The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange

Responses of Membranes and the Photosynthetic Apparatus to Salt Stress in Cyanobacteria

Cyanobacteria are autotrophs whose photosynthetic process is similar to that of higher plants, although the photosynthetic apparatus is slightly different. They have been widely used for decades as model systems for studying the principles of photosynthesis, especially the effects of environmental stress on photosynthetic activities. Salt stress, which is the most common abiotic stress in nature, combines ionic and osmotic stresses. High cellular ion concentrations and osmotic stress can alter normal metabolic processes and photosynthesis. Additionally, salt stress increases the intracellular reactive oxygen species (ROS) contents. Excessive amounts of ROS will damage the photosynthetic apparatus, inhibit the synthesis of photosystem-related proteins, including the D1 protein, and destroy the thylakoid membrane structure, leading to inhibited photosynthesis. In this review, we mainly introduce the effects of salt stress on the cyanobacterial membranes and photosynthetic apparatus. We also describe specific salt tolerance mechanisms. A thorough characterization of the responses of membranes and photosynthetic apparatus to salt stress may be relevant for increasing agricultural productivity.

Protein arrangement factor: a new photosynthetic parameter characterizing the organization of thylakoid membrane proteins

A proper spatial distribution of photosynthetic pigment-protein complexes - PPCs (photosystems, light-harvesting antennas) is crucial for photosynthesis. In plants, photosystems I and II (PSI and PSII) are heterogeneously distributed between granal and stromal thylakoids. Here we have described similar heterogeneity in the PSI, PSII and phycobilisomes (PBSs) distribution in cyanobacteria thylakoids into microdomains by applying a new image processing method suitable for the Synechocystis sp. PCC6803 strain with yellow fluorescent protein-tagged PSI. The new image processing method is able to analyze the fluorescence ratios of PPCs on a single-cell level, pixel per pixel. Each cell pixel is plotted in CIE1931 color space by forming a pixel-color distribution of the cell. The most common position in CIE1931 is then defined as protein arrangement (PA) factor with xy coordinates. The PA-factor represents the most abundant fluorescence ratio of PSI/PSII/PBS, the 'mode color' of studied cell. We proved that a shift of the PA-factor from the center of the cell-pixel distribution (the 'median' cell color) is an indicator of the presence of special subcellular microdomain(s) with a unique PSI/PSII/PBS fluorescence ratio in comparison to other parts of the cell. Furthermore, during a 6-h high-light (HL) treatment, 'median' and 'mode' color (PA-factor) of the cell changed similarly on the population level, indicating that such microdomains with unique PSI/PSII/PBS fluorescence were not formed during HL (i.e. fluorescence changed equally in the whole cell). However, the PA-factor was very sensitive in characterizing the fluorescence ratios of PSI/PSII/PBS in cyanobacterial cells during HL by depicting a 4-phase acclimation to HL, and their physiological interpretation has been discussed.

Influence of nitrogen source on photochemistry and antenna size of the photosystems in marine green macroalgae, Ulva lactuca

Ulva lactuca is regarded as a prospective energy crop for biorefinery owing to its affluent biochemical composition and high growth rate. In fast-growing macroalgae, biomass development strictly depends on external nitrogen pools. Additionally, nitrogen uptake rates and photosynthetic pigment content vary with type of nitrogen source and light conditions. However, the combined influence of nitrogen source and light intensity on photosynthesis is not widely studied. In present study, pale green phenotype of U. lactuca was obtained under high light (HL) condition when inorganic nitrogen (nitrate) in the media was substituted with organic nitrogen (urea). Further, pale green phenotype survived the saturating light intensities in contrast to the normal pigmented control which bleached in HL. Detailed analysis of biochemical composition and photosynthesis was performed to understand functional antenna size and photoprotection in pale green phenotype. Under HL, urea-grown cultures exhibited increased growth rate, carbohydrate and lipid content while substantial reduction in protein, chlorophyll content and PSII antenna size was observed. Further, in vivo slow and polyphasic chlorophyll a (Chl a) fluorescence studies revealed reduction in excitation pressure on PSII along with low non-photochemical quenching thus, transmitting most of the absorbed energy into photochemistry. The results obtained could be correlated to previous report on cultivation of U. lactuca through saturating summer intensities (1000 µmole photons m^-2 s^-1) in urea based: poultry litter extract (PLE). Having proved critical role of urea in conforming photoprotection, the application PLE was authenticated for futuristic, sustainable and year-round biomass cultivation.

