Photoinhibition and repair in Dunaliella salina acclimated to different growth irradiances

Deciphering the β-carotene hyperaccumulation in Dunaliella by the comprehensive analysis of Dunaliella salina and Dunaliella tertiolecta under high light conditions

The green microalga Dunaliella salina hyperaccumulates β-carotene in the chloroplast, which turns its cells orange. This does not occur in the sister species Dunaliella tertiolecta. However, the molecular mechanisms of β-carotene hyperaccumulation were still unclear. Here, we discovered the reasons for β-carotene hyperaccumulation by comparing the morphology, physiology, genome, and transcriptome between the carotenogenic D. salina and the noncarotenogenic D. tertiolecta after transfer to high light. The differences in photosynthetic capacity, cell growth, and the concentration of stored carbon suggest that these species regulate the supply and utilization of carbon differently. The number of β-carotene-containing plastid lipid globules increased in both species, but much faster and to a greater extent in D. salina than in D. tertiolecta. Consistent with the accumulation of plastid lipid globules, the expression of the methyl-erythritol-phosphate and carotenoid biosynthetic pathways increased only in D. salina, which explains the de novo synthesis of β-carotene. In D. salina, the concomitantly upregulated expression of the carotene globule proteins suggests that hyperaccumulation of β-carotene also requires a simultaneous increase in its sink capacity. Based on genomic analysis, we propose that D. salina has genetic advantages for routing carbon from growth to carotenoid metabolism.

Scale-up of a Fibonacci-Type Photobioreactor for the Production of Dunaliella salina

In this work, the previously proposed Fibonacci-type photobioreactor is scaled up and evaluated to produce Dunaliella salina. First, the composition of the culture medium was optimized to achieve maximal productivity. Next, the Fibonacci-type reactor was scaled up to 1250 L maintaining high solar radiation interception capacity of this type of reactor. Finally, the performance of the reactor for the production of green cells of Dunaliella salina at the environmental conditions prevailing in the Atacama Desert was evaluated. Data demonstrated that the proposed photobioreactor allows the temperature, pH and dissolved oxygen concentration to be maintained within the optimal ranges recommended for the selected strain. Both better exposure to solar radiation and photonic flow dilution avoids the use of cooling systems to prevent overheating under outdoor conditions. The system allows up to 60% more solar radiation to be intercepted than does the horizontal surface, likewise, allowing to maintain the pH efficiently through CO₂ injection and to keep the dissolved oxygen concentration in acceptable ranges, thanks to its adequate mass transfer capacity. The biomass concentration reached up to 0.96 g L^-1, three times higher than that obtained in a raceway reactor under the same environmental conditions, whereas productivity was up to 0.12 g L^-1 day (2.41 g m^-2 day). Maximum specific outdoor growth rates reached up to 0.17 day^-1. Undoubtedly, this technology scaled up constitutes a new type of photobioreactor for use at the industrial scale since it is capable of maximizing biomass productivity under high light conditions.

Strain specific differences in rates of Photosystem II repair in picocyanobacteria correlate to differences in FtsH protein levels and isoform expression patterns

Picocyanobacteria are the numerically dominant photoautotrophs of the oligotrophic regions of Earth’s oceans. These organisms are characterized by their small size and highly reduced genomes. Strains partition to different light intensity and nutrient level niches, with differing photosynthetic apparatus stoichiometry, light harvesting machinery and susceptibility to photoinactivation. In this study, we grew three strains of picocyanobacteria: the low light, high nutrient strain Prochlorococcus marinus MIT 9313; the high light, low nutrient Prochlorococcus marinus MED 4; and the high light, high nutrient marine Synechococcus strain WH 8102; under low and high growth light levels. We then performed matched photophysiology, protein and transcript analyses. The strains differ significantly in their rates of Photosystem II repair under high light and in their capacity to remove the PsbA protein as the first step in the Photosystem II repair process. Notably, all strains remove the PsbD subunit at the same rate that they remove PsbA. When grown under low light, MIT 9313 loses active Photosystem II quickly when shifted to high light, but has no measurable capacity to remove PsbA. MED 4 and WH 8102 show less rapid loss of Photosystem II and considerable capacity to remove PsbA. MIT 9313 has less of the FtsH protease thought to be responsible for the removal of PsbA in other cyanobacteria. Furthermore, by transcript analysis the predominant FtsH isoform expressed in MIT 9313 is homologous to the FtsH 4 isoform characterized in the model strain Synechocystis PCC 6803, rather than the FtsH 2 and 3 isoforms thought to be responsible for PsbA degradation. MED 4 on the other hand shows high light inducible expression of the isoforms homologous to FtsH 2 and 3, consistent with its faster rate of PsbA removal. MIT 9313 has adapted to its low light environment by diverting resources away from Photosystem II content and repair.

