Functional evolution of photochemical energy transformations in oxygen-producing organisms

Functional Plant Biology (Impact Factor: 2.57). 01/2009; 36(6). DOI: 10.1071/FP09087

ABSTRACT Chlorophyll a is the photochemical agent accounting for most oxygenic photosynthesis, that is, over 99.9% of photosynthetic primary activity on Earth. The spectral and energetic properties of chlorophyll a can, at least in part, be rationalised interms of the solar spectral output and the energetics of oxygenproduction and carbon dioxide reduction with twophotochemicalreactions.Thelongwavelengthlimitoninvivochlorophyllaabsorptionisprobablyclosetotheenergetic limit: longer wavelengths could not support a high rate and efficiency of oxygenic photosynthesis. Retinal, a b-carotene derivative that is the chromophore of rhodopsin, acts not only as a sensory pigment, but also as an ion-pumping photochemical transducer. Both sensory and energy-transforming rhodopsins occur in oxygenic phototrophs, although the extent of expression and the function of the latter are not well understood.

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    ABSTRACT: Photosynthetic life requires sufficient photosynthetically active radiation (PAR) to metabolise. On Earth, plant behaviour, physiology and metabolism are sculpted around the night-day cycle by an endogenous biological circadian clock. The evolution of life was influenced by the Earth-Sun orbital dynamic, which generates the photo-environment incident on the planetary surface. In this work the unusual photo-environment of an Earth-like planet (ELP) in 3:2 spin orbit resonance is explored. Photo-environments on the ELP are longitudinally differentiated, in addition to differentiations relating to latitude and depth (for aquatic organisms) which are familiar on Earth. The light environment on such a planet could be compatible with Earth's photosynthetic life although the threat of atmospheric freeze-out and prolonged periods of darkness would present significant challenges. We emphasise the relationship between the evolution of life on a planetary body with its orbital dynamics.
    International Journal of Astrobiology 02/2014; · 0.83 Impact Factor
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    ABSTRACT: We studied the photocurrents of a cyanobacterial rhodopsin Gloeobacter violaceus (GR) in Xenopus laevis oocytes and HEK-293 cells. This protein is a light-driven proton pump with striking similarities to marine proteorhodopsins, including the D121-H87 cluster of the retinal Schiff base counterion and a glutamate at position 132 that acts as a proton donor for chromophore reprotonation during the photocycle. Interestingly, at low extracellular pHo and negative voltage, the proton flux inverted and directed inward. Using electrophysiological measurements of wild-type and mutant GR, we demonstrate that the electrochemical gradient limits outward-directed proton pumping and converts it into a purely passive proton influx. This conclusion contradicts the contemporary paradigm that at low pH, proteorhodopsins actively transport H(+) into cells. We identified E132 and S77 as key residues that allow inward directed diffusion. Substitution of E132 with aspartate or S77 with either alanine or cysteine abolished the inward-directed current almost completely. The proton influx is likely caused by the pKa of E132 in GR, which is lower than that of other microbial ion pumping rhodopsins. The advantage of such a low pKa is an acceleration of the photocycle and high pump turnover at high light intensities.
    Biophysical Journal 11/2013; 105(9):2055-63. · 3.83 Impact Factor
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    ABSTRACT: Direct measurement of gross photosynthesis and primary productivity The rate of photosynthesis is a primary constraint on the availability of energy for the production of organic matter within aquatic ecosystems. Gross photosynthesis (GP) is defined as the rate at which reducing power is generated through the conversion of absorbed light energy, with the assumption that most of this energy is used for organic matter production (gross primary productivity (GPP); Begon et al. 2006). For example, Raven (2009) estimated that oxygenic photosynthesis contributes greater than 99% of global GPP. The distinction between GPP and GP is necessary, as a number of processes operating between O 2 evolution at photosystem II (PSII) and CO 2 assimilation by the Calvin cycle can uncouple GPP from GP (Geider and MacIntyre 2002; Behrenfeld et al. 2004; Halsey et al. 2010; Suggett et al. 2010a). GPP or GP is often reported per unit area of lake, stream, or ocean surface, or per unit volume of water. Within the oxygenic photoau-totrophs, which includes the phytoplankton that dominate most aquatic environments, O 2 is generated as a by-product of GP. Thus, GPP can be reported as carbon assimilation (e.g., as mol C m –3 d –1 or mol C m –2 d –1), whereas GP can be reported as oxygen evolution (e.g., as mol O 2 m –3 d –1 or mol O 2 m –2 d –1) Abstract Phytoplankton primary productivity is most commonly measured by 14 C assimilation although less direct methods, such as O 2 exchange, have also been employed. These methods are invasive, requiring bottle incuba-tion for up to 24 h. As an alternative, Fast Repetition Rate fluorometry (FRRf) has been used, on wide temporal and spatial scales within aquatic systems, to estimate photosystem II (PSII) electron flux per unit volume (JV PSII), which generally correlates well with photosynthetic O 2 evolution. A major limitation of using FRRf aris-es from the need to employ an independent method to determine the concentration of functional photosys-tem II reaction centers ([RCII]); a requirement that has prevented FRR fluorometers being used, as stand-alone instruments, for the estimation of electron transport. Within this study, we have taken a new approach to the analysis of FRRf data, based on a simple hypothesis; that under a given set of environmental conditions, the ratio of rate constants for RCII fluorescence emission and photochemistry falls within a narrow range, for all groups of phytoplankton. We present a simple equation, derived from the established FRRf algorithm, for deter-mining [RCII] from dark FRRf data alone. We also describe an entirely new algorithm for estimating JV PSII , which does not require determination of [RCII] and is valid for a heterogeneous model of connectivity among RCIIs. Empirical supporting evidence is presented. These data are derived from FRR measurements across a diverse range of microalgae, in parallel with independent measurements of [RCII]. Possible sources of error, particular-ly under nutrient stress conditions, are discussed.
    Limnology and oceanography, methods 01/2012; 10:142-154. · 1.68 Impact Factor


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