Regulation of PSII function in Cyanothece sp. ATCC 51142 during a light-dark cycle

Cyanobacteria, as well as green algae and higher plants, have highly conserved photosynthetic machinery. Cyanothece sp. ATCC 51142 is a unicellular, aerobic, diazotrophic cyanobacterium that fixes N₂ in the dark. In Cyanothece, the psbA gene family is composed of five members, encoding different isoforms of the D1 protein. A new D1 protein has been postulated in the literature, which blocks PSII during the night and allows the fixation of nitrogen. We present data showing changes in PSII function in cells grown in cycles alternating between 12 h of light and dark, respectively, at Cyanothece sp. ATCC 51142. Cyanothece sp. ATCC 51142 uses intrinsic mechanisms to protect its nitrogenase activity in a two-stage process. In Stage I, immediately after the onset of darkness, the cells lose photosynthetic activity in a reversible process, probably by dissociation of water oxidation complex from photosystem II via a mechanism that does not require de novo protein synthesis. In Stage II, a more severe disruption of photosystem II function occurs is in part protein synthesis dependent and it could be a functional signature of the presence of sentinel D1 in a limited number of reaction centers still active or not yet inactivated by the mechanism described in Stage I. This process of inhibition uses light as a triggering signal for both the inhibition of photosynthetic activity and recovery when light returns. The intrinsic mechanism of photosynthetic inactivation during darkness with the interplay of the two mechanisms requires further studies.

Sensitivity of cold acclimation to elevated autumn temperature in field-grown Pinus strobus seedlings

Climate change will increase autumn air temperature, while photoperiod decrease will remain unaffected. We assessed the effect of increased autumn air temperature on timing and development of cold acclimation and freezing resistance in Eastern white pine (EWP, Pinus strobus) under field conditions. For this purpose we simulated projected warmer temperatures for southern Ontario in a Temperature Free-Air-Controlled Enhancement (T-FACE) experiment and exposed EWP seedlings to ambient (Control) or elevated temperature (ET, +1.5°C/+3°C during day/night). Photosynthetic gas exchange, chlorophyll fluorescence, photoprotective pigments, leaf non-structural carbohydrates (NSC), and cold hardiness were assessed over two consecutive autumns. Nighttime temperature below 10°C and photoperiod below 12 h initiated downregulation of assimilation in both treatments. When temperature further decreased to 0°C and photoperiod became shorter than 10 h, downregulation of the light reactions and upregulation of photoprotective mechanisms occurred in both treatments. While ET seedlings did not delay the timing of the downregulation of assimilation, stomatal conductance in ET seedlings was decreased by 20-30% between August and early October. In both treatments leaf NSC composition changed considerably during autumn but differences between Control and ET seedlings were not significant. Similarly, development of freezing resistance was induced by exposure to low temperature during autumn, but the timing was not delayed in ET seedlings compared to Control seedlings. Our results indicate that EWP is most sensitive to temperature changes during October and November when downregulation of photosynthesis, enhancement of photoprotection, synthesis of cold-associated NSCs and development of freezing resistance occur. However, we also conclude that the timing of the development of freezing resistance in EWP seedlings is not affected by moderate temperature increases used in our field experiments.

A unique regulation of the expression of the psbA, psbD, and psbE genes, encoding the 01, 02 and cytochrome b559 subunits of the Photosystem II complex in the chlorophyll d containing cyanobacterium Acaryochloris marina

Photosynthetic electron transport, chromatic photoacclirnation and expression of the genes encoding the 01, 02, and cytochrome b559 subunits of the Photosystem II complex were studied in the chlorophyll d containing cyanobacterium Acaryochloris marina MBIC11017 under various environmental conditions. During oxygen deprivation and inhibition of photosynthetic electron transport by dibromothymoquinone the psbA1 gene encoding a 01' isoform was induced. All of the three psbA and one of the three psbD (psbD2) genes, encoding two different isoforms of the 01 and the abundant isoform of the 02 proteins, respectively were induced under exposure to UV-B radiation and high intensity visible light. Under far red light the amount of Photosystem II complexes increased, and expression of the psbE2 gene encoding the alpha-subunit of cytochrome b559 was enhanced. However, the psbF and psbE1 genes encoding the beta- and another isoform of alpha-cytochrome b559, respectively remained lowly expressed under all conditions. Far red light also induced the psbD3 gene encoding a 02' isoform whose primary structure is different from the abundant 02 isoform. psbD3 was also induced under low intensity visible light, when chromatic photoacclimation was indicated by a red-shifted absorption of chlorophyll d. Our results show that differential expression of multigene families encoding different isoforms of 01 and 02 plays an important role in the acclimation of A. marina to contrasting environmental conditions. Moreover, the disproportionate quantity of transcripts of the alpha and beta subunits of cytochrome b559 implies the existence of an alpha-alpha homodimer organization of cytochrome b559 in Photosystem II complexes.

