Control in the dark of the plastoquinone redox state by mitochondrial activity in Chlamydomonas reinhardtii

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.

Isolation of Chlamydomonas reinhardtii mutants with altered mitochondrial respiration by chlorophyll fluorescence measurement

The unicellular green alga Chlamydomonas reinhardtii is a model organism for studying energetic metabolism. Most mitochondrial respiratory-deficient mutants characterized to date have been isolated on the basis of their reduced ability to grow in heterotrophic conditions. Mitochondrial deficiencies are usually partly compensated by adjustment of photosynthetic activity and more particularly by transition to state 2. In this work, we explored the opportunity to select mutants impaired in respiration and/or altered in dark metabolism by measuring maximum photosynthetic efficiency by chlorophyll fluorescence analyses (FV/FM). Out of about 2900 hygromycin-resistant insertional mutants generated from wild type or from a mutant strain deficient in state transitions (stt7 strain), 22 were found to grow slowly in heterotrophic conditions and 8 of them also showed a lower FV/FM value. Several disrupted coding sequences were identified, including genes coding for three different subunits of respiratory-chain complex I (NUO9, NUOA9, NUOP4) or for isocitrate lyase (ICL1). Overall, the comparison of respiratory mutants obtained in wild-type or stt7 genetic backgrounds indicated that the FV/FM value can be used to isolate mutants severely impaired in dark metabolism.

Algae after dark: mechanisms to cope with anoxic/hypoxic conditions

Chlamydomonas reinhardtii is a unicellular, soil-dwelling (and aquatic) green alga that has significant metabolic flexibility for balancing redox equivalents and generating ATP when it experiences hypoxic/anoxic conditions. The diversity of pathways available to ferment sugars is often revealed in mutants in which the activities of specific branches of fermentative metabolism have been eliminated; compensatory pathways that have little activity in parental strains under standard laboratory fermentative conditions are often activated. The ways in which these pathways are regulated and integrated have not been extensively explored. In this review, we primarily discuss the intricacies of dark anoxic metabolism in Chlamydomonas, but also discuss aspects of dark oxic metabolism, the utilization of acetate, and the relatively uncharacterized but critical interactions that link chloroplastic and mitochondrial metabolic networks.

Oxidation of plastohydroquinone by photosystem II and by dioxygen in leaves

In sunflower leaves linear electron flow LEF=4O2 evolution rate was measured at 20 ppm O2 in N2. PSII charge separation rate CSRII=aII∙PAD∙(Fm-F)/Fm, where aII is excitation partitioning to PSII, PAD is photon absorption density, Fm and F are maximum and actual fluorescence yields. Under 630 nm LED+720 nm far-red light (FRL), LEF was equal to CSRII with aII=0.51 to 0.58. After FRL was turned off, plastoquinol (PQH2) accumulated, but LEF decreased more than accountable by F increase, indicating PQH2-oxidizing cyclic electron flow in PSII (CEFII). CEFII was faster under conditions requiring more ATP, consistent with CEFII being coupled with proton translocation. We propose that PQH2 bound to the QC site is oxidized, one e- moving to P680+, the other e- to Cyt b559. From Cyt b559 the e- reduces QB- at the QB site, forming PQH2. About 10-15% electrons may cycle, causing misses in the period-4 flash O2 evolution and lower quantum yield of photosynthesis under stress. We also measured concentration dependence of PQH2 oxidation by dioxygen, as indicated by post-illumination decrease of Chl fluorescence yield. After light was turned off, F rapidly decreased from Fm to 0.2 Fv, but further decrease to F0 was slow and O2 concentration dependent. The rate constant of PQH2 oxidation, determined from this slow phase, was 0.054 s(-1) at 270 μM (21%) O2, decreasing with Km(O2) of 60 μM (4.6%) O2. This eliminates the interference of O2 in the measurements of CEFII.

