Nitrate and nitrite uptake by free-living and immobilized N-started cells ofPhormidium laminosum

N-starved free-living and polyvinyl-immobilized cells ofPhormidium laminosum (strain OH-1-pCl₁) have been investigated in relation to their nitrate and nitrite uptake characteristics. N-deficient cells showed higher inorganic N-uptake rates than N-sufficient ones. The photosynthetic activities of the cells decreased progressively with the time of N-starvation. N-starved cells produced high amounts of exopolysaccharides, which appear to assist the immobilization process. Inorganic N-uptake by N-starved cells occurred in both light and dark under aerobic conditions. In anaerobiosis light was required for the uptake, confirming that the necessary energy might perhaps be derived from the respiratory electron transport chain under aerobiosis. Ammonium inhibited nitrate uptake but did not affect the uptake of nitrite. Initial nitrate and nitrite uptake rates were temperature-dependent and yielded hyperbolic curves when plotted against the N source concentration, indicating the existence of saturable transport system(s).     

BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

In the non-N₂-fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH-I-pCl₁), N starvation induced an increase in the rate of respiration and a decrease in the rate of O₂ evolution. When NO₃^? was added to illuminated N-starved cells, O₂ evolution immediately increased to levels shown by NO₃^? grown cells, even though N-starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light-saturating conditions and after 2–3 days of N starvation. The respiratory rate of N-starved cells was stimulated by the addition of NO₃^? or NH₄⁺ and partially inhibited at very low irradiances, even in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). Results indicate that N-starved cells obtain the energy supply for N assimilation through a process different from that used by N-sufficient cells. N-starved cells were able to take up NO₃^? in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO₃^? addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH₄⁺ addition. Addition of NO₃^? or NH₄⁺ favored photoinhibition of photosystem II, the effect being faster after NH₄⁺ addition.     

Prolonged high light treatment of plant cells results in changes of the amount,the localization and the electrophoretic behavior of several thylakoid membrane proteins

The effect of a 30 h high light treatment on the amount and the localization of thylakoid proteins was analysed in low light grown photoautotrophic cells of Marchantia polymorpha and Chenopodium rubrum. High light treatment resulted in a net loss of D1 protein which was accompanied by comparable losses of other proteins of the PS II core (reaction center with inner antenna). LHC II proteins were not reduced correspondingly, indicating that these complexes are less affected by prolonged high light. High light influenced the distribution of PS II components between the grana and the stroma region of the thylakoid membrane, probably by translocation of the respective PS II proteins. Additionally, modifications of several thylakoid proteins were detected in high light treated cells of C. rubrum. These effects are discussed in relation to photoinhibitory damage and repair processes.Abbreviations BCA bioinchonic acid - chl chlorophyll - CF₁ coupling factor - CYC cycloheximide - GT grana thylakoids - HL high light - LL low light - PAGE polyacrylamide gel electrophoresis - PFD photon flux density - PS I Photosystem I - PS II Photosystem II - RC reaction center - SDS sodium dodecylsulfate - ST stroma thylakoids - Thyl unfractionated thylakoids     

Diurnal variations of intracellular nitrate storage by marine diatoms: effects of nutritional state

Nitrate uptake and accumulation were measured in N-sufficient, N-limited, and 24 h N-starved cells of Phaeodactylum tricornutum Bohlin and Skeletonema costatum Grev., growing under a light-dark cycle. In N-sufficient cells the uptakerate was reduced at night and showed possible variation during the light period. In N-limited and N-starved cells such diurnal changes in uptake were absent, except extremely rapid, but short-lived nitrate uptake was observed early in the morning in N-limited cells. The nitrate accumulation inside the cells reflects a transient uncoupling between uptake and reduction mostly due to the light-dark cycle and strongly influenced by the physiological state of the cells. This accumulation is high during the night and at the beginning of the day, but decreases during the light period in N-sufficient cells. On the other hand, nitrate storage in N-sufficient and N-limited cultures shows a strong diurnal pattern, with maximum accumulation, suggesting the greatest uncoupling between uptake and assimilation, in the morning. In N-starved cells, accumulation is high and constant during the entire light period. Consequently, the uncoupling between nitrate uptake and reduction decreases during the light period but increases with N deficiency. These results indicate the importance of light periodicity and nutritional state of the cells on the nitrate utilization. They reveal the need for more systematic studies on N dynamics in relation to nutrient-light regimes.     

