Involvement of state transitions in the switch between linear and cyclic electron flow in Chlamydomonas reinhardtii

The energetic metabolism of photosynthetic organisms is profoundly influenced by state transitions and cyclic electron flow around photosystem I. The former involve a reversible redistribution of the light-harvesting antenna between photosystem I and photosystem II and optimize light energy utilization in photosynthesis whereas the latter process modulates the photosynthetic yield. We have used the wild-type and three mutant strains of the green alga Chlamydomonas reinhardtii—locked in state I (stt7), lacking the photosystem II outer antennae (bf4) or accumulating low amounts of cytochrome b₆f complex (A-AUU)—and measured electron flow though the cytochrome b₆f complex, oxygen evolution rates and fluorescence emission during state transitions. The results demonstrate that the transition from state 1 to state 2 induces a switch from linear to cyclic electron flow in this alga and reveal a strict cause–effect relationship between the redistribution of antenna complexes during state transitions and the onset of cyclic electron flow.     

Towards efficient hydrogen production: the impact of antenna size and external factors on electron transport dynamics in <Emphasis Type="Italic">Synechocystis</Emphasis> PCC 6803

Three Synechocystis PCC 6803 strains with different levels of phycobilisome antenna-deficiency have been investigated for their impact on photosynthetic electron transport and response to environmental factors (i.e. light-quality, -quantity and composition of growth media). Oxygen yield and P₇₀₀ reduction kinetic measurements showed enhanced linear electron transport rates—especially under photoautotrophic conditions—with impaired antenna-size, starting from wild type (WT) (full antenna) over ΔapcE- (phycobilisomes functionally dissociated) and Olive (lacking phycocyanin) up to the PAL mutant (lacking the whole phycobilisome). In contrast to mixotrophic conditions (up to 80% contribution), cyclic electron transport plays only a minor role (below 10%) under photoautotrophic conditions for all the strains, while linear electron transport increased up to 5.5-fold from WT to PAL mutant. The minor contribution of the cyclic electron transport was proportionally increased with the linear one in the ΔapcE and Olive mutant, but was not altered in the PAL mutant, indicating that upregulation of the linear route does not have to be correlated with downregulation of the cyclic electron transport. Antenna-deficiency involves higher linear electron transport rates by tuning the PS2/PS1 ratio from 1:5 in WT up to 1:1 in the PAL mutant. While state transitions were observed only in the WT and Olive mutant, a further ~30% increase in the PS2/PS1 ratio was achieved in all the strains by long-term adaptation to far red light (720 nm). These results are discussed in the context of using these cells for future H₂ production in direct combination with the photosynthetic electron transport and suggest both Olive and PAL as potential candidates for future manipulations toward this goal. In conclusion, the highest rates can be expected if mutants deficient in phycobilisome antennas are grown under photoautotrophic conditions in combination with uncoupling of electron transport and an illumination which excites preferably PS1.     

Hydrogen production by <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: an elaborate interplay of electron sources and sinks

The unicellular green alga Chlamydomonas reinhardtii possesses a [FeFe]-hydrogenase HydA1 (EC 1.12.7.2), which is coupled to the photosynthetic electron transport chain. Large amounts of H₂ are produced in a light-dependent reaction for several days when C. reinhardtii cells are deprived of sulfur. Under these conditions, the cells drastically change their physiology from aerobic photosynthetic growth to an anaerobic resting state. The understanding of the underlying physiological processes is not only important for getting further insights into the adaptability of photosynthesis, but will help to optimize the biotechnological application of algae as H₂ producers. Two of the still most disputed questions regarding H₂ generation by C. reinhardtii concern the electron source for H₂ evolution and the competition of the hydrogenase with alternative electron sinks. We analyzed the H₂ metabolism of S-depleted C. reinhardtii cultures utilizing a special mass spectrometer setup and investigated the influence of photosystem II (PSII)- or ribulosebisphosphate-carboxylase/oxygenase (Rubisco)-deficiency. We show that electrons for H₂-production are provided both by PSII activity and by a non-photochemical plastoquinone reduction pathway, which is dependent on previous PSII activity. In a Rubisco-deficient strain, which produces H₂ also in the presence of sulfur, H₂ generation seems to be the only significant electron sink for PSII activity and rescues this strain at least partially from a light-sensitive phenotype. The latter indicates that the down-regulation of assimilatory pathways in S-deprived C. reinhardtii cells is one of the important prerequisites for a sustained H₂ evolution.     

