EARLY TOXIC EFFECTS OF ZINC ON PSII OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE)1

Zinc toxicity on photosynthetic activity in cells of Synechocystis aquatilis f. aquatilis Sauvageau was investigated by monitoring Hill activity and fluorescence. The oxygen‐evolving activity decreased to about 80% of the initial value after exposure to 0.1 mM ZnSO₄ for 1 h. The PSII activity was inhibited by 40% in the presence of zinc concentrations ranging from 0.5 to 5.0 mM, suggesting that the metal effect is limited by zinc uptake. The fluorescence capacity (F_max–F/F_max) decreased from 0.57 to 0.35 and 0.20 in Zn‐treated cells for 15 and 60 min, respectively, thus providing evidence for rapid inactivation of electron transport at PSII. Zinc treatment promoted a rapid increase in PSII fluorescence that was counteracted by addition of 1,4‐benzoquinone, indicating that electron transfer at the reducing side of the PSII reaction center is arrested by zinc. Furthermore, a decline in the fluorescence yield could be observed after 1 h of zinc treatment as well as when Zn‐treated cells were excited in presence of 3‐(3′,4′‐dichlorophenyl)‐1,1‐dimethylurea. Under these conditions, zinc did not affect energy transfer from phycobilisomes to PSII, and the gradual quenching of PSII fluorescence may be due to a decrease in electron flow on the donor side of PSII. However, the 20% increase in the minimal fluorescence intensity (F_o) in parallel to the absence of changes in the maximal fluorescence intensity (F_max), observed in the first hour of zinc treatment, could also suggest a metal‐induced decline in the energy transfer from PSII‐chl a antenna to the PSII reaction center.     

DIFFERENTIAL IMPACTS OF PHOTOACCLIMATION AND THERMAL STRESS ON THE PHOTOBIOLOGY OF FOUR DIFFERENT PHYLOTYPES OF SYMBIODINIUM (PYRRHOPHYTA)1

The capacity for photoacclimation to light at 100 or 600 μmol photons·m^?2·s^?1 and the subsequent response to thermal stress was examined in four genetically distinct cultures of symbiotic dinoflagellates in the genus Symbiodinium with the ITS2 designations A1, A1.1, B1, and F2. While all algal types showed typical signs of photoacclimation to high light via a reduction in chl a, there was a differential response in cellular growth, photosystem II (PSII) activity, and the chl a‐specific absorption coefficient between cultures. When maintained at 32°C for up to 10 days, significant variation in the susceptibility to thermal stress was observed in the rate of loss in PSII activity and electron transport, PSII reaction center degradation, and cellular growth. The order of thermal tolerance did not change between the two light levels. However, as expected, loss in photosynthetic function was exacerbated in the thermally sensitive phylotypes (B1 and A1.1) when acclimated to the higher light intensity. There was no consistent relationship between thermal tolerance and changes in light energy dissipation via non‐photochemical pathways. Phylotypes F2 and A1 showed a high degree of thermal tolerance, yet the cellular responses to light and temperature were markedly different between these algae. The F2 isolate showed the greatest capacity for photoacclimation and growth at high light and temperature, while the A1 isolate appeared to adjust to thermal stress by a slight decline in PSII activity and a significant decline in growth, possibly at the expense of increased photosystem and cellular repair rates.     

Acclimation to low ultraviolet-B radiation increases photosystem I abundance and cyclic electron transfer with enhanced photosynthesis and growth in the cyanobacterium Nostoc sphaeroides

Ultraviolet-B radiation is known to harm most photosynthetic organisms with the exception of several studies of photosynthetic eukaryotes in which UV-B showed positive effects. In this study, we investigated the effect of acclimation to low UV-B radiation on growth and photosynthesis of the cyanobacterium Nostoc sphaeroides. Exposure to 0.08 W m⁻² UV-B plus low visible light for 14 d significantly increased the growth rate and biomass production by 16% and 30%, respectively, compared with those under visible light alone. The UV-B acclimated cells showed an approximately 50% increase in photosynthetic efficiency (α) and photosynthetic capacity (P_max), a higher PSI/PSII fluorescence ratio, an increase in PSI content and consequently enhanced cyclic electron flow, relative to those of non-acclimated cells. Both the primary quinone-type acceptor and plastoquinone pool re-oxidation were up-regulated in the UV-B acclimated cells. In parallel, the UV-B acclimated colonies maintained a higher rate of D1 protein synthesis following exposure to elevated intensity of UV-B or visible light, thus functionally mitigating photoinhibition. The present data provide novel insight into photosynthetic acclimation to low UV-B radiation and suggest that UV-B may act as a positive ecological factor for the productivity of some photosynthetic prokaryotes, especially during twilight periods or in shaded environments.     

Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton,and the relevance for pulse‐amplitude‐modulated and fast‐repetition‐rate fluorometry1

We studied the variability of in vivo absorption coefficients and PSII‐scaled fluorescence excitation (fl‐ex) spectra of high light (HL) and low light (LL) acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and 10 different phytoplankton classes. By scaling fl‐ex spectra to the corresponding absorption spectra by matching them in the 540–650 nm range, we obtained estimates for the fraction of total chl a that resided in PSII, the absorption of light by PSII, PSI, and photoprotective carotenoids. The in vivo red peak absorption maxima ranged from 673 to 679 nm, reflecting bonding of chl a to different pigment proteins. A simple approach is presented for quantifying intracellular self‐shading and evaluating the impact of photoacclimation on biooptical characteristics of the different PGs examined. In view of these results, parameters used in the calculation of oxygenic photosynthesis based on pulse‐amplitude‐modulated (PAM) and fast‐repetition‐rate (FRR) fluorometers are discussed, showing that the ratio between light available to PSII and total absorption, essential for the calculation of the oxygen release rate (using the PSII‐scaled fluorescence spectrum as a proxy) was dependent on species and photoacclimation state. Three subgroups of chromophytes exhibited 70%–80%, 60%–80%, and 50%–60% chl a in PSII‐LHCII; the two subgroups of chlorophytes, 70% or 80%; and cyanobacteria, only 12%. In contrast, the mean fraction for chromo‐ and chlorophytes of quanta absorbed by PSII was 73% in LL‐ and 55% in HL‐acclimated cells; thus, the corresponding ratios 0.55 and 0.73 might be used as correction factors adjusting for quanta absorbed by PSII for PAM and FRR measurements.     

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^?2 s^?1 with faster growth and more biomass production when compared to cells at 50 µmol m^?2 s^?1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein–pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII–LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

     

VARIATION OF STOMATAL DISTRIBUTION IN CAREX AQUATILIS (CYPERACEAE)

The density and distribution of stomates in Carex aquatilis Wahl. in the Pacific Northwest were examined using epidermal peels of samples of leaves from natural populations, from greenhouse-grown transplants and from seedling families grown under controlled conditions. These were compared to stomatal distributions of populations in eastern North America. Populations of Carex aquatilis Wahl. form 2 groups based on the distribution and density of stomates. Carex aquatilis var. dives (Holm) Kükenthal is epistomatic, with adaxial stomatal densities of 28.7–48.5/0.1 mm². The C. aquatilis var. aquatilis is amphistomatic, with adaxial stomatal densities of 8.1–22.2/0.1 mm² and abaxial densities of 11.3–24.5/0.1 mm² in the Pacific Northwest. Total stomatal frequencies are similar in both groups. Stomatal distribution and densities are here shown to not vary significantly within populations and appear to be genetically determined, as shown by progeny tests and growth of seedlings under uniform and experimental conditions. Stomatal distribution in Carex aquatilis appears to be adaptive, and intraspecific variation provides a system for determining the adaptive significance of differences in stomatal patterns.     

