Photosynthesis Research (v.120, #3)

Wolfgang Haehnel 20.1.1944-13.5.2013 by Hugo Scheer (247-248).
Almost 1 year has passed since the death of Wolfgang Haehnel on 13.5.2013, after a long fight against cancer. With his death, the photosynthesis community has lost an excellent scientist, a critical peer, and a stimulating sparring partner in discussions: I also lost a good friend.

Metabolic design for cyanobacterial chemical synthesis by John W. K. Oliver; Shota Atsumi (249-261).
Photosynthetic chemical production in cyanobacteria is a promising technology for renewable energy, CO2 mitigation, and fossil fuel replacement. Metabolic engineering has enabled a direct biosynthetic process from CO2 fixation to chemical feedstocks and biofuels, without requiring costly production, storage, and breakdown of cellulose or sugars. However, direct production technology is challenged by a need to achieve high-carbon partitioning to products in order to be competitive. This review discusses principles for the design of biosynthetic pathways in cyanobacteria and describes recent advances in relevant genetic tools. This field is at a critical juncture in assessing the strength and feasibility of carbon partitioning. To address this, we have included the stoichiometry of reducing equivalents and carbon conservation for heterologous pathways, and a method for calculating product yields against a theoretical maximum.
Keywords: Metabolic engineering; Cyanobacteria; Carbon fixation; Chemical feedstocks; Biofuel

Leaf chlorophyll content provides valuable information about physiological status of plants; it is directly linked to photosynthetic potential and primary production. In vitro assessment by wet chemical extraction is the standard method for leaf chlorophyll determination. This measurement is expensive, laborious, and time consuming. Over the years alternative methods, rapid and non-destructive, have been explored. The aim of this work was to evaluate the applicability of a fast and non-invasive field method for estimation of chlorophyll content in quinoa and amaranth leaves based on RGB components analysis of digital images acquired with a standard SLR camera. Digital images of leaves from different genotypes of quinoa and amaranth were acquired directly in the field. Mean values of each RGB component were evaluated via image analysis software and correlated to leaf chlorophyll provided by standard laboratory procedure. Single and multiple regression models using RGB color components as independent variables have been tested and validated. The performance of the proposed method was compared to that of the widely used non-destructive SPAD method. Sensitivity of the best regression models for different genotypes of quinoa and amaranth was also checked. Color data acquisition of the leaves in the field with a digital camera was quick, more effective, and lower cost than SPAD. The proposed RGB models provided better correlation (highest R 2) and prediction (lowest RMSEP) of the true value of foliar chlorophyll content and had a lower amount of noise in the whole range of chlorophyll studied compared with SPAD and other leaf image processing based models when applied to quinoa and amaranth.
Keywords: Leaf image analysis; RGB multi-regression model; Chenopodium quinoa ; Amaranthus sp.

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria by Takatoshi Fujita; Joonsuk Huh; Semion K. Saikin; Jennifer C. Brookes; Alán Aspuru-Guzik (273-289).
We present a theoretical study of excitation dynamics in the chlorosome antenna complex of green photosynthetic bacteria based on a recently proposed model for the molecular assembly. Our model for the excitation energy transfer (EET) throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of the supramolecular structure and electronic structure calculations of the excited states. We characterized the optical properties of the chlorosome with absorption, circular dichroism and fluorescence polarization anisotropy decay spectra. The simulation results for the excitation dynamics reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to several tens of ps. We assign the time scales of the EET to specific physical processes by comparing our results with the data obtained from time-resolved spectroscopy experiments.
Keywords: Excitation energy transfer; Chlorosome; Exciton diffusion; Exciton–vibration coupling; Light-harvesting antenna system; Green sulfur bacteria

Expression and characterization of cytochrome c 553 from Heliobacterium modesticaldum by Trevor S. Kashey; John B. Cowgill; Michael D. McConnell; Marco Flores; Kevin E. Redding (291-299).
Cytochrome c553 of Heliobacterium modesticaldum is the donor to P800 +, the primary electron donor of the heliobacterial reaction center (HbRC). It is a membrane-anchored 14-kDa cytochrome that accomplishes electron transfer from the cytochrome bc complex to the HbRC. The petJ gene encoding cyt c 553 was cloned and expressed in Escherichia coli with a hexahistidine tag replacing the lipid attachment site to create a soluble donor that could be made in a preparative scale. The recombinant cytochrome had spectral characteristics typical of a c-type cytochrome, including an asymmetric α-band, and a slightly red-shifted Soret band when reduced. The EPR spectrum of the oxidized protein was characteristic of a low-spin cytochrome. The midpoint potential of the recombinant cytochrome was +217 ± 10 mV. The interaction between soluble recombinant cytochrome c 553 and the HbRC was also studied. Re-reduction of photooxidized P800 + was accelerated by addition of reduced cytochrome c 553. The kinetics were characteristic of a bimolecular reaction with a second order rate of 1.53 × 104 M−1 s−1 at room temperature. The rate manifested a steep temperature dependence, with a calculated activation energy of 91 kJ mol−1, similar to that of the native protein in Heliobacillus gestii cells. These data demonstrate that the recombinant soluble cytochrome is comparable to the native protein, and likely lacks a discrete electrostatic binding site on the HbRC.
Keywords: Cytochrome c ; Heliobacterium modesticalcum ; Photosynthetic reaction center; Transient spectroscopy