Deactivation processes in PsbA1-Photosystem II and PsbA3-Photosystem II under photoinhibitory conditions in the cyanobacterium Thermosynechococcus elongatus

The sensitivity to high light conditions of Photosystem II with either PsbA1 (WT*1) or PsbA3 (WT*3) as the D1 protein was studied in whole cells of the thermophilic cyanobacterium Thermosynechococcus elongatus. When the cells are cultivated under high light conditions the following results were found: (i) The O(2) evolution activity decreases faster in WT*1 cells than in WT*3 cells both in the absence and in the presence of lincomycin, a protein synthesis inhibitor; (ii) In WT*1 cells, the rate constant for the decrease of the O(2) evolution activity is comparable in the presence and in the absence of lincomycin; (iii) The D1 content revealed by western blot analysis decays similarly in both WT*1 and WT*3 cells and much slowly than O(2) evolution; (iv) The faster decrease in O(2) evolution in WT*1 than in WT*3 cells correlates with a much faster inhibition of the S(2)-state formation; (v) The shape of the WT*1 cells is altered. All these results are in agreement with a photo-inhibition process resulting in the loss of the O(2) activity much faster than the D1 turnover in PsbA1-PSII and likely to a greater production of reactive oxygen species under high light conditions in WT*1 than in WT*3. This latter result is discussed in view of the known effects of the PsbA1 to PsbA3 substitution on the redox properties of the Photosystem II cofactors. The observation that under low light conditions WT*3 cells are able to express the psbA(3) gene, whereas under similar conditions wild type cells are expressing mainly the psbA(1) gene is also discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Salt stress induces a decrease in excitation energy transfer from phycobilisomes to photosystem II but an increase to photosystem I in the cyanobacterium Spirulina platensis

The effects of salt stress (0-0.8M NaCl) on excitation energy transfer from phycobilisomes to photosystem I (PSI) and photosystem II (PSII) in the cyanobacterium Spirulina platensis were investigated. Salt stress resulted in a significant decrease in photosynthetic oxygen evolution activity and PSII electron transport activity, but a significant increase in PSI electron transport activity. Analyses of the polyphasic fluorescence transients (OJIP) showed that, with an increase in salt concentration, the fluorescence yield at the phases J, I and P declined considerably and the transient almost leveled off at 0.8M NaCl. Analyses of the JIP test demonstrated that salt stress led to a decrease in the maximal efficiency of PSII photochemistry, the probability of electron transfer beyond Q(A), and the yield of electron transport beyond Q(A). In addition, salt stress resulted in a decrease in the electron transport per PSII reaction center, but an increase in the absorption per PSII reaction center. However, there was no significant change in the trapping per PSII reaction center. Furthermore, there was a decrease in the concentration of the active PSII reaction centers. Analyses of 77K chlorophyll fluorescence emission spectra excited either at 436 or 580nm showed that salt stress inhibited excitation energy transfer from phycobilisomes to PSII but induced an increase in the efficiency of energy transfer from phycobilisomes to PSI. Based on these results, it is suggested that, through a down-regulation of PSII reaction centers and a shift of excitation energy transfer in favor of PSI, the PSII apparatus was protected from excess excitation energy.

Energetics in photosystem II from Thermosynechococcus elongatus with a D1 protein encoded by either the psbA1 or psbA3 gene

