Background With limited agricultural land and increasing human population, it is essential to enhance overall photosynthesis, and thus, productivity. Oxygenic photosynthesis begins with light absorption, followed by excitation energy transfer to the reaction centers, primary photochemistry, electron and proton transport, ATP synthesis, and then CO2 fixation (Calvin-Benson cycle, as well as Hatch-Slack cycle). We mention here some of the discoveries related to this process, such as the existence of two light reactions and two photosystems connected by an electron transport ‘chain’ (the Z-scheme), chemiosmotic hypothesis for ATP synthesis, water oxidation clock for oxygen evolution, steps for carbon fixation, and finally the diverse mechanisms of regulatory processes, such as “state transitions” and “non-photochemical quenching” of the excited state of chlorophyll a. Scope In this review, we emphasize that mathematical modeling is a highly valuable tool in understanding and making predictions on photosynthesis. Different mathematical models have been used to examine current theories on diverse photosynthetic processes; these have been validated through simulation(s) of available experimental data, such as chlorophyll a fluorescence induction, measured with fluorometers using continuous (or modulated) exciting light, and absorbance changes at 820 nm (ΔA820 ) related to redox changes in P700, the reaction center of Photosystem I. Conclusions We show in this review the important role of modeling in deciphering and untangling complex photosynthesis processes taking place simultaneously, as well as in predicting possible ways to obtain higher biomass and productivity in plants, algae and cyanobacteria.
Summary One of the major factors limiting biomass productivity in algae is the low thermodynamic efficiency of photosynthesis. The greatest thermodynamic inefficiencies in photosynthesis occur during the conversion of light into chemical energy. At full sunlight the light‐harvesting antenna captures photons at a rate nearly 10 times faster than the rate‐limiting step in photosynthetic electron transport. Excess captured energy is dissipated by non‐productive pathways including the production of reactive oxygen species. Substantial improvements in photosynthetic efficiency have been achieved by reducing the optical cross‐section of the light‐harvesting antenna by selectively reducing chlorophyll b levels and peripheral light‐harvesting complex subunits. Smaller light‐harvesting antenna, however, may not exhibit optimal photosynthetic performance in low or fluctuating light environments. We describe a translational control system to dynamically adjust light‐harvesting antenna sizes for enhanced photosynthetic performance. By expressing a chlorophyllide a oxygenase (CAO) gene having a 5′ mRNA extension encoding a Nab1 translational repressor binding site in a CAO knockout line it was possible to continuously alter chlorophyll b levels and correspondingly light‐harvesting antenna sizes by light‐activated Nab1 repression of CAO expression as a function of growth light intensity. Significantly, algae having light‐regulated antenna sizes had substantially higher photosynthetic rates and two‐fold greater biomass productivity than the parental wild‐type strains as well as near wild‐type ability to carry out state transitions and non‐photochemical quenching. These results have broad implications for enhanced algae and plant biomass productivity.
Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.
Selective inhibition of photosynthesis is a fundamental strategy to solve the global challenge caused by harmful cyanobacterial blooms. However, there is a lack of specificity of the currently used cyanocides, because most of them act on cyanobacteria by generating nontargeted oxidative stress. Here, for the first time, we find that the simplest β-diketone, acetylacetone, is a promising specific cyanocide, which acts on Microcystis aeruginosa through targeted binding on bound iron species in the photosynthetic electron transport chain, rather than by oxidizing the components of the photosynthetic apparatus. The targeted binding approach outperforms the general oxidation mechanism in terms of specificity and eco-safety. Given the essential role of photosynthesis in both natural and artificial systems, this finding not only provides a unique solution for the selective control of cyanobacteria but also sheds new light on the ways to modulate photosynthesis.
Photosystem II (PSII) of plants, algae and cyanobacteria is a specialised protein complex that uses light energy to transfer electrons from water to plastoquinone, producing molecular oxygen and reduced plastoquinone. The PSII complex includes a peripheral antenna containing chlorophyll and other pigments to absorb light, a reaction centre that utilises the excitation energy transferred to it for charge separation, cofactors that stabilise the charge pair via electron transfer reactions, a Mn 4 CaO 5 cluster that oxidises water, and a binding pocket where plastoquinone is reduced. The electrons and protons that PSII extracts from water are employed in the overall photosynthetic process for the reduction of CO 2 , which provides the chemical energy for most life on Earth. PSII is the only known biological source of O 2 produced from water and is responsible for the molecular oxygen in the atmosphere. Key Concepts Photosystem II (PSII) is a membrane‐embedded pigment–protein complex, containing more than 20 subunits and approximately 100 cofactors. The PSII antenna and the PSII reaction centre are distinct protein complexes. Light is absorbed by chlorophyll, carotenoid and phycobilin pigments in the antenna and the excitation energy is rapidly transferred to the reaction centre. At the reaction centre, light‐induced charge separation takes place resulting in the formation of a chlorophyll cation and a pheophytin anion that are approximately 10 Å apart; this charge separation is rapidly stabilised by the transfer of the charges to more distant cofactors with smaller differences in redox potentials. The oxidation of water occurs at the Mn 4 CaO 5 cluster, which is embedded in the two protein subunits D1 and CP43 on the luminal side of PSII. To oxidise two molecules of water, four oxidising equivalents must be accumulated in the Mn 4 CaO 5 cluster by four consecutive light‐induced charge separations. Water oxidation by PSII occurs at the Mn 4 CaO 5 cluster, likely via oxo‐oxyl radical coupling in the so‐called S 4 state. Hydrogen‐bonding networks surrounding the Mn 4 CaO 5 cluster are crucial for its catalytic activity, as well as its structural flexibility. Bicarbonate ions play regulatory roles for electron transfer through PSII. The electrons and protons extracted from water by PSII drive the reduction of NADP + via Photosystem I, and the production of ATP, respectively.
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