Integration of carbon assimilation modes with photosynthetic light capture in the green alga Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii is capable of using organic and inorganic carbon sources simultaneously, which requires the adjustment of photosynthetic activity to the prevailing mode of carbon assimilation. We obtained novel insights into the regulation of light-harvesting at photosystem II (PSII) following altered carbon source availability. In C. reinhardtii, synthesis of PSII-associated light-harvesting proteins (LHCBMs) is controlled by the cytosolic RNA-binding protein NAB1, which represses translation of particular LHCBM isoform transcripts. This mechanism is fine-tuned via regulation of the nuclear NAB1 promoter, which is activated when linear photosynthetic electron flow is restricted by CO(2)-limitation in a photoheterotrophic context. In the wild-type, accumulation of NAB1 reduces the functional PSII antenna size, thus preventing a harmful overexcited state of PSII, as observed in a NAB1-less mutant. We further demonstrate that translation control as a newly identified long-term response to prolonged CO(2)-limitation replaces LHCII state transitions as a fast response to PSII over-excitation. Intriguingly, activation of the long-term response is perturbed in state transition mutant stt7, suggesting a regulatory link between the long- and short-term response. We depict a regulatory circuit operating on distinct timescales and in different cellular compartments to fine-tune light-harvesting in photoheterotrophic eukaryotes.

Central carbon metabolism and electron transport in Chlamydomonas reinhardtii: metabolic constraints for carbon partitioning between oil and starch

The metabolism of microalgae is so flexible that it is not an easy task to give a comprehensive description of the interplay between the various metabolic pathways. There are, however, constraints that govern central carbon metabolism in Chlamydomonas reinhardtii that are revealed by the compartmentalization and regulation of the pathways and their relation to key cellular processes such as cell motility, division, carbon uptake and partitioning, external and internal rhythms, and nutrient stress. Both photosynthetic and mitochondrial electron transfer provide energy for metabolic processes and how energy transfer impacts metabolism and vice versa is a means of exploring the regulation and function of these pathways. A key example is the specific chloroplast localization of glycolysis/gluconeogenesis and how it impacts the redox poise and ATP budget of the plastid in the dark. To compare starch and lipids as carbon reserves, their value can be calculated in terms of NAD(P)H and ATP. As microalgae are now considered a potential renewable feedstock, we examine current work on the subject and also explore the possibility of rerouting metabolism toward lipid production.

SHORT-TERM EFFECTS OF ACETATE AND MICROAEROBIC CONDITIONS ON PHOTOSYNTHESIS AND RESPIRATION IN CHLORELLA SOROKINIANA GXNN 01 (CHLOROPHYTA)(1)

The culture of microalgae using organic carbon sources decreases the cost of operation in closed systems. The effect of carbon sources on microalgae is thus an interesting problem in not only theoretical research but also practical production. The short-term effects of acetate and microaerobic conditions on the growth, photosynthesis, and respiration of the green microalga Chlorella sorokiniana I. Shihira & R.W. Krauss GXNN 01 were described after acetate addition to autotrophic cultures. As the acetate concentration increased, cells needed a longer lag phase to grow, and 243.8 mM acetate completely inhibited growth. Acetate addition induced an immediate response in photosynthesis and respiration. The activity of PS II and PS I were impaired and declined with different rates, and then recovered compared with autotrophic cells. Carbonic anhydrase and Rubisco activities were also inhibited at the beginning, and respiration was increased. We propose that ATP consumption for acetate assimilation results in surplus NADPH, and then accumulated reducing power over-reduces inter-photosystem components and raises the transthylakoid proton gradient, which redistributes energy between PS I and PS II, and leads to a decrease in the PS II/PS I ratio and O2 evolution. An apparent cyclic electron flow was also observed, which may be mainly mediated by NAD(P)H dehydrogenase-dependent pathway since NADPH was in excess. These observations pointed to an acclimation process after acetate addition, and suggested the interaction between photosynthesis and respiration involving ATP and reducing power.

Function of the chloroplastic NAD(P)H dehydrogenase Nda2 for H₂ photoproduction in sulphur-deprived Chlamydomonas reinhardtii