The main cofactors involved in the function of Photosystem II (PSII) are borne by the D1 and D2 proteins. In some cyanobacteria, the D1 protein is encoded by different psbA genes. In Thermosynechococcus elongatus the amino acid sequence deduced from the psbA3 gene compared to that deduced from the psbA1 gene points a difference of 21 residues. In this work, PSII isolated from a wild type T. elongatus strain expressing PsbA1 or from a strain in which both the psbA1 and psbA2 genes have been deleted were studied by a range of spectroscopies in the absence or the presence of either a urea type herbicide, DCMU, or a phenolic type herbicide, bromoxynil. Spectro-electrochemical measurements show that the redox potential of PheoD1 is increased by 17 mV from -522 mV in PsbA1-PSII to -505 mV in PsbA3-PSII. This increase is about half that found upon the D1-Q130E single site directed mutagenesis in Synechocystis PCC 6803. This suggests that the effects of the D1-Q130E substitution are, at least partly, compensated for by some of the additional amino-acid changes associated with the PsbA3 for PsbA1 substitution. The thermoluminescence from the S2QA-* charge recombination and the C identical with N vibrational modes of bromoxynil detected in the non-heme iron FTIR difference spectra support two binding sites (or one site with two conformations) for bromoxynil in PsbA3-PSII instead of one in PsbA1-PSII which suggests differences in the QB pocket. The temperature dependences of the S2QA-* charge recombination show that the strength of the H-bond to PheoD1 is not the only functionally relevant difference between the PsbA3-PSII and PsbA1-PSII and that the environment of QA (and, as a consequence, its redox potential) is modified as well. The electron transfer rate between P680+* and YZ is found faster in PsbA3 than in PsbA1 which suggests that the redox potential of the P680/P680+* couple (and hence that of 1P680*/P680+*) is tuned as well when shifting from PsbA1 to PsbA3. In addition to D1-Q130E, the non-conservative amongst the 21 amino acid substitutions, D1-S270A and D1-S153A, are proposed to be involved in some of the observed changes.

Minimal genomes, maximal productivity: comparative genomics of the photosystem and light-harvesting complexes in the marine cyanobacterium, Prochlorococcus

Although Prochlorococcus isolates possess the smallest genomes of any extant photosynthetic organism, this genus numerically dominates vast regions of the world's subtropical and tropical open oceans and has evolved to become an important contributor to global biogeochemical cycles. The sequencing of 12 Prochlorococcus genomes provides a glimpse of the extensive genetic heterogeneity and, thus, physiological potential of the lineage. In this study, we present an up-to-date comparative analysis of major proteins of the photosynthetic apparatus in 12 Prochlorococcus genomes. Our analyses reveal a striking diversity within the Prochlorococcus lineage in the major protein complexes of the photosynthetic apparatus. The heterogeneity that has evolved in the photosynthetic apparatus suggests versatility in strategies for optimizing photosynthesis under conditions of environmental variability and stress. This diversity could be particularly important in ensuring the survival of a lineage whose individuals have evolved minimal genomes and, thus, relatively limited repertoires for responding to environmental challenges.

Salt stress inhibits photosystems II and I in cyanobacteria

Recent studies of responses of cyanobacterial cells to salt stress have revealed that the NaCl-induced decline in the photosynthetic activities of photosystems II and I involves rapid and slow changes. The rapid decreases in the activities of both photosystems, which occur within a few minutes, are reversible and are associated with osmotic effects, which induce the efflux of water from the cytosol through water channels and rapidly increase intracellular concentrations of salts. Slower decreases in activity, which occur within hours, are irreversible and are associated with ionic effects that are due to the influx of Na(+) and Cl(-) ions through K(+)(Na(+)) channels and, probably, Cl(-) channels, with resultant dissociation of extrinsic proteins from photosystems. In combination with light stress, salt stress significantly stimulates photoinhibition by inhibiting repair of photodamaged photosystem II. Tolerance of photosystems to salt stress can be enhanced by genetically engineered increases in the unsaturation of fatty acids in membrane lipids and by intracellular synthesis of compatible solutes, such as glucosylglycerol and glycinebetaine. In this review, we summarize recent progress in research on the effects of salt stress on photosynthesis in cyanobacteria.

Differential regulation of psbA and psbD gene expression, and the role of the different D1 protein copies in the cyanobacterium Thermosynechococcus elongatus BP-1

