Photosynthesis Research

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The growth and O2-evolving activity of HC-requiring mutants (a). Cells were diluted to the indicated optical density (OD700 = 0.15, 0.07, or 0.03), 3 μl of the cell suspensions were spotted on agar plates (pH 6.2, 7.0, 7.8, and 9.0) and incubated for 4 days under 5% [v/v] CO2, 0.04% [v/v] CO2, or 0.01% [v/v] CO2 conditions with continuous light at 120 μmol photons m⁻² s⁻¹. b K0.5 (Ci) values and maximum rates of O2-evolving activity (Vmax) of C9, KO-60, and KO-62 cells grown in LC conditions for 12 h at pH 6.2, 7.0, 7.8, and 9.0. The K0.5 (Ci) values were calculated as the Ci concentration required for half-maximum oxygen-evolving activity. Data from all experiments show mean values ± SD from three biological replicates. *P < 0.05 by Student’s t-test
Accumulation of CCM-related proteins in WT, ccm1, H82 (cas), KO-60 KO-62 (a), CC-5325, saga1 (CC-5420), and SAGA1-Venus/saga1 (CC-5422) (b). Cells were first grown under HC conditions for 24 h and shifted to HC or LC conditions for 12 h. Histone H3 was used as a loading control. Asterisks indicate the non-specific protein bands
Differentially Expressed Genes (DEGs) in KO-60 and saga1 in LC condition for 2 h in light from RNA-seq analysis. The trimmed mean of M values (TMM) normalization method was used to compare the expression levels among two strains with a False Discovery Rate (FDR) < 0.01. Volcano plots comparing the transcriptomes of KO-60 with C9 and of saga1 with CC-5325 and SAGA1-Venus/saga1. The X- and Y-axes represent log2 (FC) and –log10 (FDR), respectively. The blue and red dots represent downregulated and upregulated DEGs with FDR < 0.01. The gray dots represent no significant difference in transcriptomes
Subcellular localization of CAS and LCIB in saga1 mutants. Localization of CAS (a) and LCIB (b) in C9, KO-60, CC-5325, saga1, and SAGA1-Venus/saga1 was assessed using an indirect immunofluorescence assay with an anti-CAS and anti-LCIB antibody. Cells were grown for 2 h (a) or 12 h (b), aerating with air containing 0.04% (v/v) CO2. Scale bars = 5 µm
Model of Ci-uptake in wild type and saga1 mutant. a In wild-type, Ci-transporters and carbonic anhydrase work to concentrate inorganic carbon in the pyrenoid. The putative retrograde signal from CAS to the nucleus required for the expression of HLA3 and LCIA is shown as a red arrow. b In saga1 mutants, the accumulation of Ci-transporters is reduced, and the pyrenoid morphology is changed
Microalgae induce a CO2-concentrating mechanism (CCM) to maintain photosynthetic affinity for dissolved inorganic carbon (Ci) under CO2-limiting conditions. In the model alga Chlamydomonas reinhardtii, the pyrenoid-localized Ca²⁺-binding protein CAS is required to express genes encoding the Ci-transporters, high-light activated 3 (HLA3), and low-CO2-inducible protein A (LCIA). To identify new factors related to the regulation or components of the CCM, we isolated CO2-requiring mutants KO-60 and KO-62. These mutants had insertions of a hygromycin-resistant cartridge in the StArch Granules Abnormal 1 (SAGA1) gene, which is necessary to maintain the number of pyrenoids and the structure of pyrenoid tubules in the chloroplast. In both KO-60 and the previously identified saga1 mutant, expression levels of 532 genes were significantly reduced. Among them, 10 CAS-dependent genes, including HLA3 and LCIA, were not expressed in the saga1 mutants. While CAS was expressed normally at the protein levels, the localization of CAS was dispersed through the chloroplast rather than in the pyrenoid, even under CO2-limiting conditions. These results suggest that SAGA1 is necessary not only for maintenance of the pyrenoid structure but also for regulation of the nuclear genes encoding Ci-transporters through CAS-dependent retrograde signaling under CO2-limiting stress.
 
Selected residues and water molecules in the vicinity of the Mn4CaO5 cluster and D1-D170 based on the 1.93 Å cryo-EM structural model of PSII from Synechocytis sp. PCC 6803, 7n8o (Gisriel et al. 2022). Residues from the D1, D2, and CP43 subunits are depicted in green; yellow, and pink, respectively. Manganese, Ca, and Cl ions are depicted in purple, yellow, and green, respectively. Water molecules and μ-oxo bridges are depicted as red spheres. For the CP43 subunit, the residue numbering for Synechocystis sp. PCC 6803 is used (in other reported structures, E341 and R344 are labeled E354 and R357, respectively (see page 142 of Bricker and Frankel 2002))
Upper traces: comparison of X-band CW EPR spectra of dark-adapted (blue) and illuminated (green) D1-D170E PSII core complexes. Illumination was for 30 s at 273 K. Lower traces: comparison of the light-minus-dark (S2-minus-S1) EPR signal of D1-D170E (black) with a simulation (red). Experimental conditions: temperature, 5.0 K; microwave frequency, 9.40 GHz; microwave power, 2.0 mW; modulation amplitude, 1.0 mT; modulation frequency, 100 kHz, conversion time, 49 ms. The narrow radical signal of Tyr YD⋅ at g = 2 has been excised for clarity
Comparison of light-minus-dark S2-minus-S1 EPR spectra of WT (black) and D1-D170E (red) PSII complexes measured at 10 K. Illumination was for 30 s at 273 K. Asterisk denotes the reduction of Fe³⁺ in the QAFe complex (100 mT). Cross denotes the presence of rhombic Fe³⁺ ions (150 mT). Experimental conditions: temperature, 10 K; microwave frequency, 9.37 GHz; microwave power, 2.0 mW; modulation amplitude, 1.0 mT; modulation frequency, 100 kHz; conversion time, 49 ms. The narrow radical signal of Tyr YD⋅ at g = 2 has been excised for clarity
The residue D1-D170 bridges Mn4 with the Ca ion in the O2-evolving Mn4CaO5 cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the Sn+1-minus-Sn FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn–Teller conformers of the S1 state such that (i) a different S1 state conformer is stabilized in D1-D170E than in wild-type and (ii) the S1 to S2 transition in D1-D170E produces a high-spin form of the S2 state rather than the low-spin form that is produced in wild-type. In this study, we employed EPR spectroscopy to test if a high-spin form of the S2 state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S2 state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S1 and S2 states in a site-directed mutant that retains substantial O2 evolving activity.
 
Nitrogen allocated to the photosynthetic apparatus and its partitioning into different photosynthetic components is crucial for understanding plant carbon gain and plant productivity. It is known that photosynthetic nitrogen content and partitioning are controlled by both environmental and vegetation factors and have versatile and dynamic responses. However, such responses are greatly simplified in most current gas exchange models, in which only a prescribed relationship is commonly applied to describe the effect of nitrogen on photosynthesis and with limited model performance. While within-canopy variation at a specific time in leaf photosynthetic nitrogen content and partitioning has been studied previously, far less attention has been paid to the seasonal dynamics of photosynthetic nitrogen content and partitioning, which is especially critical to deciduous forests. In this study, we integrated long-term field observations in deciduous forests in Japan to determine seasonal patterns of photosynthetic nitrogen content and partitioning (rubisco, electron transport, and light capture) and to examine how photosynthetic nitrogen content and partitioning varied seasonally in deciduous forest canopies growing at different altitudes. The results demonstrated that there were remarkable seasonal variations in both photosynthetic nitrogen content and partitioning in deciduous forests along the altitudinal gradient. Moreover, photosynthetic nitrogen use efficiency was well explained by nitrogen partitioning rather than total leaf nitrogen. These results suggest that seasonal patterns of nitrogen partitioning should be integrated into ecosystem models to accurately project emergent properties of ecosystem productivity on local, regional, and global scales.
 
Recently, the long-standing paradigm of variable chlorophyll (Chl) fluorescence (Fv) in vivo originating exclusively from PSII was challenged, based on measurements with green algae and cyanobacteria (Schreiber and Klughammer 2021, PRES 149, 213-231). Fv(I) was identified by comparing light-induced changes of Fv > 700 nm and Fv < 710 nm. The Fv(I) induced by strong light was about 1.5 × larger in Fv > 700 nm compared to Fv < 710 nm. In the present communication, concentrating on the model green alga Chlorella vulgaris , this work is extended by comparing the light-induced changes of long-wavelength fluorescence (> 765 nm) that is excited by either far-red light (720 nm, mostly absorbed in PSI) or visible light (540 nm, absorbed by PSI and PSII). Polyphasic rise curves of Fv induced by saturating 540 nm light are measured, which after normalization of the initial O-I 1 rises, assumed to reflect Fv(II), display a 2 × higher I 2 -P transient with 720 nm excitation (720ex) compared with 540ex. Analysis of the Fo(I) contributions to Fo(720ex) and Fo(540ex) reveals that also Fo(I)720ex is 2 × higher than Fo(I)540ex, which supports the notion that the whole I 2 -P transient is due to Fv(I). The twofold increase of the excitation ratio of F(I)/F(II) from 680 to 720 nm is much smaller than the eight–tenfold increase of PSI/PSII known from action spectra. It is suggested that the measured F > 765 nm is not representative for the bulk chlorophyll of PSI, but rather reflects a small fraction of far-red absorbing chlorophyll forms (“red Chls”) with particular properties. Based on the same approach (comparison of polyphasic rise curves measured with 720ex and 540ex), the existence of Fv(I) is confirmed in a variety of other photosynthetic organisms (cyanobacteria, moss, fern, higher plant leaves).
 
System description. a Illustration of the processes determining the concentration of functional PSII in the cell. b In bulk cultures, light penetration is attenuated due to cell–cell shading, resulting in uneven light exposure. c In our approach, cells are grown in a single 2D layer on a microscope, allowing uniform and controlled delivery of light for photosynthetic experiments. d Illustration of the different phases of the experiment. Cells were initially incubated for 30 min to allow for acclimation to the microscope environment prior to imaging with an inverted 100× objective (1.45 NA). They were then filmed for 10 h to obtain initial growth measurements. The cells were then irradiated with pulses of UV-A (395/25 nm) light, then imaged for a further 22 h to observe recovery. Growth light (610–650 nm LED; ~ 157 µmol photons m⁻² s⁻¹) was provided from above during acclimation, growth, and recovery. e The duration and intensity of the irradiation pulse were varied over 35 physically separated x–y locations on the same agar pad, allowing different experimental conditions to be carried out simultaneously. f Representative images showing the transmitted light (brightfield), the chlorophyll fluorescence, and the phycobilin fluorescence of WT and ∆cpc strains. The color bar indicates the scale of the fluorescence intensities, which were normalized to the same relative scale for each image. Scale bar is 5 µm
Low and high UV-A irradiance conditions. a Lineage tree of a representative WT colony in the low irradiation condition. Each branch represents a cell, and each bifurcation indicates a division event. The length of each branch is the time taken before the cell divides. Representative images of the colony at equally spaced timepoints are shown to the right of the tree. The intensity of these images is normalized to the maximum intensity of the sequence. b The sum of cell lengths, c the mean chlorophyll intensity (Chl), d the mean phycobilin intensity (Pcb) of all WT lineages (N = 184) in the low irradiation movie. e Lineage tree of a representative Δcpc colony in the low irradiation condition. f–g Cell length, mean chlorophyll intensity, and phycobilin intensity of all Δcpc (N = 125) in the low irradiation movie. i, Lineage of representative WT colony in the high irradiation condition, with corresponding data of all WT colonies (N = 82) in the movie (j–l). m Lineage of representative Δcpc colony in the high irradiation condition, with corresponding data from all Δcpc colonies (N = 17) in the movie (n–p). The dotted gray lines indicate the time when UV-A irradiation was applied
Asymmetric survival in response to intermediate UV-A irradiation. a Representative lineage of a WT colony, where all cells stopped growing after irradiation. b Representative lineage of a WT colony showing asymmetric survival. c Representative lineage of a Δcpc colony. Despite one of the branches not dividing over the duration of the time course, the cell still appears to be growing in length. d–e Sum of cell lengths, chlorophyll, and phycobilin intensity of WT lineages (N = 139). g–i, Sum of cell lengths, chlorophyll, and phycobilin intensity of Δcpc lineages (N = 44). In d–i, the green traces are cells which stopped growing, while the orange traces indicate cells which continued to grow after irradiation. The dotted gray lines indicate the time when UV-A irradiation was applied
Identification and model of photoendosome formation. a Representative images showing internalized vesicle-like structures (photoendosome) with chlorophyll (Chl) and phycobilin (Pcb) fluorescence in WT cells. b A similar structure was found in the only Δcpc cell that stopped growing after irradiation. The white arrows indicate the position of the fluorescent puncta. Scale bars are 2 μm. c Proposed model of photoendosome formation after UV-A-induced photodamage
Oxygenic photosynthesis is driven by the coupled action of the light-dependent pigment–protein complexes, photosystem I and II, located within the internal thylakoid membrane system. However, photosystem II is known to be prone to photooxidative damage. Thus, photosynthetic organisms have evolved a repair cycle to continuously replace the damaged proteins in photosystem II. However, it has remained difficult to deconvolute the damage and repair processes using traditional ensemble approaches. Here, we demonstrate an automated approach using time-lapse fluorescence microscopy and computational image analysis to study the dynamics and effects of photodamage in single cells at subcellular resolution in cyanobacteria. By growing cells in a two-dimensional layer, we avoid shading effects, thereby generating uniform and reproducible growth conditions. Using this platform, we analyzed the growth and physiology of multiple strains simultaneously under defined photoinhibitory conditions stimulated by UV-A light. Our results reveal an asymmetric cellular response to photodamage between sibling cells and the generation of an elusive subcellular structure, here named a ‘photoendosome,’ derived from the thylakoid which could indicate the presence of a previously unknown photoprotective mechanism. We anticipate these results to be a starting point for further studies to better understand photodamage and repair at the single-cell level.
 
