The major light-harvesting pigment protein of Acaryochloris marina

Presence of low-energy chlorophylls d in photosystem I trimer and monomer cores isolated from Acaryochloris sp. NBRC 102871

Acaryochloris species belong to a special category of cyanobacteria possessing chlorophyll (Chl) d. One of the photosynthetic characteristics of Acaryochloris marina MBIC11017 is that the absorption spectra of photosystem I (PSI) showed almost no bands and shoulders of low-energy Chls d over 740 nm. In contrast, the absorption spectra of other Acaryochloris species showed a shoulder around 740 nm, suggesting that low-energy Chls d within PSI are diversified among Acaryochloris species. In this study, we purified PSI trimer and monomer cores from Acaryochloris sp. NBRC 102871 and examined their protein and pigment compositions and spectral properties. The protein bands and pigment compositions of the PSI trimer and monomer of NBRC102871 were virtually identical to those of MBIC11017. The absorption spectra of the NBRC102871 PSIs exhibited a shoulder around 740 nm, whereas the fluorescence spectra of PSI trimer and monomer displayed maximum peaks at 754 and 767 nm, respectively. These spectral properties were different from those of MBIC11017, indicating the presence of low-energy Chls d within the NBRC102871 PSIs. Moreover, we analyzed the NBRC102871 genome to identify amino acid sequences of PSI proteins and compared them with those of the A. marina MBIC11017 and MBIC10699 strains whose genomes are available. The results showed that some of the sequences in NBRC102871 were distinct from those in MBIC11017 and MBIC10699. These findings provide insights into the variety of low-energy Chls d with respect to the protein environments of PSI cores among the three Acaryochloris strains.

Structure of a unique PSII-Pcb tetrameric megacomplex in a chlorophyll d-containing cyanobacterium

Acaryochloris marina is a unique cyanobacterium using chlorophyll d (Chl d) as its major pigment and thus can use far-red light for photosynthesis. Photosystem II (PSII) of A. marina associates with a number of prochlorophyte Chl-binding (Pcb) proteins to act as the light-harvesting system. We report here the cryo-electron microscopic structure of a PSII-Pcb megacomplex from A. marina at a 3.6-angstrom overall resolution and a 3.3-angstrom local resolution. The megacomplex is organized as a tetramer consisting of two PSII core dimers flanked by sixteen symmetrically related Pcb proteins, with a total molecular weight of 1.9 megadaltons. The structure reveals the detailed organization of PSII core consisting of 15 known protein subunits and an unknown subunit, the assembly of 4 Pcb antennas within each PSII monomer, and possible pathways of energy transfer within the megacomplex, providing deep insights into energy transfer and dissipation mechanisms within the PSII-Pcb megacomplex involved in far-red light utilization.

Phycobilisomes and Phycobiliproteins in the Pigment Apparatus of Oxygenic Photosynthetics: From Cyanobacteria to Tertiary Endosymbiosis

Eukaryotic photosynthesis originated in the course of evolution as a result of the uptake of some unstored cyanobacterium and its transformation to chloroplasts by an ancestral heterotrophic eukaryotic cell. The pigment apparatus of Archaeplastida and other algal phyla that emerged later turned out to be arranged in the same way. Pigment-protein complexes of photosystem I (PS I) and photosystem II (PS II) are characterized by uniform structures, while the light-harvesting antennae have undergone a series of changes. The phycobilisome (PBS) antenna present in cyanobacteria was replaced by Chl a/b- or Chl a/c-containing pigment-protein complexes in most groups of photosynthetics. In the form of PBS or phycobiliprotein aggregates, it was inherited by members of Cyanophyta, Cryptophyta, red algae, and photosynthetic amoebae. Supramolecular organization and architectural modifications of phycobiliprotein antennae in various algal phyla in line with the endosymbiotic theory of chloroplast origin are the subject of this review.

Comparative Genomic Analysis of the Marine Cyanobacterium Acaryochloris marina MBIC10699 Reveals the Impact of Phycobiliprotein Reacquisition and the Diversity of Acaryochloris Plasmids

Acaryochloris is a marine cyanobacterium that synthesizes chlorophyll d, a unique chlorophyll that absorbs far-red lights. Acaryochloris is also characterized by the loss of phycobiliprotein (PBP), a photosynthetic antenna specific to cyanobacteria; however, only the type-strain A. marina MBIC11017 retains PBP, suggesting that PBP-related genes were reacquired through horizontal gene transfer (HGT). Acaryochloris is thought to have adapted to various environments through its huge genome size and the genes acquired through HGT; however, genomic information on Acaryochloris is limited. In this study, we report the complete genome sequence of A. marina MBIC10699, which was isolated from the same area of ocean as A. marina MBIC11017 as a PBP-less strain. The genome of A.marina MBIC10699 consists of a 6.4 Mb chromosome and four large plasmids totaling about 7.6 Mb, and the phylogenic analysis shows that A.marina MBIC10699 is the most closely related to A. marina MBIC11017 among the Acaryochloris species reported so far. Compared with A. marina MBIC11017, the chromosomal genes are highly conserved between them, while the genes encoded in the plasmids are significantly diverse. Comparing these genomes provides clues as to how the genes for PBPs were reacquired and what changes occurred in the genes for photosystems during evolution.