In Thermosynechococcus elongatus BP-1, which is the preferred organism in recent structural studies of PSII, three psbA and two psbD genes code for three D1 and one D2 protein isoforms, respectively. The regulation and function of these genes and protein products is largely unknown. Therefore, we used quantitative RT-PCR to follow changes in the mRNA level of the respective genes, in combination with biophysical measurements to detect changes in the electron transport activity of Photosystem II under exposure to different visible and UV light, and temperature conditions. In cells which are acclimated to 40 micromol m(-2)s(-1) growth light conditions at 40 degrees C the main populations of the psbA and psbD transcripts arise from the psbA1 and psbD1 genes, respectively. When the temperature is raised to 60 degrees C psbA1 becomes the single dominating psbA mRNA species. Upon exposure of the cells to 500 micromol m(-2)s(-1) intensity visible light psbA3 replaces psbA1 as the dominating psbA mRNA species, and psbD2 increases at the expense of psbD1. UV-B radiation also increases the abundance of psbA3, and psbD2 at the expense of psbA1 and psbD1, respectively. From the different extent of total D1 protein loss in the absence and presence of lincomycin it was estimated that the PsbA3 protein isoform replaces PsbA1 in about 65% of PSII centers after 2 h of high light acclimation. Under the conditions of different psbA transcript distributions chlorophyll fluorescence and thermoluminescence measurements were applied to monitor charge recombination characteristics of the S2Q(A)(-) and S2Q(B)(-) states. We obtained faster decay of flash-induced chlorophyll fluorescence in the presence of DCMU, as well as lower peak temperature of the Q and B thermoluminescence bands when PsbA3 replaced PsbA1 as the main D1 protein isoform. The relevance of dynamic changes in the abundance of psbA and psbD transcript levels, as well as D1 protein isoforms in the acclimation of T. elongatus to changing environmental conditions is discussed.

UVB Effects on the Photosystem II-D1 Protein of Phytoplankton and Natural Phytoplankton Communities

The reaction center of photosystem II is susceptible to photodamage. In particular the D1 protein located in the photosystem II core has a rapid, light-dependent turnover termed the photosystem II repair cycle that, under illumination, degrades and resynthesizes D1 protein to limit accumulation of photodamaged photosystem II. Most studies concerning the effects of UVB (280-320 nm) on this cycle have been on cyanobacteria or specific phytoplankton species rather than on natural communities of phytoplankton. During a 5-year multidisciplinary project on the effects of UV radiation (200-400 nm) on natural systems, the effects of UVB on the D1 protein of natural phytoplankton communities were assessed. This review provides an overview of photoinhibitory effects of light on cultured and natural phytoplankton, with an emphasis on the interrelation of UVB exposure, D1 protein degradation and the repair of photosystem II through D1 resynthesis. Although the UVB component of the solar spectrum contributes to the primary photoinactivation of photosystem II, we conclude that, in natural communities, inhibition of the rate of the photosystem II repair cycle is a more important influence of UVB on primary productivity. Indeed, exposing tropical and temperate phytoplankton communities to supplemented UVB had more inhibitory effect on D1 synthesis than on the D1 degradation process itself. However, the rate of net D1 damage was faster for the tropical communities, likely because of the effects of high ambient light and water temperature on mechanisms of protein degradation and synthesis.

Synechocystis 6803 mutants expressing distinct forms of the Photosystem II D1 protein from Synechococcus 7942: relationship between the psbA coding region and sensitivity to visible and UV-B radiation

Synechocystis PCC 6803 mutants expressing either the "low light" (D1:1) or the "high light" (D1:2) form of the Photosystem II (PSII) D1 protein from Synechococcus PCC 7942 were constructed and characterized with respect to properties of PSII and sensitivity to visible and UV-B radiation. The AI and AIII mutants (containing only the D1:1 and D1:2 forms, respectively) exhibited very similar PSII characteristics as the control strain and they differed only in the accelerated decay kinetics of flash-induced variable fluorescence measured in the presence of DCMU. However, the mutants showed increased sensitivity to photodamage induced by visible and UV-B radiation, with higher loss of PSII activity in the AI than in the AIII strain. Thus, the difference between strains containing D1:1 and D1:2 found previously in Synechococcus 7942 is maintained after transfer of corresponding psbA genes into Synechocystis 6803 and is directly related to the coding region of these genes. The higher light sensitivity of the AI mutant is caused partly by the higher rate of photodamage and partly by the less efficient PSII repair.

A transient exchange of the photosystem II reaction center protein D1:1 with D1:2 during low temperature stress of Synechococcus sp. PCC 7942 in the light lowers the redox potential of QB