The relative contributions of the PSII-dependent and Nda2-dependent pathways for H₂ photoproduction were investigated in the green microalga Chlamydomonas reinhardtii after suphur-deprivation. For this purpose, H₂ gas production was compared for wild-type and Nda2-deficient cells with or without DCMU (a PSII-inhibitor) in the same experimental conditions. Nda2-deficiency caused a 30% decrease of the maximal H₂ photoevolution rate observed shortly after the establishment of anoxia, and an acceleration of the decline of H₂ photoevolution rate with time. DCMU addition to Nda2-deficient cells completely inhibited H₂ photoproduction, showing that the PSII-independent H₂ photoproduction relies on the presence of Nda2, which feeds the photosynthetic electron transport chain with electrons derived from oxidative catabolism. Nda2-protein abundance increased as a result of sulphur deprivation and further during the H₂ photoproduction process, resulting in high rates of non-photochemical plastoquinone reduction in control cells. Nda2-deficiency had no significant effect on photosynthetic and respiratory capacities in sulphur-deprived cells, but caused changes in the cell energetic status (ATP and NADPH/NADP+ ratio). The rapid decline of H₂ photoevolution rate with time in Nda2-deficient cells revealed a more pronounced inhibition of H₂ photoproduction by accumulated H₂ in the absence of non-photochemical plastoquinone reduction. Nda2 is therefore important for linking H₂ photoproduction with catabolism of storage carbon compounds, and seems also involved in regulating the redox poise of the photosynthetic electron transport chain during H₂ photoproduction.

Temperature responses of dark respiration in relation to leaf sugar concentration

Changes in leaf sugar concentrations are a possible mechanism of short-term adaptation to temperature changes, with natural fluctuations in sugar concentrations in the field expected to modify the heat sensitivity of respiration. We studied temperature-response curves of leaf dark respiration in the temperate tree Populus tremula (L.) in relation to leaf sugar concentration (1) under natural conditions or (2) leaves with artificially enhanced sugar concentration. Temperature-response curves were obtained by increasing the leaf temperature at a rate of 1°C min⁻¹. We demonstrate that respiration, similarly to chlorophyll fluorescence, has a break-point at high temperature, where respiration starts to increase with a faster rate. The average break-point temperature (T(RD) ) was 48.6 ± 0.7°C at natural sugar concentration. Pulse-chase experiments with ¹⁴CO₂ demonstrated that substrates of respiration were derived mainly from the products of starch degradation. Starch degradation exhibited a similar temperature-response curve as respiration with a break-point at high temperatures. Acceleration of starch breakdown may be one of the reasons for the observed high-temperature rise in respiration. We also demonstrate that enhanced leaf sugar concentrations or enhanced osmotic potential may protect leaf cells from heat stress, i.e. higher sugar concentrations significantly modify the temperature-response curve of respiration, abolishing the fast increase of respiration. Sugars or enhanced osmotic potential may non-specifically protect respiratory membranes or may block the high-temperature increase in starch degradation and consumption in respiratory processes, thus eliminating the break-points in temperature curves of respiration in sugar-fed leaves.

Cyclic electron flow around photosystem I in unicellular green algae

Cyclic electron flow around PSI, or cyclic photophosphorylation, is the photosynthetic process which recycles the reducing equivalents produced by photosystem I in the stroma towards the plastoquinone pool. Through the activity of cytochrome b(6)f, which also transfers protons across the membrane, it promotes the synthesis of ATP. The literature dealing with cyclic electron flow in unicellular algae is far less abundant than it is for plants. However, in the chloroplast of algae such as Chlorella or Chlamydomonas, an efficient carbohydrate catabolism renders the redox poise much more reducing than in plant chloroplasts. It is therefore worthwhile highlighting the specific properties of unicellular algae because cyclic electron flow is highly dependent upon the accumulation of these stromal reducing equivalents. Such an increase of reducing power in the stroma stimulates the reduction of plastoquinones, which is the limiting step of cyclic electron flow. In anaerobic conditions in the dark, this reaction can lead to a fully reduced plastoquinone pool and induce state transitions, the migration of 80% of light harvesting complexes II and 20% of cytochrome b(6)f complex from the PSII-enriched grana to the PSI-enriched lamella. These ultrastructural changes have been proposed to further enhance cyclic electron flow by increasing PSI antenna size, and forming PSI-cyt b(6)f supercomplexes. These hypotheses are discussed in light of recently published data.

Loss of mitochondrial ATP synthase subunit beta (Atp2) alters mitochondrial and chloroplastic function and morphology in Chlamydomonas

Mitochondrial F1FO ATP synthase (Complex V) catalyses ATP synthesis from ADP and inorganic phosphate using the proton-motive force generated by the substrate-driven electron transfer chain. In this work, we investigated the impact of the loss of activity of the mitochondrial enzyme in a photosynthetic organism. In this purpose, we inactivated by RNA interference the expression of the ATP2 gene, coding for the catalytic subunit beta, in the green alga Chlamydomonas reinhardtii. We demonstrate that in the absence of beta subunit, complex V is not assembled, respiratory rate is decreased by half and ATP synthesis coupled to the respiratory activity is fully impaired. Lack of ATP synthase also affects the morphology of mitochondria which are deprived of cristae. We also show that mutants are obligate phototrophs and that rearrangements of the photosynthetic apparatus occur in the chloroplast as a response to ATP synthase deficiency in mitochondria. Altogether, our results contribute to the understanding of the yet poorly studied bioenergetic interactions between organelles in photosynthetic organisms.

