Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a three-fold reduction in photosystems I and II (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P₇₀₀) and PSII (chlorophyll/Q_A) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in β-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.     

Effective Absorption Cross-Sections in Porphyridium cruentum: Implications for Energy Transfer between Phycobilisomes and Photosystem II Reaction Centers

Effective absorption cross-sections for O₂ production by Porphyridium cruentum were measured at 546 and 596 nanometers. Although all photosystem II reaction centers are energetically coupled to phycobilisomes, any single phycobilisome acts as antenna for several photosystem II reaction centers. The cross-section measured in state I was 50% larger than that measured in state II.     

An Evidence Indicating That a Weak Orange Light Absorbed by Phycobilisomes Causes Inactivation of PSII in Cells of the Red Alga Porphyridium cruentum Grown under a Weak Red Light Preferentially Exciting Chl a

Changes in the PSII fluorescence upon shift of light qualitywere studied with the red alga Porphyridium cruentum IAM R-1and supplementarily with P. cruentum ATCC 50161, the cyanophytesSynechocystis spp. PCC6714 and PCC6803 and Synechococcus sp.NIBB1071. When Porphyridium cruentum grown under a weak redlight (PSI light) preferentially absorbed by Chl a was illuminatedwith a weak orange light (PSII light) mainly absorbed by phycobilisomes(PBS), a change of PSII fluorescence at room temperature wasinduced. The ratio of Fvm (Fm— Fo) to Fm was reduced rapidlyaccompanying the increase in Fo (T^1/2 ca. 3 min). The effectsof DCMU and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinoneindicated that the fluorescence change is induced when plastoquinonepool is highly reduced. The fluorescence change after a shortPSII light illumination was reversible; it rapidly recoveredin the dark (T ^1/2 ca. 3 min). The reversibility was graduallyreduced and disappeared after 40 h under PSII light accompanyingdecrease in PSII activity per PBS down to almost 50%. Sincethe pattern of the fluorescence change resembles that observablewhen PSII is photoinactivated, PSII light probably induces thephotoinactivation of PSII, possibly reversibly at first andirreversibly after prolonged illumination. Such a rapid fluorescencechange was insignificant in Synechocystis sp. either PCC6714or PCC6803. Only a slow and small decrease in Fvm/Fm level appearedafter prolonged PSII light illumination (the reduction of PSIIactivity per PBS was around 20%). In Porphyridium, shift fromPSII light to PSI light caused a rapid and chloramphenicol-sensitiveFvm/Fm elevation during the first 10 h while the increase inPSH activity per PBS was only 10% of that before the light shift.Then, a gradual elevation followed up to the level at the steadystate under PSI light. A similar rapid increase in Fvm/Fm wasobserved with Synechocystis PCC6714, in which the synthesisof PSII is not regulated, suggesting that a rapid increase inFvm/Fm does not reflect the acceleration of the synthesis ofPSII. Results were interpreted as that (1) PSII light causesphotoinactivation of PSII. Such a photoinactivation is markedin Prophyridium cells grown under PSI light. (2) In Porphyridium,changes in the abundance of PSII upon shift of light qualityare largely attributed to the photoinactivation of this type. (Received February 19, 1999; Accepted June 14, 1999)     

Carbonic Anhydrase of a Unicellular Red Alga Porphyridium cruentum R-l. II. Distribution and Role in Photosynthesis

Antibody was raised against Porphyridium carbonic anhydrase(CA) which was electrophoretically recovered from the gel afterSDS-polyacrylamide slab gel electrophoresis (SDS-PAGE) of thepartially purified enzyme. The antiserum reacted with CA ofPorphyridium, but not with that of Chlamydomonas reinhardtii.Even though the antiserum did not react with CA from P. cruentumR-l in Ouchterlony's double immunodiffusion, it blocked theenzyme activity in the presence of 1% Nonidet P-40 and 1% TritonX-100. After Western blotting and enzyme-linked immunostaining(ELIS), only one band which reacted with the antiserum was detectedin the extract of low-CO₂ cells (grown under ordinary air) ofP cruentum, while no significant band was detected in that ofhigh-CO₂ cells (grown under air enriched with 1–5% CO₂).Immunogold electron microscopy of low-CO₂ cells of P. cruentumR-l using this antibody revealed that most of the CA was localizedin the chloroplast, with some in the cytoplasm. No specificbinding of gold particles was observed in the high-CO₂ cells. ¹Present address: National Institute for Basic Biology, Myodaiji,Okazaki 444, Japan (Received May 18, 1987; Accepted September 7, 1987)     

