insufficient to cover the Calvin cycle energy requirements, depending on the metabolism of the cell. Moreover, disturbance in metabolism homeostasis, caused by environmental stress conditions, increases ATP demand, which cannot be covered by LET. Thus, in photosynthetic apparatus must exist alternative electron transport pathways, these include: cyclic electron transport (CET) mediated by NDH complex or PGR5/PGRL1 proteins, water-water cycle and PTOX enzyme. Activity of alternative pathways can optimize ratio in production of ATP/NADPH, appropriately to requirements, which allows to achieve redox balance and ATP contents.

Chlorella vulgaris cultivation under super high light intensity: An application of the flashing light effect

Chlorella vulgaris was grown under super high illumination (7000 μmolPhotonPAR/m2/s). Continuous and flashing light patterns were applied to investigate potential gain originating from the flashing light effect (square pattern, duty cycle of 0.5). Three control configurations were tested, all under continuous light: 200 μmolPhotonPAR/m2/s (conventional cultivation conditions), 4000 μmolPhotonPAR/m2/s (about the same average amount of energy), and 7000 μmolPhotonPAR/m2/s (maximum incident light intensity, cells did not grow). The experiments were conducted in ultra-thin flat-panel photobioreactors to ensure isoactinic growth conditions. After acclimation, the monitored outcomes were: growth rate, macronutrient composition (proteins, carbohydrates, lipids), pigment content, and transient fluorescence (OJIP assays). The results showed that Chlorella vulgaris can grow at a similar rate under 200 μmolPhotonPAR/m2/s of continuous light and under 7000 μmolPhotonPAR/m2/s of flashing light for frequencies between 0.1 and 100 Hz. Acclimation mechanisms did not alter cell macronutrient composition, yet, chlorophyll contents decreased while carotenoids increased (allegedly linked to an increased expression of the VAZ cycle). OJIP tests also revealed potential up-regulation of the water-water cycle (featuring PTOX enzyme), which would allow faster repletion of the PQ pool, delaying photosynthetic apparatus saturation. Above 100 Hz (1000 Hz in this study), cells exhibited the same growth rate as under the equivalent amount of continuous light. It suggests that light alternation is too fast for the cells to perceive it. On the contrary, too low frequencies (0.01 Hz) showed lower performances than under the same average intensity. In this condition, during the light phase, cells are exposed to harmful conditions (continuous 7000 μmolPhotonPAR/m2/s), and the too-long dark phase only allows the cells some rest but does not bring any benefits. In terms of applicability, these results pave the way for the use of super high light (either artificial or by sunlight concentration) to overcome the energy limitation burdening photoautotrophic microalgae cultivation.

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 (AN), stomatal conductance (gs) and mesophyll conductance (gm) under high light in tobacco leaves. However, this exogenous melatonin strongly delayed the induction kinetics of gs and gm, leading to the slower induction speed of AN. During photosynthetic induction, AN 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 AN 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.

Cooperative pathway of O2 reduction to H2O2 in chloroplast thylakoid membrane: new insight into the Mehler reaction.

Oxygen reduction in chloroplasts in the light was discovered by (Mehler Arch Biochem Biophys 33:65-77, 1951) as production of hydrogen peroxide. Later, it was shown that the primary product of the oxygen reduction is superoxide radical produced in thylakoids by one-electron transfer from reduced components of photosynthetic electron transport chain to O2 molecule. For a long time, the formation of hydrogen peroxide was considered to be a result of disproportionation of superoxide radicals in chloroplast stroma. Here, we overview a growing number of evidence indicating on another one, additional to disproportionation, pathway of hydrogen peroxide formation in chloroplasts, namely its formation in thylakoid membrane due to reaction of superoxide radical generated in the membrane with the reduced plastoquinone molecule, plastohydroquinone. Since various components of photosynthetic electron transport chain (primarily photosystem I) can supply superoxide radicals to this reaction, we refer this two-step O2 photoreduction to H2O2 as a cooperative process. The significance of hydrogen peroxide production via this pathway for redox signaling and scavenging of reactive oxygen species is discussed.

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.

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.