Glycogen synthesis initiated by glucose-1-phosphate adenylyltransferase (glgC) represents a major carbon storage route in cyanobacteria which could divert a significant portion of assimilated carbon. Significant growth retardation in cyanobacteria with glgC knocked out (ΔglgC) has been reported in high light conditions. Here, we knocked out the glgC gene and analyzed its effects on carbon distribution in an isobutanol-producing strain of Synechococcus elongatus PCC7942 and its parental wild-type strain. We showed that isobutanol production was able to partially rescue the growth of ΔglgC mutant where the growth rescue effect positively correlated with the rate of isobutanol production. Using NaH14CO3 incorporation analysis, we observed a 28 % loss of total carbon fixation rate in the ΔglgC mutant compared to the wild-type. Upon expression of the isobutanol production pathway in ΔglgC mutant, the total carbon fixation rate was restored to the wild-type level. Furthermore, we showed that 52 % of the total carbon fixed was redirected into isobutanol biosynthesis in the ΔglgC mutant expressing enzymes for isobutanol production, which is 2.5 times higher than that of the wild-type expressing the same enzymes. These results suggest that biosynthesis of non-native product such as isobutanol can serve as a metabolic sink for replacing glycogen to rescue growth and restore carbon fixation rate. The rescue effect may further serve as a platform for cyanobacteria energy and carbon metabolism study.
Keywords: Glycogen mutant; Growth rescue; Cyanobacteria carbon sink; Carbon flux distribution; Isobutanol production

Triton X-100 as an effective surfactant for the isolation and purification of photosystem I from Arthrospira platensis by Daoyong Yu; Guihong Huang; Fengxi Xu; Mengfei Wang; Shuang Liu; Fang Huang (311-321).
Surfactants play important roles in the preparation, structural, and functional research of membrane proteins, and solubilizing and isolating membrane protein, while keeping their structural integrity and activity intact is complicated. The commercial n-Dodecyl-β-D-maltoside (DDM) and Triton X-100 (TX) were used as solubilizers to extract and purify trimeric photosystem I (PSI) complex, an important photosynthetic membrane protein complex attracting broad interests. With an optimized procedure, TX can be used as an effective surfactant to isolate and purify PSI, as a replace of the much more expensive DDM. A mechanism was proposed to interpret the solubilization process at surfactant concentrations lower than the critical solubilization concentration. PSI-TX and PSI-DDM had identical polypeptide bands, pigment compositions, oxygen consumption, and photocurrent activities. This provides an alternative procedure and paves a way for economical and large-scale trimeric PSI preparation.
Keywords: Photosystem I; Arthrospira platensis ; Triton X-100; Structural integrity; Activity

On destabilization of the Fenna–Matthews–Olson complex of Chlorobaculum tepidum by Adam Kell; Khem Acharya; Robert E. Blankenship; Ryszard Jankowiak (323-329).
The Fenna–Matthews–Olson (FMO) complex from the green sulfur bacterium Chlorobaculum tepidum was studied with respect to its stability. We provide a critical assessment of published and recently measured optical spectra. FMO complexes were found to destabilize over time producing spectral shifts, with destabilized samples having significantly higher hole-burning efficiencies; indicating a remodeled protein energy landscape. Observed correlated peak shifts near 825 and 815 nm suggest possible correlated (protein) fluctuations. It is proposed that the value of 35 cm−1 widely used for reorganization energy (E λ ), which has important implications for the contributions to the coherence rate (Kreisbeck and Kramer 3:2828–2833, 2012), in various modeling studies of two-dimensional electronic spectra is overestimated. We demonstrate that the value of E λ is most likely about 15–22 cm−1 and suggest that spectra reported in the literature (often measured on different FMO samples) exhibit varied peak positions due to different purification/isolation procedures or destabilization effects.
Keywords: FMO; Light harvesting; Photosynthesis; Spectral hole burning

Seasonal, diurnal and vertical variation in photosynthetic parameters in Phyllostachys humilis bamboo plants by Davina Van Goethem; Geert Potters; Sebastiaan De Smedt; Lianhong Gu; Roeland Samson (331-346).
In recent years, temperate bamboo species have been introduced in Europe for multiple uses such as renewable bio-based materials (wood, composites, fibres, biochemicals…) and numerous ecological functions (soil and water conservation, erosion control, phytoremediation…). Despite their interesting potential, little is known on the ecophysiology of these plants in their new habitat. Therefore, we studied gas exchange parameters on a full soil bamboo plantation of Phyllostachys humilis on a test field in Ireland (Europe). We evaluated the seasonal, diurnal and vertical variation of the parameters of two commonly used photosynthetic models, i.e. the light response curve (LRC) model and the model of Farquhar, von Caemmerer and Berry (FvCB). Furthermore, we tested if there were environmental effects on the photosynthetic parameters of these models and if a correlation between photosynthetic parameters and fluorescence parameters was present, fluorescence parameters can be easily and fast determined. Our results show that the gas exchange parameters do not vary diurnally or vertically. Only seasonal variations were found and should, therefore, be taken into account when using the LRC or FvCB model when modelling canopy growth. Therefore, a big-leaf model or a sunlit-shade model can be used for modelling bamboo growth in Western Europe. There is no straightforward relation between environmental variables and the photosynthetic parameters. Although fluorescence parameters showed a correlation with the photosynthetic parameters, application of such correlation may be limited.
Keywords: A/C i curves; A/PAR curves; Bamboo; Chlorophyll fluorescence; Dark respiration

We provide here a brief News Report on the 100th birth anniversary of Academician Alexander Abramovich Krasnovsky, one of the greatest photobiochemists of our time, who was born on August 26, 1913 and died on May 16, 1993. We provide here a short description of his research, followed by some photographs. He was a pioneering intellectual in the area of chlorophyll photochemistry, and was always ahead of his time; he, indeed, was a remarkable human being.
Keywords: Alexander Abramovich Krasnovsky; Chlorophyll; Krasnovsky reaction; Photobiochemistry; Photochemistry