Identification of the special pair of photosystem II in a chlorophyll d-dominated cyanobacterium

A Novel Epiphytic Chlorophyll d-containing Cyanobacterium Isolated from a Mangrove-associated Red Alga

A new habitat and a new chlorophyll (Chl) d-containing cyanobacterium belonging to the genus Acaryochloris are reported in this study. Hyperspectral microscopy showed the presence of Chl d-containing microorganisms in epiphytic biofilms on a red alga (Gelidium caulacantheum) colonizing the pneumato-phores of a temperate mangrove (Avicennia marina). The presence of Chl d was further proven by high performance liquid chromatography (HPLC)-based pigment analysis and by confocal imaging of cultured cells. Enrichment of mangrove biofilm samples under near-infrared radiation (NIR) yielded the new Acaryochloris sp. MPGRS1, which was closely related in terms of 16S rRNA gene sequence to an isolate from the hypertrophic Salton Sea, USA. The new isolate used Chl d as its major photopigment; Chl d and Chl a contents were ~98% and 1%-2% of total cellular chlorophyll, respectively. These findings expand the variety of ecological niches known to harbor Chl d-containing cyanobacteria and support our working hypothesis that such oxyphototrophs may be ubiquitous in habitats depleted of visible light, but with sufficient NIR exposure.

Structure and function of photosystem I and its application in biomimetic solar-to-fuel systems

Photosystem I (PSI) is one of the most efficient biological macromolecular complexes that converts solar energy into condensed energy of chemical bonds. Despite high structural complexity, PSI operates with a quantum yield close to 1.0 and to date, no man-made synthetic system approached this remarkable efficiency. This review highlights recent developments in dissecting molecular structure and function of the prokaryotic and eukaryotic PSI. It also overviews progress in the application of this complex as a natural photocathode for production of hydrogen within the biomimetic solar-to-fuel nanodevices.

Non-enzymatic conversion of chlorophyll-a into chlorophyll-d in vitro: a model oxidation pathway for chlorophyll-d biosynthesis

Chlorophyll-a (Chl-a) was readily converted into Chl-d under mild conditions without any enzymes. Treatment of Chl-a dissolved in dry tetrahydrofuran (THF) with thiophenol and acetic acid at room temperature successfully produced Chl-d in 31% yield. During the acidic oxidation, removal of the central magnesium, pheophytinization, was sufficiently suppressed. This mild pathway can give insights into the yet unidentified Chl-d biosynthesis.

Artificially produced [7-formyl]-chlorophyll d functions as an antenna pigment in the photosystem II isolated from the chlorophyllide a oxygenase-expressing Acaryochloris marina

Acaryochloris marina, a chlorophyll (Chl) d-dominated cyanobacterium, is a model organism for studying photosynthesis driven by far-red light using Chl d. Furthermore, studies on A. marina may provide insights into understanding how the oxygenic photosynthetic organisms adapt after the acquisition of new Chl. To solve the reaction mechanism of its unique photosynthesis, photosystem (PS) II complexes were isolated from A. marina and analyzed. However, the lack of a molecular genetic method for A. marina prevented us from conducting further studies. We recently developed a transformation system for A. marina and we introduced a chlorophyllide a oxygenase gene into A. marina. The resultant transformant accumulated [7-formyl]-Chl d, which has never been found in nature. In the current study, we isolated PS II complexes that contained [7-formyl]-Chl d. The pigment composition of the [7-formyl]-Chl d-containing PS II complexes was 1.96±0.04 Chl a, 53.21±1.00 Chl d, and 5.48±0.33 [7-formyl]-Chl d per two pheophytin a molecules. In contrast, the composition of the control PS II complexes was 2.01±0.06 Chl a and 62.96±2.49 Chl d. The steady-state fluorescence and excitation spectra of the PS II complexes revealed that energy transfer occurred from [7-formyl]-Chl d to the major Chl d species; however, the electron transfer was not affected by the presence of [7-formyl]-Chl d. These findings demonstrate that artificially produced [7-formyl]-Chl d molecules that are incorporated into PS II replace part of the Chl d molecules and function as the antenna. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

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.

