Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

An analysis of major U.S. crops shows that there is a large genetic potential for yield that is unrealized because of the need for better adaptation of the plants to the environments in which they are grown. Evidence from native populations suggests that high productivity can occur in these environments and that opportunities for improving production in unfavorable environments are substantial. Genotypic selection for adaptation to such environments has already played an important role in agriculture, but the fundamental mechanisms are poorly understood. Recent scientific advances make exploration of these mechanisms more feasible and could result in large gains in productivity.

http://science.sciencemag.org/content/sci/218/4571/443.full.pdf

Plant Productivity and Environment

Microalgae represent an exceptionally diverse but highly specialized group of micro-organisms adapted to various ecological habitats. Many microalgae have the ability to produce substantial amounts (e.g. 20-50% dry cell weight) of triacylglycerols (TAG) as a storage lipid under photo-oxidative stress or other adverse environmental conditions. Fatty acids, the building blocks for TAGs and all other cellular lipids, are synthesized in the chloroplast using a single set of enzymes, of which acetyl CoA carboxylase (ACCase) is key in regulating fatty acid synthesis rates. However, the expression of genes involved in fatty acid synthesis is poorly understood in microalgae. Synthesis and sequestration of TAG into cytosolic lipid bodies appear to be a protective mechanism by which algal cells cope with stress conditions, but little is known about regulation of TAG formation at the molecular and cellular level. While the concept of using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for biofuels has been explored over the past few decades, a scalable, commercially viable system has yet to emerge. Today, the production of algal oil is primarily confined to high-value specialty oils with nutritional value, rather than commodity oils for biofuel. This review provides a brief summary of the current knowledge on oleaginous algae and their fatty acid and TAG biosynthesis, algal model systems and genomic approaches to a better understanding of TAG production, and a historical perspective and path forward for microalgae-based biofuel research and commercialization.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1365-313X.2008.03492.x

Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances

1. Light and Energy. 2. Organization and Structure of Photosynthetic Systems. 3. History and Development of Photosynthesis. 4. Photosynthetic Pigments-Structure and Spectroscopy. 5. Antenna Complexes and Energy Transfer Processes. 6. Reaction Center Complexes. 7. Electron Transfer Pathways and Components. 8. Chemiosmotic Coupling and ATP Synthesis. 9. Carbon Metabolism. 10. Genetics, Assembly and Regulation of Photosynthetic. Systems. 11. Origin and Evolution of Photosynthesis. Appendix 1. Light, Energy and Kinetics

Molecular mechanisms of photosynthesis

INTRODUCTION . PHOTOINHIBITION FROM EXPOSURE TO A HIGH PFD WITHOUT ADDITIONAL STRESS . Aquatic Plants . Shade Plants . Sun Plants... .. . . . . . . . .. . . . .. . . . .... . . . . . . . . . . . . Damage to Photosynthesis Caused by an Excessive PFD . PHOTOINHIBITION INDUCED BY INTERACTION BETWEEN LIGHT AND OTHER ENVIRONMENTAL STRESS FACTORS .

Photoinhibition of Photosynthesis Induced by Visible Light

Light Emission By Plants and Bacteria

The use of fluorescence induction measurements in leaves infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea has been evaluated as a routine method for estimation of the concentration of the reaction centers of photosystem II relative to total chlorophyll in a wide variety of plant species. The procedure is based on a simple theory that takes into account the attenuation of light in passing through the leaf and the linear dependence of the fluorescence induction time from different parts of the leaf on the inverse of the local light intensity. A formula to calculate the reaction center concentration of photosystem II was obtained. The effect of the light attenuation is accounted for by a correction factor which could become practically insignificant by an optimal choice of the excitation and emission wavelengths and the geometry of the photodetector with respect to the sample. Estimation of quantum yields for primary photochemistry and influence of light scattering were considered. The results demonstrate the effect of the above factors under various circumstances and are in agreement, to a first approximation, with the theory. The utility of the method is demonstrated by a detailed study of four desert plant species: estimation of reaction center concentrations of both photosystem I (by estimation of P700) and photosystem II (by the fluorescence induction method) were made and were compared to the rates of CO 2 fixation. There was a good quantitative correlation between the photosynthetic rates and the concentration of photosystem II reaction centers (expressed as per chlorophyll or per unit area of the leaf), but no such correlation was found with photosystem I reaction centers. The ratio of total chlorophyll per reaction centers II varied in the range of about 200 to 800 in different species, but there was no variation of this parameter in any single species.

