Photosynthetic Unit Research Articles

Computer simulation of migration processes in the reaction center of PS2 during stationary laser excitation

We observed experimentally the nonlinear dependence of fluorescence of photosynthesizing organisms on the density of the flux of photons of exciting stationary radiation from an He−Cd-laser (λ=440 nm). A model of a photosynthetic unit was developed that takes account of the structure of the reacting center (RC) of PS2, the processes of migration of the energy of laser excitation from a light-collection antenna (LCA) to the reaction center with subsequent separation of charges at it, the processes of recombination of charges in the RC, and the reverse migration of energy from the RC to the LCA. Within the framework of this model we explained the experimentally observed nonlinear dependence of fluorescence on the density of the flux of exciting-radiation photons and evaluated the contribution of recombination fluorescence to the overall fluorescence of the LCA.

Chloroplast Acclimation in Leaves of Guzmania monostachia in Response to High Light

Acclimation of leaves to high light (HL; 650 micromol m(-2) s(-1)) was investigated in the long-lived epiphytic bromeliad Guzmania monostachia and compared with plants maintained under low light (LL; 50 micromol m(-2) s(-1)). Despite a 60% decrease in total chlorophyll in HL-grown plants, the chlorophyll a/b ratio remained stable. Additionally, chloroplasts from HL-grown plants had a much lower thylakoid content and reduced granal stacking. Immunofluorescent labeling techniques were used to quantify the level of photosynthetic polypeptides. HL-grown plants had 30% to 40% of the content observed in LL-grown plants for the light-harvesting complex associated with photosystems I and II, the 33-kD photosystem II polypeptide, and Rubisco. These results were verified using conventional biochemical techniques, which revealed a comparable 60% decrease in Rubisco and total soluble protein. When expressed on a chlorophyll basis, the amount of protein and Rubisco was constant for HL- and LL-grown plants. Acclimation to HL involves a tightly coordinated adjustment of photosynthesis, indicating a highly regulated decrease in the number of photosynthetic units manifested at the level of the content of light-harvesting and electron transport components, the amount of Rubisco, and the induction of Crassulacean acid metabolism. This response occurs in mature leaves and may represent a strategy that is optimal for the resource-limited epiphytic niche.

Chloroplast biogenesis 80. Proposal of a unified multibranched chlorophyll a/b biosynthetic pathway

A unified multibranched chlorophyll (Chl) biosynthetic pathway is proposed. The proposed pathway takes into account the following considerations: (a) that the earliest putative precursor of monovinyl Chl b that has been detected in higher plants is monovinyl protochlorophyllide b, (b) that in most cases, Chl b biosynthesis has its roots in the Chl a biosynthetic pathway, (c) that the Chl a biosynthetic pathway exhibits extensive biosynthetic heterogeneity, (d) that Chl biosynthesis may proceed differently at different stages of greening and in different greening groups of plants. Integration of the Chl a and b biosynthetic pathways into a unified multibranched pathway offers the functional flexibility to account for the structural and biosynthetic complexity of photosynthetic membranes. In this context, it is proposed that the unified, multibranched Chl a/b biosynthetic pathway represents the template of a Chl-protein biosynthesis center where photosystem (PS) 1, PS2, and light-harvesting Chl-protein complexes are assembled into functional photosynthetic units. The individual biosynthetic routes or groups of two to three adjacent biosynthetic routes may constitute Chl-protein biosynthesis subcenters, where specific Chl-protein complexes are assembled.

