Cyanobacteria can rapidly regulate the relative activity of their photosynthetic complexes photosystem I and II (PSI and PSII) in response to changes in the illumination conditions. This process is known as state transitions. If PSI is preferentially excited, they go to state I whereas state II is induced either after preferential excitation of PSII or after dark adaptation. Different underlying mechanisms have been proposed in literature, in particular i) reversible shuttling of the external antenna complexes, the phycobilisomes, between PSI and PSII, ii) reversible spillover of excitation energy from PSII to PSI, iii) a combination of both and, iv) increased excited-state quenching of the PSII core in state II. Here we investigated wild-type and mutant strains of Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803 using time-resolved fluorescence spectroscopy at room temperature. Our observations support model iv, meaning that increased excited-state quenching of the PSII core occurs in state II thereby balancing the photochemistry of photosystems I and II.

蓝细菌可以根据光照条件的变化快速调节其光合复合物光系统I和II（PSI和PSII）的相对活性。此过程称为状态转换。如果优先激发PSI，它们将进入状态I，而优先激发PSII之后或在暗适应之后，状态II就会被诱导。文献中已经提出了不同的潜在机制，特别是i）PSI和PSII之间的外部天线复合体，藻胆体的可逆穿梭，ii）激发能从PSII可逆向PSI的溢出，iii）两者的组合，以及iv ）在状态II中增加了PSII核的激发态猝灭。在这里，我们调查了Synechococcus sp。的野生型和突变株。PCC 7942和集胞藻 PCC 6803在室温下使用时间分辨荧光光谱法。我们的观察结果支持模型iv，这意味着PSII核的激发态猝灭在状态II中发生，从而平衡了光系统I和II的光化学。