Luminescence of singlet oxygen in photosystem II complexes isolated from cyanobacterium Synechocystis sp. PCC6803 containing monovinyl or divinyl chlorophyll a

The luminescence spectrum of singlet oxygen produced upon excitation at 674nm in the photochemically active photosystem II (PS II) complexes isolated from cyanobacterium Synechocystis sp. PCC 6803 containing different types of chlorophyll, i.e., monovinyl (wild-type) or divinyl (genetically modified) chlorophyll a. The yield of singlet oxygen, estimated using methylene blue as the standard, from the divinyl-chlorophyll PS II complex was more than five times greater than that from the monovinyl-chlorophyll PS II complex. These results are consistent with the observed difference in the sensitivity towards high intensity of light between the two cyanobacterial strains. The yield of singlet oxygen appeared to increase with the level of triplet chlorophyll, in the divinyl-chlorophyll PS II complex. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Selective detection and phylogenetic diversity of Acaryochloris spp. that exist in association with didemnid ascidians and sponge

Acaryochloris spp. are unique cyanobacteria which contain chlorophyll d as the predominant pigment. The phylogenetic diversity of Acaryochloris spp. associated with 7 Prochloron- or Synechocystis-containing didemnid ascidians and 1 Synechococcus-containing sponge obtained from the coast of the Republic of Palau was analyzed; we established a PCR primer set designed to selectively amplify the partial 16S rRNA gene of Acaryochloris spp. even in DNA samples containing a large amount of other cyanobacterial and algal DNAs. Polymerase chain reaction-denaturing gradient gel electrophoresis with this primer set enabled detection of the phyogenetic diversity of Acaryochloris spp. All the ascidian and sponge samples contained Acaryochloris spp. Fourteen phylotypes that were highly homologous (98–100%) with A. marina MBIC11017 were detected, while only 2 phylotypes were detected with our previously developed method for detecting cyanobacteria. The results also revealed that many uncultured phylotypes of Acaryochloris spp. were associated with those didemnid ascidians, since a clonal culture of only 1 phylotype has been established thus far. No specific relationship was found among the Acaryochloris phylotypes and the genera of the ascidians even when sample localities were identical; therefore, these invertebrates may provide a favorable habitat for Acaryochloris spp. rather than hosts showing any specific symbiotic relationships.

Structure and Function of Photosynthetic Reaction Centres

Extensive biochemical, biophysical, molecular biological and structural studies on a wide range of prokaryotic and eukaryotic photosynthetic organisms has revealed common features of their reaction centres where light induced charge separation and stabilization occurs. There is little doubt that all reaction centres have evolved from a common ancestor and have been optimized to maximum efficiency. As such they provide principles that can be used as a blueprint for developing artificial photo-electrochemical catalytic systems to generate solar fuels. This chapter summarises the common features of the organization of cofactors, electron transfer pathways and protein environments of reaction centres of anoxygenic and oxygenic phototrophs. In particular, the latest molecular details derived from X-ray crystallography are discussed in context of the specific catalytic functions of the Type I and Type II reaction centres.

Cationic state distribution over the chlorophyll d-containing P(D1)/P(D2) pair in photosystem II

Most of the chlorophyll (Chl) cofactors in photosystem II (PSII) from Acaryochloris marina are Chld, although a few Chla molecules are also present. To evaluate the possibility that Chla may participate in the P(D1)/P(D2) Chl pair in PSII from A. marina, the P(D1)(•+)/P(D2)(•+) charge ratio was investigated using the PSII crystal structure analyzed at 1.9-Å resolution, while considering all possibilities for the Chld-containing P(D1)/P(D2) pair, i.e., Chld/Chld, Chla/Chld, and Chld/Chla pairs. Chld/Chld and Chla/Chld pairs resulted in a large P(D1)(•+) population relative to P(D2)(•+), as identified in Chla/Chla homodimer pairs in PSII from other species, e.g., Thermosynechococcus elongatus PSII. However, the Chld/Chla pair possessed a P(D1)(•+)/P(D2)(•+) ratio of approximately 50/50, which is in contrast to previous spectroscopic studies on A. marina PSII. The present results strongly exclude the possibility that the Chld/Chla pair serves as P(D1)/P(D2) in A. marina PSII. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization

Chlorophyll d (Chl d) is the major pigment in both photosystems (PSI and II) of the cyanobacterium Acaryochloris marina, whose pigment composition represents an interesting alternative in oxygenic photosynthesis. While abundant information is available relative to photophysical properties of Chl a , the understanding of Chl d photophysics is still incomplete. In this paper, we present for the first time a characterization of Chl d phosphorescence, which accompanies radiative deactivation of the photoexcited triplet state of this pigment. Reliable information was obtained on the energy and lifetime of the Chl d triplet state in frozen solutions at 77 K using diethyl ether and aqueous dispersions of Triton X100 as solvents. It is shown that triplet Chl d is effectively populated upon photoexcitation of pigment molecules and efficiently sensitizes singlet oxygen phosphorescence in aerobic solutions under ambient conditions. The data obtained are compared with the previous results of the phosphorescence studies of Chl a and Pheo a, and their possible biological implications are discussed.