Uphill energy transfer in LH2-containing purple bacteria at room temperature

Uphill energy transfer in the LH2-containing purple bacteria Rhodopseudomonas acidophila, Rhodopseudomonas palustris, Rhodobacter sphaeroides, Chromatium vinosum and Chromatium purpuratum was studied by stationary fluorescence spectroscopy at room temperature upon selective excitation of the B800 pigments of LH2 and the B880 pigments of LH1 at 803 nm and 900 nm, respectively. The resulting fluorescence spectra differed significantly at wavelengths shorter than the fluorescence maximum but agreed at longer wavelengths. The absorption spectra of the species studied were decomposed into five bands at approx. 800, 820, 830, 850 and 880 nm using the shapes of the absorption spectra of the LH1-RC only species Rhodospirillum rubrum and the isolated B800-850 complex from Rps. acidophila strain 10050 as guide spectra. This allowed a quantification of the number of pigments in each pigment group and, consequently, the antenna size of the photosynthetic unit assuming 36 bacteriochlorophyll a molecules in an LH1-RC complex. In most of the LH2-containing purple bacterial strains the number of LH2 rings per LH1-RC was less than the idealized number of eight (Papiz et al., Trends Plant Sci. 1 (1996) 198–206), which was achieved only by C. purpuratum. Uphill energy transfer was assayed by comparing the theoretical fluorescence spectrum obtained from a Boltzmann equilibrium with the measured fluorescence spectrum obtained by 900 nm excitation. The good match of both spectra in all the purple bacteria studied indicates that uphill energy transfer occurs practically up to its thermodynamically maximal possible extent. All strains studied contained a small fraction of either poorly connected or unconnected LH2 complexes as indicated by higher fluorescence yields from the peripheral complexes than predicted by thermal equilibration or kinetic modeling. This impedes generally the quantitative analysis of blue-excited fluorescence spectra.

Photosynthetic Light-Harvesting: Reconciling Dynamics and Structure of Purple Bacterial LH2 Reveals Function of Photosynthetic Unit

Great progress in the study of structure and dynamics of photosynthetic light-harvesting pigment−protein complexes has recently resulted in detailed understanding of the light-harvesting and light-...

Supramolecular organization of the photosynthetic apparatus of Rhodobacter sphaeroides.

Native tubular membranes were purified from the purple non-sulfur bacterium Rhodobacter sphaeroides. These tubular structures contain all the membrane components of the photosynthetic apparatus, in the relative ratio of one cytochrome bc1 complex to two reaction centers, and approximately 24 bacteriochlorophyll molecules per reaction center. Electron micrographs of negative-stained membranes diffract up to 25 A and allow the calculation of a projection map at 20 A. The unit cell (a = 198 A, b = 120 A and gamma = 103 degrees) contains an elongated S-shaped supercomplex presenting a pseudo-2-fold symmetry. Comparison with density maps of isolated reaction center and light-harvesting complexes allowed interpretation of the projection map. Each supercomplex is composed of light-harvesting 1 complexes that take the form of two C-shaped structures of approximately 112 A in external diameter, facing each other on the open side and enclosing the two reaction centers. The remaining positive density is tentatively attributed to one cytochrome bc1 complex. These features shed new light on the association of the reaction center and the light-harvesting complexes. In particular, the organization of the light-harvesting complexes in C-shaped structures ensures an efficient exchange of ubihydroquinone/ubiquinone between the reaction center and the cytochrome bc1 complex.

Theories for kinetics and yields of fluorescence and photochemistry: how, if at all, can different models of antenna organization be distinguished experimentally?

The models most commonly used to describe the antenna organization of the photosynthetic membrane are the connected units model and the domain model. The theoretical descriptions of the exciton dynamics according to these models are reviewed with emphasis on a common nomenclature. Based on this nomenclature we compare for the two models the kinetics and yields of photochemistry and fluorescence under non-annihilation and annihilation conditions both under continuous light and under flash excitation. The general case is considered, that all initially open reaction centers become gradually closed and that exciton transfer between photosynthetic units (PSUs) is possible. Then, calculated kinetics and yields depend on the model assumptions made to account for the exciton transfer between PSUs. Here we extend the connected units model to flash excitation including exciton-exciton annihilation, and present a new simple mathematical formalism of the domain model under continuous light and flash excitation without annihilation. Product and fluorescence yields predicted by the connected units model for different degrees of connectivity are compared with those predicted by the domain model using the same sets of rate constants. From these calculations we conclude that it is hardly possible to distinguish experimentally between different models by any current method. If at all, classical fluorescence induction measurements are more suited for assessing the excitonic connectivity between PSUs than ps experiments.