An LED-based multi-actinic illumination system for the high throughput study of photosynthetic light responses

The responses of photosynthetic organisms to light stress are of interest for both fundamental and applied research. Functional traits related to the photoinhibition, the light-induced loss of photosynthetic efficiency, are particularly interesting as this process is a key limiting factor of photosynthetic productivity in algae and plants. The quantitative characterization of light responses is often time-consuming and calls for cost-effective high throughput approaches that enable the fast screening of multiple samples. Here we present a novel illumination system based on the concept of ‘multi-actinic imaging’ of in vivo chlorophyll fluorescence. The system is based on the combination of an array of individually addressable low power RGBW LEDs and custom-designed well plates, allowing for the independent illumination of 64 samples through the digital manipulation of both exposure duration and light intensity. The illumination system is inexpensive and easily fabricated, based on open source electronics, off-the-shelf components, and 3D-printed parts, and is optimized for imaging of chlorophyll fluorescence. The high-throughput potential of the system is illustrated by assessing the functional diversity in light responses of marine macroalgal species, through the fast and simultaneous determination of kinetic parameters characterizing the response to light stress of multiple samples. Although the presented illumination system was primarily designed for the measurement of phenotypic traits related to photosynthetic activity and photoinhibition, it can be potentially used for a number of alternative applications, including the measurement of chloroplast phototaxis and action spectra, or as the basis for microphotobioreactors.

A chlorophyll fluorescence-based method for the integrated characterization of the photophysiological response to light stress

This work introduces a new experimental method for the comprehensive description of the physiological responses to light of photosynthetic organisms. It allows the integration in a single experiment of the main established manipulative chlorophyll fluorescence-based protocols. It enables the integrated characterization of the photophysiology of samples regarding photoacclimation state (generating non-sequential light-response curves of effective PSII quantum yield, electron transport rate or non-photochemical quenching), photoprotection capacity (running light stress-recovery experiments, quantifying non-photochemical quenching components) and the operation of photoinactivation and photorepair processes (measuring rate constants of photoinactivation and repair for different light levels and the relative quantum yield of photoinactivation). The new method is based on a previously introduced technique, combining the illumination of a set of replicated samples with spatially separated actinic light beams of different intensity, and the simultaneous measurement of the fluorescence emitted by all samples using an imaging fluorometer. The main novelty described here is the independent manipulation of light intensity and duration of exposure for each sample, and the control of the cumulative light dose applied. The results demonstrate the proof of concept for the method, by comparing the responses of cultures of Chlorella vulgaris acclimated to low and high light regimes, highlighting the mapping of light stress responses over a wide range of light intensity and exposure conditions, and the rapid generation of paired light-response curves of photoinactivation and repair rate constants. This approach represents a chlorophyll fluorescence 'protocol of everything', contributing towards the high throughput characterization of the photophysiology of photosynthetic organisms.

On the use of the serial dilution culture method to enumerate viable phytoplankton in natural communities of plankton subjected to ballast water treatment

Discharge standards for ballast water treatment (BWT) systems are based on concentrations of living cells, for example, as determined with vital stains. Ultraviolet radiation (UV) stops the reproduction of microorganisms without killing them outright; they are living, but not viable, and ecologically as good as dead. Consequently, UV-treated discharge can be compliant with the intent of regulation while failing a live/dead test. An alternative evaluation of BWT can be proposed based on the assessment of viable, rather than living, cells in discharge water. In principle, the serial dilution culture-most probable number (SDC-MPN) method provides the appropriate measure for phytoplankton. But, the method has been criticized, particularly because it is thought that many phytoplankton species cannot be cultured. A review of the literature shows that although SDC-MPN has been used for more than 50 years-generally to identify and count phytoplankton species that cannot be preserved-its application to enumerate total viable phytoplankton seems to be new, putting past criticisms of the method in a different light. Importantly, viable cells need to grow only enough to be detected, not to be brought into sustained culture, and competition between species in a dilution tube is irrelevant as long as the winner is detectable. Thorough consideration of sources of error leads to recommendations for minimizing and quantifying uncertainties by optimizing growth conditions and conducting systematic comparisons. We conclude that with careful evaluation, SDC-MPN is potentially an effective method for assessing the viability of phytoplankton after BWT.