Excitation energy transfer in intact cells and in the phycobiliprotein antennae of the chlorophyll d containing cyanobacterium Acaryochloris marina

The cyanobacterium Acaryochloris marina is unique because it mainly contains Chlorophyll d (Chl d) in the core complexes of PS I and PS II instead of the usually dominant Chl a. Furthermore, its light harvesting system has a structure also different from other cyanobacteria. It has both, a membrane-internal chlorophyll containing antenna and a membrane-external phycobiliprotein (PBP) complex. The first one binds Chl d and is structurally analogous to CP43. The latter one has a rod-like structure consisting of three phycocyanin (PC) homohexamers and one heterohexamer containing PC and allophycocyanin (APC). In this paper, we give an overview on the investigations of excitation energy transfer (EET) in this PBP-light-harvesting system and of charge separation in the photosystem II (PS II) reaction center of A. marina performed at the Technische Universität Berlin. Due to the unique structure of the PBP antenna in A. marina, this EET occurs on a much shorter overall time scale than in other cyanobacteria. We also briefly discuss the question of the pigment composition in the reaction center (RC) of PS II and the nature of the primary donor of the PS II RC.

Expanding the solar spectrum used by photosynthesis

A limiting factor for photosynthetic organisms is their light-harvesting efficiency, that is the efficiency of their conversion of light energy to chemical energy. Small modifications or variations of chlorophylls allow photosynthetic organisms to harvest sunlight at different wavelengths. Oxygenic photosynthetic organisms usually utilize only the visible portion of the solar spectrum. The cyanobacterium Acaryochloris marina carries out oxygenic photosynthesis but contains mostly chlorophyll d and only traces of chlorophyll a. Chlorophyll d provides a potential selective advantage because it enables Acaryochloris to use infrared light (700-750 nm) that is not absorbed by chlorophyll a. Recently, an even more red-shifted chlorophyll termed chlorophyll f has been reported. Here, we discuss using modified chlorophylls to extend the spectral region of light that drives photosynthetic organisms.

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.

Redox potentials of primary electron acceptor quinone molecule (QA)- and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E(m)) values of -478 mV for A. marina and -536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (Q(A)), i.e., E(m)(Q(A)/Q(A)(-)), of PS II and the energy difference between [P680·Phe a(-)·Q(A)] and [P680·Phe a·Q(A)(-)], i.e., ΔG(PhQ). The E(m)(Q(A)/Q(A)(-)) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be -66 to -86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The E(m)(Phe a/Phe a(-)) in Synechocystis was measured to be -525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately -77 mV in Synechocystis, and this value was applied to A. marina (-478 mV); the E(m)(Phe a/Phe a(-)) was estimated to be approximately -401 mV. These values gave rise to a ΔG(PhQ) of -325 mV for A. marina and -383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of Q(A)(-) and Phe a(-) were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.

Constitution and energetics of photosystem I and photosystem II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina

This mini review presents current topics of discussion about photosystem (PS) I and PS II of photosynthesis in the Acaryochloris marina. A. marina is a photosynthetic cyanobacterium in which chlorophyll (Chl) d is the major antenna pigment (>95%). However, Chl a is always present in a few percent. Chl d absorbs light with a wavelength up to 30 nm red-shifted from Chl a. Therefore, the chlorophyll species of the special pair in PS II has been a matter of debate because if Chl d was the special pair component, the overall energetics must be different in A. marina. The history of this field indicates that a purified sample is necessary for the reliable identification and characterization of the special pair. In view of the spectroscopic data and the redox potential of pheophytin, we discuss the nature of special pair constituents and the localization of the enigmatic Chl a.