Upon exposure to low temperature under constant light conditions, the cyanobacterium Synechococcus sp. PCC 7942 exchanges the photosystem II reaction center D1 protein form 1 (D1:1) with D1 protein form 2 (D1:2). This exchange is only transient, and after acclimation to low temperature the cells revert back to D1:1, which is the preferred form in acclimated cells (Campbell, D., Zhou, G., Gustafsson, P., Oquist, G., and Clarke, A. K. (1995) EMBO J. 14, 5457-5466). In the present work we use thermoluminescence to study charge recombination events between the acceptor and donor sides of photosystem II in relation to D1 replacement. The data indicate that in cold-stressed cells exhibiting D1:2, the redox potential of Q(B) becomes lower approaching that of Q(A). This was confirmed by examining the Synechococcus sp. PCC 7942 inactivation mutants R2S2C3 and R2K1, which possess only D1:1 or D1:2, respectively. In contrast, the recombination of Q(A)(-) with the S(2) and S(3) states did not show any change in their redox characteristics upon the shift from D1:1 to D1:2. We suggest that the change in redox properties of Q(B) results in altered charge equilibrium in favor of Q(A). This would significantly increase the probability of Q(A)(-) and P680(+) recombination. The resulting non-radiative energy dissipation within the reaction center of PSII may serve as a highly effective protective mechanism against photodamage upon excessive excitation. The proposed reaction center quenching is an important protective mechanism because antenna and zeaxanthin cycle-dependent quenching are not present in cyanobacteria. We suggest that lowering the redox potential of Q(B) by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light.

Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0-0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport (PhiPSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (qN) and a decrease in photochemical quenching (qP). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from QA- to QB. The slope at the origin of the relative variable fluorescence curves (dV/dto) and the relative variable fluorescence at phase J (VJ) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.

Differential expression of antenna and core genes in Prochlorococcus PCC 9511 (Oxyphotobacteria) grown under a modulated light-dark cycle

The continuous changes in incident solar light occurring during the day oblige oxyphototrophs, such as the marine prokaryote Prochlorococcus, to modulate the synthesis and degradation rates of their photosynthetic components finely. How this natural phenomenon influences the diel expression of photosynthetic genes has never been studied in this ecologically important oxyphotobacterium. Here, the high light-adapted strain Prochlorococcus sp. PCC 9511 was grown in large-volume continuous culture under a modulated 12 h-12 h light-dark cycle mimicking the conditions found in the upper layer of equatorial oceans. The pcbA gene encoding the major light-harvesting complex showed strong diel variations in transcript levels with two maxima, one before the onset of illumination and the other near the end of the photoperiod. In contrast, the mRNA level of psbA (encoding the reaction centre II subunit D1), the monocistronic transcript of psbD (encoding D2) and the dicistronic transcript of psbDC were all tightly correlated with light irradiance, with a minimum at night and a maximum at noon. The occurrence of a second peak during the dark period for the monocistronic transcript of psbC (encoding one of the PS II core Chl a antenna proteins) suggested the involvement of post-transcriptional regulation. Differential expression of the external antenna and core genes may constitute a mechanism of regulation of the antenna size to cope with the excess photon fluxes that Prochlorococcus cells experience in the upper layer of oceans around midday. The 5' ends of all transcripts were mapped, and a conserved motif, 5'-TTGATGA-3', was identified within the putative psbA and pcbA promoters.

Ionic and osmotic effects of NaCl-induced inactivation of photosystems I and II in Synechococcus sp

We report here that osmotic effects and ionic effects are both involved in the NaCl-induced inactivation of the photosynthetic machinery in the cyanobacterium Synechococcus sp. PCC 7942. Incubation of the cyanobacterial cells in 0.5 M NaCl induced a rapid and reversible decline and subsequent slow and irreversible loss of the oxygen-evolving activity of photosystem (PS) II and the electron transport activity of PSI. An Na(+)-channel blocker protected both PSII and PSI against the slow, but not the rapid, inactivation. The rapid decline resembled the effect of 1.0 M sorbitol. The presence of both an Na(+)-channel blocker and a water-channel blocker protected PSI and PSII against the short- and long-term effects of NaCl. Salt stress also decreased cytoplasmic volume and this effect was enhanced by the Na(+)-channel blocker. Our observations suggested that NaCl had both osmotic and ionic effects. The osmotic effect decreased the amount of water in the cytosol, rapidly increasing the intracellular concentration of salts. The ionic effect was caused by an influx of Na(+) ions through potassium/Na(+) channels that also increased concentrations of salts in the cytosol and irreversibly inactivated PSI and PSII.