The action spectrum of Photosystem II photoinactivation in visible light

Photosynthesis is always accompanied by light induced damage to the Photosystem II (PSII) which is compensated by its subsequent repair. Photoinhibition of PSII is a complex process, balancing between photoinactivation, protective and repair mechanisms. Current understanding of photoinactivation is limited with competing hypotheses where the photosensitiser is either photosynthetic pigments or the Mn4CaO5 cluster itself, with little consensus on the mechanisms and consequences of PSII photoinactivation. The mechanism of photoinactivation should be reflected in the action spectrum of PSII photoinactivation, but there is a great diversity of the action spectra reported thus far. The only consensus is that PSII photoinactivation is greatest in the UV region of the electromagnetic spectrum. In this review, the authors revisit the methods, technical constraints and the different action spectra of PSII photoinactivation reported to date and compare them against the diverse mechanisms proposed. Upon critical examination of the reported action spectra, a hybrid mechanism of photoinactivation, sensitised by both photosynthetic pigments and the Mn4CaO5 appears to be the most plausible rationalisation.

Light stress and photoprotection in Chlamydomonas reinhardtii

Plants and algae require light for photosynthesis, but absorption of too much light can lead to photo-oxidative damage to the photosynthetic apparatus and sustained decreases in the efficiency and rate of photosynthesis (photoinhibition). Light stress can adversely affect growth and viability, necessitating that photosynthetic organisms acclimate to different environmental conditions in order to alleviate the detrimental effects of excess light. The model unicellular green alga, Chlamydomonas reinhardtii, employs diverse strategies of regulation and photoprotection to avoid, minimize, and repair photo-oxidative damage in stressful light conditions, allowing for acclimation to different and changing environments.

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton(1)

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)-replete growth rate increased monotonically with light until it reached a threshold intensity (~200 μmol photons · m(-2)  · s(-1) ). For Chlorella sp., the nitrogen quota (pg · μm(-3) ) was greatest below this threshold, beyond which it was reduced by the effect of N-stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons(-1) ) over the range of light and N-gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g(-1) ) increased as a function of light and N-stress, while C:N (mol · mol(-1) ) and relative neutral lipid:C (rel. lipid · g(-1) ) were most strongly influenced by N-stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N-availability) and energy gradients influenced C-allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.

Photosystem II photoinactivation, repair, and protection in marine centric diatoms

Diatoms are important contributors to aquatic primary production, and can dominate phytoplankton communities under variable light regimes. We grew two marine diatoms, the small Thalassiosira pseudonana and the large Coscinodiscus radiatus, across a range of temperatures and treated them with a light challenge to understand their exploitation of variable light environments. In the smaller T. pseudonana, photosystem II (PSII) photoinactivation outran the clearance of PSII protein subunits, particularly in cells grown at sub- or supraoptimal temperatures. In turn the absorption cross section serving PSII photochemistry was down-regulated in T. pseudonana through induction of a sustained phase of nonphotochemical quenching that relaxed only slowly over 30 min of subsequent low-light incubation. In contrast, in the larger diatom C. radiatus, PSII subunit turnover was sufficient to counteract a lower intrinsic susceptibility to photoinactivation, and C. radiatus thus did not need to induce sustained nonphotochemical quenching under the high-light treatment. T. pseudonana thus incurs an opportunity cost of sustained photosynthetic down-regulation after the end of an upward light shift, whereas the larger C. radiatus can maintain a balanced PSII repair cycle under comparable conditions.

Modeling the dynamic modulation of light energy in photosynthetic algae

An integrated cell-based dynamic mathematical model that take into account the role of the photon absorbing process, the partition of excitation energy, and the photoinactivation and repair of photosynthetic units, under variable light and dissolved inorganic carbon (DIC) availability is proposed. The modeling of the photon energy absorption and the energy dissipation is based on the photoadaptive changes of the underlying mechanisms. The partition of the excitation energy is based on the relative availability of light and DIC to the cell. The modeling of the photoinactivation process is based on the common aspect that it occurs under any light intensity and the modeling of the repair process is based on the evidence that it is controlled by chloroplast and nuclear-encoded enzymes. The present model links the absorption of photons and the partitioning of excitation energy to the linear electron flow and other quenchers with chlorophyll fluorescence emission parameters, and the number of the functional photosynthetic units with the photosynthetic oxygen production rate. The energy allocation to the LEF increases as DIC availability increases and/or light intensity decreases. The rate of rejected energy increases with light intensity and with DIC availability. The resulting rate coefficient of photoinactivation increases as light intensity and/or as DIC concentration increases. We test the model against chlorophyll fluorescence induction and photosynthetic oxygen production rate measurements, obtained from cultures of the unicellular green alga Scenedesmus obliquus, and find a very close quantitative and qualitative correspondence between predictions and data.