Effect of growth conditions on accumulation of internal nitrate,ammonium, amino acids,and protein in three marine diatoms

The capacity of marine phytoplankton to change their cellular content of nitrate, ammonium, amino acids, and protein in response to different growth conditions was systematically investigated. Cellular concentrations of these compounds were measured in N-starved, N-deficient, and N-sufficient Skeletonema costatum (Grev.) Cleve and in N-deficient Chaetoceros debilis Cleve and Thalassiosira gravida Cleve, both before and after the addition of a pulse of nitrogen.N-sufficient Skeletonema costatum contains high concentrations of protein, large persistent pools of amino acids, and, if it is growing on nitrate, sizeable amounts of nitrate. As it becomes N-starved, the total cellular nitrogen decreases, the internal nitrate and amino acids become entirely depleted, and the protein content is drastically reduced. After nitrogen additions to N-deficient and N-starved cultures, transient pools of unassimilated nitrogen form which can account for a large fraction of newly taken up nitrogen. The size and kind of pool which accumulates is determined by the preconditioning of the cells, the nitrogen compound which is added, and the species identity. The pools which form in S. costatum indicate that nitrate reduction is the slowest step in nitrogen assimilation, the synthesis of protein from amino acids is the next slowest, and the incorporation of ammonium into amino acid is the fastest. However, the rate limiting steps may vary between diatom species.For the first time, measurements of the variation in cellular nitrogen compounds over a wide range of environmental conditions reveal the ability of some phytoplankton to buffer the effects of a changing, and sometimes growth-limiting, nitrogen supply. They accomplish this by utilizing stored internal nitrogen for growth when the external supply is low and by quickly storing unassimilated nitrogen when the external supply is suddenly increased beyond their ability to immediately assimilate it. The accumulation of large pools of unassimilated nitrogen compounds can explain the often observed difference between nitrogen uptake rates and growth rates.     

Inhibition of Water Splitting Increases the Susceptibility of Photosystem II to Photoinhibition

Photosystem II (PSII)-enriched membrane particles were isolated from peas (Pisum sativum L.) and treated in several different ways to inhibit the water oxidation reactions, but not reaction center function itself, as judged by the light-induced rate of reduction of 2,6-dichlorophenol indophenol with and without the artificial electron donor, diphenyl carbazide. It was shown that such treatments increased the susceptibility of the PSII-enriched membranes to photoinhibition. This trend was further observed if 2,6-dichlorophenol indophenol was present during the illumination with photoinhibitory light. On the other hand, protection against the enhanced photoinhibition was found when the water-splitting activity was reconstituted or when the artificial electron donor diphenyl carbazide was present during the preillumination. The results indicate that irreversible photodamage occurred within the PSII reaction center as a consequence of illumination with strong light and that the rate of this damage was enhanced under conditions that are expected to give rise to a photoaccumulation of oxidizing species such as P680⁺ on the donor side of PSII. This mechanism of photoinhibitory damage occurred under both aerobic and anaerobic conditions.     

The stress-induced SCP/HLIP family of small light-harvesting-like proteins (ScpABCDE) protects Photosystem II from photoinhibitory damages in the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

Small CAB-like proteins (SCPs) are single-helix light-harvesting-like proteins found in all organisms performing oxygenic photosynthesis. We investigated the effect of growth in moderate salt stress on these stress-induced proteins in the cyanobacterium Synechocystis sp. PCC 6803 depleted of Photosystem I (PSI), which expresses SCPs constitutively, and compared these cells with a PSI-less/ScpABCDE^? mutant. SCPs, by stabilizing chlorophyll-binding proteins and Photosystem II (PSII) assembly, protect PSII from photoinhibitory damages, and in their absence electrons accumulate and will lead to ROS formation. The presence of 0.2 M NaCl in the growth medium increased the respiratory activity and other PSII electron sinks in the PSI-less/ScpABCDE^? strain. We postulate that this salt-induced effect consumes the excess of PSII-generated electrons, reduces the pressure of the electron transport chain, and thereby prevents ¹O₂ production.     

