Search

Photosystem II (PSII) uses light energy to oxidize water and to reduce plastoquinone in the photosynthetic electron transport chain. O 2 is produced as a byproduct. While most members of the PSII research community agree that O 2 originates from water molecules, alternative hypotheses involving bicarbonate persist in the literature. In this perspective, we provide an overview of the important roles of bicarbonate in regulating PSII activity and assembly. Further, we emphasize that biochemistry, spectroscopy, and structural biology experiments have all failed to detect bicarbonate near the active site of O 2 evolution. While thermodynamic arguments for oxygen-centered bicarbonate oxidation are valid, the claim that bicarbonate is a substrate for photosynthetic O 2 evolution is challenged., (© 2024. The Author(s).)

This chapter compares two different techniques for monitoring photosynthetic O 2 production; the wide-spread Clark-type O 2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O 2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

This chapter compares two different techniques for monitoring photosynthetic O 2 production: the widespread Clark-type O 2 electrode and the more sophisticated membrane inlet mass spectrometry (MIMS) technique. We describe how a simple membrane inlet for MIMS can be made out of a commercial Clark-type cell, and outline the advantages and drawbacks of the two techniques to guide researchers in deciding which method to use. Protocols and examples are given for measuring O 2 evolution rates and for determining the number of chlorophyll molecules per active photosystem II reaction center.

• A novel g-C 3 N 4 with Na, o co-doping and cyano groups photocatalyst was successfully prepared. • NOC CN catalyst had stronger oxidation capacity. • NOC CN exhibits a narrower band gap. • NOC CN exhibits boosted photocatalytic oxygen evolution without any co-catalyst. Tuning the electronic structure is fundamental to improve the optical character and redox ability of photocatalysts. Thus, in this article, g-C 3 N 4 with sodium and oxygen co-doping and cyano groups (NOC CN) was successfully synthesized by a brief one-step thermopolymerization. The results indicate that introduction of sodium and oxygen dopant and cyano groups bring a narrower band gap for NOC CN, adjust the overall electronic structure of NOC CN in the positive direction, and facilitate more efficient electron migration ability, which promoted NOC CN achieving photocatalytic water splitting oxygen evolution rate of 1.26 mmolh−1 g −1 without the assistance of any co-catalyst. Furthermore, DFT calculation and related experimental results was also combined to confirm the chemical structure of NOC CN. Obviously, this paper provides a feasible tactic to inflect the electronic construction of g-C 3 N 4 , improving its oxidizing ability. Na, O co-doping and cyano groups synergistically adjust the band structure of graphite carbon nitride for photocatalytic oxygen evolution without co-catalyst. [Display omitted]. [ABSTRACT FROM AUTHOR]

Photosynthetic rates of the large centric diatom Coscinodiscus granii were measured by means of multicolor variable chlorophyll fluorescence imaging, single cell 14C assimilation, and optical O2 sensor measurements during light acclimatization of cultures grown at five different irradiances: 50, 150, 235, 332, and 450 μmol photons m-2 s-1. Photo-acclimatization was evident from changes of cellular chlorophyll a content, growth rates, and light response curves. Each of the applied methods evaluates different parts and reactions in the photosynthetic apparatus, which makes a direct quantitative comparison of rates difficult, although a different degree of correlation were found between all three methods. However, when used in combination, they provide information about the internal relationship of photosynthetic pathways as well as the variation in photosynthetic capacity between individual cells within a single algal culture. [ABSTRACT FROM AUTHOR]

Light response (at 300 ppm CO2 and 10–50 ppm O2 in N2) and CO2 response curves [at absorbed photon fluence rate (PAD) of 550 μmol m−2 s−1] of O2 evolution and CO2 uptake were measured in tobacco (Nicotiana tabacum L.) leaves grown on either NO3− or NH4+ as N source and in potato (Solanum tuberosum L.), sorghum (Sorghum bicolor L. Moench), and amaranth (Amaranthus cruentus L.) leaves grown on NH4NO3. Photosynthetic O2 evolution in excess of CO2 uptake was measured with a stabilized zirconia O2 electrode and an infrared CO2 analyser, respectively, and the difference assumed to represent the rate of electron flow to acceptors alternative to CO2, mainly NO2−, SO42−, and oxaloacetate. In NO3−-grown tobacco, as well as in sorghum, amaranth, and young potato, the photosynthetic O2–CO2 flux difference rapidly increased to about 1 μmol m−2 s−1 at very low PADs and the process was saturated at 50 μmol quanta m−2 s−1. At higher PADs the O2–CO2 flux difference continued to increase proportionally with the photosynthetic rate to a maximum of about 2 μmol m−2 s−1. In NH4+-grown tobacco, as well as in potato during tuber filling, the low-PAD component of surplus O2 evolution was virtually absent. The low-PAD phase was ascribed to photoreduction of NO2− which successfully competes with CO2 reduction and saturates at a rate of about 1 μmol O2 m−2 s−1 (9% of the maximum O2 evolution rate). The high-PAD component of about 1 μmol O2 m−2 s−1, superimposed on NO2− reduction, may represent oxaloacetate reduction. The roles of NO2−, oxaloacetate, and O2 reduction in the regulation of ATP/NADPH balance are discussed. [ABSTRACT FROM PUBLISHER]