Okadaic acid inhibits cell growth and photosynthetic electron transport in the alga Dunaliella tertiolecta

Okadaic acid (OA), which is produced by several dinoflagellate species, is a phycotoxin known to induce a decrease of biomass production in phytoplankton. However, the mechanisms of OA cytotoxicity are still unknown in microalgae. In this study, we exposed the green microalga Dunaliella tertiolecta to OA concentrations of 0.05 to 0.5 μM in order to evaluate its effects on cell division, reactive oxygen species production and photosynthetic electron transport. After 72 h of treatment under continuous illumination, OA concentrations higher than 0.10 μM decreased culture cell density, induced oxidative stress and inhibited photosystem II electron transport capacity. OA effect in D. tertiolecta was strongly light dependent since no oxidative stress was observed when D. tertiolecta was exposed to OA in the dark. In the absence of light, the effect of OA on culture cell density and photosystem II activity was also significantly reduced. Therefore, light appears to have a significant role in the toxicity of OA in microalgae. Our results indicate that the site of OA interaction on photosynthetic electron transport is likely to be at the level of the plastoquinone pool, which can lead to photo-oxidative stress when light absorbed by the light-harvesting complex of photosystem II cannot be dissipated via photochemical pathways. These findings allowed for a better understanding of the mechanisms of OA toxicity in microalgae.

He-Ne laser-induced improvement in biochemical, physiological, growth and yield characteristics in sunflower (Helianthus annuus L.)

The water-soaked seeds of sunflower were exposed to low power continuous wave He-Ne laser irradiation of energies 0, 100, 300 and 500 mJ to evaluate the effect on various biochemical, physiological, growth and yield parameters of sunflower. The experiments which consisted of four replicates arranged in a completely randomized design (CRD) were carried out under the greenhouse conditions. The physiological attributes like, photosynthetic rate (A), transpiration rate (E), intrinsic CO(2) concentration (C(i) ), stomatal conductance (g(s)), chlorophyll a and b contents, relative membrane permeability and leaf water (ψ(w)), osmotic (ψ(s)) and turgor (ψ(p)) potentials, relative water contents and leaf area increased significantly as compared to control due to He-Ne treatment of seeds. The activities of superoxide dismutase, peroxidase and catalases and contents of total soluble proteins, malondialdehyde, proline and leaf total phenolic also increased due to laser treatment. Significant increase in growth parameters of sunflower like shoot fresh and dry masses, root fresh and dry masses, root and shoot lengths, number of leaves per plant and stem diameter has also been observed. The contents of K, Ca and Mg in shoot and root were also increased and an overall increase of up to 28.12% was observed due to laser treatment.

Distinctive photosystem II photoinactivation and protein dynamics in marine diatoms

Diatoms host chlorophyll a/c chloroplasts distinct from green chloroplasts. Diatoms now dominate the eukaryotic oceanic phytoplankton, in part through their exploitation of environments with variable light. We grew marine diatoms across a range of temperatures and then analyzed their PSII function and subunit turnover during an increase in light to mimic an upward mixing event. The small diatom Thalassiosira pseudonana initially responds to increased photoinactivation under blue or white light with rapid acceleration of the photosystem II (PSII) repair cycle. Increased red light provoked only modest PSII photoinactivation but triggered a rapid clearance of a subpool of PsbA. Furthermore, PsbD and PsbB content was greater than PsbA content, indicating a large pool of partly assembled PSII repair cycle intermediates lacking PsbA. The initial replacement rates for PsbD (D2) were, surprisingly, comparable to or higher than those for PsbA (D1), and even the supposedly stable PsbB (CP47) dropped rapidly upon the light shift, showing a novel aspect of rapid protein subunit turnover in the PSII repair cycle in small diatoms. Under sustained high light, T. pseudonana induces sustained nonphotochemical quenching, which correlates with stabilization of PSII function and the PsbA pool. The larger diatom Coscinodiscus radiatus showed generally similar responses but had a smaller allocation of PSII complexes relative to total protein content, with nearly equal stiochiometries of PsbA and PsbD subunits. Fast turnover of multiple PSII subunits, pools of PSII repair cycle intermediates, and photoprotective induction of nonphotochemical quenching are important interacting factors, particularly for small diatoms, to withstand and exploit high, fluctuating light.

Mechanism of REP27 protein action in the D1 protein turnover and photosystem II repair from photodamage

