Consistent Sets of Spectrophotometric Chlorophyll Equations for Acetone, Methanol and Ethanol Solvents

A set of equations for determining chlorophyll a (Chl a) and accessory chlorophylls b, c ₂ , c ₁ + c ₂ and the special case of Acaryochloris marina, which uses Chl d as its primary photosynthetic pigment and also has Chl a, have been developed for 90% acetone, methanol and ethanol solvents. These equations for different solvents give chlorophyll assays that are consistent with each other. No algorithms for Chl c compounds (c ₂ , c ₁ + c ₂) in the presence of Chl a have previously been published for methanol or ethanol. The limits of detection (and inherent error, ± 95% confidence limit), for chlorophylls in all organisms tested, was generally less than 0.1 μg/ml. The Chl a and b algorithms for green algae and land plants have very small inherent errors (< 0.01 μg/ml). Chl a and d algorithms for Acaryochloris marina are consistent with each other, giving estimates of Chl d/a ratios which are consistent with previously published estimates using HPLC and a rarely used algorithm originally published for diethyl ether in 1955. The statistical error structure of chlorophyll algorithms is discussed. The relative error of measurements of chlorophylls increases hyperbolically in diluted chlorophyll extracts because the inherent errors of the chlorophyll algorithms are constants independent of the magnitude of absorbance readings. For safety reasons, efficient extraction of chlorophylls and the convenience of being able to use polystyrene cuvettes, the algorithms for ethanol are recommended for routine assays of chlorophylls. The methanol algorithms would be convenient for assays associated with HPLC work.     

Theoretical study on the thermodynamic properties of chlorophyll d-peptides coordinating ligand

The chlorophyll d containing cyanobacterium, Acaryochloris marina has provided a model system for the study of chlorophyll replacement in the function of oxygenic photosynthesis. Chlorophyll d replaces most functions of chlorophyll a in Acaryochloris marina. It not only functions as the major light-harvesting pigment, but also acts as an electron transfer cofactor in the primary charge separation reaction in the two photosystems. The Mg-chlorophyll d-peptide coordinating interaction between the amino acid residues and chlorophylls using the latest semi-empirical PM5 method were examined. It is suggested that chlorophyll d possesses similar coordination ligand properties to chlorophyll a, but chlorophyll b possesses different ligand properties. Compared with other studies involving theoretical correlation and our prior experiments, this study suggests that the chlorophyll a-bound proteins will bind chlorophyll d without difficulty when chlorophyll d is available.     

Extinction coefficient for red-shifted chlorophylls: Chlorophyll d and chlorophyll f

Both chlorophyll f and chlorophyll d are red-shifted chlorophylls in oxygenic photosynthetic organisms, which extend photon absorbance into the near infrared region. This expands the range of light that can be used to drive photosynthesis. Quantitative determination of chlorophylls is a crucial step in the investigation of chlorophyll-photosynthetic reactions in the field of photobiology and photochemistry. No methods have yet been worked out for the quantitative determination of chlorophyll f. There is also no method available for the precise quantitative determination of chlorophyll d although it was discovered in 1943. In order to obtain the extinction coefficients (ε) of chlorophyll f and chlorophyll d, the concentrations of chlorophylls were determined by Inductive Coupled Plasma Mass Spectrometry according to the fact that each chlorophyll molecule contains one magnesium (Mg) atom. Molar extinction coefficient ε(chl f) is 71.11×10(3)Lmol(-1)A(707nm)cm(-1) and ε(chl d) is 63.68×10(3)Lmol(-1)A(697nm)cm(-1) in 100% methanol. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Molecular structure, localization and function of biliproteins in the chlorophyll a/d containing oxygenic photosynthetic prokaryote Acaryochloris marina.

We investigated the localization, structure and function of the biliproteins of the oxygenic photosynthetic prokaryote Acaryochloris marina, the sole organism known to date that contains chlorophyll d as the predominant photosynthetic pigment. The biliproteins were isolated by means of sucrose gradient centrifugation, ion exchange and gel filtration chromatography. Up to six biliprotein subunits in a molecular mass range of 15.5-18.4 kDa were found that cross-reacted with antibodies raised against phycocyanin or allophycocyanin from a red alga. N-Terminal sequences of the alpha- and beta-subunits of phycocyanin showed high homogeneity to those of cyanobacteria and red algae, but not to those of cryptomonads. As shown by electron microscopy, the native biliprotein aggregates are organized as rod-shaped structures and located on the cytoplasmic side of the thylakoid membranes predominantly in unstacked thylakoid regions. Biochemical and spectroscopic analysis revealed that they consist of four hexameric units, some of which are composed of phycocyanin alone, others of phycocyanin together with allophycocyanin. Spectroscopic analysis of isolated photosynthetic reaction center complexes demonstrated that the biliproteins are physically attached to the photosystem II complexes, transferring light energy to the photosystem II reaction center chlorophyll d with high efficiency.     