One of the main barriers to making efficient Photosystem I-based biohybrid solar cells is the need for an electrochemical pathway to facilitate electron transfer between the P700 reaction center of Photosystem I and an electrode. To this end, nature provides inspiration in the form of cytochrome c6, a natural electron donor to the P700 site. Its natural ability to access the P700 binding pocket and reduce the reaction center can be mimicked by employing cytochrome c, which has a similar protein structure and redox chemistry while also being compatible with a variety of electrode surfaces. Previous research has incorporated cytochrome c to improve the photocurrent generation of Photosystem I using time consuming and/or specialized electrode preparation. While those methods lead to high protein areal density, in this work we use the quick and facile vacuum-assisted drop-casting technique to construct a Photosystem I/cytochrome c photoactive composite film with micron-scale thickness. We demonstrate that this simple fabrication technique can result in high cytochrome c loading and improvement in cathodic photocurrent over a drop-casted Photosystem I film without cytochrome c. In addition, we analyze the behavior of the cytochrome c/Photosystem I system at varying applied potentials to show that the improvement in performance can be attributed to enhancement of the electron transfer rate to P700 sites and therefore the PSI turnover rate within the composite film.
 
Cellular levels of protein complexes involved in the conversion of solar to chemical energy. Consensus cpc ranges for the subunits are condensed from the different quantification methods, as described in Results and Discussion, with individual data-points shown in boxplots in Supplementary Fig. S2. The minimum and maximum cpc values are rounded to the nearest 10 (< 1000), 50 (1000–10,000) or 500 (> 10,000) and displayed as bars, shaded according to cpc range, for photosystem II (PSII, PsbA-H), photosystem I (PSI, PsaA-F) (a), cytochrome b6f (cytb6f), PetP, phycocyanin (PC), photosynthetic NAD(P)H dehydrogenase-like complex type-1 (NDH-1) (b) and ATP synthase (c). Abundance levels corresponding to the largest extent of overlap between subunits, shown by the horizontal dashed lines, are taken to represent probable ranges for the complexes, explained in Results and Discussion: PSII (24,000–44,000 cpc), PSI (86,000–118,500 cpc), cytb6f (3350–8450 or 14,000–28,000 cpc), NDH-1 (7000–13,500 cpc), ATP synthase (α, β: 67,000–83,500 cpc; γ, δ, ε, a, b, b': 12,000–25,000 cpc)
Cellular levels of carboxysomal proteins and enzymes of the Calvin–Benson–Bassham cycle. Consensus cpc ranges are derived from data-points shown in Supplementary Fig. S3 and displayed as in Fig. 1. Structural carboxysomal proteins (a), Calvin–Benson–Bassham cycle enzymes: carbonic anhydrase (CcaA), ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO: large, RbcL and small, RbcS subunits), glycerate-3-phosphate kinase (Pgk), glycerate-1,3-phosphate dehydrogenase (Gap1, 2), transketolase (TktA), phosphoketolase (Xfp) (b), triose phosphate isomerase (TpiA), fructose-1,6-bisphosphate aldolase (Fba1, 2), fructose-1,6- and sedoheptulose-1,7-bisphosphatase (Fbp1, 2), ribose-5-phosphate isomerase (RpiA), ribulose-3-phosphate epimerase (Rpe), ribulose-5-phosphate kinase (Prk) (c). The horizontal dashed lines in (b) and (c) define a 25,000–60,000 cpc range, explained in Results and Discussion
Cellular levels of enzymes and auxiliary proteins involved in biosynthesis. Consensus cpc ranges are derived from data-points shown in Supplementary Fig. S4 and displayed as in Fig. 1. Ferredoxin (Fd) and Fd-dependent enzymes: ferredoxin-NADP ⁺ reductase (FNR), glutamate synthase 2 (GlsF), flavodiiron protein (Flv1/3), bidirectional hydrogenase (HoxF, U, H), sulphite reductase (Sir), nitrite reductase (NirA) (a), enzymes and auxiliary proteins in the Mg-branch of the chlorophyll a biosynthesis pathway: Mg-chelatase (ChlI, ChlD, ChlH, Gun4), Mg-protoporphyrin IX methyltransferase (ChlM), O2-dependent Mg-protoporphyrin IX methyl ester cyclase (CycI, Ycf54), light-dependent protochlorophyllide oxidoreductase (LPOR), 8-vinyl reductase (DVR), geranylgeranyl reductase (ChlP), chlorophyll a synthase (ChlG) (b), enzymes in the carotenoid biosynthesis pathway: geranylgeranyl pyrophosphate synthase (CrtE), phytoene desaturase (CrtP), prolycopene isomerase/CRTISO (CrtH), ζ-carotene desaturase (CrtQ) (c), enzymes in the phycobilin biosynthesis pathway: ferrochelatase (FeCh), heme oxygenase 1 (PbsA1), chromophore lyase (CpcS1, CpcT) (d). The horizontal dashed lines, explained in Results and Discussion, define in: (a) 740–1400 and 3150–6100 cpc, (b) 1150–3550 cpc, (c) 380–1100 cpc and (d) 2300–3800 cpc
Cellular levels of assembly factors and enzymes involved in thylakoid membrane biogenesis and photosystem assembly/repair. Consensus cpc ranges are derived from data-points shown in Supplementary Fig. S5 and displayed as in Fig. 1. Proteins involved in the coordination of chlorophyll and PSII biosynthesis (a), proteins involved in thylakoid membrane biogenesis and PSII assembly (b), proteins involved in thylakoid membrane biogenesis and PSI assembly (c), and ATP-dependent zinc metalloproteases: membrane protein quality control and PSII repair (d)
Diagrammatic summary of the proteins quantified in this study, showing the range of biological processes, such as biosynthetic and assembly pathways, covered by the quantitative mass spectrometry analysis. Abbreviations are defined in Figs. 1, 2,3, 4. Proteins and subunits of complexes that have been quantified are colored in blue and shaded according their abundance levels; those not quantified are in white. Complexes such as photosystem I, photosystem II and cytochrome b6f are drawn as monomers for simplicity. Thylakoids are drawn as elongated tubular structures, which converge on a thylakoid convergence zone that appears to connect plasma and thylakoid membranes (Stengel et al. 2012; Heinz et al. 2016). The photosystem II assembly intermediates are based on those in Konert et al. (2022) and Rahimzadeh-Karvansara (2022), with the exception of the PSII-I assembly complex, the structure of which was determined by Zabret et al. (2021)
Quantifying cellular components is a basic and important step for understanding how a cell works, how it responds to environmental changes, and for re-engineering cells to produce valuable metabolites and increased biomass. We quantified proteins in the model cyanobacterium Synechocystis sp. PCC 6803 given the general importance of cyanobacteria for global photosynthesis, for synthetic biology and biotechnology research, and their ancestral relationship to the chloroplasts of plants. Four mass spectrometry methods were used to quantify cellular components involved in the biosynthesis of chlorophyll, carotenoid and bilin pigments, membrane assembly, the light reactions of photosynthesis, fixation of carbon dioxide and nitrogen, and hydrogen and sulfur metabolism. Components of biosynthetic pathways, such as those for chlorophyll or for photosystem II assembly, range between 1000 and 10,000 copies per cell, but can be tenfold higher for CO 2 fixation enzymes. The most abundant subunits are those for photosystem I, with around 100,000 copies per cell, approximately 2 to fivefold higher than for photosystem II and ATP synthase, and 5–20 fold more than for the cytochrome b 6 f complex. Disparities between numbers of pathway enzymes, between components of electron transfer chains, and between subunits within complexes indicate possible control points for biosynthetic processes, bioenergetic reactions and for the assembly of multisubunit complexes.
 
We performed active and passive measurements of diurnal cycles of chlorophyll fluorescence on potato crops at canopy level in outdoors conditions for 26 days. Active measurements of the stationary fluorescence yield (Fs) were performed using Ledflex, a fluorescence micro-LIDAR described in Moya et al. (Photosynth Res 142:1–15, 2019), capable of remote measurements of chlorophyll fluorescence under full sun-light in the wavelength range from 650 to 800 nm. Passive measurements of solar-induced fluorescence (SIF) fluxes were performed with Spectroflex, an instrument based on the method of filling-in in the O 2 A and O 2 B absorption bands at 760 nm (F760) and 687 nm (F687), respectively. Diurnal cycles of Fs showed significant variations throughout the day, directly attributed to changes in photosystem II yield. Contrasting patterns were observed according to illumination conditions. Under cloudy sky, Fs varied in parallel with photosynthetically active radiation (PAR). By contrast, during clear sky days, the diurnal cycle of Fs showed a “M” shape pattern with a minimum around noon. F687 and F760 showed different patterns, according to illumination conditions. Under low irradiance associated with cloudy conditions, F687 and F760 followed similar diurnal patterns, in parallel with PAR. Under high irradiance associated with clear sky we observed an increase of the F760/F687 ratio, which we attributed to the contributions in the 760 nm emission of photosystem I fluorescence from deeper layers of the leaves, on one end, and by the decrease of 687 nm emission as a result of red fluorescence re-absorption, on the other end. We defined an approach to derive a proxy of fluorescence yield (FYSIF) from SIF measurements as a linear combination of F687 and F760 normalized by vegetation radiance, where the coefficients of the linear combination were derived from the spectral transmittance of Ledflex. We demonstrated a close relationship between diurnal cycles of FYSIF and Fs, which outperformed other approaches based on normalization by incident light.
 
The production of reactive oxygen species (ROS) is an unavoidable consequence of oxygenic photosynthesis and represents a major cause of oxidative stress in phototrophs, having detrimental effects on the photosynthetic apparatus, limiting cell growth, and productivity. Several methods have been developed for the quantification of cellular ROS, however, most are invasive, requiring the destruction of the sample. Here, we present a new methodology that allows the concurrent quantification of ROS and photosynthetic activity, using the fluorochrome dichlorofluorescein (DCF) and in vivo chlorophyll a fluorescence, respectively. Both types of fluorescence were measured using an imaging Pulse Amplitude Modulation (PAM) fluorometer, modified by adding a UVA-excitation light source (385 nm) and a green bandpass emission filter (530 nm) to enable the sequential capture of red chlorophyll fluorescence and green DCF fluorescence in the same sample. The method was established on Phaeodactylum tricornutum Bohlin, an important marine model diatom species, by determining protocol conditions that permitted the detection of ROS without impacting photosynthetic activity. The utility of the method was validated by quantifying the effects of two herbicides (DCMU and methyl viologen) on the photosynthetic activity and ROS production in P. tricornutum and of light acclimation state in Navicula cf. recens Lange-Bertalot, a common benthic diatom. The developed method is rapid and non-destructive, allowing for the high-throughput screening of multiple samples over time.
 