Chlorophyll cycle regulates the construction and destruction of the light-harvesting complexes

Chlorophyll a and chlorophyll b are the major constituents of the photosynthetic apparatus in land plants and green algae. Chlorophyll a is essential in photochemistry, while chlorophyll b is apparently dispensable for their photosynthesis. Instead, chlorophyll b is necessary for stabilizing the major light-harvesting chlorophyll-binding proteins. Chlorophyll b is synthesized from chlorophyll a and is catabolized after it is reconverted to chlorophyll a. This interconversion system between chlorophyll a and chlorophyll b refers to the chlorophyll cycle. The chlorophyll b levels are determined by the activity of the three enzymes participating in the chlorophyll cycle, namely, chlorophyllide a oxygenase, chlorophyll b reductase, and 7-hydroxymethyl-chlorophyll reductase. This article reviews the recent progress on the analysis of the chlorophyll cycle and its enzymes. In particular, we emphasize the impact of genetic modification of chlorophyll cycle enzymes on the construction and destruction of the photosynthetic machinery. These studies reveal that plants regulate the construction and destruction of a specific subset of light-harvesting complexes through the chlorophyll cycle. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Adventures with cyanobacteria: a personal perspective

Cyanobacteria, or the blue-green algae as they used to be called until 1974, are the oldest oxygenic photosynthesizers. We summarize here adventures with them since the early 1960s. This includes studies on light absorption by cyanobacteria, excitation energy transfer at room temperature down to liquid helium temperature, fluorescence (kinetics as well as spectra) and its relationship to photosynthesis, and afterglow (or thermoluminescence) from them. Further, we summarize experiments on their two-light reaction - two-pigment system, as well as the unique role of bicarbonate (hydrogen carbonate) on the electron-acceptor side of their photosystem II, PSII. This review, in addition, includes a discussion on the regulation of changes in phycobilins (mostly in PSII) and chlorophyll a (Chl a; mostly in photosystem I, PSI) under oscillating light, on the relationship of the slow fluorescence increase (the so-called S to M rise, especially in the presence of diuron) in minute time scale with the so-called state-changes, and on the possibility of limited oxygen evolution in mixotrophic PSI (minus) mutants, up to 30 min, in the presence of glucose. We end this review with a brief discussion on the position of cyanobacteria in the evolution of photosynthetic systems.

18O labeling of chlorophyll d in Acaryochloris marina reveals that chlorophyll a and molecular oxygen are precursors

The cyanobacterium Acaryochloris marina was cultured in the presence of either H(2)(18)O or (18)O(2), and the newly synthesized chlorophylls (Chl a and Chl d) were isolated using high performance liquid chromatography and analyzed by mass spectroscopy. In the presence of H(2)(18)O, newly synthesized Chl a and d, both incorporated up to four isotopic (18)O atoms. Time course H(2)(18)O labeling experiments showed incorporation of isotopic (18)O atoms originating from H(2)(18)O into Chl a, with over 90% of Chl a (18)O-labeled at 48 h. The incorporation of isotopic (18)O atoms into Chl d upon incubation in H(2)(18)O was slower compared with Chl a with approximately 50% (18)O-labeled Chl d at 115 h. The rapid turnover of newly synthesized Chl a suggested that Chl a is the direct biosynthetic precursor of Chl d. In the presence of (18)O(2) gas, one isotopic (18)O atom was incorporated into Chl a with approximately the same kinetic incorporation rate observed in the H(2)(18)O labeling experiment, reaching over 90% labeling intensity at 48 h. The incorporation of two isotopic (18)O atoms derived from molecular oxygen ((18)O(2)) was observed in the extracted Chl d, and the percentage of double isotopic (18)O-labeled Chl d increased in parallel with the decrease of non-isotopic-labeled Chl d. This clearly indicated that the oxygen atom in the C3(1)-formyl group of Chl d is derived from dioxygen via an oxygenase-type reaction mechanism.

Redox potential of pheophytin a in photosystem II of two cyanobacteria having the different special pair chlorophylls

Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to the production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe) a. However, the reported values for Phe a are still controversial. Here, we measured the redox potential of Phe a under physiological conditions (pH 7.0; 25 degrees C) in two cyanobacteria with different special pair chlorophylls (Chls): Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chl a, and Acaryochloris marina MBIC 11017, whose special pair for PS II consists of Chl d. We obtained redox potentials of -536 +/- 8 mV for Synechocystis sp. PCC 6803 and -478 +/- 24 mV for A. marina on PS II complexes in the presence of 1.0 M betaine. The difference in the redox potential of Phe a between the two species closely corresponded with the difference in the light energy absorbed by Chl a versus Chl d. We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V for Synechocystis sp. PCC 6803 (P680) and A. marina (P713), respectively. This clearly indicates conservation in the properties of water-oxidation systems in oxygenic photosynthetic organisms, irrespective of the special-pair chlorophylls.

