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Acclimation of corals to light is known to rely on multiple strategies working at different timescales. Among them, photosynthetic alternative electron flows (AEFs) could act as photoprotective mechanisms under fluctuating light intensities. In this work, we first compared the use of AEFs in shallow and mesophotic colonies of the coral Stylophora pistillata by carrying out joint measurements of oxygen exchange and photosystems quantum yields. We observed similar capacities to re-route photosynthetically derived electrons toward oxygen (Mehler reaction) and to perform cyclic electron flow around photosystem I under high light intensity in both colony types. But in contrast to mesophotic colonies that hosted Cladocopium, the photosynthetic apparatus of Symbiodinium microadriaticum hosted by their shallow counterparts was notably able to drive a higher number of electrons, displayed a higher thermal dissipation of absorbed light energy. Then, a short-term light stress was applied to evaluate the plasticity of the photosynthetic apparatus. Both shallow and mesophotic colonies showed fast acclimation to the low light regime. In contrast, under the high light regime, mesophotic colonies showed a limited capacity to dissipate light energy and were strongly photoinhibited, though their PSI activity was partly preserved and likely involved cyclic electron flow. This study shows how important the photosynthetic alternative electron flows are in acclimation processes to light and how the plasticity of the photosynthetic processes in Symbiodiniaceae may shape the vertical distribution of the coral holobionts. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

• Hyperoxia immediately lowered PSII activity under sub-saturating and saturating light in Chlamydomonas reinhardtii. • Hyperoxia stimulated ROS production from PSII, as well as the Mehler reaction from PSI under saturating light. • Flavodiiron proteins prevented PSI-mediated Mehler reaction, but not ROS production from PSII in the light. • Lack of flavodiiron proteins led to lower PSII activity under sub-saturating and saturating light. [ABSTRACT FROM AUTHOR]

The Mehler reaction is the major source of superoxide anion radicals (O2•−) and hydrogen peroxide molecules in the chloroplast. In terms of mechanism, the Mehler reaction starts as a one-electron oxidation of electron transfer components of the photosynthetic apparatus by molecular oxygen producing O2•−, which is further reduced to H2O2, the stable product of the Mehler reaction. This review covers recent data on the mechanisms of O2 photoreduction in the photosynthetic electron transport chain, with particular emphasis on photosystem I (PSI), which has long been considered as the major site of O2•− production in the chloroplast. Despite the long history of the study, until recently, there was some contradiction as to the precise location(s) of O2 photoreduction within PSI. Here, I explain some controversies that still exist and discuss evolutionary trends toward optimizing PSI activity in the presence of O2. [ABSTRACT FROM AUTHOR]

Alternative electron fluxes such as the cyclic electron flux (CEF) around photosystem I (PSI) and Mehler reaction (Me) are essential for efficient photosynthesis because they generate additional ATP and protect both photosystems against photoinhibition. The capacity for Me can be estimated by measuring O2 exchange rate under varying irradiance and CO2 concentration. In this study, mass spectrometric measurements of O2 exchange were made using leaves of representative species of C3 and C4 grasses grown under natural light (control; PAR ~ 800 µmol quanta m−2 s−1) and shade (~ 300 µmol quanta m−2 s−1), and in representative species of gymnosperm, liverwort and fern grown under natural light. For all control grown plants measured at high CO2, O2 uptake rates were similar between the light and dark, and the ratio of Rubisco oxygenation to carboxylation (Vo/Vc) was low, which suggests little potential for Me, and that O2 uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO2 stimulated O2 uptake in the light, Vo/Vc and Me in all species. The C3 species had similar Vo/Vc, but Me was highest in the grass and lowest in the fern. Among the C4 grasses, shade increased O2 uptake in the light, Vo/Vc and the assimilation quotient (AQ), particularly at low CO2, whilst Me was only substantial at low CO2 where it may contribute 20–50% of maximum electron flow under high light. [ABSTRACT FROM AUTHOR]