Effects of Light and Oxygen on Chlorophyll d Biosynthesis in a Marine Cyanobacterium Acaryochloris marina

A marine cyanobacterium Acaryochloris marina synthesizes chlorophyll (Chl) d as a major Chl. Chl d has a formyl group at its C3 position instead of a vinyl group in Chl a. This modification allows Chl d to absorb far-red light addition to visible light, yet the enzyme catalyzing the formation of the C3-formyl group has not been identified. In this study, we focused on light and oxygen, the most important external factors in Chl biosynthesis, to investigate their effects on Chl d biosynthesis in A. marina. The amount of Chl d in heterotrophic dark-grown cells was comparable to that in light-grown cells, indicating that A. marina has a light-independent pathway for Chl d biosynthesis. Under anoxic conditions, the amount of Chl d increased with growth in light conditions; however, no growth was observed in dark conditions, indicating that A. marina synthesizes Chl d normally even under such “micro-oxic” conditions caused by endogenous oxygen production. Although the oxygen requirement for Chl d biosynthesis could not be confirmed, interestingly, accumulation of pheophorbide d was observed in anoxic and dark conditions, suggesting that Chl d degradation is induced by anaerobicity and darkness.

Acclimation process of the chlorophyll d-bearing cyanobacterium Acaryochloris marina to an orange light environment revealed by transcriptomic analysis and electron microscopic observation

The cyanobacterium Acaryochloris marina MBIC 11017 (A. marina 11017) possesses chlorophyll d (Chl. d) peaking at 698 nm as photosystem reaction center pigments, instead of chlorophyll a (Chl. a) peaking at 665 nm. About 95% of the total chlorophylls is Chl. d in A. marina 11017. In addition, A. marina 11017 possesses phycobilisome (PBS) supercomplex to harvest orange light and to transfer the absorbing energy to the photosystems. In this context, A. marina 11017 utilizes both far-red and orange light as the photosynthetic energy source. In the present study, we incubated A. marina 11017 cells under monochromatic orange and far-red light conditions and performed transcriptional and morphological studies by RNA-seq analysis and electron microscopy. Cellular absorption spectra, transcriptomic profiles, and microscopic observations demonstrated that PBS was highly accumulated under an orange light condition relative to a far-red light condition. Notably, transcription of one cpcBA operon encoding the phycobiliprotein of the phycocyanin was up-regulated under the orange light condition, but another operon was constitutively expressed under both conditions, indicating functional diversification of these two operons for light harvesting. Taking the other observations into consideration, we could illustrate the photoacclimation processes of A. marina 11017 in response to orange and far-red light conditions in detail.

The amazing phycobilisome

Cyanobacteria and red-algae share a common light-harvesting complex which is different than all other complexes that serve as photosynthetic antennas - the Phycobilisome (PBS). The PBS is found attached to the stromal side of thylakoid membranes, filling up most of the gap between individual thylakoids. The PBS self assembles from similar homologous protein units that are soluble and contain conserved cysteine residues that covalently bind the light absorbing chromophores, linear tetra-pyrroles. Using similar construction principles, the PBS can be as large as 16.8 MDa (68×45×39nm), as small as 1.2 MDa (24 × 11.5 × 11.5 nm), and in some unique cases smaller still. The PBS can absorb light between 450 nm to 650 nm and in some cases beyond 700 nm, depending on the species, its composition and assembly. In this review, we will present new observations and structures that expand our understanding of the distinctive properties that make the PBS an amazing light harvesting system. At the end we will suggest why the PBS, for all of its excellent properties, was discarded by photosynthetic organisms that arose later in evolution such as green algae and higher plants.

The identification of IsiA proteins binding chlorophyll d in the cyanobacterium Acaryochloris marina