Ledflex is a fluorometer adapted to measure chlorophyll fluorescence at the canopy level. It has been described in detail by Moya et al. (2019), Photosynthesis Research. https://doi.org/10.1007/s11120-019-00642-9. We used this instrument to determine the effect of water stress on the fluorescence of a fescue field under extreme temperature and light conditions through a 12 days campaign during summer in a Mediterranean area. The fescue field formed part of a lysimeter station in "las Tiesas," near Albacete-Spain. In addition to the fluorescence data, the surface temperature was measured using infrared radiometers. Furthermore, "Airflex," a passive fluorometer measuring the filling-in of the atmospheric oxygen absorption band at 760 nm, was installed in an ultralight plane and flown during the most critical days of the campaign. We observed with the Ledflex fluorometer a considerable decrease of about 53% of the stationary chlorophyll fluorescence level at noon under water stress, which was well correlated with the surface temperature difference between the stressed and control plots. Airflex data also showed a decrease in far-red solar-induced fluorescence upon water stress in agreement with surface temperature data and active fluorescence measurements after correction for PS I contribution. Notwithstanding, the results from airborne remote sensing are not as precise as in situ active data.
 
Ultrapurified Photosystem II complexes crystalize as uniform microcrystals (PSIIX) of unprecedented homogeneity that allow observation of details previously unachievable, including the longest sustained oscillations of flash-induced O2 yield over > 200 flashes and a novel period-4.7 water oxidation cycle. We provide new evidence for a molecular-based mechanism for PSII-cyclic electron flow that accounts for switching from linear to cyclic electron flow within PSII as the downstream PQ/PQH2 pool reduces in response to metabolic needs and environmental input. The model is supported by flash oximetry of PSIIX as the LEF/CEF switch occurs, Fourier analysis of O2 flash yields, and Joliot-Kok modeling. The LEF/CEF switch rebalances the ratio of reductant energy (PQH2) to proton gradient energy (H⁺o/H⁺i) created by PSII photochemistry. Central to this model is the requirement for a regulatory site (QC) with two redox states in equilibrium with the dissociable secondary electron carrier site QB. Both sites are controlled by electrons and protons. Our evidence fits historical LEF models wherein light-driven water oxidation delivers electrons (from QA⁻) and stromal protons through QB to generate plastoquinol, the terminal product of PSII-LEF in vivo. The new insight is the essential regulatory role of QC. This site senses both the proton gradient (H⁺o/H⁺i) and the PQ pool redox poise via e⁻/H⁺ equilibration with QB. This information directs switching to CEF upon population of the protonated semiquinone in the Qc site (Q⁻H⁺)C, while the WOC is in the reducible S2 or S3 states. Subsequent photochemical primary charge separation (P⁺QA⁻) forms no (QH2)B, but instead undergoes two-electron backward transition in which the QC protons are pumped into the lumen, while the electrons return to the WOC forming (S1/S2). PSII-CEF enables production of additional ATP needed to power cellular processes including the terminal carboxylation reaction and in some cases PSI-dependent CEF.
 
Microalgae cultivation utilizes the energy of sunlight to reduce carbon dioxide (CO2) for producing renewable energy feedstock. The commercial success of the biological fixation of carbon in a consistent manner depends upon the availability of a robust microalgae strain. In the present work, we report the identification of a novel marine Nannochloris sp. through multiparametric photosynthetic evaluation. Detailed photobiological analysis of this strain has revealed a smaller functional antenna, faster relaxation kinetics of non-photochemical quenching, and a high photosynthetic rate with increasing light and temperatures. Furthermore, laboratory scale growth assessment demonstrated a broad range halotolerance of 10–70 parts per thousand (PPT) and high-temperature tolerance up to 45 °C. Such traits led to the translation of biomass productivity potential from the laboratory scale (0.2–3.0 L) to the outdoor 50,000 L raceway pond scale (500-m2) without any pond crashes. The current investigation revealed outdoor single-day peak areal biomass productivity of 43 g m−2 d−1 in summer with an annual (March 2019–February 2020) average productivity of 20 g m−2 d−1 in seawater. From a sustainability perspective, this is the first report of successful round-the-year (> 347 days) multi-season (summer, monsoon, and winter) outdoor cultivation of Nannochloris sp. in broad seawater salinity (1–57 PPT), wide temperature ranges (15–40 °C), and in fluctuating light conditions. Concurrently, outdoor cultivation of this strain demonstrated conducive fatty acid distribution, including increased unsaturated fatty acids in winter. This inherent characteristic might play a role in protecting photosynthesis machinery at low temperatures and in high light stress. Altogether, our marine Nannochloris sp. showed tremendous potential for commercial scale cultivation to produce biofuels, food ingredients, and a sustainable source for vegetarian protein.
 
In this work, tuning oxygen tension was targeted to improve hydrogen evolution. To achieve such target, various consortia of the chlorophyte Coccomyxa chodatii with a newly isolated photosynthetic purple non-sulfur bacterium (PNSB) strain Rhodobium gokarnense were set up, sulfur replete/deprived, malate/acetate fed, bicarbonate/sulfur added at dim/high light. C. chodatii and R. gokarnense are newly introduced to biohydrogen studies for the first time. Dim light was applied to avoid the inhibitory drawbacks of photosynthetic oxygen evolution, values of hydrogen are comparable with high light or even more and thus economically feasible to eliminate the costs of artificial illumination. Particularly, the consortium of 2n- (n = 1.9 × 105 cell/ml, sulfur deprived) demonstrated its perfection for the target, i.e., the highest possible cumulative hydrogen. This consortium exhibited negative photosynthesis, i.e., oxygen uptake in the light. Most hydrogen in consortia is from bacterial origin, although algae evolved much more hydrogen than bacteria on per cell basis, but for only one day (the second 24 h), as kinetics revealed. The higher hydrogen in unibacterial culture or consortia results from higher bacterial cell density (20 times). Consortia evolved more hydrogen than their respective separate cultures, further enhanced when bicarbonate and sulfur were supplemented at higher light. The share of algae relatively increased as bicarbonate or sulfur were added at higher light intensity, i.e., PSII activity partially recovered, resulting in a transient autotrophic hydrogen evolution. The addition of acetic acid in mixture with malic acid significantly enhanced the cumulative hydrogen levels, mostly decreased cellular ascorbic acid indicating less oxidative stress and relief of PSII, relative to malic acid alone. Starch, however, decreased, indicating the specificity of acetic acid. Exudates (reducing sugars, amino acids, and soluble proteins) were detected, indicating mutual utilization. Yet, hydrogen evolution is limited; tuning PSII activity remains a target for sustainable hydrogen production.
 
Metabolic pathways involved in NO3⁻ assimilation into asparagine in roots (lower panel) and shoots (upper panel). Root assimilation of NO3⁻ involves energization and C skeleton synthesis from sucrose moved to the roots in the phloem. A fraction of the resulting asparagine is moved to the shoot in the xylem, and acid–base regulation involves H⁺ entry from soil, neutralising OH⁻ generated by nitrate assimilation. Assimilation of soil NH4⁺ (not shown) in roots resembles root assimilation, but without the reductive steps, and with acid–base regulation by excretion to the soil of the H⁺ generated in production of asparagine. Shoot NO3⁻ assimilation, using root-derived NO3⁻ transported up the xylem with root-derived K⁺/Ca²⁺, involves reduction using photoproduced reductant, ATP from photophosphorylation, and carbon skeletons from the photosynthetic carbon reduction cycle. Acid–base regulation involves malic or oxalic acid synthesis with the H⁺ neutralising the OH⁻ from NO3⁻ assimilation, and the malate and oxalate are accumulated in vacuoles with the cations that accompanied NO3⁻ up the xylem. The alternative acid–base regulation mechanism (not shown) involves malic acid production, with the malate resulting from OH⁻ neutralisation moving down the phloem with K⁺ that accompanied NO3⁻ up the xylem. In the roots, malate catabolism generates OH⁻, neutralised by H⁺ accompanying NO3⁻ entry from the soil
The photon costs of photoreduction/assimilation of nitrate (NO3⁻) into organic nitrogen in shoots and respiratory driven NO3⁻ and NH4⁺ assimilation in roots are compared for terrestrial vascular plants, considering associated pH regulation, osmotic and ontogenetic effects. Different mechanisms of neutralisation of the hydroxyl (OH⁻) ion necessarily generated in shoot NO3⁻ assimilation are considered. Photoreduction/assimilation of NO3⁻ in shoots with malic acid synthesis and either accumulation of malate in leaf vacuoles or transport of malate to roots and catabolism there have a similar cost which is around 35% less than that for root NO3⁻ assimilation and around 20% less than that for photoreduction/assimilation of NO3⁻, oxalate production and storage of Ca oxalate in leaf vacuoles. The photon cost of root NH4⁺ assimilation with H⁺ efflux to the root medium is around 70% less than that of root NO3⁻ assimilation. These differences in photon cost must be considered in the context of the use of a combination of locations of NO3⁻ assimilation and mechanisms of acid–base regulation, and a maximum of 4.9–9.1% of total photon absorption needed for growth and maintenance that is devoted to NO3⁻ assimilation and acid–base regulation.
 
qE-type NPQ temperature mapping. Samples grown in different temperatures (19, 26 and 29 °C) in low-light (A) or high-light (B) measured using the Phenoplate across multiple temperature gradients. Coloured stars on the temperature axis indicate the growth temperature of the three cultures. Lines indicate fitted data with a 3rd order polynomial function using OriginPro. Data presented are results from four Phenoplate measurements using different temperature gradients, and four biological replicates for each acclimated condition (n = 4). The experiment has been replicated after 6 and 12 months and yielded similar results
Slow-relaxing NPQ and zeaxanthin production temperature mapping. Samples acclimated to 26 °C were measured using the Phenoplate and analysed subsequently using HPLC in order to quantify their zeaxanthin content (A, B). Slow-relaxing NPQ and zeaxanthin are shown for selected samples across the entire tested temperature range for low (A) and high (B) light acclimated cultures. Samples grown in different temperatures (19, 26 and 29 °C) in low-light (C) or high-light (D) measured using the Phenoplate across multiple temperature gradients. Coloured stars on the temperature axis indicate the growth temperature of the three cultures. Lines indicate fitted data with a Biphasic Hill Growth function using OriginPro. Data presented are results from four Phenoplate measurements using different temperature gradients, and four biological replicates for each acclimated condition (n = 4)
qT2-type NPQ temperature mapping. State transition to state 2 of samples acclimated to different temperatures (19, 26 and 29 °C) in low-light (A) or high-light (B) with far-red turned on during dark recovery. qT2-type NPQ of samples acclimated to different temperatures (19, 26 and 29 °C) in low-light (C) or high-light (D) with far-red turned off during dark recovery measurements. Coloured stars on the temperature axis indicate the growth temperature of the three cultures. Lines indicate fitted data with a Biphasic Hill Growth function using OriginPro. Data presented are results from four Phenoplate measurements using different temperature gradients, and four biological replicates for each acclimated condition (n = 4)
Y(II) temperature mapping. Data points represent Y(II) values collected after five minutes of illumination with high-light (500 μmol photons m⁻² s⁻¹) at the indicated temperatures on the vertical axis. Coloured stars on the temperature axis indicate the growth temperature of the three cultures. Lines indicate fitted data with a Biphasic Hill Growth function using OriginPro. Data presented are results from four Phenoplate measurements using different temperature gradients, and four biological replicates for each acclimated condition (n = 4)
Global view of temperature response kinetics of photoprotection and PS(II) quantum yield. Data fittings only of recorded data from C. vulgaris cultures acclimated to 26 °C and low-light (A) or high-light (B). Fitting parameters are as described in legends of preceding figures
Light intensity and temperature independently impact all parts of the photosynthetic machinery in plants and algae. Yet to date, the vast majority of pulse amplitude modulated (PAM) chlorophyll a fluorescence measurements have been performed at well-defined light intensities, but rarely at well-defined temperatures. In this work, we show that PAM measurements performed at various temperatures produce vastly different results in the chlorophyte Chlorella vulgaris . Using a recently developed Phenoplate technique to map quantum yield of Photosystem II (Y(II)) and non-photochemical quenching (NPQ) as a function of temperature, we show that the fast-relaxing NPQ follows an inverse normal distribution with respect to temperature and appears insensitive to previous temperature acclimation. The slow-relaxing or residual NPQ after 5 minutes of dark recovery follows a normal distribution similar to Y(II) but with a peak in the higher temperature range. Surprisingly, higher slow- and fast-relaxing NPQ values were observed in high-light relative to low-light acclimated cultures. Y(II) values peaked at the adaptation temperature regardless of temperature or light acclimation. Our novel findings show the complete temperature working spectrum of Y(II) and how excess energy quenching is managed across a wide range of temperatures in the model microalgal species C. vulgaris . Finally, we draw attention to the fact that the effect of the temperature component in PAM measurements has been wildly underestimated, and results from experiments at room temperature can be misleading.
 