The redox state of the plastoquinone pool directly modulates minimum chlorophyll fluorescence yield in Chlamydomonas reinhardtii

The effect of the plastoquionone (PQ) pool oxidation state on minimum chlorophyll fluorescence was studied in the green alga Chlamydomonas reinhardtii. In wild type and a mutant strain that lacks both photosystems but retains light harvesting complexes, oxygen depletion induced a rise in minimum chlorophyll fluorescence. An increase in minimum fluorescence yield is also observed when the PQ pool becomes reduced in the presence of oxygen and after application of an ionophore that collapses the transmembrane proton gradient. Together these results indicate that minimum chlorophyll fluorescence is modulated by the PQ oxidation state.

Salinity affects the photoacclimation of Chlamydomonas raudensis Ettl UWO241

Chlamydomonas raudensis Ettl UWO241, a natural variant of C. raudensis, is deficient in state transitions. Its habitat, the deepest layer of Lake Bonney in Antarctica, features low irradiance, low temperature, and high salinity. Although psychrophily and low-light acclimation of this green alga has been described, very little information is available on the effect of salinity. Here, we demonstrate that this psychrophile is halotolerant, not halophilic, and it shows energy redistribution between photosystem I and II based on energy spillover under low-salt conditions. Furthermore, we revealed that C. raudensis exhibits higher non-photochemical quenching in comparison with the mesophile Chlamydomonas reinhardtii, when grown with low-salt, which is due to the lower proton conductivity across the thylakoid membrane. Significance of the C. raudensis UWO241 traits found in the low salinity culture are implicated with their natural habitats, including the high salinity and extremely stable light environments.

Characterization of Nda2, a plastoquinone-reducing type II NAD(P)H dehydrogenase in chlamydomonas chloroplasts

Electron transfer pathways associated to oxygenic photosynthesis, including cyclic electron flow around photosystem I and chlororespiration, rely on non-photochemical reduction of plastoquinones (PQs). In higher plant chloroplasts, a bacterial-like NDH complex homologous to complex I is involved in PQ reduction, but such a complex is absent from Chlamydomonas plastids where a type II NAD(P)H dehydrogenase activity has been proposed to operate. With the aim to elucidate the nature of the enzyme-supporting non-photochemical reduction of PQs, one of the type II NAD(P)H dehydrogenases identified in the Chlamydomonas reinhardtii genome (Nda2) was produced as a recombinant protein in Escherichia coli and further characterized. As many type II NAD(P)H dehydrogenases, Nda2 uses NADH as a preferential substrate, but in contrast to the eukaryotic enzymes described so far, contains non-covalently bound FMN as a cofactor. When expressed at a low level, Nda2 complements growth of an E. coli lacking both NDH-1 and NDH-2, but is toxic at high expression levels. Using an antibody raised against the recombinant protein and based on its mass spectrometric identification, we show that Nda2 is localized in thylakoid membranes. Chlorophyll fluorescence measurements performed on thylakoid membranes show that Nda2 is able to interact with thylakoid membranes of C. reinhardtii by reducing PQs from exogenous NADH or NADPH. We discuss the possible involvement of Nda2 in cyclic electron flow around PSI, chlororespiration, and hydrogen production.