CYANOBACTERIAL ACCLIMATION TO RAPIDLY FLUCTUATING LIGHT IS CONSTRAINED BY INORGANIC CARBON STATUS1

Acclimation to rapidly fluctuating light, simulating shallow aquatic habitats, is altered depending on inorganic carbon (C_i) availability. Under steady light of 50 μmol photons·m^?2·s^?1, the growth rate of Synechococcus elongatus PCC7942 was similar in cells grown in high C_i (4 mM) and low C_i (0.02 mM), with induced carbon concentrating mechanisms compensating for low C_i. Growth under fluctuating light of a 1‐s period averaging 50 μmol photons·m^?2·s^?1 caused a drop in growth rate of 28%±6% in high C_i cells and 38%±8% in low C_i cells. In high C_i cells under fluctuating light, the PSI/PSII ratio increased, the PSII absorption cross‐section decreased, and the PSII turnover rate increased in a pattern similar to high‐light acclimation. In low C_i cells under fluctuating light, the PSI/PSII ratio decreased, the PSII absorption cross‐section decreased, and the PSII turnover remained slow. Electron transport rate was similar in high and low C_i cells but in both was lower under fluctuating than under steady light. After acclimation to a 1‐s period fluctuating light, electron transport rate decreased under steady or long‐period fluctuating light. We hypothesize that high C_i cells acclimated to exploit the bright phases of the fluctuating light, whereas low C_i cells enlarged their PSII pool to integrate the fluctuating light and dampen the variation of the electron flux into a rate‐restricted C_i pool. Light response curves measured under steady light, widely used to predict photosynthetic rates, do not properly predict photosynthetic rates achieved under fluctuating light, and exploitation of fluctuating light is altered by C_i status.     

The Accumulation of Protochlorophyllide in Cells of Synechocystis PCC 6714 with a Low PSI/PSII Stoichiometry

Changes in intracellular levels of Chl a precursors were examinedin relation to changes in the PSI/PSII stoichiometry in thecyanophyte Synechocystis PCC 6714. Protochlorophyllide (Pchlide)accumulated markedly in cells with a low PSI/PSII stoichiometrygrown under light that is absorbed by Chl a (PSI light) whereasno accumulation occurred in cells with a high PSI/PSII stoichiometrygrown under light absorbed by phycobilisomes (PSII light). Levelsof Pchlide in cells grown under PSI light decreased rapidlyupon a shift to PSII light. The rapid decrease in Pchlide accompanieda transient increase in chlorophyllide a, indicating that reductionof Pchlide was enhanced by shift to PSII light. The action spectrumindicated that the Pchlide decrease upon the shift to PSII lightdepended on excitation of Pchlide, suggesting that the accumulationof Pchllide was due to limited excitation of Pchlide, so thatPchlide photoreduction, under PSI light. However, comparisonof levels of Pchlide and the photosystem complexes in wild-typePlectonema boryanum with those in a mutant that lacked the darkPchlide reductase (YFC 1004) indicated that dark reduction compensatedfor the limited photoreduction under PSI light. Similar compensationby dark reduction was confirmed with Synechocystis PCC 6714.In cultures of Synechocystis under conditions where Pchlidecould not be photoreduced, accumulation of Pchlide and low PSI/PSIIstoichiometry occurred only when cells were illuminated withlight that preferentially excited PSI. The results indicatethat the low PSI/PSII stoichiometry in cells grown under PSIlight is not a result of inefficient synthesis of Chl a witha reduced rate of Pchlide photoreduction. They suggest furtherthat accumulation of Pchlide under PSI light results from retardationof the Chl a synthesis due to suppression of PSI synthesis. ¹Present address: Tsurukawa 5-15-11, Machida, Tokyo, 195 Japan.     

Different strategies of photoacclimation by two strains of Emiliania huxleyi (Haptophyta)1

Photoacclimation involves the modification of components of the light and dark reactions to optimize photosynthesis following changes in available light. All of the energy required for photosynthesis comes from linear electron transport through PSII and PSI and is dependent upon the amount of light harvested by PSII relative to PSI (a*_PSII and a*_PSI). The amount of light harvested is determined by the effective absorption cross‐sections (σ_PSII, σ_PSI) and cellular contents of the PSII and PSI reaction center complexes (RCII, RCI). Here, we examine the effective absorption cross‐sections and reaction center contents for calcifying (B11) and noncalcifying (B92) strains of the globally important coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler when grown under various photon flux densities (PFDs). The two strains displayed different “strategies” of acclimation. As growth PFD increased, B11 preferentially changed σ and the cellular content of chl a per cell over PSU “size” (the total cellular chl a content associated with the reaction center complexes); strain B92 preferentially changed PSU size over the cellular content of reaction complexes. Neither strategy was specifically consistent with the majority of previous studies from other microalgal species. For both strains, cellular light absorption for PSII and PSI was maintained close to unity across the range of growth PFDs since changes of σ_PSII and σ_PSI were reciprocated by those of RCIIs and RCIs per cell. Our results demonstrate a significant adaptive flexibility of E. huxleyi to photoacclimate. Finally, we calculated the amount of chl a associated with either photosystem to consider our interpretations of photoacclimation based on conventional determinations of PSU size.     