Photoinactivation of the oxygen-evolving complex regulates the photosynthetic strategy of the seagrass Zostera marina

Zostera marina, a widespread seagrass, evolved from a freshwater ancestor of terrestrial monocots and successfully transitioned into a completely submerged seagrass. We found that its oxygen-evolving complex (OEC) was partially inactivated in response to light exposure, as evidenced by both the increment of the relative variable fluorescence at the K-step and the downregulation of the OEC genes and proteins. This photosynthetic regulation was further addressed at both proteome and physiology levels by an in vivo study. The unchanged content of the ΔpH sensor PsbS protein and the non-photochemical quenching induction dynamics, described by a single exponential function, verified the absence of the fast qE component. Contents and activities of chlororespiration, Mehler reaction, malic acid synthesis, and photorespiration key enzymes were not upregulated, suggesting that alternative electron flows remained unactivated. Furthermore, neither significant production of singlet oxygen nor increment of total antioxidative capacity indicated that reactive oxygen species were not produced during light exposure. In summary, these low electron consumptions may allow Z. marina to efficiently use the limited electrons caused by partial OEC photoinactivation to maintain a normal carbon assimilation level.

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.

Key photoprotective pathways of a shade-tolerant plant (Alpinia oxyphylla) for precipitation patterns change during the dry season: thermal energy dissipation and water-water cycle

Alpinia oxyphylla Miq is a shade-tolerant plant that grows under rubber trees. The decreased precipitation often leads to limitation of photosynthesis and productivity of A. oxyphylla during the dry season. However, the effects of the precipitation patterns on the photosynthesis and the photoprotective strategies of A. oxyphylla remains unclear. Herein, we examined how CO2 assimilation, light energy partitioning and reactive oxygen species (ROS) metabolism are affected by simulated precipitation patterns change. The change of precipitation patterns induced water deficits of different intensities and durations. With the decreased precipitation, stomatal conductance (Gs) and net CO2 assimilation rate (Pn) significantly decreased, resulting in excess excitation energy and over-accumulation of ROS, and further caused lipid peroxidation of photosynthetic apparatus. The phenomenon was exacerbated by extending the interval between precipitation. In addition, the decreased precipitation induced the xanthophyll cycle effectively dissipating excess excitation energy. Through the water-water cycle, a greater proportion of electron flux was shuffled to molecular oxygen (O2) to produce superoxide anion (O2•−), especially in plants under natural precipitation (W; 25 mm/month). This may be because the energy partitioning mechanism of A. oxyphylla can efficiently dissipate excessive excitation energy and reduce PSⅡ photoinhibition, thus rapidly adapting to precipitation patterns change. On the other hand, the decreased precipitation improved the capacity of ROS detoxification in the water-water cycle and reduced lipid peroxidation in W plants, which were reflected by the increase of SOD and CAT activities, and a partial enhancement of antioxidant enzymes and antioxidant metabolism in the water-water cycle. However, the down-regulation of antioxidant systems in W- plants (decreased precipitation; 10 mm/month) further accelerated ROS accumulation and the ability of ROS-scavenging might be partially compensated by ROS accumulation, which may had been a primary cause for limited the antioxidant protection. Overall, when CO2 assimilation reduced with the decreased precipitation, A. oxyphylla can form a photoprotective mechanism, both thermal energy dissipation and water-water cycle are crucial photoprotective pathways against photodamage in photosynthetic apparatus.

Phylloquinone is the principal Mehler reaction site within photosystem I in high light

Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

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.

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.

Photosynthetic performance of Chlorella vulgaris R117 mass culture is moderated by diurnal oxygen gradients in an outdoor thin layer cascade

Dissolved oxygen concentration is a critical point for microalgae in large scale cultivation systems. Highly productive cultures inevitably generate a build-up of oxygen gradients along the reactor which can affect photosynthetic performance. In this study, a fast-growing strain of Chlorella vulgaris R117 was cultured outdoors in a thin-layer cascade during a one-week trial reaching a biomass productivity of 4 g DW L−1 d−1 (27 g DW m−2 d−1). High photosynthetic activity led to oxygen oversaturation of up to 400% in some parts along the culture units at midday. The aim was to examine the effect of high dissolved oxygen concentration on diurnal changes in the photosynthetic performance and growth of the Chlorella culture using multi-technique approach. Photosynthetic activity of Chlorella R117 culture was estimated in situ and ex situ using oxygen production and in vivo Chl a fluorescence measurements, which showed good correlation. The rates of electron transport and of oxygen production were related, but the values of the μmolelectrons/μmolO2 ratio was higher than predicted, suggesting the probable involvement of electron and oxygen consuming processes such as photorespiration and Mehler reaction. These processes probably function as photoprotective mechanisms, since no photodamage was observed in the Chlorella R-117 cultures. Depression (down-regulation) of photosynthetic activity due to the exposition to high dissolved oxygen concentration along the cascade area over time was observed. The usefulness of on-line measurements was demonstrated to obtain immediate and in-situ information on the physiological status of the culture. This data can be used in models of operation control for large-scale microalgae production units.

