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_PSI) were applied to determine electron flow through PSI. Counting electrons from hydrogen oxidation as electron donor to photosystem I in parallel to DIRK_PSI 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_PSI. Combination of DIRK_PSI 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.

由光系统I和II驱动的光合电子流为固碳提供化学能。除线性模式外，还存在第二条循环路径，该路径仅涉及光系统I。线性和循环传输的确切作用仍是一个有争议的问题。在这里，我们描述了一种方法的发展，该方法首次允许通过光合作用系统中的光系统I以绝对值对电子流进行量化。特定的体内协议允许辨别质体蓝素，P700的氧化还原状态和FES-簇包括在PSI中的蓝藻的受体位点铁氧还蛋白集胞藻sp。带有近红外光谱仪Dual-KLAS / NIR的PCC 6803。用Dual-KLAS / NIR测定的P700吸光度变化与使用EPR直接测定PSI浓度线性相关。暗间隔弛豫动力学测量（DIRK _PSI）用于确定通过PSI的电子流量。平行于DIRK _PSI测量，计算从氢氧化作为电子给体到光系统I的电子，证实了该方法的有效性。与DIRK _PSI相比，通过经典PSI产量测量确定的电子流量高估了低光强度下的电子流量，并且饱和更早。DIRK _PSI的组合氧释放量测量结果显示，与PSII相比，通过PSI的剩余电子比例为35％。我们将这些电子归因于循环电子传输，这是植物假设的两倍。对流经光系统的电子进行计数可以确定光合作用所需的量子数，以产生的每个氧气为11个，这与已公布的值接近。