Efficiency of photosynthesis in a Chl d-utilizing cyanobacterium is comparable to or higher than that in Chl a-utilizing oxygenic species

The cyanobacterium Acaryochloris marina uses chlorophyll d to carry out oxygenic photosynthesis in environments depleted in visible and enhanced in lower-energy, far-red light. However, the extent to which low photon energies limit the efficiency of oxygenic photochemistry in A. marina is not known. Here, we report the first direct measurements of the energy-storage efficiency of the photosynthetic light reactions in A. marina whole cells, and find it is comparable to or higher than that in typical, chlorophyll a-utilizing oxygenic species. This finding indicates that oxygenic photosynthesis is not fundamentally limited at the photon energies employed by A. marina, and therefore is potentially viable in even longer-wavelength light environments.     

Effect of iron on growth and ultrastructure of Acaryochloris marina

The cyanobacterial genus Acaryochloris is the only known group of oxygenic phototrophs that contain chlorophyll d rather than chlorophyll a as the major photosynthetic pigment. Studies on this organism are still in their earliest stages, and biochemical analysis has rapidly outpaced growth optimization. We have investigated culture growth of the major strains of Acaryochloris marina (MBIC11017 and MBIC10697) by using several published and some newly developed growth media. It was determined that heavy addition of iron significantly enhanced culture longevity. These high-iron cultures showed an ultrastructure with thylakoid stacks that resemble traditional cyanobacteria (unlike previous studies). These cultures also show a novel reversal in the pigment ratios of the photosystem II signature components chlorophyll a and pheophytin a, as opposed to those in previous studies.     

Both chlorophylls a and d are essential for the photochemistry in photosystem II of the cyanobacteria, Acaryochloris marina

We have measured the flash-induced absorbance difference spectrum attributed to the formation of the secondary radical pair, P(+)Q(-), between 270 nm and 1000 nm at 77 K in photosystem II of the chlorophyll d containing cyanobacterium, Acaryochloris marina. Despite the high level of chlorophyll d present, the flash-induced absorption difference spectrum of an approximately 2 ms decay component shows a number of features which are typical of the difference spectrum seen in oxygenic photosynthetic organisms containing no chlorophyll d. The spectral shape in the near-UV indicates that a plastoquinone is the secondary acceptor molecule (Q(A)). The strong C-550 change at 543 nm confirms previous reports that pheophytin a is the primary electron acceptor. The bleach at 435 nm and increase in absorption at 820 nm indicates that the positive charge is stabilized on a chlorophyll a molecule. In addition a strong electrochromic band shift, centred at 723 nm, has been observed. It is assigned to a shift of the Qy band of the neighbouring accessory chlorophyll d, Chl(D1). It seems highly likely that it accepts excitation energy from the chlorophyll d containing antenna. We therefore propose that primary charge separation is initiated from this chlorophyll d molecule and functions as the primary electron donor. Despite its lower excited state energy (0.1 V less), as compared to chlorophyll a, this chlorophyll d molecule is capable of driving the plastoquinone oxidoreductase activity of photosystem II. However, chlorophyll a is used to stabilize the positive charge and ultimately to drive water oxidation.     

Modification of photosystem I reaction center by the extraction and exchange of chlorophylls and quinones.