Acclimation of<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis> to the light environment: the relationship between photosynthetic function and chloroplast composition

Plants respond to growth under different environmental conditions by adjusting the composition of the photosynthetic apparatus. To investigate the consequences of the acclimation strategies adopted by Arabidopsis thaliana, we have assessed the functioning of the photosynthetic apparatus in plants with very different chloroplast compositions. Using chlorophyll fluorescence analysis, we have determined the efficiency of, and capacity for, electron transport, assessed the ability to undergo state transitions, and measured non-photochemical quenching over a range of actinic irradiances followed by its resolution into fast- and slow-relaxing components; parallel measurements of leaf carotenoid composition were also carried out. The data clearly show that acclimation serves to maintain the electron transport chain in an oxidised state, ensuring efficient photochemistry. Furthermore, plants grown in high light have a greater capacity for energy-dependent feedback de-excitation, but this is not correlated with xanthophyll cycle pigment levels or de-epoxidation state. Surprisingly, even plants with very low levels of light-harvesting complexes were able to undergo state transitions. We also show that apparent discrepancies between chloroplast composition and photosynthetic function can be attributed to varying degrees of light penetration through the leaf. Thus, leaf chlorophyll content is an important factor influencing acclimation within the leaf.     

Interplay between non-photochemical plastoquinone reduction and re-oxidation in pre-illuminated <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: a chlorophyll fluorescence study

In photosynthetic eukaryotes, the redox state of the plastoquinone (PQ) pool is an important sensor for mechanisms that regulate the photosynthetic electron transport. In higher plants, a multimeric nicotinamide adenine dinucleotide (phosphate) (NAD(P))H dehydrogenase (NDH) complex and a plastid terminal oxidase (PTOX) are involved in PQ redox homeostasis in the dark. We recently demonstrated that in the microalgae Chlamydomonas reinhardtii, which lacks the multimeric NDH complex of higher plants, non-photochemical PQ reduction is mediated by a monomeric type-II NDH (Nda2). In this study, we further explore the nature and the importance of non-photochemical PQ reduction and oxidation in relation to redox homeostasis in this alga by recording the ‘dark’ chlorophyll fluorescence transients of pre-illuminated algal samples. From the observation that this fluorescence transient is modified by addition of propyl gallate, a known inhibitor of PTOX, and in a Nda2-deficient strain we conclude that it reflects post-illumination changes in the redox state of PQ resulting from simultaneous PTOX and Nda2 activity. We show that the post-illumination fluorescence transient can be used to monitor changes in the relative rates of the non-photochemical PQ reduction and reoxidation in response to different physiological situations. We study this fluorescence transient in algae acclimated to high light and in a mutant deficient in mitochondrial respiration. Some of our observations indicate that the chlororespiratory pathway participates in redox homeostasis in C. reinhardtii.     

Particle aggregation in photosynthetic membranes of the blue-green alga Anacystis nidulans

Freeze-fracture electron microscopy demonstrates that in photosynthetic membranes of the blue-green alga Anacystis nidulans quenched from a temperature below growth temperature, areas devoid of membrane particles occur. We suggest that this phenomenon is related to phase transitions in the photosynthetic membrane.     

State transitions at the crossroad of thylakoid signalling pathways

In order to maintain optimal photosynthetic activity under a changing light environment, plants and algae need to balance the absorbed light excitation energy between photosystem I and photosystem II through processes called state transitions. Variable light conditions lead to changes in the redox state of the plastoquinone pool which are sensed by a protein kinase closely associated with the cytochrome b ₆ f complex. Preferential excitation of photosystem II leads to the activation of the kinase which phosphorylates the light-harvesting system (LHCII), a process which is subsequently followed by the release of LHCII from photosystem II and its migration to photosystem I. The process is reversible as dephosphorylation of LHCII on preferential excitation of photosystem I is followed by the return of LHCII to photosystem II. State transitions involve a considerable remodelling of the thylakoid membranes, and in the case of Chlamydomonas, they allow the cells to switch between linear and cyclic electron flow. In this alga, a major function of state transitions is to adjust the ATP level to cellular demands. Recent studies have identified the thylakoid protein kinase Stt7/STN7 as a key component of the signalling pathways of state transitions and long-term acclimation of the photosynthetic apparatus. In this article, we present a review on recent developments in the area of state transitions.     

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q_B-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.     