Carbonic Anhydrase of a Unicellular Red Alga Porphyridium cruentum R-l. I. Purification and Properties of the Enzyme

Cells of Porphyridium cruentum R-l, a unicellular red alga,grown under ordinary air (0.04% CO₂) showed much higher activityof carbonic anhydrase (CA) than those grown under CCvenrichedair (2% CO₂). CA activity was not detected in a suspension ofintact cells, and was detectable only after the cells had beenhomogenized, indicating that this enzyme was localized onlywithin the algal cells. After partial purification of Porphyridium CA, its mol wt wasestimated as 59 kDa by SDS-PAGE and 55 kDa by gelfiltration.This suggests that the native enzyme is a monomer. Its activitywas not affected by benzensulfonamides, potent inhibitors ofCAs isolated from Chlamydomonas and other organisms. Chloride(or bromide) ions was essential for CA activity. CA activitymarkedly decreased when the cell extract had been incubatedat pH lower than 7 before assay. Upon readjusting the pH ofthe preincubation medium to 9 or higher, the enzyme activitywas restored, indicating that the inactivation is reversible. (Received April 17, 1987; Accepted July 21, 1987)     

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of porphyridium cruentum which had been frozen to -196 degrees C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at -196 degrees C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.     

Irradiance Effects on Thylakoid Membranes of the Red Alga Porphyridium cruentum. An Immunocytochemical Study

Immunogold labelling on ultrathin sections of the red alga Porphyridiumcruentum (ATCC 50161) was used to assess changes in the densityand distribution of polypeptide components of photosystem I,photosystem II, phycobilisomes, and ATP synthase within thethylakoid membrane as a function of growth irradiance. In cellsgrown under a low, limiting quantum flux (6 microeinsteins persquare meter per second of continuous white light) thylakoidmembrane density and total thylakoid area per cell are 2 1/2times greater than in cells grown under a high, saturating quantumflux (280 microeinsteins per square meter per second). Immunogoldlabelling data indicate that concentrations of photosystem I,photosystem II and phycobilisomes in thylakoids of low light-growncells are only slightly greater than in cells grown under highlight. In contrast, the concentration of ATP synthase withinthe thylakoid membrane is nearly ten times greater in high light-growncells. Photosystem I polypeptides were detected in those portionsof the thylakoid membrane which traverse the pyrenoid, but photosystemII and phycobilisomes appeared to be absent from these membranes.Ribulose-l,5-bisphosphate carboxylase was restricted primarilyto the pyrenoid, and its concentration in the stroma or pyrenoidwas little affected by the photon flux density. Quantitativeestimates of photosystems I and II, phycobilisomes, and ATPsynthase by spectroscopy or by immunoelectrophoresis are inaccord with the immunogold results and lend support to the useof immunogold labelling for quantifying changes in relativeamounts of membrane proteins. (Received October 29, 1990; Accepted February 4, 1991)     

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

Changes in Quantum Yield of Photosynthesis in the Red Alga Porphyridium cruentum Caused by Stepwise Reduction in the Intensity of Light Preferentially Absorbed by the Phycobilins

This paper describes the relation between the quantum yield of photosynthesis in the red alga Porphyridium cruentum, and the spectral composition of light, changed by filtering white light through aqueous phycobilin solutions of increasing optical density. At sufficiently high densities of the filter solution, no measurable photosynthesis can be observed, although chlorophyll a molecules are still being excited at a significant rate, as can be proved by calculations from spectral distribution curves, and is confirmed by the occurrence of a “second Emerson effect” upon addition of orange light. An interpretation of this result, based on other experiments, will be given in a subsequent paper. A modification of the opal glass technique for reducing the effect of scattering when measuring absorption, was developed in connection with this research, and also is described in the paper.     