INORGANIC CARBON REPLETION CONSTRAINS STEADY‐STATE LIGHT ACCLIMATION IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS1

Cyanobacteria show high metabolic plasticity by re‐allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50‐fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon‐concentrating mechanism (CCM) or a dissolved [Ci] of ～4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m^?2·s^?1. Cells acclimated to low Ci significantly re‐allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re‐organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell^?1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus.     

Fast cadmium inhibition of photosynthesis in cyanobacteria in vivo and in vitro studies using perturbed angular correlation of γ-rays

The effect of cadmium on the photosynthetic activity of Synechocystis PCC 6803 was monitored in this study. The oxygen evolving capacity of Synechocystis treated with 40 μM CdCl₂ was depressed to 10% of the maximum in 15 min, indicating that Cd²⁺ penetrated rapidly into the cells and blocked the photosynthetic activity. However, neither photosystem II (PSII) nor photosystem I (PSI) activity showed a significant short-term decrease which would explain this fast decrease in the whole-chain electron transport. Thermoluminescence measurements have shown that the charge separation and stabilization in PSII remains essentially unchanged during the first few hours following the Cd²⁺ treatment. The electron flow through PSI was monitored by following the redox changes of the P700 reaction centers of PSI. Alterations in the oxidation kinetics of P700 in the Cd²⁺-treated cells indicated that Cd²⁺ treatment might affect the available electron acceptor pool of P700, including the CO₂ reduction and accumulation in the cells. Perturbed angular correlation of γ-rays (PAC) using the radioactive ^111mCd isotope was used to follow the Cd²⁺ uptake at a molecular level. The most plausible interpretation of the PAC data is that Cd²⁺ is taken up by one or more Zn proteins replacing Zn²⁺ in Synechocystis PCC 6803. Using the radioactive ¹⁰⁹Cd isotope, a protein of approximately 30 kDa that binds Cd²⁺ could be observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. The results indicate that Cd²⁺ might inactivate different metal-containing enzymes, including carbonic anhydrase, by replacing the zinc ion, which would explain the rapid and almost full inhibition of the photosynthetic activity in cyanobacteria.     

Activities of Photosystems I and II in Chlamydomonas segnis Adapted and Adapting to Air and Air-enriched with Carbon Dioxide*

Activities of photosystems I and II were compared at a saturating irradiance in air- and 5% CO₂-adapted and adapting Chlamydomonas segnis at the active phase of photosynthesis during the cell cycle. PSII activity was 200% greater in air- than in 5% CO₂-adapted cells, while PSI activity was similar in both types of cells and matched the level of PSII activity in air-adapted cells. As a result, air- and 5% CO₂-adapted cells were characterized by low and high PSI/PSII ratios, respectively. In air-adapted cells, the greater PSII activity (rate of O₂ evolved) exceeded that of photosynthetic (Ps) O₂ evolution, resulting in a Ps/PSII ratio below unity. This was associated with higher levels of catalase activity, lower l -ascorbate content, and higher dehydro-l -ascorbate content than in 5% CO₂-adapted cells. During adaptation to air or 5% CO₂ for 6 h in light, PSI rather than PSII was sensitive to changes in the concentration of CO₂, and the adapting cells acquired the characteristics of air- and 5% CO₂-adapted cells as indicated by PSI/PSII, Ps/PSII, catalase activity, l -ascorbate and dehydro-l -ascorbate contents. The results are discussed in the light of changes in the molecular organization of the thylakoid membranes and enhanced non-cyclic electron transport coupled with O₂-uptake (Mehler reaction) for the generation of the ATP required for CO₂/HCO^?₃-transport in air-adapted and adapting cells.     