Pitcher plants (Nepenthes sp.) are insectivorous angiosperm plants with modified leaves known as pitchers best known as acting as traps for insects. Pitcher plants are typically found under boggy conditions under both forest cover and open areas with very poor nutrient status, particularly N-status. The pitchers have low photosynthetic activity. The Chl a content of the pitcher tissue of both Nepenthes mirabilis (green and red) varieties was very low. Chl b/a ratios of the green variety phyllodes (lamina) and pitchers were ≈ 0.24 to 0.29. In the red variety, the mature phyllodes had a Chl b/a ratio ≈ 0.28 but both the pitchers and the young phyllodes had Chl b/a ratios of nearly 0.5. Photosynthetic electron transport (ETR) was measured using PAM technology. Phyllodes of both varieties showed photoinhibition at supra-optimal irradiances [Nepenthes mirabilis (green variety), Eopt ≈ 200–250 µmol photon m⁻² s⁻¹; red variety, Eopt ≈ 100–150 µmol photon m⁻² s⁻¹]. Pitchers had low optimum irradiances (Eopt ≈ 40–90 µmol photon m⁻² s⁻¹). Maximum ETR (ETRmax) of phyllodes of both varieties was low (ETRmax ≈ 50 µmol e⁻ g⁻¹ Chl a s⁻¹); ETRmax was higher for pitchers on a Chl a basis (ETRmax ≈ 80–100 µmol e⁻ g⁻¹ Chl a s⁻¹); a consequence of their low Chl a content on a surface area basis. ETRmax of cut disks of phyllodes did not respond strongly to incubation in NH4⁺, glutamate or aspartate as N-sources but did respond positively to added urea.
 
Lhca1 is one of the four pigment-protein complexes composing the outer antenna of plant Photosystem I-light-havesting I supercomplex (PSI-LHCI). It forms a functional dimer with Lhca4 but, differently from this complex, it does not contain ‘red-forms,’ i.e., pigments absorbing above 700 nm. Interestingly, the recent PSI-LHCI structures suggest that Lhca1 is the main point of delivering the energy harvested by the antenna to the core. To identify the excitation energy pathways in Lhca1, we developed a structure-based exciton model based on the simultaneous fit of the low-temperature absorption, linear dichroism, and fluorescence spectra of wild-type Lhca1 and two mutants, lacking chlorophylls contributing to the long-wavelength region of the absorption. The model enables us to define the locations of the lowest energy pigments in Lhca1 and estimate pathways and timescales of energy transfer within the complex and to the PSI core. We found that Lhca1 has a particular energy landscape with an unusual (compared to Lhca4, LHCII, and CP29) configuration of the low-energy states. Remarkably, these states are located near the core, facilitating direct energy transfer to it. Moreover, the low-energy states of Lhca1 are also coupled to the red-most state (red forms) of the neighboring Lhca4 antenna, providing a pathway for effective excitation energy transfer from Lhca4 to the core.
 
Vcmax and Jmax generated by steady-state A–Ci and RACiR fitting curves for different evergreen broadleaved species. a represented Vcmax of C. eyrei, b represented Jmax of C. eyrei, c represented Vcmax of C. sclerophylla, d represented Jmax of C. sclerophylla, e represented Vcmax of V. odoratissimum, f represented Jmax of V. odoratissimum, g represented Vcmax of C. nocturnum, h represented Jmax of C. nocturnum. SS represented steady-state A–Ci curves. R1 represented the [CO2] gradient in the RACiR curves of 400–1500 ppm, R2 was 400–200–800 ppm, R3 was 420–20–620 ppm, R4 was 420–20–820 ppm, R5 was 420–20–1020 ppm, R6 was 420–20–1220 ppm, R7 was 420–20–1520 ppm, R8 was 420–20–1820 ppm, R9 was 450–50–650 ppm, R10 was 650–50 ppm, and R11 was 650–50–650 ppm (n = 3). Steady-state A–Ci curves were shown in green; RACiR curves were shown in grey. Different lowercase letters indicated significant differences (Tukey’s HSD) of photosynthetic parameters derived by steady-state A–Ci and RACiR curves. The difference was insignificant without letter markers
Scatter plots of the RACiR and steady-state A–Ci of C. eyrei under different [CO2] gradients. Steady-state A–Ci curves were shown in green; RACiR curves were shown in grey. (a1) to (k1) indicated the measurements of the first leaf under 11 different [CO2] gradients. (a2) to (k2) indicated the measurements of the second leaf under 11 different [CO2] gradients. (a3) to (k3) indicated the measurements of the third leaf under 11 different [CO2] gradients. Results of the other three studied species were shown in the supplementary materials
Using steady-state photosynthesis–intercellular CO2 concentration (A–Ci) response curves to obtain the maximum rates of ribulose-1,5-bisphosphate carboxylase oxygenase carboxylation (Vcmax) and electron transport (Jmax) is time-consuming and labour-intensive. Instead, the rapid A–Ci response (RACiR) technique provides a potential, high-efficiency method. However, efficient parameter settings of RACiR technique for evergreen broadleaved species remain unclear. Here, we used Li-COR LI-6800 to obtain the optimum parameter settings of RACiR curves for evergreen broadleaved trees and shrubs. We set 11 groups of CO2 gradients ([CO2]), i.e. R1 (400–1500 ppm), R2 (400–200–800 ppm), R3 (420–20–620 ppm), R4 (420–20–820 ppm), R5 (420–20–1020 ppm), R6 (420–20–1220 ppm), R7 (420–20–1520 ppm), R8 (420–20–1820 ppm), R9 (450–50–650 ppm), R10 (650–50 ppm) and R11 (650–50–650 ppm), and then compared the differences between steady-state A–Ci and RACiR curves. We found that Vcmax and Jmax calculated by steady-state A–Ci and RACiR curves overall showed no significant differences across 11 [CO2] gradients (P > 0.05). For the studied evergreens, the efficiency and accuracy of R2, R3, R4, R9 and R10 were higher than the others. Hence, we recommend that the [CO2] gradients of R2, R3, R4, R9 and R10 could be applied preferentially for measurements when using the RACiR technique to obtain Vcmax and Jmax of evergreen broadleaved species.
 
Herein, the effect of cationic antiseptics (chlorhexidine, picloxidine, miramistin, octenidine) on the initial processes of the delivery of light energy and its efficient use by the reaction centers in intact spinach photosystem II core complexes has been investigated. The characteristic effects—an increase in the fluorescence yield of light-harvesting pigments and a slowdown in the rate of energy migration in bacterial photosynthetic chromatophores has been recently demonstrated mainly in the presence of octenidine (Strakhovskaya et al., in Photosynth Res 147:197–209, 2021; Knox et al., in Photosynth Res, https://doi.org/10.1007/s11120-022-00909-8, 2022). In this study, we also observed that in the presence of octenidine, the fluorescence intensity of photosystem II core complexes increases by 5–10 times, and the rate of energy migration from antennae to the reaction centers decreases by 3 times. In addition, with an increase in the concentration of this antiseptic, a new effect related to a shift of the spectrum, absorption and fluorescence to the short-wavelength region has been found. Similar effects were observed when detergent Triton X-100 was added to photosystem II samples. We concluded that the antiseptic primarily affects the structure of the internal light-harvesting antenna (CP43 and CP47), through which the excitation energy is delivered to the reaction center. As a result of such an impact, the chlorophyll molecules in this structure are destabilized and their optical and functional characteristics change.
 
Chlorophyll biosynthetic pathway in higher plants and the steps were highlighted with yellow which are modified for enhancing photosynthetic efficiency
Chlorophyll degradation pathway and reutilization of the chlorophyll catabolite phytol in the chlorophyll synthesis (genes engineered for photosynthesis enhancement are highlighted in yellow)
Major pathways and enzymes targeted for enhancing the photosynthetic efficiency in different crops. The targeted metabolic pathways and enzymes are tagged with the red stars, which includes RuBisCO and RuBisCo activase enzyme, manipulation of the chlorophyll metabolism, electron transport chain, light-harvesting complex, C3 and C4 cycle enzymes, bioengineering of the stomatal response (enhancing stomatal conductance), the photorespiration pathway and introduction of the aquaporins importing CO2
Various factors working to defend the plant during the abiotic stress
Current global agricultural production needs to be increased to feed the unconstrained growing population. The changing climatic condition due to anthropogenic activities also makes the conditions more challenging to meet the required crop productivity in the future. The increase in crop productivity in the post green revolution era most likely became stagnant, or no major enhancement in crop productivity observed. In this review article, we discuss the emerging approaches for the enhancement of crop production along with dealing to the future climate changes like rise in temperature, increase in precipitation and decrease in snow and ice level, etc. At first, we discuss the efforts made for the genetic manipulation of chlorophyll metabolism, antenna engineering, electron transport chain, carbon fixation, and photorespiratory processes to enhance the photosynthesis of plants and to develop tolerance in plants to cope with changing environmental conditions. The application of CRISPR to enhance the crop productivity and develop abiotic stress-tolerant plants to face the current changing climatic conditions is also discussed.
 