Thermal and pH Stability of the B-Phycoerythrin from the Red Algae Porphyridium cruentum

Phycoerythrin is the major light-harvesting pigment-protein of the red algae Porphyridium cruentum and is widely used as fluorescent probe and analytical reagent. Additionally this protein has a potential application as natural dye in food industry. Nevertheless the knowledge of the functional properties of this alga protein is limited, hindering its application as food additive. In this article we report a biophysical characterization of B-phycoerythrin from Porphyridium cruentum (B-PE) in order to study its stability and spectral properties in a broad range of pHs. This information can help in its potential application as colorant in the food industry. Spectroscopic data obtained in this work show that B-PE has a stronger functional stability in the pH range 4.0–10.0, and Size Exclusion Chromatography suggests that the protein maintains a (αβ)₆-γ oligomeric structure in that range of pHs. At pH 7.0, an apparent T _m value of 77.5?±?0.5 °C was calculated. At this pH, the protein is highly stable with a loss of only 20 % of its spectral properties (absorbance and fluorescence) after 25 days at room temperature. These results indicate that B-PE is more stable in a broad range of pHs than other phycoerythrin proteins, which would facilitate its use in the food industry.     

Dynamics of Photosystem II and Its Light Harvesting System in Response to Light Changes in the Halotolerant Alga Dunaliella salina

A photosystem two (PSII) core complex consisting of five major polypeptides (47, 40, 32, 30, and 10 kilodaltons) and a light harvesting chlorophyll a/b complex (LHC-2) have been isolated from the halotolerant alga Dunaliella salina. The chlorophyll and polypeptide composition of both complexes were compared in illuminated and dark-adapted cultures. Dark adaptation is accompanied by a decrease in the chlorophyll a to chlorophyll b (Chl a/Chl b) ratio of intact thylakoids without any change in total chlorophyll. These changes occur with a half-time of 3 hours and are reversed upon reillumination. Analyses of PSII enriched membrane fragments suggest that the decrease in the Chl a/Chl b is due partly to an increase in the Chl b content of LHC-2 and partly to changes in the relative levels of the two complexes. Apparently during dark adaptation there is: (a) a net synthesis of chlorophyll b, (b) removal of PSII core complexes resulting in a 2-fold drop in the PSII cores to LHC-2 chlorophyll ratio. These changes should dramatically increase the light harvesting capacity of the remaining PSII reaction centers. Presumably this adjustment of antenna size and composition is a physiological mechanism necessary for responding to shade conditions. Also detected, using ³²P, are light-induced phosphorylation of the LHC-2 (consistent with the ability to undergo State transitions) and of the 40 and 30 kilodalton subunits of the PSII core complex. These observations indicate that additional mechanisms may also exist to help optimize the interception of quanta during rapid changes in illumination conditions.     

Energy Transfer from the Phycobilisomes to Photosystem II Reaction Centers in Wild Type Cyanidium caldarium

Nonsaturating light at 600 or 436 nanometers was used to excite specifically phycocyanin or chlorophyll a, respectively, both of which participate in light capture in photosystem II of Cyanidium caldarium. The ratio of absorption of light by phycocyanin to chlorophyll in photosystem II in this organism is >20 at 600 nanometers and ≤0.2 at 436 nanometers.     

Steady-State Phosphorylation of Light-Harvesting Complex II Proteins Preserves Photosystem I under Fluctuating White Light

