Photosynthesis Research (v.112, #1)

Can 50 years of research, performed between ignorance and the wish to know, and executed between hope, despair, satisfaction and pain, be compressed into an abstract? What has been done in more than 50 years may be expressed in four words: it was worth it. If I had another life, I would do it again. In the beginning of my career, life was an enigma. It still is. Molecular details of the workings of life had been largely unknown when I began. Now, at the end, I still wish to know details: how is light, master of life, manipulated to either support life, when photosynthesis is possible, or to protect it when light endangers it. What is the molecular and the physical nature of the biological mechanisms which control both, energy conservation and energy dissipation, in photosynthesis?
Keywords: Chlorophyll fluorescence; Desiccation tolerance; Energy conservation; Energy dissipation; Photoprotection; Photosynthesis

Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria by Stefan G. H. Simis; Yannick Huot; Marcel Babin; Jukka Seppälä; Liisa Metsamaa (13-30).
Excitation–emission fluorescence matrices of phytoplankton communities were simulated from laboratory-grown algae and cyanobacteria cultures, to define the optical configurations of theoretical fluorometers that either minimize or maximize the representation of these phytoplankton groups in community variable fluorescence measurements. Excitation sources that match the photosystem II (PSII) action spectrum of cyanobacteria do not necessarily lead to equal representation of cyanobacteria in community fluorescence. In communities with an equal share of algae and cyanobacteria, inducible PSII fluorescence in algae can be retrieved from community fluorescence under blue excitation (450–470 nm) with high accuracy (R 2 = 1.00). The highest correlation between community and cyanobacterial variable fluorescence is obtained under orange-red excitation in the 590–650 nm range (R 2 = 0.54). Gaussian band decomposition reveals that in the presence of cyanobacteria, the emission detection slit must be narrow (up to 10 nm) and centred on PSII chlorophyll-a emission (~683 nm) to avoid severe dampening of the signal by weakly variable phycobilisomal fluorescence and non-variable photosystem I fluorescence. When these optimizations of the optical configuration of the fluorometer are followed, both cyanobacterial and algal cultures in nutrient replete exponential growth exhibit values of the maximum quantum yield of charge separation in PSII in the range of 0.65–0.7.
Keywords: Phytoplankton; Cyanobacteria; Algae; Variable fluorescence; Excitation–emission matrix; Fluorometry; Photosynthesis

A mathematical formulation of the relationship between optical absorption coefficient of photosynthetic pigment molecules and light intensity was developed. It showed that physical parameters of photosynthetic pigment molecule (i.e., light absorption cross-section of photosynthetic pigment molecule, its average lifetime in the excited state, total photosynthetic pigment molecules, the statistical weight, or degeneracy of energy level of photosynthetic pigment molecules in the ground state and in the excited state) influenced on both the light absorption coefficient and effective light absorption cross-section of photosynthetic pigment molecules. Moreover, it also showed that both the light absorption coefficient and effective light absorption cross-section of photosynthetic pigment molecules were not constant, they decreased nonlinearly with light intensity increasing. The occupation numbers of photosynthetic pigment molecules in the excited states increased nonlinearly with light intensity increasing.
Keywords: Average lifetime of photosynthetic pigment molecule in the excited state; Effective optical absorption cross-section; Optical absorption coefficient of photosynthetic pigment molecules; Photosynthetic pigment molecule

The Chlamydomonas reinhardtii DNA-insertional transformant truncated light-harvesting antenna 1 (tla1) mutant, helped identify the novel TLA1 gene (GenBank Accession # AF534570-71) as an important genetic determinant in the chlorophyll antenna size of photosynthesis. Down-regulation in the amount of the TLA1 23 kDa protein in the cell resulted in smaller chlorophyll antenna size for both photosystems (in Tetali et al. Planta 225:813–829, 2007). Specific polyclonal antibodies, raised against the recombinant TLA1 protein, showed a cross-reaction with the predicted 23 kDa TLA1 protein in C. reinhardtii protein extracts, but also showed a strong cross-reaction with a protein band migrating to 28.5 kDa. Questions of polymorphism, or posttranslational modification of the TLA1 protein were raised as a result of the unexpected 28.5 kDa cross-reaction. Work in this paper aimed to elucidate the nature of the unexpected 28.5 kDa cross-reaction, as this was deemed to be important in terms of the functional role of the TLA1 protein in the regulation of the chlorophyll antenna size of photosynthesis. Immuno-precipitation of the 28.5 kDa protein, followed by LC–mass spectrometry, showed amino acid sequences ascribed to the psbD/D2 reaction center protein of PSII. The common antigenic determinant between TLA1 and D2 was shown to be a stretch of nine conserved amino acids V-F—L(V)LP-GNAL in the C-terminus of the two proteins, constituting a high antigenicity “GNAL” domain. Antibodies raised against the TLA1 protein containing this domain recognized both the TLA1 and the D2 protein. Conversely, antibodies raised against the TLA1 protein minus the GNAL domain specifically recognized the 23 kDa TLA1 protein and failed to recognize the 28.5 kDa D2 protein. D2 antibodies raised against an oligopeptide containing this domain also cross-reacted with the TLA1 protein. It is concluded that the 28.5 kDa cross-reaction of C. reinhardtii protein extracts with antiTLA1 antibodies is due to antibody affinity for the GNAL domain of the D2 protein and has no bearing on the identity or function of the TLA1 protein.
Keywords: Antigenic epitope; Chlamydomonas reinhardtii ; Protein domain; TLA1 protein; D2 protein; NOC4 protein; Western blot analysis