Interactive effects of external manganese, the toxic metals copper and zinc, and light incontrolling cellular manganese and growth in a coastal diatom

Manganese is a fundamental component of the photosystem 11 reaction center, and thus its requirement should be linked to light utilization. High levels of Cu and Zn antagonize Mn nutrition by inhibiting cellular Mn uptake. Experiments were conducted to examine the interactive effects of Mn, the toxic metals Cu and Zn, and light on cellular Mn, chlorophyll a (Chl a), and specific growth rate in Thalassiosiru pseudonana. Cu and Zn inhibition of growth rate occurred only at. low Mn ion concentrations, an effect that was independent of light intensity and was totally accounted for by inhibition of Mn uptake by these metals. Cells grown at growth‐limiting low light intensities contained high levels of Mn and Chl a, which are needed for the synthesis of additional photosynthetic units during low‐light acclimation. This additional Mn was not provided by higher cellular Mn uptake rates, since these rates were the same in light‐saturated and light‐limited cells. However, because steady‐state cell Mn is equal to the uptake rate divided by the specific growth rate, the cells were able to accumulate the additional Mn needed for acclimation once light limitation caused growth rate to decline, a inherent negative feedback relation. In seawater, Mn and Cu may interact with light in controlling algal growth rate, since the free ion concentration of both metals is influenced by iight within the euphotic zone.

THE KINETICS OF THE PHOTOACCLIMATION RESPONSE OF NANNOCHLOROPSIS SP. (EUSTIGMATOPHYCEAE): A STUDY OF CHANGES IN ULTRASTRUCTURE AND PSU DENSITY

In this study we report the kinetics of photoacclimation of the unicellular alga Nannochloropsis sp. grown under high light (HL), and subsequently transferred to low light (LL). We examined the changes in ultrastructural features, pigmentation, and photosynthetic parameters over short intervals until the LL steady state was reached. The ultrastructural changes were followed by quantitative morphometric measurements of transmission electron micrographs. We found that the increase in the relative volume of the chloroplast during acclimation to LL (twofold) was accompanied by an increase in number of stacks (twofold) and in the surface area of thylakoids per cell (2.5‐fold). The increase in photosynthetic unit (PSU) density was about 2.15‐fold. Maximal density was about 84 PSU·μm−2 in LL cells, and minimal density was 39 PSU·μm−2 in HL cells. The HL/LL ratio of the in vivo optical absorption cross‐section of PSU (σPSU) was 2.8, whereas in the in vivo optical absorption cross‐section of the cell (σcell), the trend of change was in the opposite direction: 1.7‐fold higher in LL‐acclimated cells than in HL‐acclimated cells. We propose a partial sequence of the photoacclimation processes based on our data and the derived rate constants.

Are the light-harvesting I complexes from Rhodospirillum rubrum arranged around the reaction centre in a square geometry?

The basic photosynthetic unit containing the reaction centre and the light-harvesting I complex (RC-LHI) of the purple non-sulphur bacterium Rhodospirillum rubrum was purified and reconstituted into two-dimensional (2D) membrane crystals. Transmission electron microscopy using conventional techniques and cryoelectron microscopy of the purified single particles and of 2D crystals yielded a projection of the RC-LHI complex at a resolution of at least 1.6 nm. In this projection the LHI ring appears to have a square symmetry and packs in a square crystal lattice. The square geometry of the LHI ring was observed also in images of single isolated particles of the RC-LHI complex. However, although the LHI units are packed identically within the crystal lattice, a new rotational analysis developed here showed that the reaction centres take up one of four possible orientations within the ring. This fourfold disorder supports our interpretation of a square ring symmetry and suggests that a hitherto undetected component may be present within the photosynthetic unit.

