Photosynthesis Research (v.132, #2)

Itzhak Ohad (1930–2016) by Noam Adir; Susana Geifman-Shochat; Dan G. Ohad; Nir Ohad; Nir Keren (107-109).

Comparison of excitation energy transfer in cyanobacterial photosystem I in solution and immobilized on conducting glass by Sebastian Szewczyk; Wojciech Giera; Sandrine D’Haene; Rienk van Grondelle; Krzysztof Gibasiewicz (111-126).
Excitation energy transfer in monomeric and trimeric forms of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 in solution or immobilized on FTO conducting glass was compared using time-resolved fluorescence. Deposition of PSI on glass preserves bi-exponential excitation decay of ~4–7 and ~21–25 ps lifetimes characteristic of PSI in solution. The faster phase was assigned in part to photochemical quenching (charge separation) of excited bulk chlorophylls and in part to energy transfer from bulk to low-energy (red) chlorophylls. The slower phase was assigned to photochemical quenching of the excitation equilibrated over bulk and red chlorophylls. The main differences between dissolved and immobilized PSI (iPSI) are: (1) the average excitation decay in iPSI is about 11 ps, which is faster by a few ps than for PSI in solution due to significantly faster excitation quenching of bulk chlorophylls by charge separation (~10 ps instead of ~15 ps) accompanied by slightly weaker coupling of bulk and red chlorophylls; (2) the number of red chlorophylls in monomeric PSI increases twice—from 3 in solution to 6 after immobilization—as a result of interaction with neighboring monomers and conducting glass; despite the increased number of red chlorophylls, the excitation decay accelerates in iPSI; (3) the number of red chlorophylls in trimeric PSI is 4 (per monomer) and remains unchanged after immobilization; (4) in all the samples under study, the free energy gap between mean red (emission at ~710 nm) and mean bulk (emission at ~686 nm) emitting states of chlorophylls was estimated at a similar level of 17–27 meV. All these observations indicate that despite slight modifications, dried PSI complexes adsorbed on the FTO surface remain fully functional in terms of excitation energy transfer and primary charge separation that is particularly important in the view of photovoltaic applications of this photosystem.
Keywords: Photosystem I; Time-resolved fluorescence; Streak camera; Excitation energy transfer; Cyanobacteria; Photovoltaics; Biophotovoltaics; Red chlorophylls

Sensing photosynthetic herbicides in an electrochemical flow cell by Tibor Szabó; Richárd Csekő; Kata Hajdu; Krisztina Nagy; Orsolya Sipos; Péter Galajda; Győző Garab; László Nagy (127-134).
Specific inhibitory reactions of herbicides with photosynthetic reaction centers bound to working electrodes were monitored in a conventional electrochemical cell and a newly designed microfluidic electrochemical flow cell. In both cases, the bacterial reaction centers were bound to a transparent conductive metal oxide, indium-tin-oxide, electrode through carbon nanotubes. In the conventional cell, photocurrent densities of up to a few μA/cm2 could be measured routinely. The photocurrent could be blocked by the photosynthetic inhibitor terbutryn (I 50 = 0.38 ± 0.14 μM) and o-phenanthroline (I 50 = 63.9 ± 12.2 μM). The microfluidic flow cell device enabled us to reduce the sample volume and to simplify the electrode arrangement. The useful area of the electrodes remained the same (ca. 2 cm2), similar to the classical electrochemical cell; however, the size of the cell was reduced considerably. The microfluidic flow control enabled us monitoring in real time the binding/unbinding of the inhibitor and cofactor molecules at the secondary quinone site.
Keywords: Photosynthesis; Biocomposites; Bioelectronics; Herbicide biosensors; Photocurrent

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is responsible for carbon dioxide conversion during photosynthesis and, therefore, is the most important protein in biomass generation. Modifications of this biocatalyst toward improvements in its properties are hindered by the complicated and not yet fully understood assembly process required for the formation of active holoenzymes. An entire set of auxiliary factors, including chaperonin GroEL/GroES and assembly chaperones RbcX or Rubisco accumulation factor 1 (RAF1), is involved in the folding and subsequent assembly of Rubisco subunits. Recently, it has been shown that cyanobacterial RAF1 acts during the formation of the large Rubisco subunit (RbcL) dimer. However, both its physiological function and its necessity in the prokaryotic Rubisco formation process remain elusive. Here, we demonstrate that the Synechocystis sp. PCC 6803 strain with raf1 gene disruption shows the same growth rate as wild-type cells under standard conditions. Moreover, the Rubisco biosynthesis process seems to be unperturbed in mutant cells despite the absence of RbcL-RAF1 complexes. However, in the tested environmental conditions, sulfur starvation triggers the degradation of RbcL and subsequent proteolysis of other polypeptides in wild-type but not Δraf1 strains. Pull-down experiments also indicate that, apart from Rubisco, RAF1 co-purifies with phycocyanins. We postulate that RAF1 is not an obligatory factor in cyanobacterial Rubisco assembly, but rather participates in environmentally regulated Rubisco homeostasis.
Keywords: RAF1; Rubisco; Assembly chaperones; Carbon fixation; Cyanobacteria