http://www.plantphysiol.org/content/plantphysiol/67/3/570.full.pdf

Photosystem II Photosynthetic Unit Sizes from Fluorescence Induction in Leaves : CORRELATION TO PHOTOSYNTHETIC CAPACITY.

Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem II is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.

Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants

https://www.life.illinois.edu/govindjee/Electronic%20Publications/1981/1981_govindjee_et_al_plant.sci.lett.%281981%29.pdf

Chlorophyll A fluorescence transient as an indicator of water potential of leaves

The transition of the physical phase of lipids in membrane fragments of a blue-green alga Anacystis nidulans was studied by a spin labeling technique. The maximum hyperfine splitting of the electron spin resonance spectrum of the N-oxyl-4', 4'-dimethyloxazolidine derivative of 5-ketostearic acid plotted against the reciprocal of the absolute temperature gave a discontinuity point that was characteristic of a transition of the physical phase of the hydrocarbon region of membrane lipids. The phase transition appeared at approximately 13 or 24 C in the organisms grown at 28 or 38 C, respectively.The temperature dependence curve of chlorophyll a fluorescence in intact cells, membrane fragments, and extracted lipids of Anacystis cells suspended in a buffer solution showed that the fluorescence yield became maximum near the phase transition temperatures. These findings suggest that chlorophyll a in the thylakoid membrane works as a native fluorescence probe for the detection of phase transition.The temperature dependence of photosynthetic electron transport reactions was studied by measuring the oxidoreductive reactions of P700 and by measuring O(2) evolution. Each of the Arrhenius plots of the reaction rates was composed of two straight lines with a break near the phase transition temperatures. The activation energy was always lower above than below the transition temperatures. It is proposed to explain these phenomena that a reaction involving plastoquinone is influenced by the physical state of membrane lipids.The shift between the pigment state 1 and state 2 measured by fluorescence transients also showed a characteristic break in the Arrhenius plots near the phase transition temperatures; below the transition temperatures the shift almost disappeared. This suggests that the configurational change of the thylakoid membrane related to the state 1 and state 2 shift is dependent on the physical state of membrane lipids. In the chloroplasts of lettuce and spinach, on the other hand, there was no break in the Arrhenius plot of the electron transport reactions or of Mg(2+)-induced changes of chlorophyll a fluorescence.It is suggested that the transitions of the hyperfine splitting of the ESR signal, electron transport, and the configurational change, as well as the appearance of the maximum of chlorophyll a fluorescence, in the thylakoid membranes of Anacystis nidulans are all related to the transition of the physical phase of membrane lipids between the liquid crystalline state and the mixed liquid crystal-solid state.

Relationships between the Transition of the Physical Phase of Membrane Lipids and Photosynthetic Parameters in Anacystis nidulans and Lettuce and Spinach Chloroplasts.

David C. Fork

Papers

Light Emission By Plants and Bacteria

Photosystem II Photosynthetic Unit Sizes from Fluorescence Induction in Leaves : CORRELATION TO PHOTOSYNTHETIC CAPACITY.

Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants

Chlorophyll A fluorescence transient as an indicator of water potential of leaves

Relationships between the Transition of the Physical Phase of Membrane Lipids and Photosynthetic Parameters in Anacystis nidulans and Lettuce and Spinach Chloroplasts.