Photoinhibitory damage is modulated by the rate of photosynthesis and by the photosystem II light-harvesting chlorophyll antenna size

We investigated the effect of photosynthetic electron transport and of the photosystem II (PSII) chlorophyll (Chl) antenna size on the rate of PSII photoinhibitory damage. To modulate the rate of photosynthesis and the light-harvesting capacity in the unicellular chlorophyte Dunaliella salina Teod., we varied the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO₃, or with supplemental CO₂ bubbled in the form of 3% CO₂ in air. The NaHCO₃-grown cells displayed slow rates of photosynthesis and had a small PSII light-harvesting Chl antenna size (60 Chl molecules). The half-time of PSII photodamage was 40 min. When switched to supplemental CO₂ conditions, the rate of photodamage was retarded to a t_1/2 = 70 min. Conversely, CO₂-supplemented cells displayed faster rates of photosynthesis and a larger PSII light-harvesting Chl antenna size (500 Chl molecules). They also showed a rate of photodamage with t_1/2 = 40 min. When depleted of CO₂, the rate of photodamage was accelerated (t_1/2  = 20 min). These results indicate that the in-vivo susceptibility to photodamage is modulated by the rate of forward electron transport through PSII. Moreover, a large Chl antenna size enhances the rate of light absorption and photodamage and, therefore, counters the mitigating effect of forward electron transport. We propose that under steady-state photosynthesis, the rate of light absorption (determined by incident light intensity and PS Chl antenna size) and the rate of forward electron transport (determined by CO₂ availability) modulate the oxidation/reduction state of the primary PSII acceptor Q_A, which in turn defines the low/high probability for photodamage in the PSII reaction center. Received: 14 August 1997 / Accepted: 26 September 1997     

MPH1 is a thylakoid membrane protein involved in protecting photosystem II from photodamage in land plants

Photosystem II (PSII) is highly susceptible to photoinhibition caused by environmental stimuli such as high light; therefore plants have evolved multifaceted mechanisms to efficiently protect PSII from photodamage. We previously published data suggesting that Maintenance of PSII under High light 1 (MPH1, encoded by AT5G07020), a PSII-associated proline-rich protein found in land plants, participates in the maintenance of normal PSII activity under photoinhibitory stress. Here we provide additional evidence for the role of MPH1 in protecting PSII against photooxidative damage. Two Arabidopsis thaliana mutants lacking a functional MPH1 gene suffer from severe photoinhibition relative to the wild-type plants under high irradiance light. The mph1 mutants exhibit significantly decreased PSII quantum yield and electron transport rate after exposure to photoinhibitory light. The mutants also display drastically elevated photodamage to PSII reaction center proteins after high-light treatment. These data add further evidence that MPH1 is involved in PSII photoprotection in Arabidopsis. MPH1 homologs are found across phylogenetically diverse land plants but are not detected in algae or prokaryotes. Taken together, these results suggest that MPH1 protein began to play a role in protecting PSII against excess light following the transition from aquatic to terrestrial conditions.     

Effect of nitrate on respiration ofMentha arvensis

The supply of nitrate nitrogen caused a marked increase in the rate of respiration of Japanese mint. Sodium azide strongly inhibited the rate of respiration at all the nitrate levels, but the inhibition was more marked at higher levels. Besides this, inhibition caused by azide treatment was less marked in older and nitrogen deficient tissues than in younger ones at higher nitrogen levels. The addition of sodium diethyl dithio-carbamate (DIECA), an inhibitor of copper containing enzymes resulted in an increase in the respiration of mint leaves which increased further with an increase in the nitrate supply. The same concentration of DIECA which stimulate the respiration of leaves caused an inhibition in the respiration of roots. The inhibition was greater at lower levels and decreased consistently as the supply of nitrate increased. The sensitivity of root respiration to DIECA observed with varying levels of nitrate indicated that unlike the leaf, the roots contain copper-containing enzymes which get decreased as the nitrate supply is increased. An increased supply of nitrogen up to 16 me NO₃ ^- -N was associated with an increase in respiratory quotient.     