Present study describes the responses of short rotation coppice (SRC) mulberry, a potential bio-energy tree, grown under interactive environment of elevated CO2 (E) and water stress (WS). Growth in E stimulated photosynthetic performance in well-watered (WW) as well as during WS with significant increases in light-saturated photosynthetic rates (A Sat), water use efficiency (WUEi), intercellular [CO2], and photosystem–II efficiency (F V/F M and ∆F/F M′) with concomitant reduction in stomatal conductance (g s) and transpiration (E) compared to ambient CO2 (A) grown plants. Reduced levels of proline, H2O2, and malondialdehyde (MDA) and higher contents of antioxidants including ascorbic acid and total phenolics in WW and WS in E plants clearly demonstrated lesser oxidative damage. Further, A plants showed higher transcript abundance and antioxidant enzyme activities under WW as well as during initial stages of WS (15 days). However, with increasing drought imposition (30 days), A plants showed down regulation of antioxidant systems compared to their respective E plants. These results clearly demonstrated that future increased atmospheric CO2 enhances the photosynthetic potential and also mitigate the drought-induced oxidative stress in SRC mulberry. In conclusion, mulberry is a potential bio-energy tree crop which is best suitable for short rotation coppice forestry-based mitigation of increased [CO2] levels even under intermittent drought conditions, projected to prevail in the fast-changing global climate.
Keywords: Antioxidant systems; Drought; Elevated CO2 ; Mulberry; Photosynthesis; Short rotation coppice

Transcriptional and post-translational control of chlorophyll biosynthesis by dark-operative protochlorophyllide oxidoreductase in Norway spruce by Tibor Stolárik; Boris Hedtke; Jiří Šantrůček; Petr Ilík; Bernhard Grimm; Andrej Pavlovič (165-179).
Unlike angiosperms, gymnosperms use two different enzymes for the reduction of protochlorophyllide to chlorophyllide: the light-dependent protochlorophyllide oxidoreductase (LPOR) and the dark-operative protochlorophyllide oxidoreductase (DPOR). In this study, we examined the specific role of both enzymes for chlorophyll synthesis in response to different light/dark and temperature conditions at different developmental stages (cotyledons and needles) of Norway spruce (Picea abies Karst.). The accumulation of chlorophyll and chlorophyll-binding proteins strongly decreased during dark growth in secondary needles at room temperature as well as in cotyledons at low temperature (7 °C) indicating suppression of DPOR activity. The levels of the three DPOR subunits ChlL, ChlN, and ChlB and the transcripts of their encoding genes were diminished in dark-grown secondary needles. The low temperature had minor effects on the transcription and translation of these genes in cotyledons, which is suggestive for post-translational control in chlorophyll biosynthesis. Taking into account the higher solubility of oxygen at low temperature and oxygen sensitivity of DPOR, we mimicked low-temperature condition by the exposure of seedlings to higher oxygen content (33%). The treatment resulted in an etiolated phenotype of dark-grown seedlings, confirming an oxygen-dependent control of DPOR activity in spruce cotyledons. Moreover, light-dependent suppression of mRNA and protein level of DPOR subunits indicates that more efficiently operating LPOR takes over the DPOR function under light conditions, especially in secondary needles.
Keywords: Chill stress; Chlorophyll; DPOR; Low temperature; Protochlorophyllide; Norway spruce

Erratum to: Transcriptional and post-translational control of chlorophyll biosynthesis by dark-operative protochlorophyllide oxidoreductase in Norway spruce by Tibor Stolárik; Boris Hedtke; Jiří Šantrůček; Petr Ilík; Bernhard Grimm; Andrej Pavlovič (181-181).