Pictorial representation of the metabolite flow and key reactions involved in concentrating CO2 around Rubisco in the bundle sheath cells of the three distinct C4 subtypes, i.e., NADP-ME, NAD-ME, and PEPCK. CO2 is converted to HCO3⁻ by βCA (1) in cytosol of the mesophyll cells. HCO3⁻ and PEP are converted to OAA by PEPC (2). In NADP-ME type OAA is converted to malate and transported to chloroplasts of BS cells for its decarboxylation by NADP-ME (4a). In NAD-ME type Aspartate is transported to mitochondria of BS cells, where it is converted to malate and decarboxylated by NAD-ME (4b). In PEPCK subtype both Asp and malate are transported to cytosol and mitochondria of BS cells respectively. Asp is converted back to OAA (6) and decarboxylated by PEPCK (4c). Malate is decarboxylated by NAD-ME (4b). Mesophyll cells are in yellow, bundle sheath cells are in light green, chloroplasts are in dark green, mitochondria are in orange and vascular bundles are in purple. MAL, malate; OAA, Oxaloacetate; PEP, Phosphoenolpyruvate; ASP, Aspartate; ALA, Alanine; PYR, Pyruvate. 1. β carbonic anhydrase 2. phosphoenolpyruvate carboxylase 3. NAD/P malate dehydrogenase 4a. NADP-malic enzyme 4b. NAD-malic enzyme 4c. phosphoenolpyruvate carboxykinase 5. pyruvate orthophosphate dikinase 6. aspartate amino acids transferases 7. alanine amino acids transferase
Key roles played by C4 enzyme isoforms in C3 plants
Pathways and reactions catalyzed by isoforms of C4 photosynthetic enzymes in C3 plants. (1) Atmospheric CO2 entering into cells is either converted to HCO3⁻ by the action of cytosolic βCA or diffuses to chloroplasts for Rubisco carboxylation reaction (red arrows). HCO3⁻ is assimilated into OAA by cytoplasmic PEPC (2). OAA so produced is converted to malate by cytosolic MDH (3). OAA and malate can also be utilized in the TCA cycle for amino acids and fatty acids biosynthesis (Fan et al. 2013; Kandoi et al. 2016), and to maintain redox homeostasis via malate valve operating in chloroplast and mitochondrial membranes (blue arrows) (Kandoi et al. 2018). Malate can be converted to pyruvate in cytosol and chloroplast by cytoplasmic and plastidic-Malic enzyme, respectively, generating CO2 and NADPH. In chloroplasts CO2 is used in the C3 cycle, whereas, NADPH can be used in a variety of biosynthetic processes and antioxidant defense pathways and production of secondary metabolites via shikimate pathway. NADPH in the cytosol is used for the synthesis of compatible solutes, membrane repair, antioxidant system etc. In the cytoplasm and chloroplast pyruvate is converted to PEP via cytsolic and plastidic PPDK (5), respectively. PPDK in C3 plants is also associated with the formation of transport amino acid glutamine (pathway shown in orange arrows) (Taylor et al. 2010). During seed germination PPDK is involved in the mobilization of protein reserves to form sugars via gluconeogenesis (pathway shown in green arrows) (Eastmond et al. 2015). (6). PEPCK is involved in the gluconeogenic pathway utilizing lipid reserves via the Glyoxylate cycle for germination and seedling establishment (route shown in purple arrows) (Rylott et al. 2003). Acetyl CoA from fatty acids breakdown enters glyoxysomes initiating the glyoxylate cycle. Glyoxylate and succinate are produced in the glyoxysomes from isocitrate. Glyoxylate is converted to malate in glyoxysomes, whereas, succinate is transported to mitochondria and is metabolized to OAA in the TCA cycle. CA, carbonic anhydrase; PEPC, phosphoenolpyruvate carboxylase; MDH, malate dehydrogenase; NAD/NADP-ME, NAD/NADP-malic enzyme; PEPCK, phosphoenolpyruvate carboxykinase; PPDK, pyruvate orthophosphate dikinase; OAA, oxaloacetic acid; NADPH, nicotinamide adenine dinucleotide phosphate, reduced; ROS, reactive oxygen species; 2-OG, 2-oxoglutarate; Glu, glutamate; Gln, glutamine; DTC, dicarboxylate-tricarboxylate carrier; DiT1; dicarboxylate transporter 1; Cys, cysteine; Thr, threonine; Trp, tryptophan; Ser, serine; Gly, glycine, SAP, shikimic acid pathway
Diagram showing re-assimilation of carbon and nitrogen compounds present in xylem stream by the C4 enzymes present in cells surrounding vascular tissues of C3 plants. Pathway for re-fixation of carbon compounds (black arrows). CO2 generated from root/stem respiration or from the soil either moves directly to the xylem or converted to OAA by root PEPC, which is subsequently converted to malate. Both malate and CO2 move up to the stem and petiole region. CA converts CO2 to HCO3⁻, which is subsequently assimilated into OAA by PEPC (1) OAA can be utilized for replenishment of TCA cycle intermediates. OAA is converted to Malate and subsequently decarboxylated to pyruvate by NADP-ME (2a) and NAD-ME (2b) in chloroplasts and mitochondria, respectively. The CO2 released in the reaction is utilized in C3 cycle to produce sugars. The chloroplastic PPDK (3) regenerates the CO2 acceptor PEP from pyruvate. Alternatively, pyruvate can be used for the synthesis of sucrose via gluconeogenesis and is transported to the developing plant tissues (Hibberd and Quick 2002; Berveiller and Damesin 2008). Pathway for re-absorption of nitrogenous compounds (blue arrows): The major amino acid, Asn, present in the xylem sap is re-cycled with the help of C4 enzyme PEPCK (4) present in parenchyma cells between xylem and phloem. Asn is converted to OAA via Asp, which eventually is converted to PEP by PEPCK. The PEP produced in both these routes could have multiple fates depending on the plant needs. It can either be used for the production of secondary metabolites such as lignin via the shikimate pathway or to generate sugar via gluconeogenesis. Otherwise, PEPC restores OAA pool from PEP and CO2 to initiate a new round of CO2 re-fixation (Bailey and Leegood, 2016; Brown Naomi et al. 2009). CA, carbonic anhydrase; PEPC, phosphoenolpyruvate carboxylase; NAD/NADP-ME, NAD/NADP-malic enzyme; PEPCK, phosphoenolpyruvate carboxykinase; PPDK, pyruvate orthophosphate dikinase; PEP, phosphoenolpyruvate; OAA, oxaloacetic acid; Asn, asparagine; Asp, aspartate
As compared to C3, C4 plants have higher photosynthetic rates and better tolerance to high temperature and drought. These traits are highly beneficial in the current scenario of global warming. Interestingly, all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), the decarboxylating enzymes NAD/NADP-malic enzyme (NAD/NADP-ME), and phosphoenolpyruvate carboxykinase (PEPCK), and finally pyruvate orthophosphate dikinase (PPDK) catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation. Consistent with this view differential expression pattern of these non-photosynthetic C3 isoforms has been observed in different tissues across the plant developmental stages, such as germination, grain filling, and leaf senescence. Also abundance of these C3 isoforms is increased considerably in response to environmental fluctuations particularly during abiotic stress. Here we review the vital roles played by C3 isoforms of C4 enzymes and the probable mechanisms by which they help plants in acclimation to adverse growth conditions. Further, their potential applications to increase the agronomic trait value of C3 crops is discussed.
 
A Distribution of unique RC protonation microstates at pH 7. The MCCE calculations were run on one snapshot of the MD trajectory SQ2 (Table 1). Each dot represents one protonation microstate, and natural log of the probability gives the fraction of times a state with this protonation distribution is accepted by MC sampling. Each protonation microstate can be found in many MCCE side chain conformation states with a range of microstates with different energy. The dot color and size report on this range of energies with bigger, darker dots having a wider energy distribution. Energies are in kcal/mol. B The running sum of the probabilities of the protonation microstates. Protonation microstates are graphed in order of decreasing probability. Each point adds the probability to that of all points to its left
The correlation of residue charge in the protonation microstates in the MCCE calculations with QB•− of a snapshot from the MD trajectory SQ2. A Residues with significant correlation. The residues names are given as Chain Designator (H, L or M) and the one letter code for the amino acids followed by the residue number. Residues with stronger correlation in sticks, more weakly interacting residues are lines. QB•−: the ubiquinone head and tail as magenta sticks, Fe: yellow sphere. B weighted Pearson’s correlation of protonation states of individual residues. Blue: positive correlation; red: negative correlation; darker color indicates stronger correlation
Hydrogen bond network leading to QB•− in a snapshot from SQ2. Residues are colored with: QB orange; Grotthuss competent hydroxyl residues greens; the bases blue, and acids red with darker shade indicating the residue is charged and the lighter shade that it is neutral. A Network connection drawn with Cytoscape (Shannon et al. 2003). Each node is a residue. Each line is a connection through two or fewer waters. B Pymol picture of the SQ2 snapshot from the network shown in (A). Residues are labeled with one letter amino acid code_subunitResidueNumber. Cytoplasm is on the left side as noted. S_H80 and S_L4 could not be shown in this orientation of the protein
Protons participate in many reactions. In proteins, protons need paths to move in and out of buried active sites. The vectorial movement of protons coupled to electron transfer reactions establishes the transmembrane electrochemical gradient used for many reactions, including ATP synthesis. Protons move through hydrogen bonded chains of waters and hydroxy side chains via the Grotthuss mechanism and by proton binding and release from acidic and basic residues. MCCE analysis shows that proteins exist in a large number of protonation states. Knowledge of the equilibrium ensemble can provide a rational basis for setting protonation states in simulations that fix them, such as molecular dynamics (MD). The proton path into the QB site in the bacterial reaction centers (RCs) of Rb. sphaeroides is analyzed by MD to provide an example of the benefits of using protonation states found by the MCCE program. A tangled web of side chains and waters link the cytoplasm to QB. MCCE analysis of snapshots from multiple trajectories shows that changing the input protonation state of a residue in MD biases the trajectory shifting the proton affinity of that residue. However, the proton affinity of some residues is more sensitive to the input structure. The proton transfer networks derived from different trajectories are quite robust. There are some changes in connectivity that are largely restricted to the specific residues whose protonation state is changed. Trajectories with QB•− are compared with earlier results obtained with QB [Wei et. al Photosynthesis Research volume 152, pages153–165 (2022)] showing only modest changes. While introducing new methods the study highlights the difficulty of establishing the connections between protein conformation.
 
Modern models for estimating canopy photosynthetic rates (Ac) can be broadly classified into two categories, namely, process-based mechanistic models and artificial intelligence (AI) models, each category having unique strengths (i.e., process-based models have generalizability to a wide range of situations, and AI models can reproduce a complex process using data without prior knowledge about the underlying mechanism). To exploit the strengths of both categories of models, a novel “hybrid” canopy photosynthesis model that combines process-based models with an AI model was proposed. In the proposed hybrid model, process-based models for single-leaf photosynthesis and image analysis first transform raw inputs (environmental data and canopy images) into the single-leaf photosynthetic rate (AL) and effective leaf area index (Lc)), after which AL and Lc are fed into an artificial neural network (ANN) model to predict Ac. The hybrid model successfully predicted the diurnal cycles of Ac of an eggplant canopy even with a small training dataset and successfully reproduced a typical Ac response to changes in the CO2 concentration outside the range of the training data. The proposed hybrid AI model can provide an effective means to estimate Ac in actual crop fields, where obtaining a large amount of training data is difficult.
 
We provide here an overview of the remarkable life and outstanding research of David (Dave) Charles Fork (March 4, 1929–December 13, 2021) in oxygenic photosynthesis. In the words of the late Jack Edgar Myers, he was a top ‘photosynthetiker’. His research dealt with novel findings on light absorption, excitation energy distribution, and redistribution among the two photosystems, electron transfer, and their relation to dynamic membrane change as affected by environmental changes, especially temperature. David was an attentive listener and a creative designer of experiments and instruments, and he was also great fun to work with. He is remembered here by his family, coworkers, and friends from around the world including Australia, France, Germany, Japan, Sweden, Israel, and USA.
 
Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.
 
Microalgae require copper (Cu) in trace levels for their growth and metabolism, it is a vital component of certain metalloproteins. Although this element has been widely studied concerning microalgae physiology, the effects of environmentally relevant levels have been less studied. We studied the photosynthesis and growth of the Chlorophyte Monoraphidium sp. exposed to Cu ranging from low (1.7 nM) to high (589.0 nM) free Cu ions (Cu²⁺) concentrations. The growth rate was unaffected by Cu concentrations in the range of 1.7–7.4 nM Cu²⁺, but decreased beyond it. The relative maximum electron transport rate (rETRm), saturation irradiance (Ek), photochemical quenching (qP and qL), and PSII operating efficiency (ΔF/Fm′)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\Delta F/F_{m}^{\prime } )$$\end{document} were stimulated in the 3.4–7.4 nM Cu²⁺ range, concentrations slightly higher than the control, whereas non-photochemical quenching (NPQ) gradually increased with increasing Cu²⁺. The photosystem II antenna size [Sigma (II)440] increased under high Cu (589.0 nM), which resulted in a decrease in the quinone A (QA) reduction time (tau). In contrast, the QA re-oxidation time was unaffected by Cu exposure. These findings show that a slight increase in Cu stimulated photosynthesis in Monoraphidium sp., whereas high Cu reduced photosynthesis and increased the dissipation of captured light energy. This research is a contribution to the understanding of the dynamic photo-physiological responses of Monoraphidium sp. to Cu ions.
 
Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.
 