Distinct physiological responses to a high light and low CO2 environment revealed by fluorescence quenching in photoautotrophically grown Chlamydomonas reinhardtii

Mechanisms for countering environmental stress are essential to photosynthetic organisms. Alteration of the photosynthetic apparatus, a mechanism for balancing the flux of light energy and carbon fixation, can be characterized by fluorescence properties. In this study, we have established a simple protocol to determine the extent of energy-dependent quenching (qE) and quenching by state transition (qT) in Chlamydomonas cells by examining their fluorescence properties under light fluctuations. We identified qE as the uncoupler-sensitive NPQ component that was rapidly relaxed upon transition to dark conditions. We characterized the qT component by determining low-temperature fluorescence spectra and analyzing a state-transition-less mutant. By these methods, we observed that similar abiotic stresses-high light conditions (where excess energy is supplied) and low CO2 conditions (where energy utilization is limited)-induced different types of NPQ. High light conditions induced mainly qE-quenching that increased gradually while low CO2 conditions induced mainly qT-quenching that peaked in 20 min and then decreased gradually. That high light and low carbon signals induced different physiological responses suggests that they triggered different genetic responses, which altered protein expression under each of the conditions.

Photosynthesis and state transitions in mitochondrial mutants of Chlamydomonas reinhardtii affected in respiration

Photosynthetic activities were analyzed in Chlamydomonas reinhardtii mitochondrial mutants affected in different complexes (I, III, IV, I + III, and I + IV) of the respiratory chain. Oxygen evolution curves showed a positive relationship between the apparent yield of photosynthetic linear electron transport and the number of active proton-pumping sites in mitochondria. Although no significant alterations of the quantitative relationships between major photosynthetic complexes were found in the mutants, 77 K fluorescence spectra showed a preferential excitation of photosystem I (PSI) compared with wild type, which was indicative of a shift toward state 2. This effect was correlated with high levels of phosphorylation of light-harvesting complex II polypeptides, indicating the preferential association of light-harvesting complex II with PSI. The transition to state 1 occurred in untreated wild-type cells exposed to PSI light or in 3-(3,4-dichlorophenyl)-1,1-dimethylureatreated cells exposed to white light. In mutants of the cytochrome pathway and in double mutants, this transition was only observed in white light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. This suggests higher rates of nonphotochemical plastoquinone reduction through the chlororespiratory pathway, which was confirmed by measurements of the complementary area above the fluorescence induction curve in dark-adapted cells. Photo-acoustic measurements of energy storage by PSI showed a stimulation of PSI-driven cyclic electron flow in the most affected mutants. The present results demonstrate that in C. reinhardtii mutants, permanent defects in the mitochondrial electron transport chain stabilize state 2, which favors cyclic over linear electron transport in the chloroplast.

In vivo interactions between photosynthesis, mitorespiration, and chlororespiration in Chlamydomonas reinhardtii

Interactions between photosynthesis, mitochondrial respiration (mitorespiration), and chlororespiration have been investigated in the green alga Chlamydomonas reinhardtii using flash illumination and a bare platinum electrode. Depending on the physiological status of algae, flash illumination was found to induce either a fast (t(1/2) approximately 300 ms) or slow (t(1/2) approximately 3 s) transient inhibition of oxygen uptake. Based on the effects of the mitorespiratory inhibitors myxothiazol and salicyl hydroxamic acid (SHAM), and of propyl gallate, an inhibitor of the chlororespiratory oxidase, we conclude that the fast transient is due to the flash-induced inhibition of chlororespiration and that the slow transient is due to the flash-induced inhibition of mitorespiration. By measuring blue-green fluorescence changes, related to the redox status of the pyridine nucleotide pool, and chlorophyll fluorescence, related to the redox status of plastoquinones (PQs) in C. reinhardtii wild type and in a photosystem I-deficient mutant, we show that interactions between photosynthesis and chlororespiration are favored when PQ and pyridine nucleotide pools are reduced, whereas interactions between photosynthesis and mitorespiration are favored at more oxidized states. We conclude that the plastid oxidase, similar to the mitochondrial alternative oxidase, becomes significantly engaged when the PQ pool becomes highly reduced, and thereby prevents its over-reduction.

Cyclic electron flow around photosystem I in C(3) plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex

