Article

Raven JA.. Functional evolution of photochemical energy transformations in oxygen-producing organisms. Funct Plant Biol 36: 505-515

Functional Plant Biology (Impact Factor: 3.15). 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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    • "These interactions of light and iron suggest that, even with the evidence from mesoscale iron fertilization experiments of stimulation of larger diatom growth by the addition of iron, light effects could be important in the iron response (Finkel et al., 2006). The low-iron-adapted F. cylindrus and Pseudo-nitzschia granii genomes code for an energy-transducing (protonpumping ) rhodopsin, which provides an iron-independent means of photon energy conversion (Marchetti et al., 2012), although the photon use efficiency is lower than cyclic electron transport, as well as involving more protein per chromophore and a lower maximum rate of proton transport per unit protein, than cyclic electron flow in thylakoids (Raven, 2009). Furthermore, it is not clear whether any proton gradient produced by an energy-transducing rhodopsin can be used to phosphorylate ADP, as it does not seem to be clear that the rhodopsin is targeted to the thylakoid membrane (or the inner mitochondrial membrane) with an ATP synthase (Marchetti et al., 2012). "
    [Show abstract] [Hide abstract] ABSTRACT: The essential element iron has a low biological availability in the surface ocean where photosynthetic organisms live. Recent advances in our understanding of iron acquisition mechanisms in brown algae and diatoms (stramenopile algae) show the importance of the reduction of ferric to ferrous iron prior to, or during, transport in the uptake process. The uses of iron in photosynthetic stramenopiles resembles that in other oxygenic organisms, although (with the exception of the diatom Thalassiosira oceanica from an iron-deficient part of the ocean) they lack plastocyanin, instead using cytochrome c 6, This same diatom further economizes genotypically on the use of iron in photosynthesis by decreasing the expression of photosystem I, cytochrome c 6, and the cytochrome b 6 f complex per cell and per photosystem II relative to the coastal Thalassiosira pseudonana; similar changes occur phenotypically in response to iron deficiency in other diatoms such as Phaeodactylum tricornutum. In some diatoms grown under iron-limiting conditions, essentially all of the iron in the cells can be accounted for by the iron occurring in catalytic proteins. However, stramenopiles can store iron. Genomic studies show that pennate, but not centric, diatoms have the iron storage protein ferritin. While Mössbauer and X-ray analysis of (57)Fe-labelled Ectocarpus siliculosus shows iron in an amorphous mineral phase resembling the core of ferritin, the genome shows no protein with significant sequence similarity to ferritin.
    Preview · Article · May 2013 · Journal of Experimental Botany
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    • "The moles of oxygen (O 2 ) consumed per mol of nitrogen (N) fixed were 9.3:1 and 3.2:1 at normal and low oxygen respectively (Table 1). Using an ATP production of 4.28 mol ATP per mole of oxygen respired (Raven, 2009), the observed oxygen drawdown during the dark phase can be converted to 40 and 14 mol ATP per mol of nitrogen (N) fixed at normal and low oxygen, respectively.Table 1 sums up the cellular budget of oxygen, nitrogen, and carbon during the light and dark cycle for low and normal oxygen. According to Eq. 1, 8 mol ATP are needed for the direct reduction of 0.5 mol N 2 to ammonium, so the nitrogenase enzyme reaction consumes 20 and 57% of the cellular energy produced in the dark at normal and low oxygen respectively. "
    [Show abstract] [Hide abstract] ABSTRACT: The recent detection of heterotrophic nitrogen (N(2)) fixation in deep waters of the southern Californian and Peruvian OMZ questions our current understanding of marine N(2) fixation as a process confined to oligotrophic surface waters of the oceans. In experiments with Crocosphaera watsonii WH8501, a marine unicellular diazotrophic (N(2) fixing) cyanobacterium, we demonstrated that the presence of high nitrate concentrations (up to 800 μM) had no inhibitory effect on growth and N(2) fixation over a period of 2 weeks. In contrast, the environmental oxygen concentration significantly influenced rates of N(2) fixation and respiration, as well as carbon and nitrogen cellular content of C. watsonii over a 24-h period. Cells grown under lowered oxygen atmosphere (5%) had a higher nitrogenase activity and respired less carbon during the dark cycle than under normal oxygen atmosphere (20%). Respiratory oxygen drawdown during the dark period could be fully explained (104%) by energetic needs due to basal metabolism and N(2) fixation at low oxygen, while at normal oxygen these two processes could only account for 40% of the measured respiration rate. Our results revealed that under normal oxygen concentration most of the energetic costs during N(2) fixation (∼60%) are not derived from the process of N(2) fixation per se but rather from the indirect costs incurred for the removal of intracellular oxygen or by the reversal of oxidative damage (e.g., nitrogenase de novo synthesis). Theoretical calculations suggest a slight energetic advantage of N(2) fixation relative to assimilatory nitrate uptake, when oxygen supply is in balance with the oxygen requirement for cellular respiration (i.e., energy generation for basal metabolism and N(2) fixation). Taken together our results imply the existence of a niche for diazotrophic organisms inside oxygen minimum zones, which are predicted to further expand in the future ocean.
    Full-text · Article · Jul 2012 · Frontiers in Microbiology
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    • "Significant contributions of horizontal/lateral gene transfer among uni-cellular [6] and multi-cellular [7] organisms during the evolution, including the evolution of photosynthesis [8,9], have been recognized. Various perspectives on evolution of photosynthesis have been reported in literature8910111213141516171819202122232425, whereas our understanding of transition from anaerobic to aerobic world is still fragmentary. The recent genomic report on Cfl. "
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