The function of the REP27 protein (GenBank accession no. EF127650) in the photosystem II (PSII) repair process was elucidated. REP27 is a nucleus-encoded and chloroplast-targeted protein containing two tetratricopeptide repeat (TPR) motifs, two putative transmembrane domains, and an extended carboxyl (C)-terminal region. Cell fractionation and western-blot analysis localized the REP27 protein in the Chlamydomonas reinhardtii chloroplast thylakoids. A folding model for REP27 suggested chloroplast stroma localization for amino- and C-terminal regions as well as the two TPRs. A REP27 gene knockout strain of Chlamydomonas, termed the rep27 mutant, was employed for complementation studies. The rep27 mutant was aberrant in the PSII-repair process and had substantially lower than wild-type levels of D1 protein. Truncated REP27 cDNA constructs were made for complementation of rep27, whereby TPR1, TPR2, TPR1+TPR2, or the C-terminal domains were deleted. rep27-complemented strains minus the TPR motifs showed elevated levels of D1 in thylakoids, comparable to those in the wild type, but the PSII photochemical efficiency of these strains was not restored, suggesting that the functionality of the PSII reaction center could not be recovered in the absence of the TPR motifs. It is suggested that TPR motifs play a role in the functional activation of the newly integrated D1 protein in the PSII reaction center. rep27-complemented strains missing the C-terminal domain showed low levels of D1 protein in thylakoids as well as low PSII photochemical efficiency, comparable to those in the rep27 mutant. Therefore, the C-terminal domain is needed for a de novo biosynthesis and/or assembly of D1 in the photodamaged PSII template. We conclude that REP27 plays a dual role in the regulation of D1 protein turnover by facilitating cotranslational biosynthesis insertion (C-terminal domain) and activation (TPR motifs) of the nascent D1 during the PSII repair process.

Analysis of non-photochemical energy dissipating processes in wild type Dunaliella salina (green algae) and in zea1, a mutant constitutively accumulating zeaxanthin

Generally there is a correlation between the amount of zeaxanthin accumulated within the chloroplast of oxygenic photosynthetic organisms and the degree of non-photochemical quenching (NPQ). Although constitutive accumulation of zeaxanthin can help protect plants from photo-oxidative stress, organisms with such a phenotype have been reported to have altered rates of NPQ induction. In this study, basic fluorescence principles and the routinely used NPQ analysis technique were employed to investigate excitation energy quenching in the unicellular green alga Dunaliella salina, in both wild type (WT) and a mutant, zea1, constitutively accumulating zeaxanthin under all growth conditions. The results showed that, in D. salina, NPQ is a multi-component process consisting of energy- or DeltapH-dependent quenching (qE), state-transition quenching (qT), and photoinhibition quenching (qI). Despite the vast difference in the amount of zeaxanthin in WT and the zea1 mutant grown under low light, the overall kinetics of NPQ induction were almost the same. Only a slight difference in the relative contribution of each quenching component could be detected. Of all the NPQ subcomponents, qE seemed to be the primary NPQ operating in this alga in response to short-term exposure to excessive irradiance. Whenever qE could not operate, i.e., in the presence of nigericin, or under conditions where the level of photon flux is beyond its quenching power, qT and/or qI could adequately compensate its photoprotective function.

PHOTOINHIBITION OF PSII IN EMILIANIA HUXLEYI (HAPTOPHYTA) UNDER HIGH LIGHT STRESS: THE ROLES OF PHOTOACCLIMATION, PHOTOPROTECTION, AND PHOTOREPAIR(1)

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low-light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in Fv /Fm , than high-light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de-epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross-section (σPSII ) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σPSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a "lake" organization of the light-harvesting complex.

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.

A simple chlorophyll fluorescence parameter that correlates with the rate coefficient of photoinactivation of photosystem II

A method of partitioning the energy in a mixed population of active and photoinactivated Photosystem II (PS II) complexes based on chlorophyll fluorescence measurements is presented. There are four energy fluxes, each with its quantum efficiency: a flux associated with photochemical electron flow in active PS II reaction centres (JPS II), thermal dissipation in photoinactivated, non-functional PS IIs (JNF), light-regulated thermal dissipation in active PS IIs (JNPQ) and a combined flux of fluorescence and constitutive, light-independent thermal dissipation (Jf,D). The four quantum efficiencies add up to 1.0, without the need to introduce an 'excess' term E, which in other studies has been claimed to be linearly correlated with the rate coefficient of photoinactivation of PS II (kpi). We examined the correlation of kpi with various fluxes, and found that the combined flux (JNPQ + Jf,D= Jpi) is as well correlated with kpi as is E. This combined flux arises from Fs/Fm ', the ratio of steady-state to maximum fluorescence during illumination, which represents the quantum efficiency of combined non-photochemical dissipation pathways in active PS IIs. Since Fs/Fm ' or its equivalent, Jpi, is a likely source of events leading to photoinactivation of PS II, we conclude that Fs/Fm ' is a simple predictor of kpi.