Cyclic electron flow around photosystem (PS) I has been widely described in vitro in chloroplasts or thylakoids isolated from C(3) plant leaves, but its occurrence in vivo is still a matter of debate. Photoacoustic spectroscopy and kinetic spectrophotometry were used to analyze cyclic PS I activity in tobacco (Nicotiana tabacum cv Petit Havana) leaf discs illuminated with far-red light. Only a very weak activity was measured in air with both techniques. When leaf discs were placed in anaerobiosis, a high and rapid cyclic PS I activity was measured. The maximal energy storage in far-red light increased to 30% to 50%, and the half-time of the P(700) re-reduction in the dark decreased to around 400 ms; these values are comparable with those measured in cyanobacteria and C(4) plant leaves in aerobiosis. The stimulatory effect of anaerobiosis was mimicked by infiltrating leaves with inhibitors of mitochondrial respiration or of the chlororespiratory oxidase, therefore, showing that changes in the redox state of intersystem electron carriers tightly control the rate of PS I-driven cyclic electron flow in vivo. Measurements of energy storage at different modulation frequencies of far-red light showed that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco mutant deficient in the plastid Ndh complex was kinetically different from that of the wild type, the cycle being slower in the former leaves. We conclude that the Ndh complex is required for rapid electron cycling around PS I.

Chlororespiration

Chlororespiration has been defined as a respiratory electron transport chain (ETC) in interaction with the photosynthetic ETC in thylakoid membranes of chloroplasts. The existence of chlororespiration has been disputed during the last decade, with the initial evidence mainly obtained with intact algal cells being possibly explained by redox interactions between chloroplasts and mitochondria. The discovery in higher-plant chloroplasts of a plastid-encoded NAD(P)H-dehydrogenase (Ndh) complex, homologous to the bacterial complex I, and of a nuclear-encoded plastid terminal oxidase (PTOX), homologous to the plant mitochondrial alternative oxidase, brought molecular support to the concept of chlororespiration. The functionality of these proteins in non-photochemical reduction and oxidation of plastoquinones (PQs), respectively, has recently been demonstrated. In thylakoids of mature chloroplasts, chlororespiration appears to be a relatively minor pathway compared to linear photosynthetic electron flow from H2O to NADP+. However, chlororespiration might play a role in the regulation of photosynthesis by modulating the activity of cyclic electron flow around photosystem I (PS I). In non-photosynthetic plastids, chlororespiratory electron carriers are more abundant and may play a significant bioenergetic role.

Chlororespiration and the process of carotenoid biosynthesis

The plastoquinone pool during dark adaptation is reduced by endogenous reductants and oxidized at the expense of molecular oxygen. We report here on the redox state of plastoquinone in darkness, using as an indicator the chlorophyll fluorescence kinetics of whole cells of a Chlamydomonas reinhardtii mutant strain lacking the cytochrome b(6)f complex. When algae were equilibrated with a mixture of air and argon at 1.45% air, plastoquinol oxidation was inhibited whereas mitochondrial respiration was not. Consequently, mitochondrial oxidases cannot be responsible for the oxygen consumption linked to plastoquinol oxidation. Plastoquinol oxidation in darkness turned out to be sensitive to n-propyl gallate (PG) and insensitive to salicylhydroxamic acid (SHAM), whereas mitochondrial respiration was sensitive to SHAM and PG. Thus, both PG treatment and partial anaerobiosis allow to draw a distinction between an inhibition of plastoquinol oxidation and an inhibition of mitochondrial respiration, indicating the presence of a plastoquinol:oxygen oxidoreductase. The possible identification of this oxidase with an oxidase involved in carotenoid biosynthesis is discussed in view of various experimental data.

Chlororespiration

The term 'chlororespiration' is used to describe the activity of a putative respiratory electron transler chain within the thylakoid membrane of chloroplasts and was originally proposed by Bennouon in 1982 to explain effects on the redox state of the plastoquinone pool in green algae in the absence of photosynthetic plastoquinone electrontransfer. In his original model, Bennoun suggested that the pool could be reduced through the action of a NAD(P) H dehydrogenase and could be oxidized by oxygen at an oxidase. At the same time an electrochemical gradient would be generated across the membrane. This review describes the current status of the chlororespiration model in light of the recent discoveries of novel respiratory components chloroplast thylakoid membrane.

