Department of Biology, Botanical Institute, Christian-Albrechts-University, 24118 Kiel, Germany.
Division of Food Technology, Institute of Human Nutrition and Food Science, Christian-Albrechts-University, 24118 Kiel, Germany.
Biochim Biophys Acta Bioenerg. 2021 Mar 1;1862(3):148353. doi: 10.1016/j.bbabio.2020.148353. Epub 2020 Dec 18.
Photosynthetic electron flow, driven by photosystem I and II, provides chemical energy for carbon fixation. In addition to a linear mode a second cyclic route exists, which only involves photosystem I. The exact contributions of linear and cyclic transport are still a matter of debate. Here, we describe the development of a method that allows quantification of electron flow in absolute terms through photosystem I in a photosynthetic organism for the first time. Specific in-vivo protocols allowed to discern the redox states of plastocyanin, P700 and the FeS-clusters including ferredoxin at the acceptor site of PSI in the cyanobacterium Synechocystis sp. PCC 6803 with the near-infrared spectrometer Dual-KLAS/NIR. P700 absorbance changes determined with the Dual-KLAS/NIR correlated linearly with direct determinations of PSI concentrations using EPR. Dark-interval relaxation kinetics measurements (DIRK) were applied to determine electron flow through PSI. Counting electrons from hydrogen oxidation as electron donor to photosystem I in parallel to DIRK measurements confirmed the validity of the method. Electron flow determination by classical PSI yield measurements overestimates electron flow at low light intensities and saturates earlier compared to DIRK. Combination of DIRK with oxygen evolution measurements yielded a proportion of 35% of surplus electrons passing PSI compared to PSII. We attribute these electrons to cyclic electron transport, which is twice as high as assumed for plants. Counting electrons flowing through the photosystems allowed determination of the number of quanta required for photosynthesis to 11 per oxygen produced, which is close to published values.
光合作用电子流由光系统 I 和 II 驱动，为碳固定提供化学能量。除了线性模式外，还存在第二种循环途径，它仅涉及光系统 I。线性和循环运输的确切贡献仍存在争议。在这里，我们首次描述了一种方法的发展，该方法首次允许在光合生物体中以绝对术语量化通过光系统 I 的电子流。特定的体内方案允许在蓝藻 Synechocystis sp. PCC 6803 中区分质体蓝蛋白、P700 和包括铁硫簇在内的铁氧还蛋白在光系统 I 受体位点的氧化还原状态，使用近红外光谱仪 Dual-KLAS/NIR。用 Dual-KLAS/NIR 确定的 P700 吸收变化与使用 EPR 直接确定 PSI 浓度呈线性相关。暗间隔弛豫动力学测量 (DIRK) 用于确定通过 PSI 的电子流。将氢氧化作为电子供体到光系统 I 的电子与 DIRK 测量同时计数，证实了该方法的有效性。与 DIRK 测量相结合的经典 PSI 产量测量高估了低光强度下的电子流，并且比 DIRK 更早达到饱和。将 DIRK 与氧气产生测量相结合，得出通过 PSI 传递的过剩电子的比例为 PSII 的 35%。我们将这些电子归因于循环电子运输，这是植物中假设的两倍。通过计算流经光合作用系统的电子数量，可以确定光合作用每产生一个氧所需的量子数为 11，这与已发表的值接近。