The central Chilean Andes are located in a Mediterranean-type climate zone, characterized by dry summers and high irradiance. This creates a contrasting elevational gradient because higher elevations get more solid precipitation and lower temperatures, resulting in higher soil humidity along the growing season compared with severe drought at lower elevations. Therefore, species with wide elevational distributions, such as Phacelia secunda, must have developed specific adaptations to cope with contrasting severity of drought stress-induced photoinhibition at different elevations. We hypothesize that P. secunda from lower elevation, is more tolerant to drought stress-induced photo-damage than plants from high elevation. This higher tolerance will be associated to a higher diversity of photoprotective strategies in plants that naturally suffers severe drought every growing season. To test this hypothesis, plants from 2700 and 3600 m in the central Chilean Andes were grown under the common garden and then subjected to water restriction. We measured stress indicators, photochemistry of PSII and PSI and estimate alternative electron sinks. Drought affected P. secunda photosynthetic performance differentially depending on the elevation of provenance. Plants from lower elevation exhibited higher drought tolerance than higher elevation ones. This was related to higher levels of heat dissipation and alternative electron sinks exhibited by plants from lower elevation under drought stress. We concluded that plants naturally subjected to recurrent drought are better adapted to respond to drought stress using additional photochemical photoprotective mechanisms and confirm the role of alternative electron sinks ameliorating photodamage. [ABSTRACT FROM AUTHOR]

Highlights • Excess NH 4 +-N induced oxidative stress in shoots of the submerged macrophyte H. verticillata. • The distribution of reactive oxygen in submerged plants was first investigated by cytochemical electron microscopic observation. • The sources of reactive oxygen in the shoots of H. verticillata under NH 4 +-N stress were related to PM NADPH oxidase and Mehler reaction. Abstract In this study, the effects of excess NH 4 +-N on the subcellular accumulation of O 2 − and H 2 O 2 in submerged plant Hydrilla verticillata (L.f.) Royle were investigated using both histochemical and cytochemical methods. Treatments with ≥ 2.00 and ≥ 5.00 mg L−1 NH 4 +-N for 5 d significantly increased production of O 2 − and H 2 O 2 , respectively. The activities of plasma membrane-bound NADPH (nicotinamide adenine dinucleotide phosphate) oxidases and antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, dehydroascorbate reductase and glutathione reductase) were also increased correspondingly. This study also provides the first cytochemical evidence of subcellular accumulation of O 2 − and H 2 O 2 in the submerged plants. In the leaves of H. verticillata treated with 20.0 mg L−1 NH 4 +-N, O 2 − dependent DAB precipitates were found primarily on the inner side of the plasma membrane, extracellular space and chloroplasts. H 2 O 2 -CeCl 3 precipitates were mainly localized on the inner side of the plasma membrane and extracellular space of the mesophyll cells. Treatments with the inhibitors of NADPH oxidase (diphenylene iodonium and imidazole) indicate that NH 4 +-N-induced production of O 2 − and H 2 O 2 in H. verticillata leaves may involve plasma membrane-bound NADPH oxidase. Moreover, low-light treatment decreased NH 4 +-induced O 2 − production, suggesting that alterations in the photosynthetic electron transfer chain due to NH 4 + toxicity could lead to O 2 − production. [ABSTRACT FROM AUTHOR]

Reduction-oxidation (redox) reactions, in which electrons move from a donor to an acceptor, are the functional heart of photosynthesis. It is not surprising therefore that reactive oxygen species (ROS) are generated in abundance by photosynthesis, providing a plethora of redox signals as well as functioning as essential regulators of energy and metabolic fluxes. Chloroplasts are equipped with an elaborate and multifaceted protective network that allows photosynthesis to function with high productivity even in resource-limited natural environments. This includes numerous antioxidants with overlapping functions that provide enormous flexibility in redox control. ROS are an integral part of the repertoire of chloroplast signals that are transferred to the nucleus to convey essential information concerning redox pressure within the electron transport chain. Current evidence suggests that there is specificity in the gene-expression profiles triggered by the different ROS signals, so that singlet oxygen triggers programs related to over excitation of photosystem (PS) II while superoxide and hydrogen peroxide promote the expression of other suites of genes that may serve to alleviate electron pressure on the reducing side of PSI. Not all chloroplasts are equal in their signaling functions, with some sub-populations appearing to have better contacts/access to the nucleus than others to promote genetic and epigenetic responses. While the concept that light-induced increases in ROS result in damage to PSII and photoinhibition is embedded in the photosynthesis literature, there is little consensus concerning the extent to which such oxidative damage happens in nature. Slowly reversible decreases in photosynthetic capacity are not necessarily the result of light-induced damage to PSII reaction centers. [ABSTRACT FROM AUTHOR]