Aerobic anoxygenic phototrophic bacteria.

The aerobic anoxygenic phototrophic bacteria are a relatively recently discovered bacterial group. Although taxonomically and phylogenetically heterogeneous, these bacteria share the following distinguishing features: the presence of bacteriochlorophyll a incorporated into reaction center and light-harvesting complexes, low levels of the photosynthetic unit in cells, an abundance of carotenoids, a strong inhibition by light of bacteriochlorophyll synthesis, and the inability to grow photosynthetically under anaerobic conditions. Aerobic anoxygenic phototrophic bacteria are classified in two marine (Erythrobacter and Roseobacter) and six freshwater (Acidiphilium, Erythromicrobium, Erythromonas, Porphyrobacter, Roseococcus, and Sandaracinobacter) genera, which phylogenetically belong to the alpha-1, alpha-3, and alpha-4 subclasses of the class Proteobacteria. Despite this phylogenetic information, the evolution and ancestry of their photosynthetic properties are unclear. We discuss several current proposals for the evolutionary origin of aerobic phototrophic bacteria. The closest phylogenetic relatives of aerobic phototrophic bacteria include facultatively anaerobic purple nonsulfur phototrophic bacteria. Since these two bacterial groups share many properties, yet have significant differences, we compare and contrast their physiology, with an emphasis on morphology and photosynthetic and other metabolic processes.

The cyanobacterium Spirulina platensis contains a long wavelength-absorbing pigment C738 (F76077K) at room temperature.

Spirulina platensis is a cyanobacterium which usually lives under high-light conditions. Nonetheless, it is thought to contain the most red-shifted antenna pigment of all known Chl a-containing phototrophic organisms, as shown by its 77 K fluorescence peaking at 760 nm. To exclude preparation artifacts and to exclude the possibility that long wavelength-absorbing pigments form only when the temperature is lowered to 77 K, we carried out experiments with whole cells at room temperature. The combined analysis of stationary absorption and fluorescence spectra as well as fluorescence induction and time-resolved fluorescence decays shows that the pigment responsible for the 77 K fluorescence at 760 nm (i) has the oscillator strength of approximately one Chl a molecule, (ii) absorbs maximally at 738 nm (), (iii) is present only in the antenna system of PS I, (iv) participates in light collection, and (v) does not entail a low photochemical quantum yield. Other, more abundant but less red-shifted Chl a antenna pigments lead to a significantly larger absorption cross section of the photosynthetic unit of PS I above 700 nm compared to units that would not possess these long wavelength-absorbing pigments. These results support the hypothesis that the physiological role of long wavelength-absorbing pigments is to increase the absorption cross section at wavelengths of >700 nm when in densely populated mats the spectrally filtered light is relatively more intense at these wavelengths [Trissl, H.-W. (1993) Photosynth. Res. 35, 247-263].

Excitons and excitation transfer in the photosynthetic unit of purple bacteria

Photosynthetic purple bacteria employ an antenna system to absorb sunlight and transfer electronic excitation to a centrally located protein, the photosynthetic reaction center. The antenna system employs for this purpose a hierarchy of circular bacteriochlorophyll aggregates. For the case of a particular light harvesting protein with known structure, one- and two-exciton spectra of its circular bacteriochlorophyll aggregate are determined by means of an effective Hamiltonian description. A detailed geometric model for the complete antenna system of the purple bacterium Rhodobacter sphaeroides is provided by molecular modelling and its bacteriochlorophyll aggregates are investigated, employing also an effective Hamiltonian. Transfer times within the complete antenna system and from the antennae to the reaction center are calculated.