The bioavailable iron in many aquatic ecosystems is extremely low, and limits the growth and photosynthetic activity of phytoplankton. In response to iron limitation, a group of chlorophyll-binding proteins known as iron stress-induced proteins are induced and serve as accessory light-harvesting components for photosystems under iron limitation. In the present study, we investigated physiological features of Acaryochloris marina in response to iron-deficient conditions. The growth doubling time under iron-deficient conditions was prolonged to ~3.4 days compared with 1.9 days under normal culture conditions, accompanied with dramatically decreased chlorophyll content. The isolation of chlorophyll-binding protein complexes using sucrose density gradient centrifugation shows six main green bands and three main fluorescence components of 712, 728, and 748 nm from the iron-deficient culture. The fluorescence components of 712 and 728 nm co-exist in the samples collected from iron-deficient and iron-replete cultures and are attributed to Chl d-binding accessory chlorophyll-binding antenna proteins and also from photosystem II. A new chlorophyll-binding protein complex with its main fluorescence peak at 748 nm was observed and enriched in the heaviest fraction from the samples collected from the iron-deficient culture only. Combining western blotting analysis using antibodies of CP47 (PSII), PsaC (PSI) and IsiA and proteomic analysis on an excised protein band at ~37 kDa, the heaviest fraction (-F6) isolated from iron-deficient culture contained Chl d-bound PSI-IsiA supercomplexes. The PSII-antenna supercomplexes isolated from iron-replete conditions showed two fluorescence peaks of 712 and 728 nm, which can be assigned as 6-transmembrane helix chlorophyll-binding antenna and photosystem II fluorescence, respectively, which is supported by protein analysis of the fractions (F5 and F6).

The wavelength of the incident light determines the primary charge separation pathway in Photosystem II

Charge separation is a key component of the reactions cascade of photosynthesis, by which solar energy is converted to chemical energy. From this photochemical reaction, two radicals of opposite charge are formed, a highly reducing anion and a highly oxidising cation. We have previously proposed that the cation after far-red light excitation is located on a component different from P_D1, which is the location of the primary electron hole after visible light excitation. Here, we attempt to provide further insight into the location of the primary charge separation upon far-red light excitation of PS II, using the EPR signal of the spin polarized ³P₆₈₀ as a probe. We demonstrate that, under far-red light illumination, the spin polarized ³P₆₈₀ is not formed, despite the primary charge separation still occurring at these conditions. We propose that this is because under far-red light excitation, the primary electron hole is localized on Chl_D1, rather than on P_D1. The fact that identical samples have demonstrated charge separation upon both far-red and visible light excitation supports our hypothesis that two pathways for primary charge separation exist in parallel in PS II reaction centres. These pathways are excited and activated dependent of the wavelength applied.

Excitation energy transfer in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by spectral hole burning

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron-phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673 nm. The electronic states at ~633 and 644 nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673 nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron-phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490-1499, 2014) based on difference fluorescence line-narrowing experiments.

The Complex Transcriptional Response of Acaryochloris marina to Different Oxygen Levels

Ancient oxygenic photosynthetic prokaryotes produced oxygen as a waste product, but existed for a long time under an oxygen-free (anoxic) atmosphere, before an oxic atmosphere emerged. The change in oxygen levels in the atmosphere influenced the chemistry and structure of many enzymes that contained prosthetic groups that were inactivated by oxygen. In the genome of Acaryochloris marina, multiple gene copies exist for proteins that are normally encoded by a single gene copy in other cyanobacteria. Using high throughput RNA sequencing to profile transcriptome responses from cells grown under microoxic and hyperoxic conditions, we detected 8446 transcripts out of the 8462 annotated genes in the Cyanobase database. Two-thirds of the 50 most abundant transcripts are key proteins in photosynthesis. Microoxic conditions negatively affected the levels of expression of genes encoding photosynthetic complexes, with the exception of some subunits. In addition to the known regulation of the multiple copies of psbA, we detected a similar transcriptional pattern for psbJ and psbU, which might play a key role in the altered components of photosystem II. Furthermore, regulation of genes encoding proteins important for reactive oxygen species-scavenging is discussed at genome level, including, for the first time, specific small RNAs having possible regulatory roles under varying oxygen levels.

Solution structure and excitation energy transfer in phycobiliproteins of Acaryochloris marina investigated by small angle scattering

The structure of phycobiliproteins of the cyanobacterium Acaryochloris marina was investigated in buffer solution at physiological temperatures, i.e. under the same conditions applied in spectroscopic experiments, using small angle neutron scattering. The scattering data of intact phycobiliproteins in buffer solution containing phosphate can be well described using a cylindrical shape with a length of about 225Å and a diameter of approximately 100Å. This finding is qualitatively consistent with earlier electron microscopy studies reporting a rod-like shape of the phycobiliproteins with a length of about 250 (M. Chen et al., FEBS Letters 583, 2009, 2535) or 300Å (J. Marquart et al., FEBS Letters 410, 1997, 428). In contrast, phycobiliproteins dissolved in buffer lacking phosphate revealed a splitting of the rods into cylindrical subunits with a height of 28Å only, but also a pronounced sample aggregation. Complementary small angle neutron and X-ray scattering experiments on phycocyanin suggest that the cylindrical subunits may represent either trimeric phycocyanin or trimeric allophycocyanin. Our findings are in agreement with the assumption that a phycobiliprotein rod with a total height of about 225Å can accommodate seven trimeric phycocyanin subunits and one trimeric allophycocyanin subunit, each of which having a height of about 28Å. The structural information obtained by small angle neutron and X-ray scattering can be used to interpret variations in the low-energy region of the 4.5K absorption spectra of phycobiliproteins dissolved in buffer solutions containing and lacking phosphate, respectively.