In this mini review, we focus on recent advances in the atomistic modeling of biological light-harvesting (LH) complexes. Because of their size and sophisticated electronic structures, multiscale methods are required to investigate the dynamical and spectroscopic properties of such complexes. The excitation energies, in this context also known as site energies, excitonic couplings, and spectral densities are key quantities which usually need to be extracted to be able to determine the exciton dynamics and spectroscopic properties. The recently developed multiscale approach based on the numerically efficient density functional tight-binding framework followed by excited state calculations has been shown to be superior to the scheme based on pure classical molecular dynamics simulations. The enhanced approach, which improves the description of the internal vibrational dynamics of the pigment molecules, yields spectral densities in good agreement with the experimental counterparts for various bacterial and plant LH systems. Here, we provide a brief overview of those results and described the theoretical foundation of the multiscale protocol.
 
Three-dimensional structure of the reaction center from R. sphaeroides. Shown are the amino acid residues that were tested for their contribution to proton release, including residues from the L subunit, Thr L130, Leu L131, Arg L135, Asp L155, Tyr L162, Ser L244, and Cys L247 (yellow) and from the M subunit, Arg M164, Asp M184, Thr M186, Ser M190, His M193, and Tyr M210 (blue). Also shown are the cofactors, including the bacteriochlorophyll dimer P (red), the primary quinone QA and secondary quinone QB (red), and the bacteriochlorophyll monomers, bacteriopheophytins, and non-heme iron (wheat)
Kinetics of the recoveries of the absorbance changes at 865 nm after 5 min of illumination for reaction centers from wild-type (black), DN(M184) (brown), DN(L155) (red), and TV(L130) (blue). Traces are normalized using the absorbance at 865 nm measured at the end of the illumination. Conditions: 1 µM reaction centers in 15 mM Tris–Cl, pH 8.0, 0.05% Triton ×-100, 100 µM terbutryn
Comparison of results after five minutes of illumination for the set of twelve mutant reaction centers relative to wild-type, showing the classification into three groups (red, wheat, and blue). a The proton release for each type of reaction center was measured as a voltage change and normalized to the voltage change observed for wild-type (Table S2). b The extent of the long-lived P⁺ signal was measured by the change in absorbance at 865 nm remaining after 5 to 10 min in the dark and normalized to the value obtained for the wild-type (Fig. 2 and Table S2). c Structure of the reaction center showing the twelve amino acid residues colored according to their grouping and the bacteriochlorophyll dimer (green). View is down the two-fold symmetry axis of the protein
Structural model of the proposed proton pathway. A hydrogen-bond network was identified near the bacteriochlorophyll dimer (green) by the effects of changing three amino acid residues, Asp L155, Ser M190, and His M193 (purple), and including contributions from four additional amino acid residues, Trp L156, Ser L158, Asn M195, and Tyr M198 (cyan) and bound water molecules (red spheres). The pathway is anchored at each end by residues Thr L130 and Arg M164 (blue)
Insight into control of proton transfer, a crucial attribute of cellular functions, can be gained from investigations of bacterial reaction centers. While the uptake of protons associated with the reduction of the quinone is well characterized, the release of protons associated with the oxidized bacteriochlorophyll dimer has been poorly understood. Optical spectroscopy and proton release/uptake measurements were used to examine the proton release characteristics of twelve mutant reaction centers, each containing a change in an amino acid residue near the bacteriochlorophyll dimer. The mutant reaction centers had optical spectra similar to wild-type and were capable of transferring electrons to the quinones after light excitation of the bacteriochlorophyll dimer. They exhibited a large range in the extent of proton release and in the slow recovery of the optical signal for the oxidized dimer upon continuous illumination. Key roles were indicated for six amino acid residues, Thr L130, Asp L155, Ser L244, Arg M164, Ser M190, and His M193. Analysis of the results points to a hydrogen-bond network that contains these residues, with several additional residues and bound water molecules, forming a proton transfer pathway. In addition to proton transfer, the properties of the pathway are proposed to be responsible for the very slow charge recombination kinetics observed after continuous illumination. The characteristics of this pathway are compared to proton transfer pathways near the secondary quinone as well as those found in photosystem II and cytochrome c oxidase.
 
There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072–1075, 2008).
 
Nitrogen (N) deficiency represents an important limiting factor affecting photosynthetic productivity and the yields of crop plants. Significant reported differences in N use efficiency between the crop species and genotypes provide a good background for the studies of diversity of photosynthetic and photoprotective responses associated with nitrogen deficiency. Using distinct wheat (Triticum aestivum L.) genotypes with previously observed contrasting responses to nitrogen nutrition (cv. Enola and cv. Slomer), we performed advanced analyses of CO2 assimilation, PSII, and PSI photochemistry, also focusing on the heterogeneity of the stress responses in the different leaf levels. Our results confirmed the loss of photosynthetic capacity and enhanced more in lower positions. Non-stomatal limitation of photosynthesis was well reflected by the changes in PSII and PSI photochemistry, including the parameters derived from the fast-fluorescence kinetics. Low photosynthesis in N-deprived leaves, especially in lower positions, was associated with a significant decrease in the activity of alternative electron flows. The exception was the cyclic electron flow around PSI that was enhanced in most of the samples with a low photosynthetic rate. We observed significant genotype-specific responses. An old genotype Slomer with a lower CO2 assimilation rate demonstrated enhanced alternative electron flow and photorespiration capacity. In contrast, a modern, highly productive genotype Enola responded to decreased photosynthesis by a significant increase in nonphotochemical dissipation and cyclic electron flow. Our results illustrate the importance of alternative electron flows for eliminating the excitation pressure at the PSII acceptor side. The decrease in capacity of electron acceptors was balanced by the structural and functional changes of the components of the electron transport chain, leading to a decline of linear electron transport to prevent the overreduction of the PSI acceptor side and related photooxidative damage of photosynthetic structures in leaves exposed to nitrogen deficiency.
 
This study investigated the effect of transient submergence on the recovery of photosynthetic activity and translocation of photosynthate in IR67520 (Sub1A genotype) and IR72442 (non-Sub1A genotype) using ¹³C-labeled tracer, coupled with some photosynthetic physiological assessments. Plant growth, photosynthetic capacity, and photosynthetic recovery were studied by treating the two rice genotypes without or completely submerged for 7 days in transparent acrylic tanks filled with water to a depth of 80 cm, followed by 7 days of reaeration. Results revealed that the IR67520 was able to obtain new carbon source for assimilation during at 7 days of recovery periods. The IR72442 genotype partitioned ¹³C to the newly developed upper leaves more than the IR67520 genotype did. This was due to its inability to obtain CO2 from other source during post submergence. Recovery of chlorophyll content, ability to retain higher biomass, and ability to grow faster at 7 days of recovery periods also indicated the ability of Sub1A genotype to reactivate its photosynthetic capacity.
 
A Gene structural overview of soybean and Arabidopsis genes that encode for Rca. Thick horizontal black and grey bars indicate untranslated and protein coding portions of the mature mRNA, respectively. Thin horizontal lines indicate introns. Vertical lines mark (in order) the following corresponding regions of the protein product: chloroplast targeting peptide, N-domain, α/ß domain, α-helical domain, C-domain, and C-terminal extension. Models with only 4 vertical lines do not encode a C-terminal extension. The inverted triangles indicate the region of sequence targeted by the RNAi construct. B mRNA sequence alignment showing RNAi-targeted regions of soybean Rcas. The cloned portion of the Glyma02g249600 mRNA used in RNAi vector construction is shown in the top line consensus sequence; the binding sites of the cloning primers are underlined. Exon junctions are indicated by the dots at positions 2 or 3. Conserved residues are omitted from non-consensus sequences
Western blot (A) and densitometry (B) based determination of Rca-α presence of wild-type (lane 1) and transgenic (lanes 3–8) soybean. M contains the ladder. Percentages in B were determined by dividing the intensity of the Rca-α band by the intensity of the Rca-ß band. The determined Rca-α status of each of the segregating transgenics is shown at the bottom of B
Photosynthetic induction curves of transgenics in A growth chambers at 26 °C, B growth chambers at 38 °C, and C the field. A step increase of 20 to 500 µmol m⁻² s⁻¹ PPFD was used for the growth chamber measurements while a step increase of 100 to 1700 µmol m⁻² s⁻¹ PPFD was used for the field measurements. Error bars represent one standard deviation. Boxes indicate the portion of the response curve that was used for fitting the time constants (τs), which are inset in each graph along with the replicate numbers
Rubisco activase (Rca) facilitates the catalytic repair of Rubisco, the CO2-fixing enzyme of photosynthesis, following periods of darkness, low to high light transitions or stress. Removal of the redox-regulated isoform of Rubisco activase, Rca-α, enhances photosynthetic induction in Arabidopsis and has been suggested as a strategy for the improvement of crops, which may experience frequent light transitions in the field; however, this has never been tested in a crop species. Therefore, we used RNAi to reduce the Rca-α content of soybean (Glycine max cv. Williams 82) below detectable levels and then characterized the growth, photosynthesis, and Rubisco activity of the resulting transgenics, in both growth chamber and field conditions. Under a 16 h sine wave photoperiod, the reduction of Rca-α contents had no impact on morphological characteristics, leaf expansion rate, or total biomass. Photosynthetic induction rates were unaltered in both chamber-grown and field-grown plants. Plants with reduced Rca-α content maintained the ability to regulate Rubisco activity in low light just as in control plants. This result suggests that in soybean, Rca-α is not as centrally involved in the regulation of Rca oligomer activity as it is in Arabidopsis. The isoform stoichiometry supports this conclusion, as Rca-α comprises only ~ 10% of the Rubisco activase content of soybean, compared to ~ 50% in Arabidopsis. This is likely to hold true in other species that contain a low ratio of Rca-α to Rca-ß isoforms.
 
Nonphotochemical quenching acts as a frontline response to prevent excitation energy from reaching the photochemical reaction center of photosystem II before photodamage occurs. Strong fluorescence quenching after merely one multi-turnover saturating light pulse characterizes a unique feature of nonphotochemical quenching in red algae. Several mechanisms underlying red algal nonphotochemical quenching have been proposed, yet which process(es) dominantly account for the strong fluorescence quenching is still under discussion. Here we assessed multiple nonphotochemical quenching processes in the extremophilic red alga Cyanidioschyzon merolae under light pulse and continuous illumination conditions. To assess the nonphotochemical quenching processes that might display different kinetics, fluorescence emission spectra at 77 K were measured after different periods of light treatments, and external fluorophores were added for normalization of the fluorescence level. The phycobilisome- and photosystem II-related nonphotochemical quenching processes were distinguished by light preferentially absorbed by phycobilisomes and photosystems, respectively. Multiple nonphotochemical quenching processes, including the energetic decoupling of phycobilisomes from photosystem II, the energy spillover from phycobilisomes to photosystem I and from photosystem II to photosystem I, were identified along with the previously identified intrinsic quenching within photosystem II. The ability to use multiple nonphotochemical quenching processes appears to maximize the light harvesting efficiency for photochemistry and to provide the flexibility of the energy redistribution between photosystem II and photosystem I. The effect of the various ionophores on the nonphotochemical quenching level suggests that nonphotochemical quenching is modulated by transmembrane gradients of protons and other cations.
 
Chlorosomes of green bacteria can be considered as a prototype of future artificial light-harvesting devices due to their unique property of self-assembly of a large number of bacteriochlorophyll (BChl) c/d/e molecules into compact aggregates. The presence of carotenoids (Cars) in chlorosomes is very important for photoprotection, light harvesting and structure stabilization. In this work, we studied for the first time the electrochromic band shift (Stark effect) in Cars of the phototrophic filamentous green bacterium Chloroflexus (Cfx.) aurantiacus induced by fs light excitation of the main pigment, BChl c. The high accuracy of the spectral measurements permitted us to extract a small wavy spectral feature, which, obviously, can be associated with the dynamic shift of the Car absorption band. A global analysis of spectroscopy data and theoretical modeling of absorption spectra showed that near 60% of Cars exhibited a red Stark shift of ~ 25 cm−1 and the remaining 40% exhibited a blue shift. We interpreted this finding as evidence of various orientations of Car in chlorosomes. We estimated the average value of the light-induced electric field strength in the place of Car molecules as ~ 106 V/cm and the average distance between Car and the neighboring BChl c as ~ 10 Å. We concluded that the dynamics of the Car electrochromic band shift mainly reflected the dynamics of exciton migration through the chlorosome toward the baseplate within ~ 1 ps. Our work has unambiguously shown that Cars are sensitive indicators of light-induced internal electric fields in chlorosomes.
 
Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-Km in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome b6f, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt b6f to P700+ and the proton-coupled linear flow from PSII to Cyt b6f to P700+. Analyzing responses of O2 evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the QA-reduced PSII units after their QB site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e−. Feedback regulation of this ratio leads to oscillations under saturating light and CO2, when the rate is Pi-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e− ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O2 reduction), are only a fraction of a per cent. The Pi limitation causes slip in the ATP synthase, slightly increasing the basic 12H+/3ATP requirement.
 
A portrait of Paul Levine in his study at home, provided by his wife Marie-Louise Rouff
I present my personal reminiscence of Paul Levine—a highly innovative scientist who did seminal work in photosynthesis. He was among the first to initiate and use a genetic approach toward photosynthesis. He greatly helped in establishing the green unicellular alga Chlamydomonas reinhardtii as a powerful model system not only for understanding the function of the photosynthetic apparatus but also for studying its biogenesis and regulation. During the period he spent at Harvard, he made several ground-breaking contributions such as identifying and establishing the order of some components of the photosynthetic electron transport chain as well as determining their genetic origin. He trained many students and post-doctoral fellows several of whom later became prominent in this field and in other areas of plant science.
 
Leaf stomatal conductance rates, relative to air and soil temperature at the experimental site
Leaf ΦPSII, air, and soil temperature at the experimental site. Strong correlations were observed (p < 0.001) between ΦPSII for the three willow cultivars and air and soil temperature and daylength (Table 3). We also observed a significant correlation between this parameter and snowfall for cultivar India (p < 0.04) and Fish Creek (p < 0.05)
Leaf transpiration (E) rates and air and soil temperature at the experimental site
Chlorophyll a, b, total chlorophyll, chlorophyll a/b ratio, and total carotenoids in the three willow cultivars during the trial
Leaf senescence at the end of the growing season is a complex process stimulated by changes in daylength and temperature that prepares deciduous trees for winter by reducing photosynthetic rates and remobilization of nutrients. Extending the duration of photosynthetic activity could have important consequences for the translocation of heavy metals in the phytoremediation of contaminated sites using deciduous trees like willow. In the present study, three Salix cultivars (‘India,’ ‘SX67,’ and ‘Fish Creek’) that were observed to maintain green leaves late into autumn were evaluated over an 11-week period extending from mid-September to mid-November on a brownfield site in Montreal, Canada. Gas exchange rates, chlorophyll fluorescence, and leaf pigments were measured weekly. A general trend of declining stomatal conductance and transpiration were observed early in the trial, followed by reductions in photosynthetic efficiency and concentrations of chl a, chl b, and carotenoids, in agreement with other studies. In particular, the cultivar ‘Fish Creek’ had higher rates of gas exchange and pigment concentrations than either ‘SX67’ or ‘India,’ but values for these parameters also declined more rapidly over the course of the trial. Both photoperiod and soil and air temperatures were strong drivers of changes in photosynthetic activity in all three of these cultivars according to correlation analyses. Further studies should focus on their biomass production and heavy metal accumulation capacity in light of the observed variation in photosynthetic activity stimulated by seasonal changes in light and temperature.
 
Although many photosynthesis related processes are known to be controlled by the circadian system, consequent changes in photosynthetic activities are poorly understood. Photosynthesis was investigated during the daily cycle by chlorophyll fluorescence using a PAM fluorometer in Pulmonaria vallarsae subsp. apennina , an understory herb. A standard test consists of a light induction pretreatment followed by light response curve (LRC). Comparison of the major diagnostic parameters collected during day and night showed a nocturnal drop of photosynthetic responses, more evident in water-limited plants and consisting of: (i) strong reduction of flash-induced fluorescence peaks (FIP), maximum linear electron transport rate (J max , ETR EM ) and effective PSII quantum yield (Φ PSII ); (ii) strong enhancement of nonphotochemical quenching ( NPQ ) and (iii) little or no change in photochemical quenching qP , maximum quantum yield of linear electron transport ( Φ ), and shape of LRC ( θ ). A remarkable feature of day/night LRCs at moderate to high irradiance was their linear-parallel course in double-reciprocal plots. Photosynthesis was also monitored in plants subjected to 2–3 days of continuous darkness (“long night”). In such conditions, plants exhibited high but declining peaks of photosynthetic activity during subjective days and a low, constant value with elevated NPQ during subjective night tests. The photosynthetic parameters recorded in subjective days in artificial darkness resembled those under natural day conditions. On the basis of the evidence, we suggest a circadian component and a biochemical feedback inhibition to explain the night depression of photosynthesis in P. vallarsae .
 
We present here a tribute to one of the foremost biophysicists of our time, Vladimir Anatolievich Shuvalov, who made important contributions in bioenergetics, especially on the primary steps of conversion of light energy into charge-separated states in both anoxygenic and oxygenic photosynthesis. For this, he and his research team exploited pico- and femtosecond transient absorption spectroscopy, photodichroism & circular dichroism spectroscopy, light-induced FTIR (Fourier-transform infrared) spectroscopy, and hole-burning spectroscopy. We remember him for his outstanding leadership and for being a wonderful mentor to many scientists in this area. Reminiscences by many [Suleyman Allakhverdiev (Russia); Robert Blankenship (USA); Richard Cogdell (UK); Arvi Freiberg (Estonia); Govindjee Govindjee (USA); Alexander Krasnovsky, jr, (Russia); William Parson (USA); Andrei Razjivin (Russia); Jian- Ren Shen (Japan); Sergei Shuvalov (Russia); Lyudmilla Vasilieva (Russia); and Andrei Yakovlev (Russia)] have included not only his wonderful personal character, but his outstanding scientific research.
 
Separation and biochemical characterizations of oligomeric form of acpPC antenna from Fugacium kawagutii. a SDG separation of solubilized thylakoid membrane. Sucrose concentration (M) of each step is labelled. Red square (dashed line) indicated sample is subjected for analysis. b Blue native PAGE analysis of the most intense band loaded directly after centrifugation (SDG, left panel) or after additional purification with ion exchange chromatography and subsequent concentration using 100 kDa (MWCO) filtration (SDG-IEC, right panel). The flowthrough (FT) from 100 kDa ultrafiltration was further concentrated using 30 kDa (MWCO) filtration, the retentate is denoted as SDG-IEFFT, but not subjected for further analysis. SDG reveals single band with MW of ~ 170 kDa. Further processing of the band through IEC/ultrafiltration altered the original oligomeric state resulting in several oligomeric forms as indicated by a ladder-like band pattern. (c) SDS-PAGE analysis of SDG- and SDG-IEC-acpPC. PSII—photosystem II from a cyanobacterium, Synechocystis sp. PCC 6803, PSI-T—photosystem I trimer from Synechocystis sp. PCC 6803, MW—molecular weight, SDG—sucrose density gradient, SDS—sodium dodecyl sulfate
a 77 K steady-state absorption (Abs) and b fluorescence emission (Fluo) spectra of trimeric and oligomeric forms (SDG-acpPC, SDG-IEC-acpPC) of the acpPC complex from Fugacium kawagutii. The absorption spectra are normalized at 590 nm upon assumption that Chl c2 content will be minimally affected by post-SDG purification/processing. Difference spectrum (red dash-dot, SDG-minus-SDG-IEC) shows that further sample processing removes some fraction of Chl a and diadinoxanthin (Diad). The difference spectrum is very similar to the sum of the reference spectra of both pigments taken in 2-MTHF at 77 K (spectrally shifted to match peak positions)
Spectral reconstruction of the absorption spectra of SDG-acpPC and its solvent extract. a Spectral reconstruction of 77 K absorption spectrum of SDG-acpPC with the absorption spectra of each individual pigment taken at 77 K in 2-MTHF and adequately shifted. For Chls, position of the Soret band and for carotenoids, position of the (0–0) vibronic band is provided. b Spectral reconstruction of absorption spectrum of methanol extract of SDG-acpPC with absorption spectra of individual pigments taken in methanol. This analysis provides the estimated pigment stoichiometry in the complex. For more details on the procedure refer to SI
77 K time-resolved fluorescence (TRF) decay imaging of oligomeric acpPC after SDG and SDG-IEC, respectively. The samples were excited at 540 nm (Per—peridinin). a, b 2D pseudo-color TRF profiles in which colors represent emission intensities (photon counts, ph. c.). c, d Global analysis of fluorescence emission decay profiles. Refer to the main text for more details. The profiles were smoothed for better clarity. SAFS—species associated fluorescence spectra (smoothed for clarity), nr—not resolved, F682–F698—various Chl a fluorescence species
Global analysis of TA datasets of a, b SDG-acpPC and c, d SDG-IEC-acpPC preparations performed according to anticipated models of excitation migration pathways presented in the above graphs. It was assumed that excitation at 540 nm will initially promote peridinin (Per) to its S2 excited singlet state. Subsequently, excitation energy quickly funnels to Chls a directly or via Chl c2. Modelling demonstrated that the last stage of the excitation migration pathway alternates at cryogenic temperature. Refer to the main text for more details. Time constants in parentheses (and associated routes) are for 77 K. SADS—species associated decay spectrum, Car—carotenoid (peridinin and/or diadinoxanthin), *—excited states
Light-harvesting antennas in photosynthesis capture light energy and transfer it to the reaction centers (RCs) where photochemistry takes place. The sustainable growth of the reef-building corals relies on a constant supply of the photosynthates produced by the endosymbiotic dinoflagellate, belonging to the family of Symbiodiniaceae. The antenna system in this group consists of the water-soluble peridinin-chlorophyll a-protein (PCP) and the intrinsic membrane chlorophyll a-chlorophyll c2-peridinin protein complex (acpPC). In this report, a nonameric acpPC is reported in a dinoflagellate, Fugasium kawagutii (formerly Symbiodinium kawagutii sp. CS-156). We found that extensive biochemical purification altered the oligomerization states of the initially isolated nonameric acpPC. The excitation energy transfer pathways in the acpPC nonamer and its variants were studied using time-resolved fluorescence and time-resolved absorption spectroscopic techniques at 77 K. Compared to the well-characterized trimeric acpPC, the nonameric acpPC contains an 11 nm red-shifted terminal energy emitter and substantially altered excited state lifetimes of Chl a. The observed energetic overlap of the fluorescence terminal energy emitters with the absorption of RCs is hypothesized to enable efficient downhill excitation energy transfer. Additionally, the shortened Chl a fluorescence decay lifetime in the oligomeric acpPC indicate a protective self-relaxation strategy. We propose that the highly-oligomerized acpPC nonamer represents an intact functional unit in the Symbiodiniaceae thylakoid membrane. They perform efficient excitation energy transfer (to RCs), and are under manageable regulations in favor of photoprotection.
 