Frost hardening and photosynthetic performance of Scots pine ( Pinus sylvestris L.) needles. I. Seasonal changes in the photosynthetic apparatus and its function

Photosynthetic CO2 uptake, the photochemical efficiency of photosystem II, the contents of chlorophyll and chlorophyll-binding proteins, and the degree of frost hardiness were determined in three-year-old Scots pine (Pinus sylvestris L.) trees growing in the open air but under controlled daylength. The following conditions were compared: 9-h light period (short day), 16-h light period (long day), and natural daylength. Irrespective of induction by short-day photoperiods or by subfreezing temperatures, frost hardening of the trees was accompanied by a long-lasting pronounced decrease in the photosynthetic rates of one-year-old needles. Under moderate winter conditions, trees adapted to a long-day photoperiod, assimilated CO2 with higher rates than the short-day-treated trees. In the absence of strong frost, photochemical efficiency was lower under short-day conditions than under a long-day photoperiod. Under the impact of strong frost, photochemical efficiency was strongly inhibited in both sets of plants. The reduction in photosynthetic performance during winter was accompanied by a pronounced decrease in the content of chlorophyll and of several chlorophyll-binding proteins [light-harvesting complex (LHC)IIb, LHC Ib, and a chlorophyll-binding protein with MW 43 kDa (CP 43)]. This observed seasonal decrease in photosynthetic pigments and in pigment-binding proteins was irrespective of the degree of frost hardiness and was apparantly under the control of the length of the daily photoperiod. Under a constant 9-h daily photoperiod the chlorophyll content of the needles was considerably lower than under long-day conditions. Transfer of the trees from short-day to long-day conditions resulted in a significantly increased chlorophyll content, whereas the chlorophyll content decreased when trees were transferred from a long-day to a short-day photoperiod. The observed changes in photosynthetic pigments and pigment-binding proteins in Scots pine needles are interpreted as a reduction in the number of photosynthetic units induced by shortening of the daily light period during autumn. This results in a reduction in the absorbing capacity during the frost-hardened state.

Photosynthetic Unit and Photosystems – History of Research and Current View. A. Wild and R. Ball

Photosynthetic Unit and Photosystems – History of Research and Current View. A. Wild and R. Ball

Action spectra and functional antenna sizes of Photosystems I and II in relation to the thylakoid membrane organization and pigment composition

The functional organization of competent photosynthetic units in developing thylakoids from intermittent-light grown pea as well as in the unstacked, stacked and phosphorylated stacked thylakoids from its mature chloroplasts was characterized by polarographic measurements of action spectra, reaction centre contents and optical cross-sections for PS I-mediated O2 uptake and PS II-mediated O2 evolution. The minimum antenna sizes of 60 and 37 chlorophyll a molecules for PS I and PS II, respectively, were determined in developing thylakoids with a ratio of Chl a/Chl b>50. In mature chloroplasts, the embedded light-harvesting chlorophyll a/b-binding (LHC) protein complexes increased the PS I and PS II effective antenna sizes by 3–6 times depending on the thylakoid membrane organization. In unstacked thylakoids, a randomization of PS I, PS II and LHC II led to the most uniform spectral distribution of light harvesting between the two photosystems but caused the maximal difference of their antenna sizes to be 370 and 100 Chls for the competent PS I and PS II units, respectively. Following the Mg2+-induced stacking of thylakoids, opposite complementary changes of the action spectra, antenna sizes and Chl a/Chl b ratios indicated a redistribution of a LHC II pool of ∼100 Chl ( a + b) molecules from PS I to PS II. Unlike to the stroma-exposed PS IIβ in unstacked thylakoids, the granal PS IIα units of ∼200 Chls demonstrated an additional 2-fold increase of the effective antenna size due to energy transfer within PS II dimers under strong background illumination, which closed >90% of reaction centres. Protein phosphorylation of the stacked thylakoids induced a significant inactivation of the O2-evolving PS II centres but did not cause complementary changes of the action spectra and antenna sizes of the competent PS I and PS II. In this case, light harvesting parameters of the O2-evolving PS II units were nearly unaffected, whereas the obvious relative increase of the PS I activity at 650 nm and its decrease at >700 nm both in the action spectrum and optical cross-section measurements might suggest a substitution of PS Iβ units in the O2-reducing fraction by another distinct fraction of α-type which in turn is not the same to PS Iα units in unstacked thylakoids.