Role of light in the response of PSII photochemistry to salt stress in the cyanobacterium Spirulina platensis

The role of light in the effect of salt stress on PSII photochemistry in the cyanobacterium Spirulina platensis grown at 50 micromol m(-2) s(-1) was investigated. The time-course of changes in PSII photochemistry in response to high salinity (0.8 M NaCl) incubated in the dark and at 30, 50 and 100 micromol m(-2) s(-1) was composed of two phases. The first phase, which was independent of light, was characterized by a rapid decrease (20-50%) in the maximal efficiency of PSII photochemistry (F:(v)/F:(m)), the efficiency of excitation energy capture by open PSII reaction centres (F(1)(v)/F(1)(m)), photochemical quenching (q(P)), and the quantum yield of PSII electron transport (Phi(PSII)) in the first 15 min, followed by a recovery of up to about 86-92% of their initial levels after 4 h of incubation. The second phase took place after 4 h, in which a further decline in the above parameters occurred only in the light but not in the dark, reaching levels as low as 32-56% of their initial levels after 12 h. Moreover, the higher incubation light intensity, the greater the decrease in the above parameters. At the same time, Q(B)-non-reducing PSII reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first phase, but increased again in the light in the second phase. Photosynthetic oxygen evolution activity decreased sharply by 70% in the first 5 min, and then kept largely constant until 12 h. The changes in oxygen evolution activity were independent of light intensity during both phases.

Inactivation of photosystems I and II in response to osmotic stress in Synechococcus. Contribution of water channels

The effects of osmotic stress due to sorbitol on the photosynthetic machinery were investigated in the cyanobacterium Synechococcus R-2. Incubation of cells in 1.0 M sorbitol inactivated photosystems I and II and decreased the intracellular solute space by 50%. These effects of sorbitol were reversible: Photosynthetic activity and cytoplasmic volume returned to the original values after removal of the osmotic stress. A blocker of water channels prevented the osmotic-stress-induced inactivation and shrinkage of the intracellular space. It also prevented the recovery of photosynthetic activity and cytoplasmic volume when applied just before release from osmotic stress. Inhibition of protein synthesis by lincomycin had no significant effects on the inactivation and recovery processes, an observation that suggests that protein synthesis was not involved in these processes. Our results suggest that osmotic stress decreased the amount of water in the cytoplasm via the efflux of water through water channels (aquaporins), with resultant increases in intracellular concentrations of ions and a decrease in photosynthetic activity.

Prochlorococcus, a marine photosynthetic prokaryote of global significance

The minute photosynthetic prokaryote Prochlorococcus, which was discovered about 10 years ago, has proven exceptional from several standpoints. Its tiny size (0.5 to 0.7 microm in diameter) makes it the smallest known photosynthetic organism. Its ubiquity within the 40 degrees S to 40 degrees N latitudinal band of oceans and its occurrence at high density from the surface down to depths of 200 m make it presumably the most abundant photosynthetic organism on Earth. Prochlorococcus typically divides once a day in the subsurface layer of oligotrophic areas, where it dominates the photosynthetic biomass. It also possesses a remarkable pigment complement which includes divinyl derivatives of chlorophyll a (Chl a) and Chl b, the so-called Chl a2 and Chl b2, and, in some strains, small amounts of a new type of phycoerythrin. Phylogenetically, Prochlorococcus has also proven fascinating. Recent studies suggest that it evolved from an ancestral cyanobacterium by reducing its cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system as a replacement for large phycobilisomes. Environmental constraints clearly played a predominant role in Prochlorococcus evolution. Its tiny size is an advantage for its adaptation to nutrient-deprived environments. Furthermore, genetically distinct ecotypes, with different antenna systems and ecophysiological characteristics, are present at depth and in surface waters. This vertical species variation has allowed Prochlorococcus to adapt to the natural light gradient occurring in the upper layer of oceans. The present review critically assesses the basic knowledge acquired about Prochlorococcus both in the ocean and in the laboratory.

Expression of the psbA gene in the marine oxyphotobacteria Prochlorococcus spp

The oxygenic photosynthetic prokaryotes Prochlorococcus marinus SS120 (CCMP1375) and Prochlorococcus sp. MED4 (CCMP 1378) were previously shown to exhibit different pigmentation and ecophysiological characteristics. The former strain has a much lower divinyl-Chl a to b ratio and is adapted to lower photon flux densities than the latter. In contrast to the cyanobacteria examined so far, both strains possess only one copy of the psbA gene, encoding the D1 protein of photosystem II core. In acclimated steady-state cultures, psbA transcript levels were always higher at high irradiances in both strains. Upon a shift from low to high light, the psbA transcript levels increased in both strains but more quickly in MED4 than in SS120. They decreased during the opposite shift. Iron-starved MED4 cells overexpressed psbA at all assayed irradiances, suggesting that this species, representative of populations from naturally iron-depleted oceanic areas, may have developed a particular compensation mechanism. The similar effects of DCMU and DBMIB on the expression of psbA suggest that light regulation of psbA in Prochlorococcus may be mediated by the electron transport chain. The energy state of cells could, however, also be involved in this regulation, since cultures of both strains subjected to darkness showed psbA levels significantly lower when glucose was added.

