Pogoryelov D, Sudhir P R, Kovács L, Gombos Z, Brown I, Garab G
Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, H-6701 Szeged, P.O. Box 521, Hungary.
J Bioenerg Biomembr. 2003 Oct;35(5):427-37. doi: 10.1023/a:1027339814544.
Arthrospira (Spirulina) platensis (A. platensis) is a model organism for investigation of adaptation of photosynthetic organisms to extreme environmental conditions: the cell functions in this cyanobacterium are optimized to high pH and high concentration (150-250 mM) of Na+. However, the mechanism of the possible fine-tuning of the photosynthetic functions to these extreme conditions and/or the regulation of the cellular environment to optimize the photosynthetic functions is poorly understood. In this work we investigated the effect of Na-ions on different photosynthetic activities: linear electron transport reactions (measured by means of polarography and spectrophotometry), the activity of photosystem II (PS II) (thermoluminescence and chlorophyll a fluorescence induction), and redox turnover of the cytochrome b6f complex (flash photolysis); and measured the changes of the intracellular pH (9-aminoacridine fluorescence). It was found that sodium deprivation of cells in the dark at pH 10 inhibited, within 40 min, all measured photosynthetic reactions, and led to an alkalinization of the intracellular pH, which rose from the physiological value of about 8.3-9.6. These were partially and totally restored by readdition of Na-ions at 2.5-25 mM and about 200 mM, respectively. The intracellular pH and the photosynthetic functions were also sensitive to monensin, an exogenous Na+/H+ exchanger, which collapses both proton and sodium gradients across the cytoplasmic membrane. These observations explain the strict Na+-dependency of the photosynthetic electron transport at high extracellular pH, provide experimental evidence on the alkalization of the intracellular environment, and support the hypothesized role of an Na+/H+ antiport through the plasma membrane in pH homeostasis (Schlesinger et al. (1996). J. Phycol. 32, 608-613). Further, we show that (i) the specific site of inactivation of the photosynthetic electron transport at alkaline pH is to be found at the water splitting enzyme; (ii) in contrast to earlier reports, the inactivation occurs in the dark and, for short periods, without detectable damage in the photosynthetic apparatus; and (iii) in contrast to high pH, Na+ dependency in the neutral pH range is shown not to originate from PSII, but from the acceptor side of PSI. These data permit us to conclude that the intracellular environment rather than the machinery of the photosynthetic electron transport is adjusted to the extreme conditions of high pH and high Na+ concentration.
钝顶节旋藻（螺旋藻）是研究光合生物如何适应极端环境条件的模式生物：这种蓝细菌的细胞功能已优化以适应高pH值和高浓度（150 - 250 mM）的Na⁺。然而，人们对光合功能如何针对这些极端条件进行可能的微调机制以及如何调节细胞环境以优化光合功能了解甚少。在这项工作中，我们研究了Na⁺离子对不同光合活性的影响：线性电子传递反应（通过极谱法和分光光度法测量）、光系统II（PS II）的活性（热发光和叶绿素a荧光诱导）以及细胞色素b6f复合物的氧化还原周转（闪光光解）；并测量了细胞内pH值的变化（9 - 氨基吖啶荧光）。结果发现，在pH 10的黑暗环境中剥夺细胞的Na⁺离子，在40分钟内会抑制所有测量的光合反应，并导致细胞内pH值碱化，从约8.3 - 9.6的生理值上升。分别在添加2.5 - 25 mM和约200 mM的Na⁺离子后，这些反应部分和完全恢复。细胞内pH值和光合功能对外源Na⁺/H⁺交换剂莫能菌素也很敏感，莫能菌素会破坏跨细胞质膜的质子和钠梯度。这些观察结果解释了在高细胞外pH值下光合电子传递对Na⁺的严格依赖性，为细胞内环境碱化提供了实验证据，并支持了质膜上Na⁺/H⁺反向转运体在pH稳态中的假设作用（施莱辛格等人（1996年）。《藻类学杂志》32卷，608 - 613页）。此外，我们表明：（i）碱性pH值下光合电子传递失活的特定部位位于水裂解酶；（ii）与早期报告相反，失活发生在黑暗中，并且在短时间内光合装置没有可检测到的损伤；（iii）与高pH值相反，中性pH范围内对Na⁺的依赖性并非源于PSII，而是源于PSI的受体侧。这些数据使我们能够得出结论，是细胞内环境而非光合电子传递机制适应了高pH值和高Na⁺浓度的极端条件。