Dissection of respiratory and cyclic electron transport in Synechocystis sp. PCC 6803

Cyclic electron transport (CET) is an attractive hypothesis for regulating photosynthetic electron transport and producing the additional ATP in oxygenic phototrophs. The concept of CET has been established in the last decades, and it is proposed to function in the progenitor of oxygenic photosynthesis, cyanobacteria. The in vivo activity of CET is frequently evaluated either from the redox state of the reaction center chlorophyll in photosystem (PS) I, P700, in the absence of PSII activity or by comparing PSI and PSII activities through the P700 redox state and chlorophyll fluorescence, respectively. The evaluation of CET activity, however, is complicated especially in cyanobacteria, where CET shares the intersystem chain, including plastoquinone, cytochrome b₆/f complex, plastocyanin, and cytochrome c₆, with photosynthetic linear electron transport (LET) and respiratory electron transport (RET). Here we sought to distinguish the in vivo electron transport rates in RET and CET in the cyanobacterium Synechocystis sp. PCC 6803. The reduction rate of oxidized P700 (P700⁺) decreased to less than 10% when PSII was inhibited, indicating that PSII is the dominant electron source to PSI but P700⁺ is also reduced by electrons derived from other sources. The oxidative pentose phosphate (OPP) pathway functions as the dominant electron source for RET, which was found to be inhibited by glycolaldehyde (GA). In the condition where the OPP pathway and respiratory terminal oxidases were inhibited by GA and KCN, the P700⁺ reduction rate was less than 1% of that without any inhibitors. This study indicate that the electron transport to PSI when PSII is inhibited is dominantly derived from the OPP pathway in Synechocystis sp. PCC 6803.

     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue–green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.     

Sodium Dependency of the Photosynthetic Electron Transport in the Alkaliphilic Cyanobacterium Arthrospira platensis

Arthrospira (Spirulina) platensis (A. platensis) is a model organism for investigation of adaptation of photosynthetic organisms to extreme environmental conditions: the cell functions in this cyanobacterium are optimized to high pH and high concentration (150–250 mM) of Na⁺. However, the mechanism of the possible fine-tuning of the photosynthetic functions to these extreme conditions and/or the regulation of the cellular environment to optimize the photosynthetic functions is poorly understood. In this work we investigated the effect of Na-ions on different photosynthetic activities: linear electron transport reactions (measured by means of polarography and spectrophotometry), the activity of photosystem II (PS II) (thermoluminescence and chlorophyll a fluorescence induction), and redox turnover of the cytochrome b ₆ f complex (flash photolysis); and measured the changes of the intracellular pH (9-aminoacridine fluorescence). It was found that sodium deprivation of cells in the dark at pH 10 inhibited, within 40 min, all measured photosynthetic reactions, and led to an alkalinization of the intracellular pH, which rose from the physiological value of about 8.3–9.6. These were partially and totally restored by readdition of Na-ions at 2.5–25 mM and about 200 mM, respectively. The intracellular pH and the photosynthetic functions were also sensitive to monensin, an exogenous Na⁺/H⁺ exchanger, which collapses both proton and sodium gradients across the cytoplasmic membrane. These observations explain the strict Na⁺-dependency of the photosynthetic electron transport at high extracellular pH, provide experimental evidence on the alkalization of the intracellular environment, and support the hypothesized role of an Na⁺/H⁺ antiport through the plasma membrane in pH homeostasis (Schlesinger et al. (1996). J. Phycol. 32, 608–613). Further, we show that (i) the specific site of inactivation of the photosynthetic electron transport at alkaline pH is to be found at the water splitting enzyme; (ii) in contrast to earlier reports, the inactivation occurs in the dark and, for short periods, without detectable damage in the photosynthetic apparatus; and (iii) in contrast to high pH, Na⁺ dependency in the neutral pH range is shown not to originate from PSII, but from the acceptor side of PSI. These data permit us to conclude that the intracellular environment rather than the machinery of the photosynthetic electron transport is adjusted to the extreme conditions of high pH and high Na⁺ concentration.     

Flash oxygen yield patterns of autotrophically and photoheterotrophically grown Chlamydobotrys stellata in the presence and absence of lipophilic acceptors

The obligate phototrophic green alga Chlamydobotrys stellata does not evolve oxygen when grown in CO₂-free atmosphere on acetate. With the application of the lipophilic acceptor 2,6-dichloro-p-benzoquinone it was investigated whether this phenomenon is caused by the inactivation of the water-splitting system or by an inhibition of the electron transport chain. It was found that in the presence of DCQ, the photoheterotrophic alga exhibited a normal period-4 flash oxygen pattern, but the steady state yield was only 25% of that measured in the autotrophic cells. After DCQ addition, the initial distribution of S-states and the values of the transition probabilities proved to be the same in the autotrophic and photoheterotrophic algae. These results indicate that photoheterotrophic growth conditions inhibit the electron transport of Chl. stellata behind the acceptor site of DCQ, but the water-splitting system remains active with a reduced oxygen evolving capacity.Abbreviations Chl chlorophyll - DCQ 2,6-dichloro-p-benzoquinone - DCMU 3-(3,4)-dichlorophenyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - pBQ 1,4-benzoquinone - PS I photosystem I - PS II photosystem II     