Spectral properties of bacteriophytochrome AM1_5894 in the chlorophyll d-containing cyanobacterium Acaryochloris marina

Acaryochloris marina, a unicellular oxygenic photosynthetic cyanobacterium, has uniquely adapted to far-red light-enriched environments using red-shifted chlorophyll d. To understand red-light use in Acaryochloris, the genome of this cyanobacterium was searched for red/far-red light photoreceptors from the phytochrome family, resulting in identification of a putative bacteriophytochrome AM1_5894. AM1_5894 contains three standard domains of photosensory components as well as a putative C-terminal signal transduction component consisting of a histidine kinase and receiver domain. The photosensory domains of AM1_5894 autocatalytically assemble with biliverdin in a covalent fashion. This assembled AM1_5894 shows the typical photoreversible conversion of bacterial phytochromes with a ground-state red-light absorbing (Pr) form with λBV max[Pr] 705 nm, and a red-light inducible far-red light absorbing (Pfr) form with λBV max[Pfr] 758 nm. Surprisingly, AM1_5894 also autocatalytically assembles with phycocyanobilin, involving photoreversible conversion of λPCB max[Pr] 682 nm and λPCB max[Pfr] 734 nm, respectively. Our results suggest phycocyanobilin is also covalently bound to AM1_5894, while mutation of a cysteine residue (Cys11Ser) abolishes this covalent binding. The physiological function of AM1_5894 in cyanobacteria containing red-shifted chlorophylls is discussed.

Establishment of the forward genetic analysis of the chlorophyll d-dominated cyanobacterium Acaryochloris marina MBIC 11017 by applying in vivo transposon mutagenesis system

Acaryochloris marina MBIC 11017 possesses chlorophyll (Chl) d as a major Chl, which enables this organism to utilize far-red light for photosynthesis. Thus, the adaptation mechanism of far-red light utilization, including Chl d biosynthesis, has received much attention, though a limited number of reports on this subject have been published. To identify genes responsible for Chl d biosynthesis and adaptation to far-red light, molecular genetic analysis of A. marina was required. We developed a transformation system for A. marina and introduced expression vectors into A. marina. In this study, the high-frequency in vivo transposon mutagenesis system recently established by us was applied to A. marina. As a result, we obtained mutants with the transposon in their genomic DNA at various positions. By screening transposon-tagged mutants, we isolated a mutant (Y1 mutant) that formed a yellow colony on agar medium. In the Y1 mutant, the transposon was inserted into the gene encoding molybdenum cofactor biosynthesis protein A (MoaA). The Y1 mutant was functionally complemented by introducing the moaA gene or increasing the ammonium ion in the medium. These results indicate that the mutation of the moaA gene reduced nitrate reductase activity, which requires molybdenum cofactor, in the Y1 mutant. This is the first successful forward genetic analysis of A. marina, which will lead to the identification of genes responsible for adaptation to far-red light.

Novel chlorophylls and new directions in photosynthesis research

Chlorophyll d and chlorophyll f are red-shifted chlorophylls, because their Qy absorption bands are significantly red-shifted compared with chlorophyll a. The red-shifted chlorophylls broaden the light absorption region further into far red light. The presence of red-shifted chlorophylls in photosynthetic systems has opened up new possibilities of research on photosystem energetics and challenged the unique status of chlorophyll a in oxygenic photosynthesis. In this review, we report on the chemistry and function of red-shifted chlorophylls in photosynthesis and summarise the unique adaptations that have allowed the proliferation of chlorophyll d- and chlorophyll f-containing organisms in diverse ecological niches around the world.

Artificially acquired chlorophyll b is highly acceptable to the thylakoid-lacking cyanobacterium, Gloeobacter violaceus PCC 7421

Unicellular cyanobacterium Gloeobacter violaceus is an only known oxygenic photosynthetic organism that lacks thylakoid membrane. Molecular phylogenetic analyses indicate that G. violaceus is an early-branching cyanobacterium within cyanobacterial clade. Therefore, the photosynthetic system of G. violaceus is considered to be partly similar to that of the ancestral cyanobacteria that would lack thylakoid membrane. G. violaceus possesses chlorophyll (Chl) a as the only chlorophyll species like most cyanobacteria. It was proposed that the ancestral oxygenic photosynthetic organism had not only Chl a and phycobilins but also Chl b. However, no organism which contains both Chl a and Chl b and lacks thylakoid membrane has been found in nature. Therefore, we introduced the chlorophyllide a oxygenase gene responsible for Chl b biosynthesis into G. violaceus. In the resultant transformant, Chl b accumulated at approximately 11% of total Chl independent of growth phase. Photosystem I complexes isolated from the transformant contained Chl b at 9.9% of total Chl. The presence of Chl b in the photosystem I complexes did not inhibit trimer formation. Furthermore, time-resolved fluorescence spectrum demonstrated that Chl b transferred energy to Chl a in the photosystem I complexes and did not disturb the energy transfer among the Chl a molecules. These results show that G. violaceus is tolerant to artificially produced Chl b and suggest the flexibility of photosystem for Chl composition in the ancestral oxygenic photosynthetic organism.