Acclimation and adaptation to irradiance in symbiotic dinoflagellates. II. Response of chlorophyll-protein complexes to different photon-flux densities

The response of chlorophyll–protein complexes to super- and sub-saturating photon-flux densities, PFD (250 μmol quanta m−2 s−1 and 40 μmol quanta m−2 s−1, respectively) were analyzed for Symbiodinium microadriaticum Freudenthal, the symbiont of the Caribbean jellyfish Cassiopeia xamachana; S. kawagutii Trench and Blank, the symbiont of the Indo-Pacific scleractinian Montipora verrucosa; and S. pilosum Trench and Blank, the symbiont of the Carribbean zoanthid Zoanthus sociatus. The results indicate that each species exhibits a quantitatively distinct chlorophyll (chl) a distribution among its chl–protein complexes when cultured under standardized high and low light conditions. In response to sub-saturating PFD, the three species differentially increased the cellular concentrations of most of the chl–protein complexes. Increases in P700 (reaction center of Photosystem I) under sub-saturating PFD correlate with an increase in the cellular concentrations of the Photosystem I-enriched complexes. Similarly, increases in photosynthetic unit (PSU) size correlate with an increase in the cellular concentrations of the water-soluble peridinin–chl-a–protein (PCP) complexes and the membrane-bound chl a–chl c2–peridinin–protein (acpPC) complexes, which together represent the light-harvesting components of this group. In S. microadriaticum, acclimation to sub-saturating PFD uniquely includes the preferential enrichment of the dimeric form of PCP. Under super-saturating PFD, an enrichment in photo-protective xanthophylls was detected in acpPC from S. microadriaticum and S. pilosum, but not from S. kawagutii. Each species demonstrated a characteristic photo-acclimatory response which correlates with its distribution as endosymbiont in nature, supporting the concept that different species of symbiotic dinoflagellates are adapted (sensu Bjorkman 1981) to different photic environments. The study was conducted between May 1992 and November 1994.

A model separating leaf structural and physiological effects on carbon gain along light gradients for the shade‐tolerant species Acer saccharum