The energy distribution between the photosystems and light-induced changes in the stoichiometry of system I and II reaction centers in the chlorophyll b-containing alga Mantoniella squamata (Prasinophyceae)

The chlorophyll b-containing alga Mantoniella squamata was analyzed with respect to its capacity to balance the energy distribution from the light-harvesting antenna to photosystem I or photosystem II. It was shown, that this alga is unable to alter the absorption cross section of the two photosystems in terms of short-time regulations (state transitions). The energy absorbed by the LHC, which contains 60% of total photosynthetic pigments, is transferred to both photosystems without any preference. The stoichiometry of the two photosystems is found to be extremely unequal and variable during light adaptation. In high light, the molar ratio of P-680 per P-700 is found to be two, whereas under low light conditions this ratio accounts to nearly four. This very unbalanced stoichiometry of the reaction centers gives some new insights into the concept of the photosynthetic unit as well as in the importance of the regulation of the energy distribution. It is assumed that the high concentration of photosystem II can be understood as a mechanism to prevent the overexcitation of photosystem I. In addition, the changes im membrane protein pattern are not accompanied by variations in the ratio of appressed to nonappressed membranes as probed by ultrastructural analysis. It is suggested that the thylakoids are organized like a homogenous pigment bed. The lack of state transitions can be interpreted as a consequence of this unusual membrane morphology.Abbreviations Chl chlorophyll - CPa chlorophyll a-protein of PSII - CPl P-700 chlorophyll a-protein - CPD Chlorophyll packing density index - cyt f cytochrome f - FP free pigments - LHC light-harvesting complex - P_max light saturated photosynthetic rates per chlorophyll - n number of experiments - PQ plastoquinone - PS photosystem - PSU photosynthetic unit - Q_E non-photochemical quenching - Q_Q photochemical quenching     

Multidisciplinary research in photosynthesis: A case history based on the green alga Chlamydomonas

This article examines the contribution of a unicellular green alga Chlamydomonas to progress in photosynthetic research. The objective is to focus on the aspects of Chlamydomonas that have provided an advantage over other photosynthetic organisms in investigating photosynthesis. To do this we discuss several examples that demonstrate the progress from a genetic study to a multidisciplinary approach that probes higher levels of complexity within the organism. These examples include the function and molecular regulation of electron transport components between photosystem II and photosystem I, the molecular genetics of the herbicide binding protein of photosystem II, and several different studies that have derived from a search for rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) mutants in Chlamydomonas, including chloroplast ribosome function, the regulation of the large subunit of rubisco, and the interaction between photosynthetic electron transport and carbon metabolism.     

SUSCEPTIBILITY AND PROTECTIVE MECHANISMS OF MOTILE AND NON MOTILE CELLS OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE) TO PHOTOOXIDATIVE STRESS1

The life cycle of the unicellular green alga Haematococcus pluvialis consists of motile and nonmotile stages under typical growing conditions. In this study, we observed that motile cells were more susceptible than nonmotile cells to high light, resulting in a decrease in population density and photo‐bleaching. Using two Haematococcus strains, CCAP 34/12 (a motile cell dominated strain) and SAG 34/1b (a nonmotile cell dominated strain), as model systems we investigated the cause of cell death and the protective mechanisms of the cells that survived high light. The death of motile cells under high light was attributed to the generation of excess reactive oxygen species (ROS), which caused severe damage to the photosynthetic components and the membrane system. Motile cells were able to dissipate excess light energy by nonphotochemical quenching and to relax ROS production by a partially up‐regulated scavenging enzyme system. However, these strategies were not sufficient to protect the motile cells from high light stress. In contrast, nonmotile cells were able to cope with and survive under high light by (i) relaxing the over‐reduced photosynthetic electron transport chain (PETC), thereby effectively utilizing PETC‐generated NADPH to produce storage starch, neutral lipid, and astaxanthin, and thus preventing formation of excess ROS; (ii) down‐regulating the linear electron transport by decreasing the level of cytochrome f; and (iii) consuming excess electrons produced by PSII via a significantly enhanced plastid terminal oxidase pathway.     