Salt-tolerant grasses of warm sub-tropical ecosystems differ in their distribution patterns with respect to salinity and moisture regimes. Experiments were conducted on CO fixation and light harvesting processes of four halophytic C grasses grown under different levels of salinity (0, 200 and 400 mM NaCl) under ambient environmental conditions. Two species were from a high saline coastal marsh ( Aeluropus lagopoides and Sporobolus tremulus) and two were from a moderate saline sub-coastal draw-down tidal marsh ( Paspalum paspalodes and Paspalidium geminatum). Analyses of the carbon isotope ratios of leaf biomass in plants indicated that carbon assimilation was occurring by C photosynthesis in all species during growth under varying levels of salinity. In the coastal species, with increasing salinity, there was a parallel decrease in rates of CO fixation ( A), transpiration ( E) and stomatal conductance ( g ), with no effect on water use efficiency ( WUE). These species were adapted for photoprotection by an increase in the Mehler reaction with an increase in activity of PSII/CO fixed accompanied by high levels of antioxidant enzymes, superoxide dismutase and ascorbate peroxidase. The sub-coastal species P. paspalodes and P. geminatum had high levels of carotenoid pigments and non-photochemical quenching by the xanthophyll cycle. [ABSTRACT FROM AUTHOR]

Polyphenol oxidases (PPOs) catalyze the oxidation of phenolics to quinones, the secondary reactions of which lead to oxidative browning and postharvest losses of many fruits and vegetables. PPOs are ubiquitous in angiosperms, are inducible by both biotic and abiotic stresses, and have been implicated in several physiological processes including plant defense against pathogens and insects, the Mehler reaction, photoreduction of molecular oxygen by PSI, regulation of plastidic oxygen levels, aurone biosynthesis and the phenylpropanoid pathway. Here we review experiments in which the roles of PPO in disease and insect resistance as well as in the Mehler reaction were investigated using transgenic tomato (Lycopersicon esculentum) plants with modified PPO expression levels (suppressed PPO and overexpressing PPO). These transgenic plants showed normal growth, development and reproduction under laboratory, growth chamber and greenhouse conditions. Antisense PPO expression dramatically increased susceptibility while PPO overexpression increased resistance of tomato plants to Pseudomonas syringae. Similarly, PPO-overexpressing transgenic plants showed an increase in resistance to various insects, including common cutworm (Spodoptera litura (F.)), cotton bollworm (Helicoverpa armigera (Hübner)) and beet army worm (Spodoptera exigua (Hübner)), whereas larvae feeding on plants with suppressed PPO activity had higher larval growth rates and consumed more foliage. Similar increases in weight gain, foliage consumption, and survival were also observed with Colorado potato beetles (Leptinotarsa decemlineata (Say)) feeding on antisense PPO transgenic tomatoes. The putative defensive mechanisms conferred by PPO and its interaction with other defense proteins are discussed. In addition, transgenic plants with suppressed PPO exhibited more favorable water relations and decreased photoinhibition compared to nontransformed controls and transgenic plants overexpressing PPO, suggesting that PPO may have a role in the development of plant water stress and potential for photoinhibition and photooxidative damage that may be unrelated to any effects on the Mehler reaction. These results substantiate the defensive role of PPO and suggest that manipulation of PPO activity in specific tissues has the potential to provide broad-spectrum resistance simultaneously to both disease and insect pests, however, effects of PPO on postharvest quality as well as water stress physiology should also be considered. In addition to the functional analysis of tomato PPO, the application of antisense/sense technology to decipher the functions of PPO in other plant species as well as for commercial uses are discussed.

Cyclic Electron Flow (CEF) is a regulator of acceptor-side limitations and has multiple functions in the green alga, Chlamydomonas reinhardtii. Here we draw on recent and historic literature and concentrate on its role in Photosystem I (PSI) photoprotection, outlining causes and consequences of damage to PSI and CEF’s role as an avoidance mechanism. We outline two functions of CEF in PSI photoprotection that are both linked to trans-thylakoidal gradient formation and luminal acidification: firstly, its action on Photosystem II non-photochemical quenching and photosynthetic control and secondly, its action in stromal poising to overcome acceptor-side limitation by rebalancing NADPH and ATP ratios for carbon fixation.