Defining the far-red limit of photosystem II in spinach

The far-red limit of photosystem II (PSII) photochemistry was studied in PSII-enriched membranes and PSII core preparations from spinach (Spinacia oleracea) after application of laser flashes between 730 and 820 nm. Light up to 800 nm was found to drive PSII activity in both acceptor side reduction and oxidation of the water-oxidizing CaMn(4) cluster. Far-red illumination induced enhancement of, and slowed down decay kinetics of, variable fluorescence. Both effects reflect reduction of the acceptor side of PSII. The effects on the donor side of PSII were monitored using electron paramagnetic resonance spectroscopy. Signals from the S(2)-, S(3)-, and S(0)-states could be detected after one, two, and three far-red flashes, respectively, indicating that PSII underwent conventional S-state transitions. Full PSII turnover was demonstrated by far-red flash-induced oxygen release, with oxygen appearing on the third flash. In addition, both the pheophytin anion and the Tyr Z radical were formed by far-red flashes. The efficiency of this far-red photochemistry in PSII decreases with increasing wavelength. The upper limit for detectable photochemistry in PSII on a single flash was determined to be 780 nm. In photoaccumulation experiments, photochemistry was detectable up to 800 nm. Implications for the energetics and energy levels of the charge separated states in PSII are discussed in light of the presented results.

A viewpoint: why chlorophyll a?

Chlorophyll a (Chl a) serves a dual role in oxygenic photosynthesis: in light harvesting as well as in converting energy of absorbed photons to chemical energy. No other Chl is as omnipresent in oxygenic photosynthesis as is Chl a, and this is particularly true if we include Chl a(2), (=[8-vinyl]-Chl a), which occurs in Prochlorococcus, as a type of Chl a. One exception to this near universal pattern is Chl d, which is found in some cyanobacteria that live in filtered light that is enriched in wavelengths >700 nm. They trap the long wavelength electronic excitation, and convert it into chemical energy. In this Viewpoint, we have traced the possible reasons for the near ubiquity of Chl a for its use in the primary photochemistry of Photosystem II (PS II) that leads to water oxidation and of Photosystem I (PS I) that leads to ferredoxin reduction. Chl a appears to be unique and irreplaceable, particularly if global scale oxygenic photosynthesis is considered. Its uniqueness is determined by its physicochemical properties, but there is more. Other contributing factors include specially tailored protein environments, and functional compatibility with neighboring electron transporting cofactors. Thus, the same molecule, Chl a in vivo, is capable of generating a radical cation at +1 V or higher (in PS II), a radical anion at -1 V or lower (in PS I), or of being completely redox silent (in antenna holochromes).

Unique photosystems in Acaryochloris marina

A short overview is given on the discovery of the chlorophyll d-dominated cyanobacterium Acaryochloris marina and the minor pigments that function as key components therein. In photosystem I, chlorophyll d', chlorophyll a, and phylloquinone function as the primary electron donor, the primary electron acceptor and the secondary electron acceptor, respectively. In photosystem II, pheophytin a serves as the primary electron acceptor. The oxidation potential of chlorophyll d was higher than that of chlorophyll a in vitro, while the oxidation potential of P740 was almost the same as that of P700. These results help us to broaden our view on the questions about the unique photosystems in Acaryochloris marina.

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.

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.

Two unique cyanobacteria lead to a traceable approach of the first appearance of oxygenic photosynthesis

The evolutionary route from anoxygenic photosynthetic bacteria to oxygenic cyanobacteria is discontinuous in terms of photochemical/photophysical reaction systems. It is difficult to describe this transition process simply because there are no recognized intermediary organisms between the two bacterial groups. Gloeobacter violaceus PCC 7421 might be a model organism that is suitable for analysis because it still possesses primordial characteristics such as the absence of thylakoid membranes. Whole genome analysis and biochemical and biophysical surveys of G. violaceus have favored the hypothesis that it is an intermediary organism. On the other hand, species differentiation is an evolutionary process that could be driven by changes in a small number of genes, and this process might give fair information more in details by monitoring of those genes. Comparative studies of genes, including those in Acaryochloris marina MBIC 11017, have provided information relevant to species differentiation; in particular, the acquisition of a new pigment, chlorophyll d, and changes in amino acid sequences have been informative. Here, based on experimental evidence from these two species, we discuss some of the evolutionary pathways for the appearance and differentiation of cyanobacteria.