Abstract: Spinel-type ternary transition metal oxides of nickel, iron and vanadium with composition NiFe2 −x V x O4 (0≤ x ≤1) have been synthesized by a hydroxide precipitation method and investigated for their physicochemical and electrochemical catalytic properties towards the oxygen evolution reaction (OER) in alkaline solutions at 25°C. The OER study indicates that substitution of V from 0.25 to 1.0mol for Fe in the spinel matrix increases the electrocatalytic activity of the oxide greatly; the activity being the greatest with x =0.5mol. At low overpotentials, the Tafel slope and the order for the OER with respect to OH− concentration were found to be ∼40mV and ∼2, respectively. [Copyright &y& Elsevier]

Abstract: Degradation mechanism of Ti/IrO2(0.7)+MnO2(0.3) anode for oxygen evolution was studied in 0.5M H2SO4 solution by field emission scanning electron microscopes (FESEM), Tafel slope, X-ray diffraction (XRD) and electrochemical impedance spectroscopic (EIS). The whole surface of the non-electrolyzed Ti/IrO2(0.7)+MnO2(0.3) anode consisted of nano-IrO2 poles array. The anode surface had hardly discovered cracks and had compact morphology that can prevent the electrolyte from entering the Ti/oxide interface and an insulating TiO2 film from growing on the Ti-based surface. An insulating TiO2 phase had not been generated on the Ti/oxide interface during the process of O2 evolution and the inactive anode surface still remained mass of the IrO2 and MnO2 oxides. The chemical dissolution of Ti/IrO2(0.7)+MnO2(0.3) anode coating was not the main reason for electrode degradation in 0.5M H2SO4 solution. The degradation mechanism of Ti/IrO2(0.7)+MnO2(0.3) anode differed from other conventional oxide electrodes for O2 evolution in acid medium. The platforms of Tafel slope and anode potential (900–1050h) indicated that the change of the oxygen evolution reaction (OER) mechanism was the main reason for degradation of Ti/IrO2(0.7)+MnO2(0.3) anode due to the change of adsorption intermediate. [Copyright &y& Elsevier]

Abstract: We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, QA. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8±0.5 kJ mol−1 and 12.5±0.7 kJ mol−1 have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol−1 between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants. [Copyright &y& Elsevier]

The effect of Cu toxicity on photosynthetic function, chlorophyll and Ca2+ content of Cu-tolerant Silene compacta plants grown in nutrient solution was studied. Since, in plants grown under 8 µM Cu, the chlorophyll and Ca2+ concentration as well as the photosystem II (PSII) photochemistry were increased, compared to the control, the development of an adaptive mechanism of the Cu-tolerant ecotype of S. compacta to 8 µM Cu is suggested. Increased Cu tolerance of the S. compacta ecotype reflects modulation of the photosynthetic apparatus to optimize photosynthesis. However, exposure of plants to 160 µM Cu resulted in a marked increase of the fraction of closed PSII centres and decreased quantum yield of PSII electron transport (ΦPSII) which was accompanied by a significant decline of relative quantum yield for O2 evolution (Aox/Apt). The concentration of chlorophyll and Ca2+ in leaves also decreased significantly under 160 µM Cu treatment. Photochemical quenching (qp) displayed a reduction as a result of perturbation of the photosynthetic electron transfer chain, while non-photochemical quenching (qN) increased. High Cu treatment reduced photosynthetic productivity of S. compacta plants which can be attributed, in part, to perturbation of photosynthetic process and photosynthetic pigments as well as to Ca2+ loss. [ABSTRACT FROM AUTHOR]

Oxygen evolution and chlorophyll fluorescence were measured in cold-hardened and unhardened leaves of barley (Hordeum vulgare L. cv. Åsa) during the induction period of photosynthesis. The lag phase of light-saturated photosynthesis was increased and steady-state rates of photosynthesis were higher in cold-hardened than in unhardened barley leaves. Fluorescence was quenched more rapidly during the first minutes of induction in hardened than unhardened leaves, largely because of greater energy-dependent quenching (qE). Also, slow fluorescence transients through the M peak were delayed and less pronounced in cold-hardened than in unhardened leaves. Based upon the combined fluorescence and oxygen evolution data it was concluded that cold-hardening delayed light activation of the energy consuming carbon reduction cycle, thereby delaying the use of ATP and NADPH formed in the light reaction. Measurements of oxygen evolution and fluorescence kinetics during photosynthetic induction under oxygenic and anoxygenic conditions suggest that oxygen photoreduction is important for additional ATP generation during both the onset of photosynthetic carbon assimilation and during steady-state photosynthesis. [ABSTRACT FROM AUTHOR]