The photosystem (PS) I photosynthetic reaction center was modified thorough the selective extraction and exchange of chlorophylls and quinones. Extraction of lyophilized photosystem I complex with diethyl ether depleted more than 90% chlorophyll (Chl) molecules bound to the complex, preserving the photochemical electron transfer activity from the primary electron donor P700 to the acceptor chlorophyll A(0). The treatment extracted all the carotenoids and the secondary acceptor phylloquinone (A(1)), and produced a PS I reaction center that contains nine molecules of Chls including P700 and A(0), and three Fe-S clusters (F(X), F(A) and F(B)). The ether-extracted PS I complex showed fast electron transfer from P700 to A(0) as it is, and to FeS clusters if phylloquinone or an appropriate artificial quinone was reconstituted as A(1). The ether-extracted PS I enabled accurate detection of the primary photoreactions with little disturbance from the absorbance changes of the bulk pigments. The quinone reconstitution created the new reactions between the artificial cofactors and the intrinsic components with altered energy gaps. We review the studies done in the ether-extracted PS I complex including chlorophyll forms of the core moiety of PS I, fluorescence of P700, reaction rate between A(0) and reconstituted A(1), and the fast electron transfer from P700 to A(0). Natural exchange of chlorophyll a to 710-740 nm absorbing chlorophyll d in PS I of the newly found cyanobacteria-like organism Acaryochloris marina was also reviewed. Based on the results of exchange studies in different systems, designs of photosynthetic reaction centers are discussed.     

Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the chlorophyll d-containing oxyphotobacterium Acaryochloris marina

Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM (Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O(2) electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206+/-51 micromol m(-2) s(-1) (PAR) (Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109+/-27 micromol m(-2) s(-1) (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73+/-7 micromol mg Chl d(-1) h(-1). The gross photosynthesis/respiration ratio (P(g)/R) of Acaryochloris under optimum conditions was about 4.02+/-1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.     

Functional and structural organization of chlorophyll in the developing photosynthetic membranes of Euglena gracilis Z. II. Formation of system II photosynthetic units during greening under optimal light intensity.

The relationships between light-harvesting chlorophyll and reaction centers in Photosystem II were analyzed during the chloroplast development of dark-grown, non-dividing Euglena gracilis Z. Comparative measurements included light saturation of photosynthesis, oxygen evolution under flashing-light and fluorescence induction. The results obtained can be summarized as follows: (1) Photosystem II photocenters are formed in parallel with chlorophyll synthesis, but after a long lag phase. (2) As a consequence, the chlorophyll reaction center ratio (Emerson's type photosynthetic unit) decreases during greening. (3) This decrease is accompanied by considerable changes in the energy transfer and trapping properties of Photosystem II. Most of the initially synthesized chlorophylls are inactive in the transfer of excitations to active photochemical centers and are shared among newly formed Photosystem II photocenters; as a consequence, the number of chlorophylls functionally connected to each Photosystem II photocenter decreases and cooperatively between these centers appears. Results are discussed in terms of chlorophyll organization in developing photosynthetic membranes with reference to the lake or puddle models of photosynthetic unit organization.     

The thermodynamics and kinetics of electron transfer between cytochrome b6f and photosystem I in the chlorophyll d-dominated cyanobacterium, Acaryochloris marina

We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.     

Theoretical study on the thermodynamic properties of chlorophyll d-peptides coordinating ligand

The chlorophyll d containing cyanobacterium, Acaryochloris marina has provided a model system for the study of chlorophyll replacement in the function of oxygenic photosynthesis. Chlorophyll d replaces most functions of chlorophyll a in Acaryochloris marina. It not only functions as the major light-harvesting pigment, but also acts as an electron transfer cofactor in the primary charge separation reaction in the two photosystems. The Mg-chlorophyll d-peptide coordinating interaction between the amino acid residues and chlorophylls using the latest semi-empirical PM5 method were examined. It is suggested that chlorophyll d possesses similar coordination ligand properties to chlorophyll a, but chlorophyll b possesses different ligand properties. Compared with other studies involving theoretical correlation and our prior experiments, this study suggests that the chlorophyll a-bound proteins will bind chlorophyll d without difficulty when chlorophyll d is available.     

Energetics of Photosystem II charge recombination in Acaryochloris marina studied by thermoluminescence and flash-induced chlorophyll fluorescence measurements

We studied the charge recombination characteristics of Photosystem II (PSII) redox components in whole cells of the chlorophyll (Chl) d-dominated cyanobacterium, Acaryochloris marina, by flash-induced chlorophyll fluorescence and thermoluminescence measurements. Flash-induced chlorophyll fluorescence decay was retarded in the mus and ms time ranges and accelerated in the s time range in Acaryochloris marina relative to that in the Chl a-containing cyanobacterium, Synechocystis PCC 6803. In the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, which blocks the Q(B) site, the relaxation of fluorescence decay arising from S(2)Q(A)(-) recombination was somewhat faster in Acaryochloris marina than in Synechocystis PCC 6803. Thermoluminescence intensity of the so called B band, arising from the recombination of the S(2)Q(B)(-) charge separated state, was enhanced significantly (2.5 fold) on the basis of equal amounts of PSII in Acaryochloris marina as compared with Synechocystis 6803. Our data show that the energetics of charge recombination is modified in Acaryochloris marina leading to a approximately 15 meV decrease of the free energy gap between the Q(A) and Q(B) acceptors. In addition, the total free energy gap between the ground state and the excited state of the reaction center chlorophyll is at least approximately 25-30 meV smaller in Acaryochloris marina, suggesting that the primary donor species cannot consist entirely of Chl a in Acaryochloris marina, and there is a contribution from Chl d as well.     