Extensive remodeling of the photosynthetic apparatus alters energy transfer among photosynthetic complexes when cyanobacteria acclimate to far-red light

Some cyanobacteria remodel their photosynthetic apparatus by a process known as Far-Red Light Photoacclimation (FaRLiP). Specific subunits of the phycobilisome (PBS), photosystem I (PSI), and photosystem II (PSII) complexes produced in visible light are replaced by paralogous subunits encoded within a conserved FaRLiP gene cluster when cells are grown in far-red light (FRL; λ = 700–800 nm). FRL-PSII complexes from the FaRLiP cyanobacterium, Synechococcus sp. PCC 7335, were purified and shown to contain Chl a, Chl d, Chl f, and pheophytin a, while FRL-PSI complexes contained only Chl a and Chl f. The spectroscopic properties of purified photosynthetic complexes from Synechococcus sp. PCC 7335 were determined individually, and energy transfer kinetics among PBS, PSII, and PSI were analyzed by time-resolved fluorescence (TRF) spectroscopy. Direct energy transfer from PSII to PSI was observed in cells (and thylakoids) grown in red light (RL), and possible routes of energy transfer in both RL- and FRL-grown cells were inferred. Three structural arrangements for RL-PSI were observed by atomic force microscopy of thylakoid membranes, but only arrays of trimeric FRL-PSI were observed in thylakoids from FRL-grown cells. Cells grown in FRL synthesized the FRL-specific complexes but also continued to synthesize some PBS and PSII complexes identical to those produced in RL. Although the light-harvesting efficiency of photosynthetic complexes produced in FRL might be lower in white light than the complexes produced in cells acclimated to white light, the FRL-complexes provide cells with the flexibility to utilize both visible and FRL to support oxygenic photosynthesis.This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.     

ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

Growth of Anacystis in the Presence of Thiosulphate and its Consequences for the Architecture of the Photosynthetic Apparatus

With the aim of obtaining information on the degree of flexibility maintained in cyanobacteria in context with their phylogenetic position, Anacystis was grown in the presence of thiosulphate, oxidized in a photosystem I (PSI) dependent reaction (K_M 7.4 × 10^?3 M thiosulfate). Besides DBMIB, only o-phenanthroline and p-hydroxymercuribenzoate blocked thiosulphate-dependent PSI activity to some extent; iodonitrothymol, DCMU and cyanide had no influence. Growth of Anacystis in the presence of thiosulphate induced a reorganization of the photosynthetic apparatus characterized by a shift in the PSII/PSI ratio in favor of PSI, comparable to low light conditions. Capability for oxygenic photosynthesis never completely disappeared; structural elements of PSII were retained in the membrane to a certain degree. The antenna pigment system signalled high light under conditions of thiosulphate oxidation as judged from the ratio of phycocyanin to chlorophyll. Besides a shift in the ratio of PSII to PSI components, the polypeptide pattern of thylakoids from thiosulphate grown cells shows several additional components compared to the controls and, moreover, higher concentrations of some polypeptides present in the controls, particularly a M_r 41000 polypeptide. The process of thiosulphate oxidation appears bound to the thylakoid membrane.     

THE CYANOBACTERIAL CHLOROPHYLL‐BINDING‐PROTEIN ISIA ACTS TO INCREASE THE IN VIVO EFFECTIVE ABSORPTION CROSS‐SECTION OF PSI UNDER IRON LIMITATION1

Iron availability limits primary production in >30% of the world’s oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron‐stress‐induced) under iron stress, which can become the most abundant chl‐binding protein in the cell. Within iron‐limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross‐section of the photosynthetic reaction center PSI (σ_PSI) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that accumulation of IsiA results in a ～60% increase in σ_PSI, in agreement with the theoretical increase in cross‐section based on the structure of the biochemically isolated IsiA‐PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light‐harvesting antenna for PSI. On progressive iron‐stress in culture, IsiA continues to accumulate without a concomitant increase in σ_PSI, suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA‐bound chl is directly involved in photosynthetic electron transport.     