Covalently linked zinc chlorophyll dimers as a model of a chlorophyllous pair in photosynthetic reaction centers

A heterodimer, where zinc pyropheophorbide-a was linked with zinc pyropheophorbide-d through ethylene glycol diester, was prepared, as well as the corresponding homodimers. The synthetic dimers were complexed with methanol in benzene to give folded dimers by mutual Zn...O(Me)-H...O=C13(1) bonding. Such complexes had furthest red (Qy) absorption bands at longer wavelengths than the monomeric species. These red-shifts were ascribable to excitonic coupling of the Qy transition states in the chlorin pi-pi stacking conformer. In the heterodimeric system, a minor band was observed at the shorter wavelength side of the main Qy band. This observation can be explained by an additional contribution of Qy vibronic state to the exciton-coupled states. Based on the experimental results, a pair of chlorophyll(Chl)-d with Chl-a as well as a Chl-d homopair were proposed as dimers in reaction centers of Chl-d dominating cyanobacteria.

Characterization of highly purified photosystem I complexes from the chlorophyll d-dominated cyanobacterium Acaryochloris marina MBIC 11017

Photochemically active photosystem (PS) I complexes were purified from the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017, and several of their properties were characterized. PS I complexes consist of 11 subunits, including PsaK1 and PsaK2; a new small subunit was identified and named Psa27. The new subunit might replace the function of PsaI that is absent in A. marina. The amounts of pigments per one molecule of Chl d' were 97.0 +/- 11.0 Chl d, 1.9 +/- 0.5 Chl a, 25.2 +/- 2.4 alpha-carotene, and two phylloquinone molecules. The light-induced Fourier transform infrared difference spectroscopy and light-induced difference absorption spectra reconfirmed that the primary electron donor of PS I (P740) was the Chl d dimer. In addition to P740, the difference spectrum contained an additional band at 728 nm. The redox potentials of P740 were estimated to be 439 mV by spectroelectrochemistry; this value was comparable with the potential of P700 in other cyanobacteria and higher plants. This suggests that the overall energetics of the PS I reaction were adjusted to the electron acceptor side to utilize the lower light energy gained by P740. The distribution of charge in P740 was estimated by a density functional theory calculation, and a partial localization of charge was predicted to P1 Chl (special pair Chl on PsaA). Based on differences in the protein matrix and optical properties of P740, construction of the PS I core in A. marina was discussed.

Niche adaptation and genome expansion in the chlorophyll d-producing cyanobacterium Acaryochloris marina

Acaryochloris marina is a unique cyanobacterium that is able to produce chlorophyll d as its primary photosynthetic pigment and thus efficiently use far-red light for photosynthesis. Acaryochloris species have been isolated from marine environments in association with other oxygenic phototrophs, which may have driven the niche-filling introduction of chlorophyll d. To investigate these unique adaptations, we have sequenced the complete genome of A. marina. The DNA content of A. marina is composed of 8.3 million base pairs, which is among the largest bacterial genomes sequenced thus far. This large array of genomic data is distributed into nine single-copy plasmids that code for >25% of the putative ORFs. Heavy duplication of genes related to DNA repair and recombination (primarily recA) and transposable elements could account for genetic mobility and genome expansion. We discuss points of interest for the biosynthesis of the unusual pigments chlorophyll d and alpha-carotene and genes responsible for previously studied phycobilin aggregates. Our analysis also reveals that A. marina carries a unique complement of genes for these phycobiliproteins in relation to those coding for antenna proteins related to those in Prochlorococcus species. The global replacement of major photosynthetic pigments appears to have incurred only minimal specializations in reaction center proteins to accommodate these alternate pigments. These features clearly show that the genus Acaryochloris is a fitting candidate for understanding genome expansion, gene acquisition, ecological adaptation, and photosystem modification in the cyanobacteria.