H 2 producing micro-algae such as Chlamydomonas reinhardtii ( C. reinhardtii ), express the strictly anaerobic enzyme FeFe-hydrogenase (HydA). In this study, we examined whether superoxide dismutase (SOD), an antioxidant, can protect HydA under aerobic conditions. We conducted in-vitro assays using purified enzymes, to analyze the activity of HydA in the presence or absence of SOD. We observed that SOD enhances HydA activity both in the presence and absence of oxygen, showing that the SOD effect on HydA activity is not strictly oxygen dependent. Furthermore, we found that SOD boosts Ferredoxin-NADP + -oxidoreductase (FNR) NDAP + reduction but not NADPH oxidation i.e. diaphorase activity. Thus, suggesting a mechanism involving proton transfer rather than electron transfer. Based on these findings, we constructed a HydA-SOD fusion protein that further boosted hydrogen production by HydA. The HydA-SOD photosynthetic activity was enhanced by 300%, reaching 700 μmol H 2 (mg [chl] hr) −1 , which is the fastest photosynthetic rate ever reported for an algal HydA. [ABSTRACT FROM AUTHOR]

This study investigates the role of O as an electron acceptor alternative to CO in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2 mM DIC and no DIC with different O levels (air-equilibrated levels, 3 % of air equilibrium and restored air-equilibrated levels). Lowering O did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2 mM DIC, air-equilibrated O; (2) saturating CO, no O; and (3) no DIC, air-equilibrated O. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor. [ABSTRACT FROM AUTHOR]

To better understand the adaptation mechanisms of the photosynthetic apparatus of cotton plants to water deficit conditions, the influence of water deficit on photosynthesis, chlorophyll a fluorescence and the activities of antioxidant systems were determined simultaneously in Gossypium hirsutum L. cv. Xinluzao 45 (upland cotton) and Gossypium barbadense L. cv. Xinhai 21 (pima cotton). Water deficit decreased photosynthesis in both cotton species, but did not decrease chlorophyll content or induce any sustained photoinhibition in either cotton species. Water deficit increased ETR/4-AG, where ETR/4 estimates the linear photosynthetic electron flux and AG is the gross rate of carbon assimilation. The increase in ETR/4-AG, which represents an increase in photorespiration and alternative electron fluxes, was particularly pronounced in Xinluzao 45. In Xinluzao 45, water deficit increased the activities of antioxidative enzymes, as well as the contents of reactive oxygen species (ROS), which are related to the Mehler reaction. In contrast, moderate water deficit particularly increased non-photochemical quenching (NPQ) in Xinhai 21. Our results suggest that Xinluzao 45 relied on enhanced electron transport such as photorespiration and the Mehler reaction to dissipate excess light energy under mild and moderate water deficit. Xinhai 21 used enhanced photorespiration for light energy utilisation under mild water deficit but, when subjected to moderate water deficit, possessed a high capacity for dissipating excess light energy via heat dissipation. [ABSTRACT FROM AUTHOR]

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 O 2 -sensitive N 2 -fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O 2 (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 CO 2 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 CO 2 fixation. Under ambient CO 2 diazotrophic conditions, Flv3A is responsible for moderate O 2 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 H 2 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., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)

The production of reactive oxygen species ( ROS) is an unavoidable part of photosynthesis. Stress that accompanies high light levels and low CO2 availability putatively includes enhanced ROS production in the so-called Mehler reaction. Such conditions are thought to encourage O2 to become an electron acceptor at photosystem I, producing the ROS superoxide anion radical ( [ABSTRACT FROM AUTHOR]

Previous studies have shown that exposure of Arabidopsis leaves to high light (HL) causes a systemic acquired acclimation (SAA) response in the vasculature. It has been postulated that C-like photosynthesis in the leaf veins triggers this response via the Mehler reaction. To investigate this proposed connection and extend SAA to other plants, we examined the redox state of NADPH, ascorbate (ASA), and glutathione (GSH) pools; levels and histochemical localization of O- and HO signals; and activities of antioxidant enzymes in the midvein and leaf lamina of rice, when they were subjected to HL and low light. The results showed that (1) high NADPH/NADP was generated by C-like photosynthesis under HL in the midvein and (2) SAA was colocally induced by HL, as indicated by the combined signaling network, including the decrease in redox status of ASA and GSH pools, accumulation of HO and O- signals, and high superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. The high correlations between these occurrences suggest that the enhanced NADPH/NADP in HL-treated midveins might alter redox status of ASA and GSH pools and trigger HO and O- signals during SAA via the Mehler reaction. These changes in turn upregulate SOD and APX activities in the midvein. In conclusion, SAA may be a common regulatory mechanism for the adaptation of angiosperms to HL. Manipulation of NADPH/NADP levels by C-like photosynthesis promotes SAA under HL stress in the midvein. [ABSTRACT FROM AUTHOR]