Chlorophyll modifications and their spectral extension in oxygenic photosynthesis

Chlorophylls are magnesium-tetrapyrrole molecules that play essential roles in photosynthesis. All chlorophylls have similar five-membered ring structures, with variations in the side chains and/or reduction states. Formyl group substitutions on the side chains of chlorophyll a result in the different absorption properties of chlorophyll b, chlorophyll d, and chlorophyll f. These formyl substitution derivatives exhibit different spectral shifts according to the formyl substitution position. Not only does the presence of various types of chlorophylls allow the photosynthetic organism to harvest sunlight at different wavelengths to enhance light energy input, but the pigment composition of oxygenic photosynthetic organisms also reflects the spectral properties on the surface of the Earth. Two major environmental influencing factors are light and oxygen levels, which may play central roles in the regulatory pathways leading to the different chlorophylls. I review the biochemical processes of chlorophyll biosynthesis and their regulatory mechanisms.

Excitation energy transfer and electron-vibrational coupling in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by site-selective spectroscopy

In adaption to its specific environmental conditions, the cyanobacterium Acaryochloris marina developed two different types of light-harvesting complexes: chlorophyll-d-containing membrane-intrinsic complexes and phycocyanobilin (PCB) - containing phycobiliprotein (PBP) complexes. The latter complexes are believed to form a rod-shaped structure comprising three homo-hexamers of phycocyanin (PC), one hetero-hexamer of phycocyanin and allophycocyanin (APC) and probably a linker protein connecting the PBPs to the reaction centre. Excitation energy transfer and electron-vibrational coupling in PBPs have been investigated by selectively excited fluorescence spectra. The data reveal a rich spectral substructure with a total of five low-energy electronic states with fluorescence bands at 635nm, 645nm, 654nm, 659nm and a terminal emitter at about 673 nm. The electronic states at ~635 and 645 nm are tentatively attributed to PC and APC, respectively, while an apparent heterogeneity among PC subunits may also play a role. The other fluorescence bands may be associated with three different isoforms of the linker protein. Furthermore, a large number of vibrational features can be identified for each electronic state with intense phonon sidebands peaking at about 31 to 37cm⁻¹, which are among the highest phonon frequencies observed for photosynthetic antenna complexes. The corresponding Huang-Rhys factors S fall in the range between 0.98 (terminal emitter), 1.15 (APC), and 1.42 (PC). Two characteristic vibronic lines at about 1580 and 1634cm⁻¹ appear to reflect CNH⁺ and CC stretching modes of the PCB chromophore, respectively. The exact phonon and vibrational frequencies vary with electronic state implying that the respective PCB chromophores are bound to different protein environments. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Response of chlorophyll d-containing cyanobacterium Acaryochloris marina to UV and visible irradiations

We have previously investigated the response mechanisms of photosystem II complexes from spinach to strong UV and visible irradiations (Wei et al J Photochem Photobiol B 104:118-125, 2011). In this work, we extend our study to the effects of strong light on the unusual cyanobacterium Acaryochloris marina, which is able to use chlorophyll d (Chl d) to harvest solar energy at a longer wavelength (740 nm). We found that ultraviolet (UV) or high level of visible and near-far red light is harmful to A. marina. Treatment with strong white light (1,200 μmol quanta m(-2) s(-1)) caused a parallel decrease in PSII oxygen evolution of intact cells and in extracted pigments Chl d, zeaxanthin, and α-carotene analyzed by high-performance liquid chromatography, with severe loss after 6 h. When cells were irradiated with 700 nm of light (100 μmol quanta m(-2) s(-1)) there was also bleaching of Chl d and loss of photosynthetic activity. Interestingly, UVB radiation (138 μmol quanta m(-2) s(-1)) caused a loss of photosynthetic activity without reduction in Chl d. Excess absorption of light by Chl d (visible or 700 nm) causes a reduction in photosynthesis and loss of pigments in light harvesting and photoprotection, likely by photoinhibition and inactivation of photosystem II, while inhibition of photosynthesis by UVB radiation may occur by release of Mn ion(s) in Mn4CaO5 center in photosystem II.