Comparison of Photoacclimation in Twelve Freshwater Photoautotrophs (Chlorophyte,Bacillaryophyte, Cryptophyte and Cyanophyte) Isolated from a Natural Community

Different representative of algae and cyanobacteria were isolated from a freshwater habitat and cultivated in laboratory to compare their photoacclimation capacity when exposed to a wide range of light intensity and to understand if this factor may modify natural community dominance. All species successfully acclimated to all light intensities and the response of phytoplankton to increased light intensity was similar and included a decrease of most photosynthetic pigments accompanied by an increase in photoprotective pigment content relative to Chl a. Most species also decreased their light absorption efficiency on a biovolume basis. This decrease not only resulted in a lower fraction of energy absorbed by the cell, but also to a lower transfer of energy to PSII and PSI. Furthermore, energy funnelled to PSII or PSI was also rearranged in favour of PSII. High light acclimated organisms also corresponded to high non-photochemical quenching and photosynthetic electron transport reduction state and to a low Φ''_M. Thus photoacclimation processes work toward reducing the excitation pressure in high light environment through a reduction of light absorption efficiency, but also by lowering conversion efficiency. Interestingly, all species of our study followed that tendency despite being of different functional groups (colonial, flagellated, different sizes) and of different phylogeny demonstrating the great plasticity and adaptation ability of freshwater phytoplankton to their light environment. These adjustments may explain the decoupling between growth rate and photosynthesis observed above photosynthesis light saturation point for all species. Even if some species did reach higher growth rate in our conditions and thus, should dominate in natural environment with respect to light intensity, we cannot exclude that other environmental factors also influence the population dynamic and make the outcome harder to predict.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

The effects of elevated CO2 (0.5%) on chloroplasts in the tetraploid black locust (Robinia pseudoacacia L.)

Some ploidy plants demonstrate environmental stress tolerance. Tetraploid (4×) black locust (Robinia pseudoacacia L.) exhibits less chlorosis in response to high CO₂ than do the corresponding diploid (2×) plants of this species. We investigated the plant growth, anatomy, photosynthetic ability, chlorophyll (chl) fluorescence, and antioxidase activities in 2× and 4× black locusts cultivated under high CO₂ (0.5%). Elevated CO₂ (0.5%) induced a global decrease in the contents of total chl, chl a, and chl b in 2× leaves, while few changes were found in the chl content of 4× leaves. Analyses of the chl fluorescence intensity, maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), K‐step (V_k), and J‐step (V_J) revealed that 0.5% CO₂ had a negative effect on the photosynthetic capacity and growth of the 2× plants, especially the performance of PSII. In contrast, there was no significant effect of high CO₂ on the growth of the 4× plants. These analyses indicate that the decreased inhibition of the growth of 4× plants by high CO₂ (0.5%) may be attributed to an improved photosynthetic capacity, pigment content, and ultrastructure of the chloroplast compared to 2× plants.     

FLUORESCENCE EMISSION SPECTRA OF MARINE AND BRACKISH‐WATER ECOTYPES OF FUCUS VESICULOSUS AND FUCUS RADICANS (PHAEOPHYCEAE) REVEAL DIFFERENCES IN LIGHT‐HARVESTING APPARATUS1

The Bothnian Sea in the northerly part of the Baltic Sea is a geologically recent brackish‐water environment, and rapid speciation is occurring in the algal community of the Bothnian Sea. We measured low‐temperature fluorescence emission spectra from the Bothnian Sea and the Norwegian Sea ecotypes of Fucus vesiculosus L., a marine macroalga widespread in the Bothnian Sea. Powdered, frozen thallus was used to obtain undistorted emission spectra. The spectra were compared with spectra measured from the newly identified species Fucus radicans Bergström et L. Kautsky, which is a close relative of F. vesiculosus and endemic to the Bothnian Sea. The spectrum of variable fluorescence was used to identify fluorescence peaks originating in PSI and PSII in this chl c–containing alga. The spectra revealed much higher PSII emission, compared to PSI emission, in the Bothnian Sea ecotype of F. vesiculosus than in F. radicans or in the Norwegian Sea ecotype of F. vesiculosus. The results suggest that more light‐harvesting chl a/c proteins serve PSII in the Bothnian Sea ecotype of F. vesiculosus than in the two other algal strains. Treatment of the Bothnian Sea ecotype of F. vesiculosus in high salinity (10, 20, and 35 practical salinity units) for 1 week did not lead to spectral changes, indicating that the measured features of the Bothnian Sea F. vesiculosus are stable and not simply a direct result of exposure to low salinity.