Abstract: The present study examines how different pCO2 acclimations affect the CO2- and light-dependence of photophysiological processes and O2 fluxes in four Southern Ocean (SO) key phytoplankton species. We grew Chaetoceros debilis (Cleve), Pseudo-nitzschia subcurvata (Hasle), Fragilariopsis kerguelensis (O'Meara) and Phaeocystis antarctica (Karsten) under low (160μatm) and high (1000μatm) pCO2. The CO2- and light-dependence of fluorescence parameters of photosystem II (PSII) were determined by means of a fluorescence induction relaxation system (FIRe). In all tested species, nonphotochemical quenching (NPQ) is the primary photoprotection strategy in response to short-term exposure to high light or low CO2 concentrations. In C. debilis and P. subcurvata, PSII connectivity (p) and functional absorption cross-sections of PSII in ambient light (σPSII′) also contributed to photoprotection while changes in re-oxidation times of Qa acceptor (τQa) were more significant in F. kerguelensis. The latter was also the only species being responsive to high acclimation pCO2, as these cells had enhanced relative electron transport rates (rETRs) and σPSII′ while τQa and p were reduced under short-term exposure to high irradiance. Low CO2-acclimated cells of F. kerguelensis and all pCO2 acclimations of C. debilis and P. subcurvata showed dynamic photoinhibition with increasing irradiance. To test for the role and presence of the Mehler reaction in C. debilis and P. subcurvata, the light-dependence of O2 fluxes was estimated using membrane inlet mass spectrometry (MIMS). Our results show that the Mehler reaction is absent in both species under the tested conditions. We also observed that dark respiration was strongly reduced under high pCO2 in C. debilis while it remained unaltered in P. subcurvata. Our study revealed species-specific differences in the photophysiological responses to pCO2, both on the acclimation as well as the short-term level. [Copyright &y& Elsevier]

Oxygen plays an important role in photosynthesis by participating in a number of O2-consuming reactions. O2 inhibits CO2 fixation by stimulating photorespiration, thus reducing plant production. O2 interacts with photosynthetic electron transport in the chloroplasts’ thylakoids in two main ways: by accepting electrons from PSI (Mehler reaction); and by accepting electrons from reduced plastoquinone (PQ) mediated by the plastid terminal oxidase (PTOX). In this study, we show, using 101 plant species, that there is a difference in the potential for photosynthetic electron flow to O2 between angiosperms and gymnosperms. We found, from measurements of Chl fluorescence and leaf absorbance at 830 nm, (i) that electron outflow from PSII, as determined by decay kinetics of Chl fluorescence after application of a saturating light pulse, is more rapid in gymnosperms than in angiosperms; (ii) that the reaction center Chl of PSI (P700) is rapidly and highly oxidized in gymnosperms during induction of photosynthesis; and (iii) that these differences are dependent on oxygen. Finally, rates of O2 uptake measured by mass spectrometry in the absence of photorespiration were significantly promoted by illumination in dark-adapted leaves of gymnosperms, but not in those of angiosperms. The light-stimulated O2 uptake was around 10% of the maximum O2 evolution in gymnosperms and 1% in angiosperms. These results suggest that gymnosperms have increased capacity for electron leakage to oxygen in photosynthesis compared with angiosperms. The involvement of the Mehler reaction and PTOX in the electron flow to O2 is discussed. [ABSTRACT FROM AUTHOR]

Cyanobacterial flavodiiron proteins (FDPs; A-type flavoprotein, Fly) comprise, besides the D-lactamase--Iike and flavodoxin domains typical for all FDPs, an extra NAD(P)H:flavin oxidoreductase module and thus differ from FDPs in other Bacteria and Archaea. Synechocystis sp. PCC 6803 has four genes encoding the FDPs. Fivi and F1v3 function as an NAD(P)H:oxygen oxidoreductase, donating electrons directly to 02 without production of reactive oxygen species. Here we show that the Flvl and F1v3 proteins are crucial for cyanobacteria under fluctuating light, a typical light condition in aquatic environments. Under constant-light conditions, regardless of light intensity, the Flvl and Flv3 proteins are dispensable. In contrast, under fluctuating light conditions, the growth and photosynthesis of the Δf!v1(A) and/or Δf!v3(A) mutants of Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120 become arrested, resulting in cell death in the most severe cases. This reaction is mainly caused by malfunction of photosystem I and oxidative damage induced by reactive oxygen species generated during abrupt short-term increases in light intensity. Unlike higher plants that lack the FDPs and use the Proton Gradient Regulation 5 to safeguard photosystem I, the cyanobacterial homolog of Proton Gradient Regulation 5 is shown not to be crucial for growth under fluctuating light. Instead, the unique FIvl/F1v3 heterodimer maintains the redox balance of the electron transfer chain in cyanobacteria and provides protection for photosystem I under fluctuating growth light. Evolution of unique cyanobacterial FDPs is discussed as a prerequisite for the development of oxygenic photosynthesis. [ABSTRACT FROM AUTHOR]