Chlorophyll d and Acaryochloris marina: current status

The discovery of the chlorophyll d-containing cyanobacterium Acaryochloris marina in 1996 precipitated a shift in our understanding of oxygenic photosynthesis. The presence of the red-shifted chlorophyll d in the reaction centre of the photosystems of Acaryochloris has opened up new avenues of research on photosystem energetics and challenged the unique status of chlorophyll a in oxygenic photosynthesis. In this review, we detail the chemistry and role of chlorophyll d in photosynthesis and summarise the unique adaptations that have allowed the proliferation of Acaryochloris in diverse ecological niches around the world.

Extending the limits of natural photosynthesis and implications for technical light harvesting

Photosynthetic organisms provide, directly or indirectly, the energy that sustains life on earth by harvesting light from the sun. The amount of light impinging on the surface of the earth vastly surpasses the energy needs of life including man. Harvesting the sun is, therefore, an option for a sustainable energy source: directly by improving biomass production, indirectly by coupling it to the production of hydrogen for fuel or, conceptually, by using photosynthetic strategies for technological solutions based on non-biological or hybrid materials. In this review, we summarize the various light climates on earth, the primary reactions responsible for light harvesting and transduction to chemical energy in photosynthesis, and the mechanisms of competitively adapting the photosynthetic apparatus to the ever-changing light conditions. The focus is on oxygenic photosynthesis, its adaptation to the various light-climates by specialized pigments and on the extension of its limits by the evolution of red-shifted chlorophylls. The implications for potential technical solutions are briefly discussed.

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.

Cyanobacteria of the genus prochlorothrix

Green cyanobacteria differ from the blue-green cyanobacteria by the possession of a chlorophyll-containing light-harvesting antenna. Three genera of the green cyanobacteria namely Acaryochloris, Prochlorococcus, and Prochloron are unicellular and inhabit marine environments. Prochlorococcus marinus attracts most attention due to its prominent role in marine primary productivity. The fourth genus Prochlorothrix is represented by the filamentous freshwater strains. Unlike the other green cyanobacteria, Prochlorothrix strains are remarkably rare: to date, living isolates have been limited to two European locations. Taking into account fluctuating blooms, morphological resemblance to Planktothrix and Pseudanabaena, and unsuccessful attempts to obtain enrichments of Prochlorothrix, the most successful strategy to search for this cyanobacterium involves PCR with environmental DNA and Prochlorothrix-specific primers. This approach has revealed a broader distribution of Prochlorothrix. Marker genes have been found in at least two additional locations. Despite of the growing evidence for naturally occurring Prochlorothrix, there are only a few cultured strains with one of them (PCC 9006) being claimed to be axenic. In multixenic cultures, Prochlorothrix is accompanied by heterotrophic bacteria indicating a consortium-type association. The genus Prochlorothrix includes two species: P. hollandica and P. scandica based on distinctions in genomic DNA, cell size, temperature optimum, and fatty acid composition of membrane lipids. In this short review the properties of cyanobacteria of the genus Prochlorothrix are described. In addition, the evolutionary scenario for green cyanobacteria is suggested taking into account their possible role in the origin of simple chloroplast.

Biofilm growth and near-infrared radiation-driven photosynthesis of the chlorophyll d-containing cyanobacterium Acaryochloris marina

The cyanobacterium Acaryochloris marina is the only known phototroph harboring chlorophyll (Chl) d. It is easy to cultivate it in a planktonic growth mode, and A. marina cultures have been subject to detailed biochemical and biophysical characterization. In natural situations, A. marina is mainly found associated with surfaces, but this growth mode has not been studied yet. Here, we show that the A. marina type strain MBIC11017 inoculated into alginate beads forms dense biofilm-like cell clusters, as in natural A. marina biofilms, characterized by strong O(2) concentration gradients that change with irradiance. Biofilm growth under both visible radiation (VIS, 400 to 700 nm) and near-infrared radiation (NIR, ∼700 to 730 nm) yielded maximal cell-specific growth rates of 0.38 per day and 0.64 per day, respectively. The population doubling times were 1.09 and 1.82 days for NIR and visible light, respectively. The photosynthesis versus irradiance curves showed saturation at a photon irradiance of E(k) (saturating irradiance) >250 μmol photons m(-2) s(-1) for blue light but no clear saturation at 365 μmol photons m(-2) s(-1) for NIR. The maximal gross photosynthesis rates in the aggregates were ∼1,272 μmol O(2) mg Chl d(-1) h(-1) (NIR) and ∼1,128 μmol O(2) mg Chl d(-1) h(-1) (VIS). The photosynthetic efficiency (α) values were higher in NIR-irradiated cells [(268 ± 0.29) × 10(-6) m(2) mg Chl d(-1) (mean ± standard deviation)] than under blue light [(231 ± 0.22) × 10(-6) m(2) mg Chl d(-1)]. A. marina is well adapted to a biofilm growth mode under both visible and NIR irradiance and under O(2) conditions ranging from anoxia to hyperoxia, explaining its presence in natural niches with similar environmental conditions.

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.

