Photosynthesis Research

Positive and negative bands in previously measured circular dichroism (CD) spectra of Chlorobium limicola chlorosomes appeared to be sign-reversed relative to those of Chloroflexus aurantiacus chlorosomes in the 740–750 nm spectral region where bacteriochlorophyll (BChl) c absorbs maximally. It was not clear, however, whether this difference was intrinsic to the chlorosomes or was due to differences in the procedures used to prepare them. We therefore repeated the CD measurements using chlorosomes isolated from both Cb. limicola f. thiosulfatophilum and Cf. aurantiacus using the method of Gerola and Olson (1986, Biochim. Biophys. Acta 848: 69–76). Contrary to the earlier results, both types of chlorosomes had very similar CD spectra, suggesting that both have similar arrangements of BChl c molecules. The previously reported difference between the CD spectra of Chlorobium and Chloroflexus chlorosomes is due to the instability of Chlorobium chlorosomes, which can undergo a hypsochromic shift in their near infrared absorption maximum accompanied by an apparent inversion in their near infrared CD spectrum during isolation. Treating isolated chlorosomes with the strong ionic detergent sodium dodecylsulfate, which removes BChl a, does not alter the arrangement of BChl c molecules in either Chloroflexus or Chlorobium chlorosomes, as indicated by the lack of an effect on their CD spectra.

To date, cyclic electron flow around PSI (PSI-CEF) has been considered the primary (if not the only) mechanism accepted to adjust the ratio of linear vs cyclic electron flow that is essential to adjust the ratio of ATP/NADPH production needed for CO2 carboxylation. Here we provide a kinetic model showing that cyclic electron flow within PSII (PSII-CEF) is essential to account for the accelerating rate of decay in flash-induced oscillations of O2 yield as the PQ pool progressively reduces to PQH2. Previously, PSII-CEF was modeled by backward transitions using empirical Markov models like Joliot-Kok (J-K) type. Here, we adapted an ordinary differential equation methodology denoted RODE1 to identify which microstates within PSII are responsible for branching between PSII-CEF and Linear Electron Flow (LEF). We applied it to simulate the oscillations of O2 yield from both Chlorella ohadii, an alga that shows strong PSII-CEF attributed to high backward transitions, and Synechococcus elongatus sp. 7002, a widely studied model cyanobacterium. RODE2 simulations reveal that backward transitions occur in microstates that possess a QB− semiquinone prior to the flash. Following a flash that forms microstates populating (QAQB)2−, PSII-CEF redirects these two electrons to the donor side of PSII only when in the oxidized S2 and S3 states. We show that this backward transition pathway is the origin of the observed period-2 oscillations of flash O2 yield and contributes to the accelerated decay of period-4 oscillations. This newly added pathway improved RODE1 fits for cells of both S. elongatus and C. ohadii. RODE2 simulations show that cellular adaptation to high light intensity growth is due to a decrease in QB availability (empty or blocked by Q2−B), or equivalently due to a decrease in the difference in reduction potential relative to QA/QA−. PSII-CEF provides an alternative mechanism for rebalancing the NADPH:ATP ratio that occurs rapidly by adjusting the redox level of the PQ:PQH2 pool and is a necessary process for energy metabolism in aquatic phototrophs.

Cyanobacteria living in the harsh environment of the desert have to protect themselves against high light intensity and prevent photodamage. These cyanobacteria are in a desiccated state during the largest part of the day when both temperature and light intensity are high. In the desiccated state, their photosynthetic activity is stopped, whereas upon rehydration the ability to perform photosynthesis is regained. Earlier reports indicate that light-induced excitations in Leptolyngbya ohadii are heavily quenched in the desiccated state, because of a loss of structural order of the light-harvesting phycobilisome structures (Bar Eyal et al. in Proc Natl Acad Sci 114:9481, 2017) and via the stably oxidized primary electron donor in photosystem I, namely P700+ (Bar Eyal et al. in Biochim Biophys Acta Bioenergy 1847:1267–1273, 2015). In this study, we use picosecond fluorescence experiments to demonstrate that a third protection mechanism exists, in which the core of photosystem II is quenched independently.