Isolation and characterization of a xanthophyll-rich fraction from the thylakoid membrane of Dunaliella salina(green algae)

Long-term acclimation to irradiance stress (HL) of the green alga Dunaliella salina Teod. (UTEX 1644) entails substantial accumulation of zeaxanthin along with a lowering in the relative amount of other pigments, including chlorophylls and several carotenoids. This phenomenon was investigated with wild type and the zea1 mutant of D. salina, grown under conditions of low irradiance (LL), or upon acclimation to irradiance stress (HL). In the wild type, the zeaxanthin to chlorophyll (Zea/Chl)(mol : mol) ratio was as low as 0.009 : 1 under LL and as high as 0.8 : 1 under HL conditions. In the zea1 mutant, which constitutively accumulates zeaxanthin and lacks antheraxanthin, violaxanthin and neoxanthin, the Zea/Chl ratio was 0.15 : 1 in LL and 0.57 : 1 in HL. The divergent Zea/Chl ratios were reflected in the coloration of the cells, which were green under LL and yellow under HL. In LL-grown cells, all carotenoids occurred in structural association with the Chl-protein complexes. This was clearly not the case in the HL-acclimated cells. A beta-carotene-rich fraction occurred as loosely bound to the thylakoid membrane and was readily isolated by flotation following mechanical disruption of D. salina. A zeaxanthin-rich fraction was specifically isolated, upon mild surfactant treatment and differential centrifugation, from the thylakoid membrane of either HL wild type or HL-zea1 mutant. Such differential extraction of beta-carotene and Zea, and their separation from the Chl-proteins, could not be obtained from the LL-grown wild type, although small amounts of Zea could still be differentially extracted from the LL-grown zea1 strain. It is concluded that, in LL-grown D. salina, xanthophylls (including most of Zea in the zea1 strain) are structurally associated with and stabilized by the Chl-proteins in the thylakoid membrane. Under HL-growth conditions, however, zeaxanthin appears to be embedded in the lipid bilayer, or in a domain of the chloroplast thylakoids that can easily be separated from the Chl-proteins upon mild surfactant treatment. In conclusion, this work provides biochemical evidence for the domain localization of accumulated zeaxanthin under irradiance-stress conditions in green algae, and establishes protocols for the differential extraction of this high-value pigment from the green alga D. salina.

Role of the reversible xanthophyll cycle in the photosystem II damage and repair cycle in Dunaliella salina

The Dunaliella salina photosynthetic apparatus organization and function was investigated in wild type (WT) and a mutant (zea1) lacking all beta,beta-epoxycarotenoids derived from zeaxanthin (Z). The zea1 mutant lacked antheraxanthin, violaxanthin, and neoxanthin from its thylakoid membranes but constitutively accumulated Z instead. It also lacked the so-called xanthophyll cycle, which, upon irradiance stress, reversibly converts violaxanthin to Z via a de-epoxidation reaction. Despite the pronounced difference observed in the composition of beta,beta-epoxycarotenoids between WT and zea1, no discernible difference could be observed between the two strains in terms of growth, photosynthesis, organization of the photosynthetic apparatus, photo-acclimation, sensitivity to photodamage, or recovery from photo-inhibition. WT and zea1 were probed for the above parameters over a broad range of growth irradiance and upon light shift experiments (low light to high light shift and vice versa). A constitutive accumulation of Z in the zea1 strain did not affect the acclimation of the photosynthetic apparatus to irradiance, as evidenced by indistinguishable irradiance-dependent adjustments in the chlorophyll antenna size and photosystem content of WT and zea1 strain. In addition, a constitutive accumulation of Z in the zea1 strain did not affect rates of photodamage or the recovery of the photosynthetic apparatus from photo-inhibition. However, Z in the WT accumulated in parallel with the accumulation of photodamaged PSII centers in the chloroplast thylakoids and decayed in tandem with a chloroplast recovery from photo-inhibition. These results suggest a role for Z in the protection of photodamaged and disassembled PSII reaction centers, apparently needed while PSII is in the process of degradation and replacement of the D1/32-kD reaction center protein.

A mechanistic model of photosynthesis in microalgae

A dynamic model of photosynthesis is developed, accounting for factors such as photoadaptation, photoinhibition, and the "flashing light effect." The model is shown to explain the reported photosynthesis-irradiance responses observed under various conditions (constant low light, constant intense irradiance, flashing light, diurnal variation in irradiance). As significant distinguishing features, the model assumes: (1) The stored photochemical energy is consumed in an enzyme-mediated process that obeys Michaelis-Menten kinetics; and (2) photoinhibition has a square-root dependence on irradiance. Earlier dynamic models of photosynthesis assumed a first-order dependence of photoinhibition on irradiance and different kinetics of consumption of the stored energy than used in this work. These earlier models could not explain the photosynthesis-irradiance behavior under the full range of irradiance scenarios-a shortcoming that is overcome in the model developed in this work.