Changes in the cyanobacterial photosynthetic apparatus during acclimation to macronutrient deprivation

When the cyanobacterium Synechococcus sp. Strain PCC 7942 is deprived of an essential macronutrient such as nitrogen, sulfur or phosphorus, cellular phycobiliprotein and chlorophyll contents decline. The level of -carotene declines proportionately to chlorophyll, but the level of zeaxanthin increases relative to chlorophyll. In nitrogen- or sulfur-deprived cells there is a net degradation of phycobiliproteins. Otherwise, the declines in cellular pigmentation are due largely to the diluting effect of continued cell division after new pigment synthesis ceases and not to net pigment degradation. There was also a rapid decrease in O₂ evolution when Synechococcus sp. Strain PCC 7942 was deprived of macronutrients. The rate of O₂ evolution declined by more than 90% in nitrogen- or sulfur-deprived cells, and by approximately 40% in phosphorus-deprived cells. In addition, in all three cases the fluorescence emissions from Photosystem II and its antennae were reduced relative to that of Photosystem I and the remaining phycobilisomes. Furthermore, state transitions were not observed in cells deprived of sulfur or nitrogen and were greatly reduced in cells deprived of phosphorus. Photoacoustic measurements of the energy storage capacity of photosynthesis also showed that Photosystem II activity declined in nutrient-deprived cells. In contrast, energy storage by Photosystem I was unaffected, suggesting that Photosystem I-driven cyclic electron flow persisted in nutrient-deprived cells. These results indicate that in the modified photosynthetic apparatus of nutrient-deprived cells, a much larger fraction of the photosynthetic activity is driven by Photosystem I than in nutrient-replete cells.Abbreviations ES energy storage - N nitrogen - P phosphorus - PBS phycobilisomes - S sulfur     

The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O₂ evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.     

Ascaris suum: Oxidative phosphorylation in mitochondria from developing eggs and adult muscle

Respiration and the phosphorylating capability of mitochondria isolated from one-celled fertilized eggs, 10-day vermiform embryos, 21-day infective larvae, and adult body wall muscle from Ascaris suum were compared with that of rat liver mitochondria. Although oligomycin-sensitive ATPase and O₂ consumption/ mitochondrion in the presence of succinate and malate was lower in eggs than in liver, other properties such as respiratory control, ADP:O and P:O ratios at sites I, II, III, and the sensitivity of respiration to cyanide, azide, oligomycin, rotenone, and malonate were similar. In muscle mitochondria, the oligomycin-sensitive ATPase and O₂ consumption/ mitochondrion were sharply reduced, respiratory control was poor, and electron transport at sites II and III in particular was inefficiently coupled with phosphorylation. In addition, about 60% of the respiration was insensitive to cyanide or azide but sensitive to salicylhydroxamic acid. The results support earlier evidence that the free-living eggs of A. suum are aerobes. The adult parasite, while continuing to ferment actively in the presence of oxygen, nevertheless possesses one or more electron transport systems that are inefficiently coupled with aerobic phosphyorylations. The physiological significance of these systems has yet to be elucidated.     

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     

Wounding Stimulates Cyanide-sensitive Respiration in the Highly Cyanide-resistant Leaves of Bryophyllum tubiflorum Harv

The rate of respiration in sectioned leaves of Bryophyllum tubiflorum Harv. increases with decreasing section thickness. The rates of uninhibited respiration in 2- and 8-millimeter-thick sections are 74 and 46 microliters of O₂ per gram fresh weight of unruptured tissue per hour at 20 C, whereas the rate in the presence of cyanide is 31 microliters of O₂ in each case. The rates are unaffected by salicylhydroxamic acid, but cyanide and salicylhydroxamic acid together completely eliminate O₂ uptake. The capacity of the alternative respiratory pathway is thus initially high (estimated at 84% of the uninhibited respiratory rate in whole leaves) and remains constant but probably unexpressed subsequent to the rapid induction of wound respiration.     