According to the “state transitions” theory, the light-harvesting complex II (LHCII) phosphorylation in plant chloroplasts is essential to adjust the relative absorption cross section of photosystem II (PSII) and PSI upon changes in light quality. The role of LHCII phosphorylation upon changes in light intensity is less thoroughly investigated, particularly when changes in light intensity are too fast to allow the phosphorylation/dephosphorylation processes to occur. Here, we demonstrate that the Arabidopsis (Arabidopsis thaliana) stn7 (for state transition7) mutant, devoid of the STN7 kinase and LHCII phosphorylation, shows a growth penalty only under fluctuating white light due to a low amount of PSI. Under constant growth light conditions, stn7 acquires chloroplast redox homeostasis by increasing the relative amount of PSI centers. Thus, in plant chloroplasts, the steady-state LHCII phosphorylation plays a major role in preserving PSI upon rapid fluctuations in white light intensity. Such protection of PSI results from LHCII phosphorylation-dependent equal distribution of excitation energy to both PSII and PSI from the shared LHCII antenna and occurs in cooperation with nonphotochemical quenching and the proton gradient regulation5-dependent control of electron flow, which are likewise strictly regulated by white light intensity. LHCII phosphorylation is concluded to function both as a stabilizer (in time scales of seconds to minutes) and a dynamic regulator (in time scales from tens of minutes to hours and days) of redox homeostasis in chloroplasts, subject to modifications by both environmental and metabolic cues. Exceeding the capacity of LHCII phosphorylation/dephosphorylation to balance the distribution of excitation energy between PSII and PSI results in readjustment of photosystem stoichiometry.Plant acclimation to different quantities and qualities of light has been extensively investigated. The light quality experiments have usually concerned the red/blue and far-red light acclimation strategies, which have been closely related to the state transitions and the phosphorylation of the light-harvesting complex II (LHCII) proteins, Lhcb1 and Lhcb2, by the state transition7 (STN7) kinase (Allen, 2003; Bellafiore et al., 2005; Bonardi et al., 2005; Tikkanen et al., 2006; Rochaix, 2007). Such studies on acclimation to different qualities of light have uncovered key mechanisms required for the maintenance of photosynthetic efficiency in dense populations and canopies (Dietzel et al., 2008). However, the role of LHCII phosphorylation under fluctuations in the quantity of white light has been scarcely investigated. Light conditions in natural environments may be very complex with respect to the quantity of white light, which constantly fluctuates both in short- and long-term durations (Smith, 1982; Külheim et al., 2002). Thus, the acclimation strategies to natural environments must concomitantly meet the challenges of both high- and low-light acclimation. Changing cloudiness, for example, would initiate both the high-light and low-light acclimation signals in the time scale of minutes and hours, whereas the movements of leaves in the wind or the rapid movement of clouds would initiate even more frequent light acclimation signals. The kinetics of reversible LHCII phosphorylation is far too slow to cope with rapid environmental changes.The phosphorylation level of LHCII proteins in the thylakoid membrane is regulated by both the STN7 kinase and the counteracting PPH1/TAP38 phosphatase (Pribil et al., 2010; Shapiguzov et al., 2010). No definite results are available about regulation of the PPH1/TAP38 phosphatase, but the STN7 kinase is strongly under redox regulation (Lemeille et al., 2009) and controls the phosphorylation level of LHCII proteins under varying white light intensities as well as according to chloroplast metabolic cues, as described already decades ago (Fernyhough et al., 1983; Rintamäki et al., 2000; Hou et al., 2003). So far, research on the role of the STN7 kinase and LHCII phosphorylation in the light acclimation of higher plants has heavily focused on reversible LHCII phosphorylation and concomitant state transitions. The state 1-to-state 2 transition, by definition, means the phosphorylation of LHCII proteins, their detachment from PSII in grana membranes, and migration to the stroma membranes to serve in the collection of excitation energy to PSI (Fork and Satoh, 1986; Williams and Allen, 1987; Wollman, 2001; Rochaix, 2007; Kargul and Barber, 2008; Murata, 2009; Lemeille et al., 2010; Minagawa, 2011). Concomitantly, the absorption cross section of PSII decreases and that of PSI increases (Canaani and Malkin, 1984; Malkin et al., 1986; Ruban and Johnson, 2009). Indeed, state transitions have been well documented when different qualities (blue/red and far red) of light, preferentially exciting either PSII or PSI, have been applied.Different from state transitions, the white light intensity-dependent reversible LHCII phosphorylation does not result in differential excitation of the two photosystems (Tikkanen et al., 2010). Instead, both photosystems remain nearly equally excited independently whether the LHCII proteins are heavily phosphorylated or strongly dephosphorylated. Moreover, it is worth noting that the different qualities of light generally used to induce reversible LHCII phosphorylation and state transitions (blue/red and far-red lights) have usually been of very low intensity (for review, see Haldrup et al., 2001), and apparently, minimal protonation of the lumen takes place under such illumination conditions. Yet another difference between induction of LHCII protein phosphorylation by different qualities of light or different quantities of white light concerns the concomitant induction of PSII core protein phosphorylation. In the former case, the level of PSII core protein phosphorylation follows the phosphorylation pattern of LHCII proteins, whereas under different quantities of white light, the phosphorylation behavior of PSII core and LHCII proteins is the opposite (Tikkanen et al., 2008b).To gain a more comprehensive understanding of the physiological role of white light-induced changes in LHCII protein phosphorylation, we have integrated Arabidopsis (Arabidopsis thaliana) LHCII phosphorylation with other light-dependent regulatory modifications of light harvesting and electron transfer in the thylakoid membrane, which include the nonphotochemical quenching of excitation energy (for review, see Niyogi, 1999; Horton and Ruban, 2005; Barros and Kühlbrandt, 2009; de Bianchi et al., 2010; Jahns and Holzwarth, 2012; Ruban et al., 2012) and the photosynthetic control of electron transfer by the cytochrome b₆f (Cytb6f) complex (Rumberg and Siggel, 1969; Witt, 1979; Tikhonov et al., 1981; Bendall, 1982; Nishio and Whitmarsh, 1993; Joliot and Johnson, 2011; Suorsa et al., 2012; for review, see Foyer et al., 1990, 2012), both strongly dependent on lumenal protonation.It is demonstrated that the steady-state LHCII phosphorylation is particularly important under rapidly fluctuating light (FL) conditions. This ensures equal energy distribution to both photosystems, prevents the accumulation of electrons in the intersystem electron transfer chain (ETC), eliminates perturbations in chloroplast redox balance, and maintains PSI functionality upon rapid fluctuations in white light intensity.     