Chlorophyll fluorescence analysis of cyanobacterial photosynthesis and acclimation

Cyanobacteria are ecologically important photosynthetic prokaryotes that also serve as popular model organisms for studies of photosynthesis and gene regulation. Both molecular and ecological studies of cyanobacteria benefit from real-time information on photosynthesis and acclimation. Monitoring in vivo chlorophyll fluorescence can provide noninvasive measures of photosynthetic physiology in a wide range of cyanobacteria and cyanolichens and requires only small samples. Cyanobacterial fluorescence patterns are distinct from those of plants, because of key structural and functional properties of cyanobacteria. These include significant fluorescence emission from the light-harvesting phycobiliproteins; large and rapid changes in fluorescence yield (state transitions) which depend on metabolic and environmental conditions; and flexible, overlapping respiratory and photosynthetic electron transport chains. The fluorescence parameters FV/FM, FV'/FM',qp,qN, NPQ, and phiPS II were originally developed to extract information from the fluorescence signals of higher plants. In this review, we consider how the special properties of cyanobacteria can be accommodated and used to extract biologically useful information from cyanobacterial in vivo chlorophyll fluorescence signals. We describe how the pattern of fluorescence yield versus light intensity can be used to predict the acclimated light level for a cyanobacterial population, giving information valuable for both laboratory and field studies of acclimation processes. The size of the change in fluorescence yield during dark-to-light transitions can provide information on respiration and the iron status of the cyanobacteria. Finally, fluorescence parameters can be used to estimate the electron transport rate at the acclimated growth light intensity.

Effects of ultraviolet-B radiation on photosystem II of the cyanobacterium Synechocystis sp. PCC 6083

The effects of ultraviolet-B radiation (280-320 nm) on photosystem II of Synechocystis sp. PCC 6303 were investigated at the functional and structural levels. Loss of oxygen-evolving and electron-transport activity, measured by various techniques including Clark electrode polarography, fluorescence induction and fluorescence relaxation after a single turnover flash, are discussed in terms of two types of damage caused by ultraviolet-B radiation: (a) depletion of the plastoquinone pool; (b) perturbation and degradation of the D1 protein, with cleavage in the second transmembrane segment. These findings are in full agreement with those obtained, both in vivo and in vitro for higher plants for which a donor-side mechanism involving the water-splitting Mn cluster has been proposed for the main cleavage of the D1 protein. At the structural level, complete disruption of the photosystem II core is documented as a consequence of (or in parallel with) degradation of the D1 protein. From this point of view, ultraviolet-B-induced photoinhibition is unlike the visible-induced type and less susceptible to repair by synthesis and reinsertion of new D1 protein.

Complementary chromatic adaptation alters photosynthetic strategies in the cyanobacterium Calothrix

The cyanobacterium Calothrix sp. strain PCC 7601 drastically changes phycobiliprotein composition and colour in response to light quality, through complementary chromatic adaptation (CCA). Red light promotes phycocyanin-II and inhibits phycoerythrin synthesis, while green light has the opposite effect, through changes in transcription regulated by a putative green/red photoreceptor(s). The effects of CCA on photosynthesis were characterized by measuring oxygen evolution and chlorophyll fluorescence parameters. Cells fully acclimated to either red or green light achieve a similar photosynthetic quantum yield of oxygen evolution (light-use efficiency). Shifting acclimated cells from green to red or from red to green light caused similar 40% drops in photosynthetic quantum yield. Therefore, full CCA significantly increases light use efficiency, which is of great importance under light-limited growth. Cells growing under red light are in state I, with very low PS II to PS I energy transfer, since red light is absorbed both by phycocyanin in the phycobilisome/PS II supracomplex and by PS I chlorophyll. Cells growing under green light are in state II, with high transfer of excitation energy from the phycobilisome/PS II supracomplex to PS I. This transfer allows green light captured by phycoerythrin to ultimately drive both PS I and PS II photochemistry.

Two forms of the Photosystem II D1 protein alter energy dissipation and state transitions in the cyanobacterium Synechococcus sp. PCC 7942

Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. D1: 1 predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only D1:1 have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing D1:2. We show that when dark-adapted or under low to moderate light, cells with D1:1 have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with D1:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697-698 nm in cells containing D1:1 than in cells with D1:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing D1:1 than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D1:1 confers capacity for extreme State II which may be of benefit under low and variable light.