Chlorophylldas the major photopigment inAcaryochloris marina

Chlorophyll (Chl) d is the major pigment in the photosystems (PS) and light-harvesting complex(es) of Acaryochloris marina. Chl a is present in small and variable amounts in PSII and in the light-harvesting complex(es). Isolated PSII complex showed a major fluorescence emission peak at 725 nm and a smaller emission peak due to Chl d at 701 nm, while the PSI complex showed two pools of Chl d, one with emission at 730 nm and the other at 709 nm at 77 K. In PSI and PSII of classical cyanobacteria and of higher plants, where Chl a is the predominant pigment rather than Chl d, these differences are not as pronounced. Light energy absorbed by phycobiliproteins was also active in these Chl d emissions. The major light-harvesting pigment protein is similar to the prochlorophyte Chl-binding protein (pcb) and had a major emission peak at 711 nm. In Cyanobacteria an iron-stress induced Chl-binding protein (isiA) forms a polymeric ring around PSI, and so the effect(s) of iron stress on A. marina where investigated. No clear evidence could be deduced for the formation of an isiA protein under iron stress and no clear changes in the proportion of Chl d :Chl a could be discerned although phycobilins showed a decreased under iron-stress conditions. That Chl d replaces Chl a in all its functions in A. marina is clear; the advantage of this evolutionary development appears to be to enable A. marina to absorb far-red light which occurs in environments where red light is filtered out by other photosynthetic organisms.

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.

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.

A new chlorophyll d-containing cyanobacterium: evidence for niche adaptation in the genus Acaryochloris

Chlorophyll d is a photosynthetic pigment that, based on chemical analyses, has only recently been recognized to be widespread in oceanic and lacustrine environments. However, the diversity of organisms harbouring this pigment is not known. Until now, the unicellular cyanobacterium Acaryochloris marina is the only characterized organism that uses chlorophyll d as a major photopigment. In this study we describe a new cyanobacterium possessing a high amount of chlorophyll d, which was isolated from waters around Heron Island, Great Barrier Reef (23° 26' 31.2″ S, 151° 54' 50.4″ E). The 16S ribosomal RNA is 2% divergent from the two previously described isolates of A. marina, which were isolated from waters around the Palau islands (Pacific Ocean) and the Salton Sea lake (California), suggesting that it belongs to a different clade within the genus Acaryochloris. An overview sequence analysis of its genome based on Illumina technology yielded 871 contigs with an accumulated length of 8 371 965 nt. Their analysis revealed typical features associated with Acaryochloris, such as an extended gene family for chlorophyll-binding proteins. However, compared with A. marina MBIC11017, distinct genetic, morphological and physiological differences were observed. Light saturation is reached at lower light intensities, Chl d/a ratios are less variable with light intensity and the phycobiliprotein phycocyanin is lacking, suggesting that cyanobacteria of the genus Acaryochloris occur in distinct ecotypes. These data characterize Acaryochloris as a niche-adapted cyanobacterium and show that more rigorous attempts are worthwhile to isolate, cultivate and analyse chlorophyll d-containing cyanobacteria for understanding the ecophysiology of these organisms.

Fitting light saturation curves measured using modulated fluorometry

A blue diode PAM (Pulse Amplitude Modulation) fluorometer was used to measure rapid Photosynthesis (P) versus Irradiance (E) curves (P vs. E curves) in Synechococcus (classical cyanobacteria), Prochlorothrix (prochlorophyta), Chlorella (chlorophyta), Rhodomonas (cryptophyta), Phaeodactylum (bacillariophyta) Acaryochloris (Chl d/a cyanobacteria) and Subterranean Clover (Trifolium subterraneum, Papilionaceae, Angiospermae). Effective quantum yield (Phi(PSII)) versus irradiance curves could be described by a simple exponential decay function (Phi(PSII) = Phi(PSII, maxe(-kE)) although Log/Log transformation was sometimes found to be necessary to obtain the best fits. Photosynthesis was measured as relative Electron Transport Rate (rETR) standardised on a chlorophyll basis. P versus E curves were fitted to the waiting-in-line function (an equation of the form P = P(max) x k x E x e(-kE)) allowing half-saturating and optimal irradiances (E(optimum)) to be estimated. The second differential of the equation shows that at twice optimal light intensities, there is a point of inflection in the P versus E curve. Photosynthesis is inhibited 26.4% at this point of inflection. The waiting-in-line model was found to be a very good descriptor of photosynthetic light saturation curves and superior to hyperbolic functions with an asymptotic saturation point (Michaelis-Menten, exponential saturation and hyperbolic tangent). The exponential constants (k) of the Phi(PSII) versus E and P versus E curves should be equal because rETR is directly proportional to Phi(PSII) x E. The conventionally calculated Non-Photochemical Quenching (NPQ) in Synechococcus was not significantly different to zero but NPQ versus E curves for the other algae could be fitted to an exponential saturation model. The kinetics of NPQ does not appear to be related to the kinetics of Phi(PSII) or rETR.