Thylakoid membrane energization and swelling in photoinhibited Chlamydomonas cells is prevented in mutants unable to perform cyclic electron flow

Photoinhibition of Photosystem II in unicellular algae in vivo is accompanied by thylakoid membrane energization and generation of a relatively high pH as demonstrated by ¹⁴C-methylamine uptake in intact cells. Presence of ammonium ions in the medium causes extensive swelling of the thylakoid membranes in photoinhibited Chlamydomonas reinhardtii but not in Scenedesmus obliquus wild type and LF-1 mutant cells. The rise in pH and the related thylakoid swelling do not occur at light intensities which do not induce photoinhibition. The rise in pH and membrane energization are not induced by photoinhibitory light in C. reinhardtii mutant cells possessing an active Photosystem II but lacking cytochrome b₆/f, plastocyanin or Photosystem I activity and thus being unable to perform cyclic electron flow around Photosystem I. In these mutants the light-induced turnover of the D1 protein of Reaction Center II is considerably reduced. The high light-dependent rise in pH is induced in the LF-1 mutant of Scenedesmus which can not oxidize water but otherwise possesses an active Reaction Center II indicating that PS II-linear electron flow activity and reduction of plastoquinone are not required for this process. Based on these results we conclude that photoinhibition of Photosystem II activates cyclic electron flow around Photosystem I which is responsible for the high membrane energization and pH rise in cells exposed to excessive light intensities.Abbreviations cyt b₆/f cytochrome b₆/f - Diuron 3-(3,4-dichlorophenyl)-1 dimethyl urea - Q_B the secondary quinone acceptor of reaction center II - DNP 2,4,Dinitrophenol - FCCP carbonyl cyanide trifluoromethoxy phenylhydrazone - SDS-PAGE sodium dodecylsulfate polyacrylamide gel electrophoresis     

Development of cyanide-resistant respiration in mitochondria from potato tubers treated with ethanol,acetaldehyde, and acetic acid

Treatment of intact potato (Solanum tuberosum L.) tubers with acetaldehyde, ethanol or acetic-acid vapors led to a respiratory upsurge which was further increased when the volatiles were applied in 100% O₂. Mitochondria from tubers held in 100% O₂ (O₂ control) displayed a substrate state, state 3, and state 4 in respiration, whereas in mitochondria from the volatile-treated tubers the respiratory rate of the different states was virtually indistinguishable. This respiratory pattern was companied by the development of a cyanide-resistant respiration since these mitochondria exhibited resistance to CN and sensitivity to CN+salicylhydroxamic acid. Acetaldehyde-treated potatoes showed a time-course development (up to 36 h) of cyanide resistance and concomitant sensitivity to salicylhydroxamic acid, indicating the onset of synthetic processes leading to the observed changes in mitochondrial respiration.Abbreviations V total respiration rate - V_cyt velocity of O₂ uptake attributable to cytochrome oxidase - V_alt velocity of O₂ uptake attributable to the alternate oxidase - RCR respiratory control ratio - SHAM salicylhydroxamic acid Paper of the Journal Series, New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Brunswick, N.J., USA     

In search of a reversible stage of photoinhibition in a higher plant: No changes in the amount of functional Photosystem II accompany relaxation of variable fluorescence after exposure of lincomycin-treated Cucurbita pepo leaves to high light

