Anastasios Melis

Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement

Comparing photosynthetic and photovoltaic efficiencies is not a simple issue. Although both processes harvest the energy in sunlight, they operate in distinctly different ways and produce different types of products: biomass or chemical fuels in the case of natural photosynthesis and nonstored electrical current in the case of photovoltaics. In order to find common ground for evaluating energy-conversion efficiency, we compare natural photosynthesis with present technologies for photovoltaic-driven electrolysis of water to produce hydrogen. Photovoltaic-driven electrolysis is the more efficient process when measured on an annual basis, yet short-term yields for photosynthetic conversion under optimal conditions come within a factor of 2 or 3 of the photovoltaic benchmark. We consider opportunities in which the frontiers of synthetic biology might be used to enhance natural photosynthesis for improved solar energy conversion efficiency.

Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage in vivo?

Organisms that rely on oxygenic photosynthesis are subject to the effects of photo-oxidative damage, which impairs the function of photosystem-II (PSII). This phenomenon has the potential to lower rates of photosynthesis and diminish plant growth. Experimental evidence shows that the steady-state oxidation-reduction level of the primary quinone acceptor (QA) of PSII is the parameter that controls photodamage under a variety of physiological and environmental conditions. When QA is reduced, excitation energy at PSII is dissipated via a charge-recombination reaction. Such non-assimilatory dissipation of excitation generates singlet oxygen that might act to covalently modify the photochemical reaction center chlorophyll. Under steady-state photosynthesis conditions, the reduction state of QA increases linearly with irradiance, thereby causing a correspondingly linear increase in the probability of photodamage. It is concluded that there is a low probability that photodamage will occur when QA is oxidized and excitation energy is utilized in electron transport, and a significantly higher probability when QA is reduced in the course of steady-state photosynthesis.

Microalgae: a green source of renewable H2

This article summarizes recent advances in the field of algal hydrogen production. Two fundamental approaches are being developed. One involves the temporal separation of the usually incompatible reactions of O(2) and H(2) production in green algae, and the second involves the use of classical genetics to increase the O(2) tolerance of the reversible hydrogenase enzyme. The economic and environmental impact of a renewable source of H(2) are also discussed.

DYNAMICS OF PHOTOSYNTHETIC MEMBRANE COMPOSITION AND FUNCTION

Significant progress has been achieved, leading to identification of functional intermediates in the thylakoid membrane and to advances on the mechanism of electron-transport and photophosphorylation. The goal of this review is to put the thylakoid membrane into perspective, to address cause and effect relationships between the different phenomena, and also to address potential mechanisms for the regulation of these phenomena at the molecular level

Green alga hydrogen production: progress, challenges and prospects

The ability of green algae to photosynthetically generate molecular hydrogen has captivated the fascination and interest of the scienti,c community for the past 60 years due to the fundamental and practical importance of the process. Historically—under proper experimental conditions— the photosynthetic activity of the green alga “hydrogenase” was only transient in nature. It lasted from several seconds to a few minutes due to the fact that photosynthesis and H2O-oxidation entail the release of molecular O2. Oxygen is a positive suppressor of hydrogenase gene expression, and a powerful inhibitor of the [Fe]-hydrogenase. Given the acute oxygen sensitivity of the hydrogenase and the prevailing oxidative environmental conditions on earth, questions have been asked as to whether the hydrogenase is anything more than a relic of the evolutionary past of the chloroplast in green algae, and whether this enzyme and the process of photosynthesis can ever be utilized to generate hydrogen for commercial purposes [1]. Deprivation of sulfur-nutrients in green algae causes a reversible inhibition in the activity of oxygenic photosynthesis. In the absence of sulfur from the growth medium, protein biosynthesis is impeded, and the green algae cannot perform the required turnover of the photosystem-II (PSII) D1=32 kDa reaction center protein [2]. Under S-deprivation, the photochemical activity of PSII declines, and rates of photosynthetic oxygen evolution drop below those of oxygen consumption by respiration [3]. In consequence, sealed cultures of the green alga Chlamydomonas reinhardtii become anaerobic in the light. Following anaerobiosis, they spontaneously induce the “hydrogenase pathway” of electron transport in the chloroplast and photosynthetically produce

Dunaliella salina (Chlorophyta) with small chlorophyll antenna sizes exhibit higher photosynthetic productivities and photon use efficiencies than normally pigmented cells

The photon use efficiencies and maximal rates of photosynthesis in Dunaliella salina (Chlorophyta) cultures acclimated to different light intensities were investigated. Batch cultures were grown to the mid-exponential phase under continuous low-light (LL: 100 μmol photon m-2 s-1) or high-light (HL: 2000 μmol photon m-2 s-1) conditions. Under LL, cells were normally pigmented (deep green) containing ∼500 chlorophyll (Chl) molecules per photosystem II (PSII) unit and ∼250 Chl molecules per photosystem I (PSI). HL-grown cells were yellow-green, contained only 60 Chl per PSII and 100 Chl per PSI and showed signs of chronic photoinhibition, i.e., accumulation of photodamaged PSII reaction centers in the chloroplast thylakoids. In LL-grown cells, photosynthesis saturated at ∼200 μmol photon m-2 s-1 with a rate (Pmax) of ∼100 mmol O2 (mol Chl)-1 s-1. In HL-grown cells, photosynthesis saturated at much higher light intensities, i.e. ∼2500 μmol photon m-2 s-1, and exhibited a three-fold higher Pmax (∼300 mmol O2 (mol Chl)-1 s-1) than the normally pigmented LL-grown cells. Recovery of the HL-grown cells from photoinhibition, occurring prior to a light-harvesting Chl antenna size increase, enhanced Pmax to ∼675 mmol O2 (mol Chl)-1 s-1. Extrapolation of these results to outdoor mass culture conditions suggested that algal strains with small Chl antenna size could exhibit 2–3 times higher productivities than currently achieved with normally pigmented cells.