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.

The water-water cycle facilitates photosynthetic regulation under fluctuating light in the epiphytic orchid Dendrobium officinale

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. However, the strategies for photosynthetic regulation under fluctuating light in wild epiphytic plants are little known. We here compared PSI and PSII performances under fluctuating light in the epiphytic orchid Dendrobium officinale and the terrestrial orchid Bletilla striata. Upon dark-to-light transition, D. officinale showed a rapid re-oxidation of P700, but this rapid re-oxidation of P700 was not observed in B. striata. Within the first 40 s after transition from dark to actinic light, B. striata showed much higher activation of cyclic electron transport (CET) than D. officinale. However, this strong activation of CET could not prevent the over-reduction of PSI. Therefore, the rapid re-oxidation of P700 in D. officinale was attributed to pseudo-cyclic electron transport (water-water cycle). Within the first 60 s after transition from low to high light, PSI was highly oxidized in D. officinale but was highly reduced in B. striata. Concomitantly, B. striata showed much higher stimulation of CET. These results indicate that water-water cycle significantly regulates PSI redox state under fluctuating light in the epiphytic orchid D. officinale. By comparison, the terrestrial orchid B. striata employs CET to cope with fluctuating light, similar to the phenotype of Arabidopsis thaliana. Taking together, these two orchids used different strategies for photosynthetic regulation to cope with fluctuating light.

The water-water cycle is more effective in regulating redox state of photosystem I under fluctuating light than cyclic electron transport

Photosynthetic electron flux from water via photosystem II (PSII) and PSI to oxygen (water-water cycle) may act as an alternative electron sink under fluctuating light in angiosperms. We measured the P700 redox kinetics and electrochromic shift signal under fluctuating light in 11 Camellia species and tobacco leaves. Upon dark-to-light transition, these Camellia species showed rapid re-oxidation of P700. However, this rapid re-oxidation of P700 was not observed when measured under anaerobic conditions, as was in experiment with tobacco performed under aerobic conditions. Therefore, photo-reduction of O2 mediated by water-water cycle was functional in these Camellia species but not in tobacco. Within the first 10 s after transition from low to high light, PSI was highly oxidized in these Camellia species but was over-reduced in tobacco leaves. Furthermore, such rapid oxidation of PSI in these Camellia species was independent of the formation of trans-thylakoid proton gradient (ΔpH). These results indicated that in addition to ΔpH-dependent photosynthetic control, the water-water cycle can protect PSI against photoinhibition under fluctuating light in these Camellia species. We here propose that the water-water cycle is an overlooked strategy for photosynthetic regulation under fluctuating light in angiosperms.

A Chlorophyte Alga Utilizes Alternative Electron Transport for Primary Photoprotection.

The green alga Desmodesmus armatus is an emerging biofuel platform that produces high amounts of lipids and biomass in mass culture. We observed D. armatus in light-limiting, excess-light, and sinusoidal-light environments to investigate its photoacclimation behaviors and the mechanisms by which it dissipates excess energy. Chlorophyll a/b ratios and the functional absorption cross section of PSII suggested a constitutively small light-harvesting antenna size relative to other green algae. In situ and ex situ measurements of photo-physiology revealed that nonphotochemical quenching is not a significant contributor to photoprotection; however, cells do not suffer substantial photoinhibition despite its near absence. We performed membrane inlet mass spectrometry analysis to show that D. armatus has a very high capacity for alternative electron transport (AET) measured as light-dependent oxygen consumption. Up to 90% of electrons generated at PSII can be dissipated by AET in a water-water cycle during growth in rapidly fluctuating light environments, like those found in industrial-scale photobioreactors. This work highlights the diversity of photoprotective mechanisms present in algal systems, indicating that nonphotochemical quenching is not necessarily required for effective photoprotection in some algae, and suggests that engineering AET may be an attractive target for increasing the biomass productivity of some strains.