Identification of a novel vinyl reductase gene essential for the biosynthesis of monovinyl chlorophyll in Synechocystis sp. PCC6803

The vast majority of oxygenic photosynthetic organisms use monovinyl chlorophyll for their photosynthetic reactions. For the biosynthesis of this type of chlorophyll, the reduction of the 8-vinyl group that is located on the B-ring of the macrocycle is essential. Previously, we identified the gene encoding 8-vinyl reductase responsible for this reaction in higher plants and termed it DVR. Among the sequenced genomes of cyanobacteria, only several Synechococcus species contain DVR homologues. Therefore, it has been hypothesized that many other cyanobacteria producing monovinyl chlorophyll should contain a vinyl reductase that is unrelated to the higher plant DVR. To identify the cyanobacterial gene that is responsible for monovinyl chlorophyll synthesis, we developed a bioinformatics tool, correlation coefficient calculation tool, which calculates the correlation coefficient between the distributions of a certain phenotype and genes among a group of organisms. The program indicated that the distribution of a gene encoding a putative dehydrogenase protein is best correlated with the distribution of the DVR-less cyanobacteria. We subsequently knocked out the corresponding gene (Slr1923) in Synechocystis sp. PCC6803 and characterized the mutant. The knock-out mutant lost its ability to synthesize monovinyl chlorophyll and accumulated 3,8-divinyl chlorophyll instead. We concluded that Slr1923 encodes the vinyl reductase or a subunit essential for monovinyl chlorophyll synthesis. The function and evolution of 8-vinyl reductase genes are discussed.

Vibrational mode frequency calculations of chlorophyll-d for assessing (P740(+)-P740) FTIR difference spectra obtained using photosystem I particles from Acaryochloris marina

Acaryochloris marina is an oxygen-evolving organism that utilizes chlorophyll-d for light induced photochemistry. In photosystem I particles from Acaryochloris marina, the primary electron donor is called P740, and it is thought that P740 consist of two chlorophyll-d molecules. (P740(+)-P740) FTIR difference spectra have been produced, and vibrational features that are specific to chlorophyll-d (and not chlorophyll-a) were observed, supporting the idea that P740 consists chlorophyll-d molecules. Although bands in the (P740(+)-P740) FTIR difference spectra were assigned specifically to chlorophyll-d, how these bands shifted, and how their intensities changed, upon cation formation was never considered. Without this information it is difficult to draw unambiguous conclusions from the FTIR difference spectra. To gain a more detailed understanding of cation induced shifting of bands associated with vibrational modes of P740 we have used density functional theory to calculate the vibrational properties of a chlorophyll-d model in the neutral, cation and anion states. These calculations are shown to be of considerable use in interpreting bands in (P740(+)-P740) FTIR difference spectra. Our calculations predict that the 3(1) formyl C-H mode of chlorophyll-d upshifts/downshifts upon cation/anion formation, respectively. The mode intensity also decreases/increases upon cation/anion formation, respectively. The cation induced bandshift of the 3(1) formyl C-H mode of chlorophyll-d is also strongly dependant on the dielectric environment of the chlorophyll-d molecules. With this new knowledge we reassess the interpretation of bands that were assigned to 3(1) formyl C-H modes of chlorophyll-d in (P740(+)-P740) FTIR difference spectra. Considering our calculations in combination with (P740(+)-P740) FTIR DS we find that the most likely conclusions are that P740 is a dimeric Chl-d species, in an environment of low effective dielectric constant ( approximately 2-8). In the P740(+) state, charge is asymmetrically distributed over the two Chl-d pigments in a roughly 60:40 ratio.

Are there ecological implications for the proposed energetic restrictions on photosynthetic oxygen evolution at high oxygen concentrations?

It has recently been shown that, in subthylakoid particles prepared using detergent, there is inhibition of oxygen production reactions in photosynthesis by thermodynamic feedback from oxygen build-up, with 50% inhibition at 230 kPa partial pressure of oxygen. This article presents a comprehensive analysis of laboratory data on the effects of high oxygen partial pressures on photosynthesis, and on photo-lithotrophic and chemo-organotrophic growth, of oxygen-producing organisms. The article also contains an analysis of the extent to which high oxygen concentrations occur at the site of photosystem II (PSII) activity under natural conditions today and in the past. The conclusion is that the oxygen concentrations found in nature are very unlikely to reach that needed to cause 50% inhibition of the photosynthetic oxygen production reaction in subthylakoid particles, but that it is just possible that a small part of the inhibition of photosynthesis and of photo-lithotrophic growth by oxygen can be attributed to inhibition of oxygen production by PSII.