Characterization of photosynthesis in Arabidopsis ER-to-plastid lipid trafficking mutants by Ziru Li; Jinpeng Gao; Christoph Benning; Thomas D. Sharkey (49-61).
Vascular plants use two pathways to synthesize galactolipids, the predominant lipid species in chloroplasts—a prokaryotic pathway that resides entirely in the chloroplast, and a eukaryotic pathway that involves assembly in the endoplasmic reticulum. Mutants deficient in the endoplasmic reticulum pathway, trigalactosyldiacylglycerol (tgd1-1 and tgd2-1) mutants, had been previously identified with reduced contents of monogalactosyldiacylglycerol and digalactosyldiacylglycerol, and altered lipid molecular species composition. Here, we report that the altered lipid composition affected photosynthesis in lipid trafficking mutants. It was found that proton motive force as measured by electrochromic shift was reduced by ~40 % in both tgd mutants. This effect was accompanied by an increase in thylakoid conductance attributable to ATPase activity and so the rate of ATP synthesis was nearly unchanged. Thylakoid conductance to ions also increased in tgd mutants. However, gross carbon assimilation in tgd mutants as measured by gas exchange was only marginally affected. Rubisco activity, electron transport rate, and photosystem I and II oxidation status were not altered. Despite the large differences in proton motive force, responses to heat and high light stress were similar between tgd mutants and the wild type.
Keywords: tgd1-1 ; tgd2-1 ; Proton motive force; Electrochromic shift; Galactolipids; Photosynthesis

Characterization of photosystem I from spinach: effect of solution pH by Jianguo Liu; Xuefang Zhang; Meng Wang; Jing Liu; Meiwen Cao; Jianren Lu; Zhanfeng Cui (63-70).
Our previous work has demonstrated the isolation of photosystem I (PSI) from spinach using ultrafiltration with a final purity of 84 %. In order to get a higher purity of PSI and more importantly to develop a practical bioseparation process, key physiochemical properties of PSI and their dependence on operational parameters must be assessed. In this study, the effect of solution pH, one of the most important operating parameters for membrane process, on the property of PSI was examined. Following the isolation of crude PSI from spinach using n-dodecyl-beta-d-maltoside as detergent, the isoelectric point, aggregation size, zeta potential, low-temperature fluorescence, atomic force microscopy imaging, secondary structure, and thermal stability were determined. Solution pH was found to have a significant effect on the activity, aggregation size and thermal stability of PSI. The results also suggested that the activity of PSI was related to its aggregation size.
Keywords: Solution pH; Property; Photosystem I; Membrane protein; Spinach

The Fenna–Matthews–Olson antenna protein from the green bacterium Pelodictyon phaeum mediates the energy transfer from a peripheral antenna complex to the membrane-bound reaction center. The three-dimensional structure of this protein has been previously modeled using X-ray diffraction to a resolution limit of 2.0 Å, with R work and R free values of 16.6 and 19.9 %, respectively (Larson et al., Photosynth Res 107:139–150, 2011). This model shows the protein as consisting of β-sheets surrounding several bacteriochlorophyll cofactors. While most of the model clearly matches the electron density maps, in this paper we re-examine the electron density for a specific feature, namely the eighth bacteriochlorophyll a cofactor. This electron density is now interpreted as arising primarily from the end of an otherwise disordered polyethylene glycol molecule. Additional electron density is present but the density is weak and cannot be unambiguously assigned. The new model has R work and R free values of 16.2 and 19.0 %, respectively.
Keywords: Energy transfer; Light harvesting complex; Three-dimensional structure; Green bacteria

The temporal profile of the phosphorescence of singlet oxygen endogenously photosensitized by photosystem II (PSII) reaction centre (RC) in an aqueous buffer has been recorded using laser excitation and a near infrared photomultiplier tube. A weak emission signal was discernible, and could be fitted to the functional form $$ a[exp ( - t/ au_{2} ) - exp ( - t/ au_{1} )] $$ , with $$ a > 0 $$ and $$ au_{2} > au_{1} $$ . The value of $$ au_{2} $$ decreased from 11.6 ± 0.5 μs under aerobic conditions to 4.1 ± 0.2 μs in oxygen-saturated samples, due to enhanced bimolecular quenching of the donor triplet by oxygen, whereas that of $$ au_{1} $$ , identifiable with the lifetime of singlet oxygen, was close to 3 μs in both cases. Extrapolations based on the low amplitude of the emission signal of singlet oxygen formed by PSII RC in the aqueous buffer and the expected values of $$ au_{1} $$ and $$ au_{2} $$ in chloroplasts indicate that attempts to analyse the temporal profile of singlet oxygen in chloroplasts are unlikely to be rewarded with success without a significant advance in the sensitivity of the detection equipment.
Keywords: Singlet oxygen; Photosystem II reaction centre; Phosphorescence emission; Temporal profile