A mechanistic model of algal photoinhibition induced by photodamage to photosystem-II

Photoinhibition is a central problem for the understanding of plasticity in photosynthesis vs. irradiance response. It effectively reduces the photosynthetic rate. In this contribution, we present a mechanistic model of algal photoinhibition induced by photodamage to photosystem-II. Photosystem-IIs (PSIIs) are assumed to exist in three states: open, closed and inhibited. Photosynthesis is closely associated with the transitions between the three states. The present model is defined by four parameters: effective cross section of PSII, number of PSIIs, turnover time of electron transfer chains and the ratio of rate constant of damage to that of repair of D1 proteins in PSIIs. It gives a photosynthetic response curve of phytoplankton to irradiance (PI-curve). Without photoinhibition, the PI-curve is in hyperbola with the first three parameters. The PI-curve with photoinhibition can be simplified to the same form as the hyperbola by replacing either the number of PSIIs with the number of functional PSIIs or the turnover time of electron transfer chains with the average turnover time.

Photosystem II damage and repair cycle in the green alga Dunaliella salina: involvement of a chloroplast-localized HSP70

The involvement of HSP70B in the photosystem II damage and repair process in Dunaliella salina was investigated. A full-length cDNA of the D. salina hsp70B gene was cloned and sequenced. Expression patterns of the hsp70B gene were investigated upon shifting a D. salina culture from low-light to high-light growth conditions, designed to significantly accelerate the rate of PSII photodamage. Northern blot analyses and nuclear run-on transcription assays revealed a significant but transient induction of hsp70B gene transcription, followed by a subsequent increase in HSP70B protein synthesis and accumulation. Mild detergent solubilization of photoinhibited thylakoid membranes, in which photodamaged PSII centers had accumulated, followed by native gel electrophoresis revealed the formation of a 320 kDa protein complex that contained, in addition to the HSP70B, the photodamaged but as yet undegraded D1 protein as well as D2 and CP47. Evidence suggested that the 320 kDa complex is a transiently forming PSII repair intermediate. Denaturing solubilization of the 320 kDa PSII repair intermediate by SDS-urea resulted in cross-linking of its polypeptide constituents, yielding a 160 kDa protein complex. The role of the HSP70B in the repair of photodamaged PSII centers, e.g. in stabilizing the disassembled PSII-core complex and in facilitating the D1 degradation and replacement process, is discussed.

Involvement of zeaxanthin and of the Cbr protein in the repair of photosystem II from photoinhibition in the green alga Dunaliella salina

A light-sensitive and chlorophyll (Chl)-deficient mutant of the green alga Dunaliella salina (dcd1) showed an amplified response to irradiance stress compared to the wild-type. The mutant was yellow-green under low light (100 micromol photons m(-2) s(-1)) and yellow under high irradiance (2000 micromol photons m(-2) s(-1)). The mutant had lower levels of Chl, lower levels of light harvesting complex II, and a smaller Chl antenna size. The mutant contained proportionately greater amounts of photodamaged photosystem (PS) II reaction centers in its thylakoid membranes, suggesting a greater susceptibility to photoinhibition. This phenotype was more pronounced under high than low irradiance. The Cbr protein, known to accumulate when D. salina is exposed to irradiance stress, was pronouncedly expressed in the mutant even under low irradiance. This positively correlated with a higher zeaxanthin content in the mutant. Cbr protein accumulation, xanthophyll cycle de-epoxidation state, and fraction of photodamaged PSII reaction centers in the thylakoid membrane showed a linear dependence on the chloroplast 'photoinhibition index', suggesting a cause-and-effect relationship between photoinhibition, Cbr protein accumulation and xanthophyll cycle de-epoxidation state. These results raised the possibility of zeaxanthin and Cbr involvement in the PSII repair process through photoprotection of the partially disassembled, and presumably vulnerable, PSII core complexes from potentially irreversible photooxidative bleaching.

Quality control of photosystem II

Photosystem II is particularly vulnerable to excess light. When illuminated with strong visible light, the reaction center D1 protein is damaged by reactive oxygen molecules or by endogenous cationic radicals generated by photochemical reactions, which is followed by proteolytic degradation of the damaged D1 protein. Homologs of prokaryotic proteases, such as ClpP, FtsH and DegP, have been identified in chloroplasts, and participation of the thylakoid-bound FtsH in the secondary degradation steps of the photodamaged D1 protein has been suggested. We found that cross-linking of the D1 protein with the D2 protein, the alpha-subunit of cytochrome b(559), and the antenna chlorophyll-binding protein CP43, occurs in parallel with the degradation of the D1 protein during the illumination of intact chloroplasts, thylakoids and photosystem II-enriched membranes. The cross-linked products are then digested by a stromal protease(s). These results indicate that the degradation of the photodamaged D1 protein proceeds through membrane-bound proteases and stromal proteases. Moreover, a 33-kDa subunit of oxygen-evolving complex (OEC), bound to the lumen side of photosystem II, regulates the formation of the cross-linked products of the D1 protein in donor-side photoinhibition of photosystem II. Thus, various proteases and protein components in different compartments in chloroplasts are implicated in the efficient turnover of the D1 protein, thus contributing to the control of the quality of photosystem II under light stress conditions.