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.

Nomenclature for membrane-bound light-harvesting complexes of cyanobacteria

Accessory chlorophyll-binding proteins (CBP) in cyanobacteria have six transmembrane helices and about 11 conserved His residues that might participate in chlorophyll binding. In various species of cyanobacteria, the CBP proteins bind different types of chlorophylls, including chlorophylls a, b, d and divinyl-chlorophyll a, b. The CBP proteins do not belong to the light-harvesting complexes (LHC) superfamily of plant and algae. The proposed new name of CBP for this class of proteins, which is a unique accessory light-harvesting superfamily in cyanobacteria, clarifies the confusion of names of prochlorophytes chlorophyll binding protein (Pcb), PSII-like light-harvesting proteins and iron-stress-induced protein A (IsiA). The CBP complexes are a member of a larger family that includes the chlorophyll a-binding proteins CP43 and CP47 that function as core antennas of photosystem II.

Association of chlorophyll a/b-binding Pcb proteins with photosystems I and II in Prochlorothrix hollandica

Action spectra for photosystem II (PSII)-driven oxygen evolution and of photosystem I (PSI)-mediated H(2) photoproduction and photoinhibition of respiration were used to determine the participation of chlorophyll (Chl) a/b-binding Pcb proteins in the functions of pigment apparatus of Prochlorothrix hollandica. Comparison of the in situ action spectra with absorption spectra of PSII and PSI complexes isolated from the cyanobacterium Synechocystis 6803 revealed a shoulder at 650 nm that indicated presence of Chl b in the both photosystems of P. hollandica. Fitting of two action spectra to absorption spectrum of the cells showed a chlorophyll ratio of 4:1 in favor of PSI. Effective antenna sizes estimated from photochemical cross-sections of the relevant photoreactions were found to be 192+/-28 and 139+/-15 chlorophyll molecules for the competent PSI and PSII reaction centers, respectively. The value for PSI is in a quite good agreement with previous electron microscopy data for isolated Pcb-PSI supercomplexes from P. hollandica that show a trimeric PSI core surrounded by a ring of 18 Pcb subunits. The antenna size of PSII implies that the PSII core dimers are associated with approximately 14 Pcb light-harvesting proteins, and form the largest known Pcb-PSII supercomplexes.

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.

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

The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43' (PcbC), D1, D2, cytochrome b(559), PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 +/- 7 Chl d, 3.0 +/- 0.4 Chl a, 17 +/- 3 alpha-carotene, and 1.4 +/- 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.

An electron paramagnetic resonance investigation of the electron transfer reactions in the chlorophyll d containing photosystem I of Acaryochloris marina

Electron paramagnetic resonance (EPR) spectroscopy reveals functional and structural similarities between the reaction centres of the chlorophyll d-binding photosystem I (PS I) and chlorophyll a-binding PS I. Continuous wave EPR spectrometry at 12K identifies iron-sulphur centres as terminal electron acceptors of chlorophyll d-binding PS I. A transient light-induced electron spin echo (ESE) signal indicates the presence of a quinone as the secondary electron acceptor (Q) between P(740)(+) and the iron-sulphur centres. The distance between P(740)(+) and Q(-) was estimated within point-dipole approximation as 25.23+/-0.05A, by the analysis of the electron spin echo envelope modulation.

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.

Evolution of the Inner Light-Harvesting Antenna Protein Family of Cyanobacteria, Algae, and Plants

Two hypotheses account for the evolution of the inner antenna light-harvesting proteins of oxygenic photosynthesis in cyanobacteria, algae, and plants: one in which the CP43 protein of photosytem II gave rise to the extrinsic CP43-like antennas of cyanobacteria (i.e. IsiA and Pcb proteins), as a late development, and the other in which CP43 and CP43-like proteins derive from an ancestral protein. In order to determine which of these hypotheses is most likely, we analyzed the family of antenna proteins by a variety of phylogenetic techniques, using alignments of the six common membrane-spanning helices, constructed using information on the antenna proteins' three-dimensional structure, and surveyed for evidence of factors that might confound inference of a correct phylogeny. The first hypothesis was strongly supported. As a consequence, we conclude that the ancestral photosynthetic apparatus, with 11 membrane-spanning helices, split at an early stage during evolution to form, on the one hand, the reaction center of photosystem II and, on the other hand, the ancestor of inner antenna proteins, CP43 (PsbC) and CP47 (PsbB). Only much later in evolution did the CP43 lineage give rise to the CP43' proteins (IsiA and Pcb) of cyanobacteria.

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.

Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S(i) state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S(i) states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y(Z) are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S(2) and S(3) states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S(2) and S(3) decay between A. marina cells and spinach thylakoids reflect a shift of the E(m) of Y(D)/Y(D)(ox) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S(0) oxidation to S(1) by Y(D)(ox). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.