Pumpkin (Cucurbita pepo L.) leaves in which chloroplast protein synthesis was inhibited with lincomycin were exposed to strong photoinhibitory light, and changes in F_O, F_M, F_V/F_M and in the amount of functional Photosystem II (O₂ evolution induced by saturating single-turnover flashes) were monitored during the high-light exposure and subsequent dark or low-light incubation. In the course of the photoinhibitory illumination, F_M, F_V/F_M and the amount of functional PS II declined continuously whereas F_O dropped rapidly to some extent and then slowly increased. If the experiments were done at room temperature, termination of the photoinhibitory illumination resulted in partial relaxation of the F_V/F_M ratio and in an increase in F_O and F_M. The relaxation was completed in 10–15 min after short-term (15 min) photoinhibitory treatment but continued 30–40 min if the exposure to high light was longer than 1 h. No changes in the amount of functional PS II accompanied the relaxation of F_V/F_M in darkness or in low light, in the presence of lincomycin. Transferring the leaves to low temperature (+4°C) after the room-temperature illumination (2 h) completely inhibited the relaxation of F_V/F_M. Low temperature did not suppress the relaxation if the photoinhibitory illumination had also been done at low temperature. The results indicate that illumination of lincomycin-poisoned pumpkin leaves at room temperature does not lead to accumulation of a reversibly photoinactivated intermediate.Abbreviations F_O, F_M chlorophyll fluorescence with all reaction centres open or closed, respectively - F_V variable fluorescence (F_V=F_M–F_O) - LHC Light-harvesting complex - PS II Photosystem II - Q_A, Q_B primary and secondary quinone electron acceptors of PS II, respectively - qN_E, qN_T, qN_I non-photochemical quenching due to high-energy state, state transition or photoinhibition, respectively     

Effects of nitrogen starvation on the photosynthetic physiology of a tropical marine microalga Rhodomonas sp. (Cryptophyceae)

The effects of nitrogen starvation on biomass composition and photosynthetic function were examined in the marine cryptophyte Rhodomonas sp. Batch-cultured cells in N-sufficient medium showed a 2.5-fold increase in total carbohydrate content, and a 33% increase in cell volume when the cultures reached the stationary growth phase. These cultures also increased the ratio of phycoerythrin (PE)/hydrosoluble proteins from 6 to 22% by the 4th and 10th day of culture, respectively. In contrast, light-saturated photosynthetic activity (P_m) progressively decreased, and the value obtained at the beginning of the stationary phase was about 45% of that obtained for cells in the late exponential growth phase. Transfer to N-lacking medium caused a 3.2-fold increase in cell volume. N starvation also triggered a rapid decline in N-containing compounds such as hydrosoluble proteins and photosynthetic pigments, causing an almost complete loss of PE. The ratio of PE/hydrosoluble proteins decreased from 6 to 1% after 6 d of N deprivation. Furthermore, the PSII fluorescence capacity declined under N-starved conditions, which caused a pronounced decrease in both the P_m (circa 90%) and the apparent photosynthetic efficiency (circa 55%). Under these conditions, photosynthetically fixed carbon was used to synthesize large amounts of carbohydrates. We suggest that, in addition to the role of phycoerythrin as a light-harvesting pigment, Rhodomonas sp. responds to N-depleted conditions by mobilizing combined nitrogen from biliproteins.     

Cobalt chloride induced stimulation of Photosystem II electron transport in Synechocystis sp. PCC 6803 cells

Synechocystis sp. PCC 6803 when grown in the presence of sublethal (M) levels of cobalt chloride shows an enhancement of Photosystem II (PS II) catalyzed Hill reaction. This stimulation seems to be induced by cobalt ions as other metal ions inhibit para-benzoquinone catalyzed Hill reaction. At saturating white light intensity, this enhancement is two times over that of the control cells on unit chlorophyll basis. Analysis of the PS II electron transport rate at varying intensities of white, blue or yellow light suggests an increased maximal rates but no change in the quantum yield or effective antenna size of CoCl₂-grown cells. There were no structural and functional changes in the phycobilisome as judged by the absence of changes in the phycocyanin/allophycocyanin ratio, fluorescence emission spectra, second derivative absorption spectra at 77 K and SDS-PAGE analysis of isolated phycobilisomes. The 77 K fluorescence emission spectra of the cells showed a decrease in the ratio of Photosystem I emission (F725) to Photosystem II emission (F685) in CoCl₂-grown cells compared to the control cells. These observations indicate three possibilities: (1) there is an increase in the number of Photosystem II units; (2) a faster turnover of Photosystem II centers; or (3) an alteration in energy redistribution between PS II and PS I in CoCl₂-grown cells which causes stimulation of Photosystem II electron transport rate.Abbreviations APC allophycocyanin - Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - EDTA ethylene diamine tetraacetic acid - PBS phycobilisome - PC phycocyanin - PSI Photosystem I - PS II Photosystem II - pBQ p-benzoquinone - PMSF phenyl methyl sulfonyl fluoride