Optimization and evolution of light harvesting in photosynthesis: the role of antenna chlorophyll conserved between photosystem II and photosystem I

The efficiency of oxygenic photosynthesis depends on the presence of core antenna chlorophyll closely associated with the photochemical reaction centers of both photosystem II (PSII) and photosystem I (PSI). Although the number and overall arrangement of these chlorophylls in PSII and PSI differ, structural comparison reveals a cluster of 26 conserved chlorophylls in nearly identical positions and orientations. To explore the role of these conserved chlorophylls within PSII and PSI we studied the influence of their orientation on the efficiency of photochemistry in computer simulations. We found that the native orientations of the conserved chlorophylls were not optimal for light harvesting in either photosystem. However, PSII and PSI each contain two highly orientationally optimized antenna chlorophylls, located close to their respective reaction centers, in positions unique to each photosystem. In both photosystems the orientation of these optimized bridging chlorophylls had a much larger impact on photochemical efficiency than the orientation of any of the conserved chlorophylls. The differential optimization of antenna chlorophyll is discussed in the context of competing selection pressures for the evolution of light harvesting in photosynthesis.     

Structure, function and regulation of plant photosystem I

Photosystem I (PSI) is a multisubunit protein complex located in the thylakoid membranes of green plants and algae, where it initiates one of the first steps of solar energy conversion by light-driven electron transport. In this review, we discuss recent progress on several topics related to the functioning of the PSI complex, like the protein composition of the complex in the plant Arabidopsis thaliana, the function of these subunits and the mechanism by which nuclear-encoded subunits can be inserted into or transported through the thylakoid membrane. Furthermore, the structure of the native PSI complex in several oxygenic photosynthetic organisms and the role of the chlorophylls and carotenoids in the antenna complexes in light harvesting and photoprotection are reviewed. The special role of the 'red' chlorophylls (chlorophyll molecules that absorb at longer wavelength than the primary electron donor P700) is assessed. The physiology and mechanism of the association of the major light-harvesting complex of photosystem II (LHCII) with PSI during short term adaptation to changes in light quality and quantity is discussed in functional and structural terms. The mechanism of excitation energy transfer between the chlorophylls and the mechanism of primary charge separation is outlined and discussed. Finally, a number of regulatory processes like acclimatory responses and retrograde signalling is reviewed with respect to function of the thylakoid membrane. We finish this review by shortly discussing the perspectives for future research on PSI.     

Functional and structural organization of chlorophyll in the developing photosynthetic membranes of Euglena gracilis Z. I. Formation of System II photosynthetic units during greening under optimal light intensity

The relationships between light-harvesting chlorophyll and reaction centers in Photosystem II were analyzed during the chloroplast development of dark-grown, non-dividing Euglena gracilis Z. Comparative measurements included light saturation of photosynthesis, oxygen evolution under flashing-light and fluorescence induction. The results obtained can be summarized as follows: (1) Photosystem II photocenters are formed in parallel with chlorophyll synthesis, but after a longer lag phase. (2) As a consequence, the chlorophyll: reaction center ratio (Emerson's type photosynthetic unit) decreases during greening. (3) This decrease is accompanied by considerable changes in the energy transfer and trapping properties of Photosystem II. Most of the initially synthesized chlorophylls are inactive in the transfer of excitations to active photochemical centers and are shared among newly formed Photosystem II photocenters; as a consequence, the number of chlorophylls functionally connected to each Photosystem II photocenter decreases and cooperativity between these centers appears. Results are discussed in terms of chlorophyll organization in developing photosynthetic membranes with reference to the lake or puddle models of photosynthetic unit organization.     