Electron flow between photosystem II and oxygen in chloroplasts of photosystem I-deficient algae is mediated by a quinol oxidase involved in chlororespiration

In Chlamydomonas reinhardtii mutants deficient in photosystem I because of inactivation of the chloroplast genes psaA or psaB, oxygen evolution from photosystem II occurs at significant rates and is coupled to a stimulation of oxygen uptake. Both activities can be simultaneously monitored by continuous mass spectrometry in the presence of (18)O(2). The light-driven O(2) exchange was shown to involve the plastoquinone pool as an electron carrier, but not cytochrome b(6)f. Photosystem II-dependent O(2) production and O(2) uptake were observed in isolated chloroplast fractions. Photosystem II-dependent oxygen exchange was insensitive to a variety of inhibitors (azide, carbon monoxide, cyanide, antimycin A, and salicylhydroxamic acid) and radical scavengers. It was, however, sensitive to propyl gallate. From inhibitors effects and electronic requirements of the O(2) uptake process, we conclude that an oxidase catalyzing oxidation of plastoquinol and reduction of oxygen to water is present in thylakoid membranes. From the sensitivity of flash-induced O(2) exchange to propyl gallate, we conclude that this oxidase is involved in chlororespiration. Clues to the identity of the protein implied in this process are given by pharmacological and immunological similarities with a protein (IMMUTANS) identified in Arabidopsis chloroplasts.

An ISFET-algal (Chlamydomonas) hybrid provides a system for eco-toxicological tests

A cellular sensoring system was designed in which metabolism-dedicated pH-ISFETs and the unicellular green alga Chlamydomonas reinhardtii as a biological component, were combined. The system permits on-line detection of pH changes caused by the metabolic and photosynthetic activities of the cells. Photosynthetic activity results in a basification of the medium caused by uptake of CO2. In darkness, an acidification of the medium, resulting from the production of CO2 by degradation of starch was observed. Both, acidification and basification, are sensitive indicators for the physiological activity of the alga. Experiments using inhibitors of energy metabolism or photosynthesis illustrate the utility of this system for an on-line monitoring of substances of eco-toxicological importance.

Limited photosynthetic electron flow but no CO2 fixation in Chlamydomonas mutants lacking photosystem I

By measuring O2 and CO2 exchange in mutants of the green alga Chlamydomonas reinhardtii in which genes encoding the reaction center of photosystem I (psaA or psaB) have been deleted, we found that a photosystem II-dependent electron flow using O2 as the final acceptor can be sustained in the light. However, in contrast with recent reports using other Chlamydomonas mutants (B4 and F8), we show here that CO2 fixation does not occur in the absence of photosystem I. By deleting the psaA gene in both B4 and F8 strains, we conclude that the ability of these mutants to fix CO2 in the light is due to the presence of residual amounts of photosystem I.

Nigericin insensitive post-illumination reduction in fluorescence yield in Dunaliella tertiolecta (chlorophyte)

Cells of the green alga Dunaliella tertiolecta grown in a light/dark cycle were exposed to high light for about 15 min. In light, energy-dependent quenching reduced fluorescence emission and decreased PS II efficiency. Within 3 minutes after darkening fluorescence quenching largely relaxed. However, PS II fluorescence emission decreased again after further darkening. Fo and Fm decreased to the same relative extent and the PS II efficiency was not reduced. This Reduction in Fluorescence yield in Darkness, termed RFD for the purpose of this paper, lasted about 20 min. The deepoxidation state of xanthophylls remained unchanged during and after the 15-min exposure to high light. We show that RFD is insensitive to the uncoupler nigericin and thus unrelated to energy-dependent quenching. RFD correlated with a reduction of the PQ pool after darkening and low levels of far red or blue light (430 nm more than 460 nm) prevented RFD. This is in contrast to observations in higher plants, where a post-illumination reduction of the PQ pool causes and increase in Fo (Groom et al. (1993) Photosynth Res 36: 205-215). Changes in the adenylate energy charge were not correlated with RFD. Antimycin A and cyanide, both inhibitors of the PQ-oxidase, caused an increase in RFD whereas SHAM, an inhibitor of the chloroplastic glycolate-quinone oxidoreductase, caused a decrease. Low CO2 concentrations, known to increase the oxygenase activity of Rubisco and to generate glycolate and P-glycolate in light, caused an increase in RFD. We propose that accumulated glycolate and P-glycolate reduce the PQ pool in darkness, leading to the formation of RFD. During RFD, 77 K fluorescence emission from PS II was more reduced than that from PS I, thus resembling a state I, state II transition. However, the reduction in fluorescence yield during RFD is much larger than the reduction previously attributed to state transitions and it is unclear whether RFD and state transitions are identical. The formation and relaxation of RFD increased with higher temperatures and the extent of RFD was largest at the growth temperature (25°C). RFD has to be taken into account when fluorescence is measured after darkening as it may be mistaken for energy-dependent quenching.