Photosynthesis Research (v.99, #2)

Wilhelm Menke (1910–2007): a pioneer in chloroplast structure by Friederike Koenig; Georg H. Schmid (81-84).
Keywords: Chloroplast structure; Thylakoids; Electron microscopy; Small angle X-ray scattering; Proteomics and lipidomics of the photosynthetic membrane; Immunology; Spectroscopy; Friedl Weber; Kurt Noack; Wilhelm Menke

A viewpoint: Why chlorophyll a? by Lars Olof Björn; George C. Papageorgiou; Robert E. Blankenship; Govindjee (85-98).
Chlorophyll a (Chl a) serves a dual role in oxygenic photosynthesis: in light harvesting as well as in converting energy of absorbed photons to chemical energy. No other Chl is as omnipresent in oxygenic photosynthesis as is Chl a, and this is particularly true if we include Chl a 2, (=[8-vinyl]-Chl a), which occurs in Prochlorococcus, as a type of Chl a. One exception to this near universal pattern is Chl d, which is found in some cyanobacteria that live in filtered light that is enriched in wavelengths >700 nm. They trap the long wavelength electronic excitation, and convert it into chemical energy. In this Viewpoint, we have traced the possible reasons for the near ubiquity of Chl a for its use in the primary photochemistry of Photosystem II (PS II) that leads to water oxidation and of Photosystem I (PS I) that leads to ferredoxin reduction. Chl a appears to be unique and irreplaceable, particularly if global scale oxygenic photosynthesis is considered. Its uniqueness is determined by its physicochemical properties, but there is more. Other contributing factors include specially tailored protein environments, and functional compatibility with neighboring electron transporting cofactors. Thus, the same molecule, Chl a in vivo, is capable of generating a radical cation at +1 V or higher (in PS II), a radical anion at −1 V or lower (in PS I), or of being completely redox silent (in antenna holochromes).
Keywords: Chemistry of chlorophylls; Chlorophyll a ; Chlorophyll d ; Chlorophylls in proteins; Color of plants; Cyanobacteria; Evolution of photosystems; Oxygenic photosynthesis; Photosystem I; Photosystem II; Reaction centers; Spectra of chlorophylls

Light state transition is a physiological function of oxygenic organisms to balance the excitation of photosystem II (PSII) and photosystem I (PSI), hence a prerequisite of oxygen-evolving photosynthesis. For cyanobacteria, phycobilisome (PBS) movement during light state transition has long been expected, but never observed. Here the dynamic behavior of PBS movement during state transition in cyanobacterium Synechocystis PCC6803 is experimentally detected via time-dependent fluorescence fluctuation. Under continuous excitation of PBSs in the intact cells, time-dependent fluorescence fluctuations resemble “damped oscillation” mode, which indicates dynamic searching of a PBS in an “overcorrection” manner for the “balance” position where PSII and PSI are excited equally. Based on the parallel model, it is suggested that the “damped oscillation” fluorescence fluctuation is originated from a collective movement of all the PBSs to find the “balance” position. Based on the continuous fluorescence fluctuation during light state transition and also variety of solar spectra, it may be deduced that light state transition of oxygen-evolution organisms is a natural behavior that occurs daily rather than an artificial phenomenon at extreme light conditions in laboratory.
Keywords: Phycobilisome; Photosystem I; Photosystem II; State transition; Mobility; Energy distribution

Monomerization and trimerization of photosystem I (PSI) in cyanobacteria are reversible to response to light switched off and on, which leads to “energy spillover” to regulate excitation of the two photosystems in balance. Considering that PSI is a trans-membrane protein embedded in thylakoid membranes, the monomerization or trimerization must involve a movement of PSI in the membranes. In this work, the mobility of PSI was demonstrated by dependence of the monomerization and trimerization on temperature for intact Spirulina platensis cells undergoing a light-to-dark or a dark-to-light transition. Based on the characteristic absorbance of monomers and trimmers, it confirms that both monomerization and trimerization are temperature-sensitive. The relative populations of the monomers and trimmers are invariable above the phase transition temperature (T PT) while directly proportional to temperature below T PT. On the other hand, the rate to reach the equilibrium population is proportional to temperature above T PT but invariable below T PT. The PSI mobility and the temperature-dependent population are contrary to those of plastoquinone (PQ) molecules because PSI is a trans-membrane protein while PQ molecules are small diffusive electron carriers in thylakoid membranes as well as their distinctive sizes and environments. The less monomerization of PSI but the invariable time constant at lower temperature below T PT may be due to that accumulation of the reduced PQ molecules results in decrease of the stromal-side H+ concentration which is a driving force of PSI monomerization.
Keywords: Cyanobacterium; Mobility; State transition; Phycobilisomes; Photosystem I; Monomerization; Trimerization

The light-induced electron transport in purple bacterium Rhodobacter sphaeroides was studied in vivo by means of kinetic difference absorption spectroscopy and kinetics of bacteriochlorophyll fluorescence yield. Measurements of redox state of the oxidised primary donor and cytochrome c and the membrane potential revealed a complex pattern of changes of the electron flow. Effects of the membrane potential on the fluorescence yield were also analysed, and a model for the fluorescence induction curve is presented. The data indicate substantial positive effect of the membrane potential on the fluorescence emission in vivo. Moreover, light-induced changes in light scattering were observed, which suggests occurrence of structural changes on the level of the photosynthetic membrane.
Keywords: Photosynthesis; Fluorescence induction; Rhodobacter sphaeroides ; Membrane potential; Light scattering; Absorbance changes

Tyrosine Z (TyrZ) oxidation observed at liquid helium temperatures provides new insights into the structure and function of TyrZ in active Photosystem II (PSII). However, it has not been reported in PSII core complex from higher plants. Here, we report TyrZ oxidation in the S1 and S2 states in PSII core complex from spinach for the first time. Moreover, we identified a 500 G-wide symmetric EPR signal (peak position g = 2.18, trough position g = 1.85) together with the g = 2.03 signal induced by visible light at 10 K in the S1 state in the PSII core complex. These two signals decay with a similar rate in the dark and both disappear in the presence of 6% methanol. We tentatively assign this new feature to the hyperfine structure of the S1TyrZ EPR signal. Furthermore, EPR signals of the S2 state of the Mn-cluster, the oxidation of the non-heme iron, and the S1TyrZ in PSII core complexes and PSII-enriched membranes from spinach are compared, which clearly indicate that both the donor and acceptor sides of the reaction center are undisturbed after the removal of LHCII. These results suggest that the new spinach PSII core complex is suitable for the electron transfer study of PSII at cryogenic temperatures.
Keywords: Photosystem II core complex; Tyrosine Z; Spinach; EPR

As an outgoing Editor of the Historical Corner of Photosynthesis Research, I present here the following list of papers of historical interest for the benefit of all. The first paper I published was: Govindjee (1988) The Discovery of Chlorophyll–protein Complex by Emil L. Smith during 1937–1941. Photosynth Res 16:285–289. In order to bring to the readers this List of references on the historical papers published in this journal (and some even elsewhere), I have organized these papers under the following headings (some are arbitrarily assigned to a particular section since they may fit in more than one section): (I) biographies (that include obituaries and tributes, arranged alphabetically, with dates of birth and death); (II) recognitions of scientists (arranged alphabetically) by others; (III) personal perspectives (arranged alphabetically); (IV) historical papers (first chronologically, by the year of publication, and then alphabetically by the names of the editors); (V) special issues of Photosynthesis Research (chronologically by the year of publication and then alphabetically by the names of editors); and lastly (VI) Conferences (available reports in Photosynthesis Research).