The xanthophyll cycle is induced by light irrespective of water status in field-grown lavender (Lavandula stoechas) plants

The water relations, the photosynthetic capacity and the pigment content of leaves, i.e. chlorophylls, carotenes and xanthophylls, were analysed during the summer drought and recovery after autumn rainfalls in lavender ( Lavandula stoechas L.) plants grown in Mediterranean field conditions. Summer drought caused photoinhibition of photosynthesis and significant decreases in chlorophylls (by ca 75%), β -carotene (by ca 65%), and lutein and neoxanthin (by ca 50%), although their contents remained unaltered between predawn and midday, suggesting a progressive decrease in response to drought. In contrast, the levels of violaxanthin decreased from predawn to midday, giving rise to enhanced formation of zeaxanthin and antheraxanthin in high light. Zeaxanthin and antheraxanthin formation was not induced by water deficit. Although the levels of photosynthetic pigments were severely affected by water deficit, carotenoids decreased less than chlorophylls, which resulted in increased levels of carotenoids per unit of chlorophyll. We conclude that the enhanced formation of zeaxanthin in high light and the increased levels of carotenoids per unit of chlorophyll observed in water-stressed plants may help to avoid photoinhibitory damage to the photosynthetic apparatus.     

Functional Organization of the Chlorophyll-Containing Complexes of Chlamydomonas reinhardi: A Study of Their Formation and Interconnection with Reaction Centers in the Greening Process of the y-1 Mutant

The stepwise synthesis and assembly of photosynthetic membrane components in the y-1 mutant of Chlamydomonas reinhardi have been previously demonstrated (Ohad 1975 In Membrane Biogenesis, Mitochondria, Chloroplasts and Bacteria, Plenum, pp 279-350). This experimental system was used here in order to investigate the process of formation and interconnection of the energy collecting chlorophylls with the reaction centers of both photosystems I and II. The following measurements were carried out: photosynthetic electron flow at various light intensities, including parts or the entire electron transfer chain; analysis of the kinetics of fluorescence emission at room temperature and fluorescence emission spectra at 77 K, and electrophoretic separation of membrane polypeptides and chlorophyll protein complexes. Based on the data obtained it is concluded that: (a) each photosystem (PSI and PSII) contains, in addition to the reaction center, an interconnecting antenna and a main or light harvesting antenna complex; (b) the formation of the light harvesting complex, interconnecting antenna, and reaction centers for each photosystem can occur independently. (c) the interconnecting antennae link the light harvesting complexes with the respective reaction centers. In their absence, energy transfer between the light harvesting chlorophylls and the reaction centers is inefficient. The formation of the interconnecting antennae and efficient assembly of photosystem components occur simultaneously with the de novo synthesis of chlorophyll and at least three polypeptides, one translated in the cytoplasm and two translated in the chloroplast. The synthesis of these polypeptides was found to be light dependent.     

Alterations in photosynthetic pigments and amino acid composition of D1 protein change energy distribution in photosystem II

The marine cyanobacterium Prochlorococcus marinus accumulates divinyl chlorophylls instead of monovinyl chlorophylls to harvest light energy. As well as this difference in its chromophore composition, some amino acid residues in its photosystem II D1 protein were different from the conserved amino acid residues in other photosynthetic organisms. We examined PSII complexes isolated from mutants of Synechocystis sp. PCC 6803, in which chromophore and D1 protein were altered (Hisashi Ito and Ayumi Tanaka, 2011) to clarify the effects of chromophores/D1 protein composition on the excitation energy distribution. We prepared the mutants accumulating divinyl chlorophyll (DV mutant). The amino acid residues of V205 and G282 in the D1 protein were substituted with M205 and C282 in the DV mutant to mimic Prochlorococcus D1 protein (DV-V205M/G282C mutant). Isolated PSII complexes were analyzed by time-resolved fluorescence spectroscopy. Energy transfer in CP47 was interrupted in PSII containing divinyl chlorophylls. The V205M/G282C mutation did not recover the energy transfer pathway in CP47, instead, the mutation allowed the excitation energy transfer from CP43 to CP47, which neighbors in the PSII dimer. Mutual orientation of the subcomplexes of PSII might be affected by the substitution. The changes of the energy transfer pathways would reduce energy transfer from antennae to the PSII reaction center, and allow Prochlorococcus to acquire light tolerance.     

Modified molecular interactions of the pheophytin and plastoquinone electron acceptors in photosystem II of chlorophyll d-containing Acaryochloris marina as revealed by FTIR spectroscopy

Photosynthesis Research - Acaryochloris marina is a unique cyanobacterium that contains chlorophyll (Chl) d as a major pigment. Because Chl d has smaller excitation energy than Chl a used in...