Photosynthetic Light Reactions in Chemically Fixed Anacystis nidulans, Chlorella pyrenoidosa, and Porphyridium cruentum

The photochemical activities of various species of unicellular algae (Anacystis nidulans, Chlorella pyrenoidosa, and Porphyridium cruentum) were studied following chemical fixation. Fixation with formaldehyde and glutaraldehyde yielded cells which retained their ability to perform photosystem I and photosystem II reactions. The photochemical efficiencies of some fixed algae are as great as those of unfixed spinach chloroplasts. Fixed algae containing accessory pigments appear to be useful models for further studies of the light reactions of photosynthesis.     

Picosecond time-resolved energy transfer in Porphyridium cruentum. Part I. In the intact alga

The wavelength-resolved fluorescence emission kinetics of the accessory pigments and chlorophyll a in Porphyridium cruentum have been studied by pico-second laser spectroscopy. Direct excitation of the pigment B-phycoerythrin with a 530 nm, 6 ps pulse produced fluorescence emission from all of the pigments as a result of energy transfer between the pigments to the reaction centre of Photosystem II. The emission from B-phycoerythrin at 576 nm follows a nonexponential decay law with a mean fluorescence lifetime of 70 ps, whereas the fluorescence from R-phycocyanin (640 nm), allophycocyanin (660 nm) and chlorophyll a (685 nm) all appeared to follow an exponential decay law with lifetimes of 90 ps, 118 ps and 175 ps respectively. Upon closure of the Photosystem II reaction centres with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination the chlorophyll a decay became non-exponential, having a long component with an apparent lifetime of 840 ps. The fluorescence from the latter three pigments all showed finite risetimes to the maximum emission intensity of 12 ps for R-phycocyanin, 24 ps for allophycocyanin and 50 ps for chlorophyll a. A kinetic analysis of these results indicates that energy transfer between the pigments is at least 99% efficient and is governed by an exp --At1/2 transfer function. The apparent exponential behaviour of the fluorescence decay functions of the latter three pigments is shown to be a direct result of the energy transfer kinetics, as are the observed risetimes in the fluorescence emissions.     