The ancestors of diatoms evolved a unique mitochondrial dehydrogenase to oxidize photorespiratory glycolate by Jessica Schmitz; Nishtala V. Srikanth; Meike Hüdig; Gereon Poschmann; Martin J. Lercher; Veronica G. Maurino (183-196).
Like other oxygenic photosynthetic organisms, diatoms produce glycolate, a toxic intermediate, as a consequence of the oxygenase activity of Rubisco. Diatoms can remove glycolate through excretion and through oxidation as part of the photorespiratory pathway. The diatom Phaeodactylum tricornutum encodes two proteins suggested to be involved in glycolate metabolism: PtGO1 and PtGO2. We found that these proteins differ substantially from the sequences of experimentally characterized proteins responsible for glycolate oxidation in other species, glycolate oxidase (GOX) and glycolate dehydrogenase. We show that PtGO1 and PtGO2 are the only sequences of P. tricornutum homologous to GOX. Our phylogenetic analyses indicate that the ancestors of diatoms acquired PtGO1 during the proposed first secondary endosymbiosis with a chlorophyte alga, which may have previously obtained this gene from proteobacteria. In contrast, PtGO2 is orthologous to an uncharacterized protein in Galdieria sulphuraria, consistent with its acquisition during the secondary endosymbiosis with a red alga that gave rise to the current plastid. The analysis of amino acid residues at conserved positions suggests that PtGO2, which localizes to peroxisomes, may use substrates other than glycolate, explaining the lack of GOX activity we observe in vitro. Instead, PtGO1, while only very distantly related to previously characterized GOX proteins, evolved glycolate-oxidizing activity, as demonstrated by in gel activity assays and mass spectrometry analysis. PtGO1 localizes to mitochondria, consistent with previous suggestions that photorespiration in diatoms proceeds in these organelles. We conclude that the ancestors of diatoms evolved a unique alternative to oxidize photorespiratory glycolate: a mitochondrial dehydrogenase homologous to GOX able to use electron acceptors other than O2.
Keywords: Diatom; Glycolate oxidase; Glycolate dehydrogenase; Phaeodactylum tricornutum ; Photorespiration; Secondary endosymbiosis

Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria by Mariann Kis; Gábor Sipka; Péter Maróti (197-209).
Mercury adsorption on the cell surface and intracellular uptake by bacteria represent the key first step in the production and accumulation of highly toxic mercury in living organisms. In this work, the biophysical characteristics of mercury bioaccumulation are studied in intact cells of photosynthetic bacteria by use of analytical (dithizone) assay and physiological photosynthetic markers (pigment content, fluorescence induction, and membrane potential) to determine the amount of mercury ions bound to the cell surface and taken up by the cell. It is shown that the Hg(II) uptake mechanism (1) has two kinetically distinguishable components, (2) includes co-opted influx through heavy metal transporters since the slow component is inhibited by Ca2+ channel blockers, (3) shows complex pH dependence demonstrating the competition of ligand binding of Hg(II) ions with H+ ions (low pH) and high tendency of complex formation of Hg(II) with hydroxyl ions (high pH), and (4) is not a passive but an energy-dependent process as evidenced by light activation and inhibition by protonophore. Photosynthetic bacteria can accumulate Hg(II) in amounts much (about 105) greater than their own masses by well-defined strong and weak binding sites with equilibrium binding constants in the range of 1 (μM)−1 and 1 (mM)−1, respectively. The strong binding sites are attributed to sulfhydryl groups as the uptake is blocked by use of sulfhydryl modifying agents and their number is much (two orders of magnitude) smaller than the number of weak binding sites. Biofilms developed by some bacteria (e.g., Rvx. gelatinosus) increase the mercury binding capacity further by a factor of about five. Photosynthetic bacteria in the light act as a sponge of Hg(II) and can be potentially used for biomonitoring and bioremediation of mercury-contaminated aqueous cultures.
Keywords: Bacterial photosynthesis; Intact cells; Spectroscopy; Hg(II) contamination; Biomediation of the environment

The classical definition of mesophyll conductance (g m) represents an apparent parameter (g m,app) as it places (photo)respired CO2 at the same compartment where the carboxylation by Rubisco takes place. Recently, Tholen and co-workers developed a framework, in which g m better describes a physical diffusional parameter (g m,dif). They partitioned mesophyll resistance (r m,dif = 1/g m,dif) into two components, cell wall and plasmalemma resistance (r wp) and chloroplast resistance (r ch), and showed that g m,app is sensitive to the ratio of photorespiratory (F) and respiratory (R d) CO2 release to net CO2 uptake (A): g m,app = g m,dif/[1 + ω(F + R d)/A], where ω is the fraction of r ch in r m,dif. We herein extend the framework further by considering various scenarios for the intracellular arrangement of chloroplasts and mitochondria. We show that the formula of Tholen et al. implies either that mitochondria, where (photo)respired CO2 is released, locate between the plasmalemma and the chloroplast continuum or that CO2 in the cytosol is completely mixed. However, the model of Tholen et al. is still valid if ω is replaced by ω(1−σ), where σ is the fraction of (photo)respired CO2 that experiences r ch (in addition to r wp and stomatal resistance) if this CO2 is to escape from being refixed. Therefore, responses of g m,app to (F + R d)/A lie somewhere between no sensitivity in the classical method (σ =1) and high sensitivity in the model of Tholen et al. (σ =0).
Keywords: CO2 transfer; Internal conductance; Mesophyll resistance