Kinetics of photoacclimation in response to a shift to high light of the red alga Rhodella violacea adapted to low irradiance

The unicellular rhodophyte Rhodella violacea can adapt to a wide range of irradiances. To create a light stress, cells acclimated to low light were transferred to higher irradiance and the kinetics of various changes produced by the light shift were analyzed. The proton gradient generated by excess light led to a non-photochemical quenching of the chlorophyll fluorescence and some photoinhibition of photosystem II centers was also produced by the light stress. After the shift to higher irradiance, the mRNA levels of three chloroplast genes that encode phycoerythrin and phycocyanin apoproteins and heme oxygenase (the first enzyme specific to the bilin synthesis) were negatively regulated. A change in the amount of thylakoids and in the total pigment content of the cells occurred during light acclimation after a light stress. The change in the size of the phycobilisome was limited to dissapearance of the terminal phycoerythrin hexamers in some of the rods. The ability of R. violacea to photoacclimate depends both on large changes in thylakoid number and pigment content and on smaller changes in the antenna size of photosystem II.

Damage to photosystem II in symbiotic dinoflagellates: a determinant of coral bleaching

Coral bleaching has been defined as a general phenomenon, whereby reef corals turn visibly pale because of the loss of their symbiotic dinoflagellates and/or algal pigments during periods of exposure to elevated seawater temperatures. During the summer of 1997, seawater temperatures in the Florida Keys remained at or above 30 degrees C for more than 6 weeks, and extensive coral bleaching was observed. Bleached colonies of the dominant Caribbean reef-building species, Montastrea faveolata and Montastrea franksi, were sampled over a depth gradient from 1 to 17 m during this period of elevated temperature and contained lower densities of symbiotic dinoflagellates in deeper corals than seen in previous "nonbleaching" years. Fluorescence analysis by pulse-amplitude modulation fluorometry revealed severe damage to photosystem II (PSII) in remaining symbionts within the corals, with greater damage indicated at deeper depths. Dinoflagellates with the greatest loss in PSII activity also showed a significant decline in the D1 reaction center protein of PSII, as measured by immunoblot analysis. Laboratory experiments on the temperature-sensitive species Montastrea annularis, as well as temperature-sensitive and temperature-tolerant cultured symbiotic dinoflagellates, confirmed the temperature-dependent loss of PSII activity and concomitant decrease in D1 reaction center protein seen in symbionts collected from corals naturally bleached on the reef. In addition, variation in PSII repair was detected, indicating that perturbation of PSII protein turnover rates during photoinhibition at elevated temperatures underlies the physiological collapse of symbionts in corals susceptible to heat-induced bleaching.

PHOTOPROTECTION REVISITED:Genetic and Molecular Approaches

The involvement of excited and highly reactive intermediates in oxygenic photosynthesis poses unique problems for algae and plants in terms of potential oxidative damage to the photosynthetic apparatus. Photoprotective processes prevent or minimize generation of oxidizing molecules, scavenge reactive oxygen species efficiently, and repair damage that inevitably occurs. This review summarizes several photoprotective mechanisms operating within chloroplasts of plants and green algae. The recent use of genetic and molecular biological approaches is providing new insights into photoprotection, especially with respect to thermal dissipation of excess absorbed light energy, alternative electron transport pathways, chloroplast antioxidant systems, and repair of photosystem II.

Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage ?

Organisms that rely on oxygenic photosynthesis are subject to the effects of photo-oxidative damage, which impairs the function of photosystem-II (PSII). This phenomenon has the potential to lower rates of photosynthesis and diminish plant growth. Experimental evidence shows that the steady-state oxidation-reduction level of the primary quinone acceptor (QA) of PSII is the parameter that controls photodamage under a variety of physiological and environmental conditions. When QA is reduced, excitation energy at PSII is dissipated via a charge-recombination reaction. Such non-assimilatory dissipation of excitation generates singlet oxygen that might act to covalently modify the photochemical reaction center chlorophyll. Under steady-state photosynthesis conditions, the reduction state of QA increases linearly with irradiance, thereby causing a correspondingly linear increase in the probability of photodamage. It is concluded that there is a low probability that photodamage will occur when QA is oxidized and excitation energy is utilized in electron transport, and a significantly higher probability when QA is reduced in the course of steady-state photosynthesis.

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O2 in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 μmol m(-2) s(-1)). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O2 concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m(-2)). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O2 concentration. The results suggest that electron transport to O2, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O2 in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO2 alone.