Structural and functional organization of the photosystems in spinach chloroplasts. Antenna size, relative electron-transport capacity, and chlorophyll composition

Abstract The structural and functional organization of the spinach chloroplast photosystems (PS) I, II α and II β was investigated. Sensitive absorbance difference spectrophotometry in the ultraviolet (▵ A 320 ) and red (▵ A 700 ) regions of the spectrum provided information on the relative concentration of PS II and PS I reaction centers. The kinetic analysis of PS II and PS I photochemistry under continuous weak excitation provided information on the number ( N ) of chlorophyll (Chl) molecules transferring excitation energy to PS II α , PS II β and PS I. Spinach chloroplasts contained almost twice as many PS II reaction centers compared to PS I reaction centers. The number N α of chlorophyll (Chl) molecules associated with PS II α was 234, while N β = 100 and N PS I = 210. Thus, the functional photosynthetic unit size of PS II reaction centers was different from that of PS I reaction centers. The relative electron-transport capacity of PS II was significantly greater than that of PS I. Hence, under light-limiting green excitation when both Chl a and Chl b molecules are excited equally, the limiting factor in the overall electron-transfer reaction was the turnover of PS I. The Chl composition of PS I, PS II α and PS II β was analyzed on the basis of a core Chl a reaction center complex component and a Chl a b- LHC component. There is a dissimilar Chl a b- LHC composition in the three photosystems with 77% of total Chl b associated with PS II α only. The results indicate that PS II α , located in the membrane of the grana partition region, is poised to receive excitation from a wider spectral window than PS II β and PS I.

Damage to functional components and partial degradation of Photosystem II reaction center proteins upon chloroplast exposure to ultraviolet-B radiation

Abstract Exposure of thylakoid membranes to ultraviolet-B radiation caused inhibition of semiquinone anion formation at Q A , inhibition of plastoquinone photoreduction and lower rates of Photosystem II electron-transport to artificial electron acceptors. The amplitude of pheophytin photoreduction was unaffected by the UV-B treatment, suggesting lack of a UV-B adverse effect on the primary charge separation reaction between the photochemical reaction center P680 and pheophytin. Under the experimental conditions employed, approx. 50% inhibition in Q A photoreduction and in the variable to maximal fluorescence ratio ( F / F max ) was observed. However, plastoquinone photoreduction was lowered by about 65% and electron-transport measurements from H 2 O to dichlorophenol indophenol were inhibited by 70–90% in the UV-B treated thylakoids. Rates of electron-transport through PS II could not be restored upon inclusion of artificial donors such as diphenyl carbazide or hydroxylamine. The results suggest a UV-B-induced damage to the primary quinone acceptor Q A and impairment in the function of plastoquinone in the thylakoid membrane. SDS-PAGE and immunoblot analysis of UV-B-exposed thylakoids revealed the appearance of small quantities of polypeptide fragments (13,11 and 5 kDa) from the Photosystem II reaction-center proteins. We suggest multiple independent targets of UV-B-irradiance in Photosystem II and point to plastoquinone, in its many different configurations in the thylakoid membrane, as a primary UV-B photosensitizer molecule.

Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae)

Unicellular green algae have the ability to operate in two distinctly different environments (aerobic and anaerobic), and to photosynthetically generate molecular hydrogen (H2). A recently developed metabolic protocol in the green alga Chlamydomonas reinhardtii permitted separation of photosynthetic O2-evolution and carbon accumulation from anaerobic consumption of cellular metabolites and concomitant photosynthetic H2-evolution. The H2 evolution process was induced upon sulfate nutrient deprivation of the cells, which reversibly inhibits photosystem-II and O2-evolution in their chloroplast. In the absence of O2, and in order to generate ATP, green algae resorted to anaerobic photosynthetic metabolism, evolved H2 in the light and consumed endogenous substrate. This study summarizes recent advances on green algal hydrogen metabolism and discusses avenues of research for the further development of this method. Included is the mechanism of a substantial tenfold starch accumulation in the cells, observed promptly upon S-deprivation, and the regulated starch and protein catabolism during the subsequent H2-evolution. Also discussed is the function of a chloroplast envelope-localized sulfate permease, and the photosynthesis–respiration relationship in green algae as potential tools by which to stabilize and enhance H2 metabolism. In addition to potential practical applications of H2, approaches discussed in this work are beginning to address the biochemistry of anaerobic H2 photoproduction, its genes, proteins, regulation, and communication with other metabolic pathways in microalgae. Photosynthetic H2 production by green algae may hold the promise of generating a renewable fuel from nature’s most plentiful resources, sunlight and water. The process potentially concerns global warming and the question of energy supply and demand.

Functional properties of photosystem IIβ in spinach chloroplasts

The activity of Photosystem (PS) IIβ was monitored for the first time in the absence of added herbicides in isolated chloroplast samples and in leaves of spinach in vivo. The new approach was implemented following identification of the initial chloroplast fluorescence rise from F0 to Fp1 (Forbush, B. and Kok, B. (1968) Biochim. Biophys. Acta 162, 243–253) as the variable fluorescence yield controlled by PS IIβ. Evidence is presented indicating the absence of the intermediate plastoquinone pool from the thylakoid membrane in which PS IIβ is localized. A two-step developmental process for PS II assembly and grana formation is proposed. According to this working hypothesis, PS IIβ is the precursor form of PS IIα, structurally and functionally complete except for the absence of the peripheral chlorophyll ab light-harvesting antenna and the complement of the plastoquinone pool. It is postulated that addition of these two hydrophobic components converts PS IIβ into PS IIα and initiates the incorporation of the thylakoid membrane into grana.