a Summed Fractional Labeling (SFL) and b R-values for the derivatized proteinogenic amino acids extracted from GTN (glucose-tolerant wild-type) Synechocystis sp. PCC6803 (black bars) and ISP (isoprene-producing transformant) Synechocystis sp. PCC6803 (grey bars). Corrected MIDs of the [M-57] fragment of alanine (Ala260), glycine (Gly246), valine (Val 288), proline (Pro 286), serine (Ser390), threonine (Thr404), phenylalanine (Phe336), aspartic acid (Asp418), glutamic acid (Glu432), and [M-85] fragment of leucine (Leu274) and isoleucine (Ile 274) was used for calculating the SFL and the R-value. Error bars represent standard error for biological triplicates. Statistically significant difference (p value < 0.01, unpaired t test) in the SFL and R-value of respective amino acids in GTN and ISP, except for Thr404
Steady state ¹³C-based metabolic flux analysis using corrected MID values of amino acids. Values indicate the predicted flux through the central carbon metabolism of a Glucose-tolerant wild-type (GTN) Synechocystis sp. PCC 6803, and b Isoprene-producing glucose-tolerant (ISP) Synechocystis sp. under mixotrophic growth conditions in BG-11 medium supplemented with 5 mM [1,2 ¹³C] glucose and grown under constant illumination of 150 μmol m–2 s–1 at 30 °C. Reactions for photorespiration are not shown in the map, as negligible flux was observed through it. Metabolite abbreviations: G6P glucose-6-phosphate, F6P fructose-6-phosphate, F16P fructose-1,6-phosphate, DHAP dihydroxyacetone phosphate, G3P glyceraldehyde-3-phospahte, E4P erythrose-4-phosphate, S7P sedoheptulose-7-phosphate, X5P xylulose-5-phosphate, R5P ribose-5-phosphate, RU5P ribulose-5-phosphate, RU15P ribulose-1,5-bisphosphate, 3-PG 3-phosphoglycerate, PEP phosphoenolpyruvate, PYR pyruvate, ACCOA acetyl-CoA, CIT citrate, ICIT isocitrate, AKG alpha-ketoglutarate, SSAD succinic semialdehyde, FUM fumarate, MAL malate, OAA oxaloacetate, ACP acetyl-phosphate, DXP deoxyxylulose-5-phosphate, MEP methylerythritol-4-phosphate, IPP isopentenyl pyrophosphate, DMPP dimethylallyl pyrophosphate, ISP isoprene, CO2 carbon dioxide. Grey arrows from key metabolites indicate drain reactions for other biomass components
The mRNA abundance levels of genes of the central carbon metabolism and the electron transport chain. Metabolite abbreviations: G6P glucose-6-phosphate, G3P glyceraldehyde-3-phospahte, RU15P ribulose-1,5-bisphosphate, PEP phosphoenolpyruvate, PYR pyruvate, ACCOA acetyl-CoA, CIT citrate, FUM fumarate, MAL malate, OAA oxaloacetate, MEP methylerythritol-4-phosphate, HMBDP (E)-4-Hydroxy-3-methyl-but-2-enyl pyrophosphate, DMPP dimethylallyl pyrophosphate, ISP isoprene, PSII photosystem II, PSI photosystem I, PQ plastoquinone, Fdx ferrodoxin, Flv Flavoprotein. * and **Represent statistically significant difference between mRNA levels in GTN and ISP with p < 0.05 and < 0.005, respectively (unpaired t test)
Cyanobacteria are photosynthetic bacteria, widely studied for the conversion of atmospheric carbon dioxide to useful platform chemicals. Isoprene is one such industrially important chemical, primarily used for production of synthetic rubber and biofuels. Synechocystis sp. PCC 6803, a genetically amenable cyanobacterium, produces isoprene on heterologous expression of isoprene synthase gene, albeit in very low quantities. Rationalized metabolic engineering to re-route the carbon flux for enhanced isoprene production requires in-dept knowledge of the metabolic flux distribution in the cell. Hence, in the present study, we undertook steady state 13C-metabolic flux analysis of glucose-tolerant wild-type (GTN) and isoprene-producing recombinant (ISP) Synechocystis sp. to understand and compare the carbon flux distribution in the two strains. The R-values for amino acids, flux analysis predictions and gene expression profiles emphasized predominance of Calvin cycle and glycogen metabolism in GTN. Alternatively, flux analysis predicted higher activity of the anaplerotic pathway through phosphoenolpyruvate carboxylase and malic enzyme in ISP. The striking difference in the Calvin cycle, glycogen metabolism and anaplerotic pathway activity in GTN and ISP suggested a possible role of energy molecules (ATP and NADPH) in regulating the carbon flux distribution in GTN and ISP. This claim was further supported by the transcript level of selected genes of the electron transport chain. This study provides the first quantitative insight into the carbon flux distribution of isoprene-producing cyanobacterium, information critical for developing Synechocystis sp. as a single cell factory for isoprenoid/terpenoid production.
 
The functions of both (bacterio) chlorophylls and carotenoids in light-harvesting complexes have been extensively studied during the past decade, yet, the involvement of BChl a high-energy Soret band in the cascade of light-harvesting processes still remains a relatively unexplored topic. Here, we present transient absorption data recorded after excitation of the Soret band in the LH2 complex from Rhodoblastus acidophilus. Comparison of obtained data to those recorded after excitation of rhodopin glucoside and B800 BChl a suggests that no Soret-to-Car energy transfer pathway is active in LH2 complex. Furthermore, a spectrally rich pattern observed in the spectral region of rhodopin glucoside ground state bleaching (420–550 nm) has been assigned to an electrochromic shift. The results of global fitting analysis demonstrate two more features. A 6 ps component obtained exclusively after excitation of the Soret band has been assigned to the response of rhodopin glucoside to excess energy dissipation in LH2. Another time component, ~ 450 ps, appearing independently of the excitation wavelength was assigned to BChl a-to-Car triplet–triplet transfer. Presented data demonstrate several new features of LH2 complex and its behavior following the excitation of the Soret band.
 
Our analysis of the X-ray crystal structure of canthaxanthin (CAN) showed that its ketolated β-ionone rings can adopt two energetically equal, but structurally distinct puckers. Quantum chemistry calculations revealed that the potential energy surface of the β-ionone ring rotation over the plane of the conjugated π-system in carotenoids depends on the pucker state of the β-ring. Considering different pucker states and β-ionone ring rotation, we found six separate local minima on the potential energy surface defining the geometry of the keto-β-ionone ring—two cis and one trans orientation for each of two pucker states. We observed a small difference in energy and no difference in relative orientation for the cis-minima, but a pronounced difference for the position of trans-minimum in alternative pucker configurations. An energetic advantage of β-ionone ring rotation from a specific pucker type can reach up to 8 kJ/mol (669cm-1). In addition, we performed the simulation of linear absorption of CAN in hexane and in a unit cell of the CAN crystal. The electronic energies of S0→S2 transition were estimated both for the CAN monomer and in the CAN crystal. The difference between them reached 690cm-1, which roughly corresponds to the energy gap between A and B pucker states predicted by theoretical estimations. Finally, we have discussed the importance of such effects for biological systems whose local environment determines conformational mobility, and optical/functional characteristics of carotenoid.
 
Trees regenerating in the understory respond to increased availability of light caused by gap formation by undergoing a range of morphological and physiological adjustments. These adjustments include the production of thick, sun-type leaves containing thicker mesophyll and longer palisade cells than in shade-type leaves. We asked whether in the shade-regenerating tree Acer pseudoplatanus , the increase in leaf thickness and expansion of leaf tissues are possible also in leaves that are already fully formed, a response reported so far only for a handful of species. We acclimated potted seedlings to eight levels (from 1 to 100%) of solar irradiance and, in late summer, transferred a subset of them to full sunlight. Within 30 days, the pre-shaded leaves increased leaf mass per area and became thicker mostly due to the elongation of palisade cells, except for the most shaded individuals which suffered irreversible photo-oxidative damage. This anatomical acclimation was accompanied by a transient decline in photosynthetic efficiency of PSII (F v /F M ), the magnitude of which was related to the degree of pre-shading. The F v /F M recovered substantially within the re-acclimation period. However, leaves of transferred plants were shed earlier in the fall, indicating that the acclimation was not fully effective. These results show that A. pseudoplatanus is one of the few known species in which mature leaves may re-acclimate anatomically to increased irradiance. This may be an important mechanism enhancing utilization of gaps created during the growing season.
 
Photosynthesis vs. light curves (LCs) have played a central role in photosynthesis research for decades. They are the commonest form of describing how photosynthesis responds to changes in light, being frequently used for characterizing photoacclimation. However, LCs are often interpreted exclusively regarding the response to light intensity, the effects of time of exposure not being explicitly considered. This study proposes the use of ‘hysteresis light curves’ (HLC), an experimental protocol focused on the cumulative effects of light exposure to obtain information on the time dependence of photosynthetic light responses. HLC are generated by exposing samples to a symmetrical sequence of increasing and decreasing light levels. The comparison of the light-increasing and the light-decreasing phases allows the quantification of the hysteresis caused by high-light exposure, the magnitude and direction of which inform on the activation, and subsequent relaxation of high-light-induced photosynthetic processes. HLCs of the chlorophyll fluorescence indices rETR (relative electron transport rate of photosystem II) and Y(NPQ) (index of non-photochemical quenching) were measured on cyanobacteria, algae, and plants, with the aim of identifying main patterns of hysteresis and their diversity. A non-parametric index is proposed to quantify the magnitude and direction of hysteresis in HLCs of rETR and Y(NPQ). The results of this study show that HLCs can provide additional relevant information on the time dependence of the light response of photosynthetic samples, not obtainable from conventional LCs, useful for phenotyping photosynthetic traits, including photoacclimation state and kinetics of light activation and relaxation of electron flow and energy dissipation processes.
 
Absorption spectra of Chl a monomer at T = 300 K without and with inhomogeneous broadening are displayed in upper and lower panel, respectively; black line: without intramolecular vibrations, excitation of Q y only; red line: with intramolecular vibrations, excitation of Q y only; green line: with intramolecular vibrations, excitation of Q y and Q x ; blue line: with additional vibronic coupling between the Q x and Q y transitions
Absorption (upper panels) and CD spectra (lower panels) of dimer at T = 300 K without and with inhomogeneous broadening (left and right panels, respectively); black line: without any vibrations, excitation of Q y only; red line: with intramolecular vibrations, excitation of Q y only; green line: with intramolecular vibrations, excitation of Q y and Q x ; blue line: with additional vibronic coupling between Q x and Q y transitions
CD spectra of dimer at T = 300 K from the lower right panel of Fig. 4 are rescaled in such a way that the positive-signed peak has the same amplitude. Different from Fig. 4, also a case with additional involvement of B x and B y is displayed as an orange dashed line
Absorption and CD spectra of dimer at T = 300 K with inhomogeneous broadening ( FWHM(Q y ) = 240 cm −1 , FWHM(Q x ) = 720 cm −1 ), excitation of Q y and Q x and vibronic coupling are displayed in upper and lower panel, respectively; if all matrix elements of the excitonic coupling are taken into account, the blue and the orange line are obtained from calculations without and with involvement of B y and B x ( FWHM(B y ) = FWHM(B x ) = 1800 cm −1 ), respectively; if only those coupling matrix elements with involvement of lowest vibrational eigenfunctions are taken into account, the violet and the red line are obtained from calculations without and with involvement of B y and B x , respectively. Please note that the absorption and CD spectra were rescaled to obtain a common maximum of the 0-0 line (the lowenergy peak).
An electron-vibrational coupling model that includes the vibronic (non-adiabatic) coupling between the Q $$_{\mathrm{y}}$$ y and Q $$_{\mathrm{x}}$$ x transitions of chlorophyll (Chl), created by Reimers and coworkers (Scientific Rep. 3, 2761, 2013) is extended here to chlorophyll dimers with interchlorophyll excitonic coupling. The model is applied to a Chl a dimer of the water-soluble chlorophyll binding protein (WSCP). As for isolated chlorophyll, the vibronic coupling is found to have a strong influence on the high-frequency vibrational sideband in the absorption spectrum, giving rise to a band splitting. In contrast, in the CD spectrum the interplay of vibronic coupling and static disorder leads to a strong suppression of the vibrational sideband in excellent agreement with the experimental data. The conservative nature of the CD spectrum in the low-energy region is found to be caused by a delicate balance of the intermonomer excitonic coupling between the purely electronic Q $$_{\mathrm{y}}$$ y transition and the Q $$_{\mathrm{y}}$$ y transition involving intramolecular vibrational excitations on one hand and the coupling to higher-energy electronic transitions on the other hand.
 
Top-cited authors
Suleyman I Allakhverdiev
  • Timiryazev Institute of Plant Physiology
Marian Brestic
  • Slovak University of Agriculture in Nitra - Slovenska posnohospodarska univerzita v Nitre
Marek Zivcak
  • Slovak University of Agriculture in Nitra - Slovenska posnohospodarska univerzita v Nitre
David Kramer
  • Michigan State University
Govindjee Govindjee
  • University of Illinois, Urbana-Champaign