We present an overview of anionic interactions with the oxidation-reduction reactions of photosystem II (PSII) acceptors. In section 1, a framework is laid for the electron acceptor side of PSII: the overview begins with a current scheme of the electron transport pathway and of the localization of components in the thylakoid membrane, which is followed by a brief description of the electron acceptor Q or QA and the various heterogeneities associated with it. In section 2, we review briefly the nature of the active species of the bicarbonate (HCO3 −) effect, the location of the site of action of HCO3 −, and its relationship to interactions with other anions. In section 3, we review data on the anion effects on the reoxidation of QA − and on the various reactions involved in the two-electron gate mechanism of PSII, and provide a hypothesis as to the action of HCO3 − on the protonation reactions. New data obtained by one of us (G) in collaboration with J.J.S. van Rensen, J.F.H Snel and W. Tonk for HCO3 −-depleted thylakoids, demonstrating the abolition of the binary oscillations contained within the periodicity of 4 observed for proton release, are also reviewed. In section 4, we comment on the measured binding constant of HCO3 − at the anion binding site. And, in section 5, we review our current concept of the mechanism of the HCO3 − effect on the electron acceptor side of PSII, and comment on the possible physiological roles for HCO3 −. Measurements of HCO3 − reversible anionic inhibition in intact cells of a green alga Scenedesmus are also reviewed.

A chlorosome is a large and efficient light-harvesting antenna system found in some photosynthetic bacteria. This system comprises self-aggregates of bacteriochlorophyll (BChl) c, d, or e possessing a chiral 1-hydroxyethyl group at the 3-position, which plays a key role in the formation of the supramolecule. Biosynthesis of chlorosomal pigments involves stereoselective conversion of 3-vinyl group to 3-(1-hydroxyethyl) group facilitated by a 3-vinyl hydratase. This 3-vinyl hydration also occurs in BChl a biosynthesis, followed by oxidation that introduces an acetyl group at the 3-position. Herein, we present in vitro enzymatic assays of paralogous 3-vinyl hydratases derived from green sulfur bacteria, Chlorobaculum tepidum and Chlorobaculum limnaeum, the filamentous anoxygenic phototroph Chloroflexus aurantiacus, and the chloracidobacterium Chloracidobacterium thermophilum. All the hydratases showed hydration activities. The biosynthetic pathway of BChl a and other chlorosomal pigments is discussed considering the substrate specificity and stereoselectivity of the present hydratases.

The psbZ gene of Synechocystis sp. PCC 6803 encodes the ∼6.6 kDa photosystem II (PSII) subunit. We here report biophysical, biochemical and in vivo characterization of Synechocystis sp. PCC 6803 mutants lacking psbZ. We show that these mutants are able to perform wild-type levels of light-harvesting, energy transfer, PSII oxygen evolution, state transitions and non-photochemical quenching (NPQ) under standard growth conditions. The mutants grow photoautotrophically; however, their growth rate is clearly retarded under low-light conditions and they are not capable of photomixotrophic growth. Further differences exist in the electron transfer properties between the mutants and wild type. In the absence of PsbZ, electron flow potentially increased through photosystem I (PSI) without a change in the maximum electron transfer capacity of PSII. Further, rereduction of P700+ is much faster, suggesting faster cyclic electron flow around PSI. This implies a role for PsbZ in the regulation of electron transfer, with implication for photoprotection.

A simple approach to determine CO2/O2 specificity factor (τ) of ribulose 1,5-bisphosphate carboxylase/oxygenase is described. The assay measures the amount of CO2 fixation at varying [CO2]/[O2] ratios after complete consumption of ribulose 1,5-bisphosphate (RuBP). Carbon dioxide fixation catalyzed by the carboxylase was monitored by directly measuring the moles of 14CO2 incorporated into 3-phosphoglycerate (PGA). This measurement at different [CO2]/[O2] ratios is used to determine graphically by several different linear plots the total RuBP consumed by the two activities and the CO2/O2 specificity factor. The assay can be used to measure the amounts of products of the carboxylase and oxygenase reactions and to determine the concentration of the substrate RuBP converted to an endpoint amount of PGA and phosphoglycolate. The assay was found to be suitable for all [CO2]/[O2] ratios examined, ranging from 14 to 215 micromolar CO2 (provided as 1–16 mM NaHCO3) and 614 micromolar O2 provided as 50% O2. The procedure described is extremely rapid and sensitive. Specificity factors for enzymes of highly divergent τ values are in good agreement with previously published data.