Photoacclimation in the Red Alga Porphyridium cruentum: Changes in Photosynthetic Enzymes, Electron Carriers, and Light-Saturated Rate of Photosynthesis as a Function of Irradiance and Spectral Quality

Acclimation of the photosynthetic apparatus to changes in the light environment was studied in the unicellular red alga Porphyridium cruentum (American Type Culture Collection No. 50161). Absolute or relative amounts of four photosynthetic enzymes and electron carriers were measured, and the data were compared with earlier observations on light-harvesting components (F.X. Cunningham, Jr., R.J. Dennenberg, L. Mustárdy, P.A. Jursinic, E. Gantt [1989] Plant Physiol 91: 1179-1187; F.X. Cunningham, Jr., R.J. Dennenberg, P.A. Jursinic, E. Gantt [1990] Plant Physiol 93: 888-895) and with measurements of photosynthetic capacity. P_max, the light-saturated rate of photosynthesis on a chlorophyll (Chl) basis, increased more than 4-fold with increase in growth irradiance from 6 to 280 μeinsteins·m⁻²·s⁻¹. Amounts of ferredoxin-NADP⁺ reductase, ribulose-1,5-bisphosphate carboxylase, and cytochrome f increased in parallel with P_max, whereas numbers of the light-harvesting complexes (photosystem [PS] I, PSII, and phycobilisomes) changed little, and ATP synthase increased 7-fold relative to Chl. The calculated minimal turnover time for PSII under the highest irradiance, 5 ms, was thus about 4-fold faster than that calculated for cultures grown under the lowest irradiance (19 ms). A change in the spectral composition of the growth light (irradiance kept constant at 15 μeinsteins·m⁻²·s⁻¹) from green (absorbed predominantly by the phycobilisome antenna of PSII) to red (absorbed primarily by the Chl antenna of PSI) had little effect on the amounts of ribulose-1,5-bisphosphate carboxylase, ATP synthase, and phycobilisomes on a Chl, protein, or thylakoid area basis. However, the number of PSI centers declined by 40%, cytochrome f increased by 40%, and both PSII and ferredoxin-NADP⁺ reductase increased approximately 3-fold on a thylakoid area basis. The substantial increase in ferredoxin-NADP⁺ reductase under PSI light is inconsistent with a PSI-mediated reduction of NADP as the sole function of this enzyme. Our results demonstrate a high degree of plasticity in content and composition of thylakoid membranes of P. cruentum.     

Photosystem I-Mediated Regulation of Water Splitting in the Red Alga, Porphyra sanjuanensis

The marine red alga, Porphyra sanjuanensis is found mainly in the high intertidal zone and at low tide subject to frequent and extreme water stress, often accompanied by high temperatures and light intensities. Such exposures can lead to severe desiccation which is accompanied by the progressive loss of photosynthetic activity. Even following the loss of more than 90% of the thallus water content the alga recovers rapidly when returned to seawater. This stress-induced, reversible inactivation of photosynthesis is believed to be a protective adaptation which prevents photodamage to the exposed alga. Effects of light, inhibitors of water splitting, and electron donors to PSI on variable fluorescence and water splitting suggest that activity of the oxygen evolving complex is regulated by the PSI-driven reduction of a component of intersystem electron transport.     

Inhibition of Photosystem II in the Green Alga Ankistrodesmus falcatus by Copper

Copper strongly inhibited 2,6-dichloroindophenol (DCIP) photoreduction in the broken cells of the green alga Ankistrodesmus falcatus (C303), and the activity lost could not be restored by adding 1,5-diphenylearbazide (DPC). Inactivation of the DCIP Hill reaction reached 45% after incubation with 10 μM cupric sulfate for 20 min. In the same time, copper (13 μg/mg chlorophyll) was bound to the broken cells. Addition of 10 mM KCl reduced copper binding by about 53%. Fluorescence intensity at room temperature decreased upon addition of cupric sulfate and was partially restored by adding 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), These results suggest that copper inactivates electron transport between the oxidizing side of the reaction center of photosystem II and the electron-donating site of DPC. Further, the effect of light intensity shows that copper mostly affected the reaction rate of the dark step and had less inhibitory effect on the quantum efficiency of the primary reaction of electron transport in photosystem II.