Interactions between inorganic phosphate (Pi) assimilation,photosynthesis and respiration in the Pi-limited green alga Selenastrum minutum*

Inorganic phosphate (P_i)-limited chemostat cultures of the green alga Selenastrum minutum were employed to investigate interactions between P_i assimilation, respiration and photosynthetic processes. Changes in net and gross gas exchange rates indicated that O₂ evolution decreases during photosynthetic P_i assimilation. Room temperature and 77K Chi a fluorescence measurements revealed that this photosynthetic suppression is correlated with a transition from state 1 to state 2. Substantial photosynthetic P_i uptake rates occur in the presence of DCMU and KCN. Additionally, the cellular ratio of ATP:NADPH increases following P_i enrichment, suggesting that the ratio of cyclic to linear electron flow is enhanced in response to the high energy requirements of P_i uptake. Net starch degradation was observed during photosynthetic P_i assimilation and the cellular pool size of 3-phosphoglycerate increased; however, gross gas exchange parameters and cellular metabolite pool sizes indicated that mitochondrial respiration plays a smaller role during P_i assimilation in the light than it does in the dark. These observations were used to formulate a model depicting possible interactions between photosynthetic electron flow, photosynthetic and respiratory carbon metabolism and metabolite exchange between the chloroplast, cytosol and mitochondrion during photosynthetic P_i assimilation.     

Isolation and characterization of mutants corresponding to the MENA,MENB, MENC and MENE enzymatic steps of 5′‐monohydroxyphylloquinone biosynthesis in Chlamydomonas reinhardtii

Phylloquinone (PhQ), or vitamin K₁, is an essential electron carrier (A₁) in photosystem I (PSI). In the green alga Chlamydomonas reinhardtii, which is a model organism for the study of photosynthesis, a detailed characterization of the pathway is missing with only one mutant deficient for MEND having been analyzed. We took advantage of the fact that a double reduction of plastoquinone occurs in anoxia in the A₁ site in the mend mutant, interrupting photosynthetic electron transfer, to isolate four new phylloquinone‐deficient mutants impaired in MENA, MENB, MENC (PHYLLO) and MENE. Compared with the wild type and complemented strains for MENB and MENE, the four men mutants grow slowly in low light and are sensitive to high light. When grown in low light they show a reduced photosynthetic electron transfer due to a specific decrease of PSI. Upon exposure to high light for a few hours, PSI becomes almost completely inactive, which leads in turn to lack of phototrophic growth. Loss of PhQ also fully prevents reactivation of photosynthesis after dark anoxia acclimation. In silico analyses allowed us to propose a PhQ biosynthesis pathway in Chlamydomonas that involves 11 enzymatic steps from chorismate located in the chloroplast and in the peroxisome.     

State transitions, cyclic and linear electron transport and photophosphorylation in Chlamydomonas reinhardtii.

The relationship between state transitions and the kinetic properties of the electron transfer chain has been studied in Chlamydomonas reinhardtii. The same turnover rate of cytochrome f was found in state 1 and 2. However, while DBMIB was inhibitory in both states, DCMU was effective only in state 1. These observations suggest that linear electron transport was active only in state 1, while a cyclic pathway around photosystem (PS) I operated in state 2. The reversible shift from linear to cyclic electron transport was modulated by changes of PSII antenna size, which inactivated the linear pathway, and by oxygen, which inhibited the cyclic one. Attainment of state 2, under anaerobiosis in the dark, was associated with the decline of the ATP/ADP ratio in the cells and the dark reduction of the intersystem carriers. Upon illumination of the cells, the ATP/ADP ratio increased in a few seconds to the aerobic level. Then, several minutes later, the F(m) returned to the state 1 level, and O(2) evolution was reactivated. This suggests that ATP, though required for photosynthesis, is not the rate-limiting factor in the reactivation of photosynthetic O(2) evolution, which is rather controlled by the redox state of the electron carriers.     

The effect of oxygen concentration on photosynthetic biomass production by algae

The effect of decreased oxygen concentration on photosynthetic biomass production was determined for Euglena gracilis Klebs strain z and Chlamydomonas reinhardtii Dangeard. At a constant carbon dioxide concentration of 0.03% (v/v), decreasing the oxygen concentration from 21% to 2% (v/v) gave a two-fold increase in dry-weight yield for E. gracilis; a result consistent with the operation of a functional glycollate pathway in this alga. A similar effect of oxygen concentration on dry-weight yield was not observed with C. reinhardtii.     

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O₂ in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b ₆-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b ₆-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and poises the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO₂ can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - cyt cytochrome - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCCD dicyclohexylcarbodiimide - DCHC dicyclohexyl-18-crown-6 - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP carbonylcyanide 4-(trifluoromethoxy) phenylhydrazone - LHC light harvesting chlorophyll - LHCP II light harvesting chlorophyll protein of Photosystem II - PQ plastoquinone - PS I, II Photosystem I, II - SHAM salicyl hydroxamic acid - TBT Tri-n-butyltin CIW/DPB Publication No. 1146