Abstract: The enhanced electricity generation in a biocathode bio-electrochemical system (BES) with Microcystis aeruginosa IPP as the cathodic microorganism under illumination is investigated. The results show that this cyanobacterium is able to act as a potential cathodic microorganism under illumination. In addition, M. aeruginosa IPP is found to produce reactive oxygen species (ROS) in its growth in the BES. ROS, as more competitive electron acceptors than oxygen, are utilized prior to oxygen. The BES current is substantially reduced when the ROS production is inhibited by mannitol, indicating that the ROS secreted by the cyanobacterium play an important role in the electricity generation of such a biocathode BES. This work demonstrates that the ROS released by cyanobacteria benefit for an enhanced electricity generation of BES. [Copyright &y& Elsevier]

This paper studied the effects of foliar spraying different concentration La(NO3)3 on the photosynthesis, chlorophyll fluorescence parameters, Mehler reaction, and xanthophyll cycle of ryegrass seedlings under the stress of 150 mmol NaHCO3 ∙L-1. Foliar spraying low concentration (0.05 mmol∙L-1) La(NO3)3 could significantly decrease the decrement of net photosynthesis rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and stomatal limited value (Ls) and the increment of intercellular CO2 concentration (Ci) under NaHCO3 stress, efficiently alleviate the inhibitory effects of NaHCO3 stress on PS II photochemical quenching (qP), actual photochemical efficiency (ΦPSII), photosynthetic carbon assimilation-dependent electron transport rate (ETRp), and Mehler reaction-dependent electron transport rate (ETRm), enhance the activities of superoxide dismutase, peroxidase, and ascorbate peroxidase, the non-photochemical energy dissipation (NPQ), the xanthophyll cycle pool (V+A+Z), and the de-epoxidation extent of xanthophyll cycle (A+Z) / (V+A+Z), and thereby, alleviate the damage of photosynthetic apparatus caused by NaH- CO3 stress. However, treating with high concentration (0.5 mmol∙L-1) La(NO3)3 had no obvious alleviation effects. It was suggested that foliar spraying an appropriate concentration La(NO3)3 could not only alleviate the decrease of ryegrass seedling's photosynthetic rate induced by non-stomata factors and the inhibition of photochemical efficiency, but also accelerate the Mehler reaction under NaHCO3 stress. With the accelerated Mehler reaction, excessive excitation energy could directly be consumed, and the xanthophyll cycle-dependent thermal dissipation could be promoted to efficiently protect the photosynthetic apparatus against photo-damage under NaHCO3 stress. Also, the active oxygen produced by the accelerated Mehler reaction could be scavenged by the enhanced anti-oxidative enzyme activities. [ABSTRACT FROM AUTHOR]

Occurrences whereby cnidaria lose their symbiotic dinoflagellate microalgae ( Symbiodinium spp.) are increasing in frequency and intensity. These so-called bleaching events are most often related to an increase in water temperature, which is thought to limit certain Symbiodinium phylotypes from effectively dissipating absorbed excitation energy that is otherwise used for photochemistry. Here, we examined photosynthetic characteristics and hydrogen peroxide (H2O2) production, a possible signal involved in bleaching, from two Symbiodinium types (a thermally “tolerant” A1 and “sensitive” B1) representative of cnidaria– Symbiodinium symbioses of reef-building Caribbean corals. Under steady-state growth at 26°C, a higher efficiency of PSII photochemistry, rate of electron turnover, and rate of O2 production were observed for type A1 than for B1. The two types responded very differently to a period of elevated temperature (32°C): type A1 increased light-driven O2 consumption but not the amount of H2O2 produced; in contrast, type B1 increased the amount of H2O2 produced without an increase in light-driven O2 consumption. Therefore, our results are consistent with previous suggestions that the thermal tolerance of Symbiodinium is related to adaptive constraints associated with photosynthesis and that sensitive phylotypes are more prone to H2O2 production. Understanding these adaptive differences in the genus Symbiodinium will be crucial if we are to interpret the response of symbiotic associations, including reef-building corals, to environmental change. [ABSTRACT FROM AUTHOR]