ABSTRACTA process‐based leaf gas exchange model for C3 plants was developed which specifically describes the effects observed along light gradients of shifting nitrogen investment in carboxylation and bioenergetics and modified leaf thickness due to altered stacking of photosynthetic units. The model was parametrized for the late‐successional, shade‐tolerant deciduous species Acer saccharum Marsh. The specific activity of ribulose‐1,5‐bisphosphate carboxylase (Rubisco) and the maximum photosynthetic electron transport rate per unit cytochrome f (cyt f) were used as indices that vary proportionally with nitrogen investment in the capacities for carboxylation and electron transport. Rubisco and cyt f per unit leaf area are related in the model to leaf dry mass per area (MA), leaf nitrogen content per unit leaf dry mass (Nm), and partitioning coefficients for leaf nitrogen in Rubisco (PR) and in bioenergetics (PB). These partitioning coefficients are estimated from characteristic response curves of photosynthesis along with information on lear structure and composition. While PR and PB determine the light‐saturated value of photosynthesis, the fraction of leaf nitrogen in thylakoid light‐harvesting components (PL) and the ratio of leaf chlorophyll to leaf nitrogen invested in light harvesting (CB), which is dependent on thylakoid stoichiometry, determine the initial photosynthetic light utilization efficiency in the model. Carbon loss due to mitochondrial respiration, which also changes along light gradients, was considered to vary in proportion with carboxylation capacity. Key model parameters ‐ Nm, PR, PB, PLCB and stomatal sensitivity with respect to changes in net photosynthesis (Gr) – were examined as a function of MA, which is linearly related to irradiance during growth of the leaves. The results of the analysis applied to A. saccharum indicate that PB and PR increase, and Gf, PL and CB decrease with increasing MA. As a result of these effects of irradiaiice on nitrogen partitioning, the slope of the light‐saturated net photosynthesis rate per unit leaf dry mass (Ammax) versus Nm relationship increased with increasing growth irradiance in mid‐season. Furthermore, the nitrogen partitioning coefficients as well as the slopes of Ammax versus Nm were independent of season, except during development of the leaf photosynthetic apparatus. Simulations revealed that the acclimation to high light increased Ammax by 40% with respect to the low light regime. However, light‐saturated photosynthesis per leaf area (Aamax) varied 3‐fold between these habitats, suggesting that the acclimation to high light was dominated by adjustments in leaf anatomy (Aamax=AmmaxMA) rather than in foliar biochemistry. This differed from adaptation to low light, where the alterations in foliar biochemistry were predicted to be at least as important as anatomical modifications. Due to the light‐related accumulation of photosynthetic mass per unit area, Aamax depended on MA and leaf nitrogen per unit area (Na). However, Na conceals the variation in both MA and Nm (Na=NmMA), and prevents clear separation of anatomical adjustments in foliage structure and biochemical modifications in foliar composition. Given the large seasonal and site nutrient availability‐related variation in Nm, and the influences of growth irradiance on nitrogen partitioning, the relationship between Aamax and Na is universal neither in time nor in space and in natural canopies at mid‐season is mostly driven by variability in MA. Thus, we conclude that analyses of the effects of nitrogen investments on potential carbon acquisition should use mass‐based rather than area‐based expressions.

Is There Any Difference in the Plastoquinone−Protein Link for QI (QA) and QII (QB) in PSU? An INDO Study

INDO/2 studies have been carried out for the elucidation of the relative environment of the plastoquinones QI and QII in thylakoid discs in chloroplast. The corresponding quinones in bacteria are QA and QB, which are known to be plastoquinones, menaquinones, or ubiquinones. We show that a major part of the free energy of reduction of QII by QI (−0.13 eV in vivo) can be accounted for if the plastoquinone of QI contains, in lieu of one of the methyl substituents, a side chain like −NHR or −NHCOR which serves as a link to the protein moiety in the photosynthetic unit (PSU). Here R represents an alkyl group. Chains like −NRR‘ and −NRCOR‘ are also possible substituents of QI. The remaining part of the free energy of reduction can be attributed to a slight difference in the polarizable environment not directly bonded to the plastoquinones.

Modeling effects of Photoadaption on the Photosynthesis–Irradiance Curve

In this article, I analyse how photoadaption affects the photosynthesis–irradiance curve in phytoplankton. Four parameters are presumably affected by photoadaption: (1) the size of a photosynthetic unit; (2) the number of photosynthetic units per cell; (3) the average turnover time of a photosynthetic unit; and (4) the chlorophyll-specific absorption cross-section. Prézelin's well-known conceptual model of photoadaption deals with variation in size and number of photosynthetic units, but not with the other two parameters. Prézelin's model predicts that the photosynthesis–irradiance curve is differentially affected by variations in size or number of photosynthetic units. By contrast, my analysis shows that if photoadaptional variation in the turnover time is also accounted for, only the cellular chlorophyll concentration is relevant; variation in the size vs. number of photosynthetic units is immaterial. The photoadaptional response of the chlorophyll-specific absorption cross-section allows a simple description consistent with experimental data. I use this description to derive a single expression for the rate of photosynthesis as dependent on irradiance and the cellular chlorophyll concentration. This model for the photosynthesis–irradiance-curve accounts for the photoadaptional alterations of all four parameters.