Photosystem I (PSI) converts photons into electrons with a nearly 100% quantum efficiency. Its minimal energy requirement for photochemistry corresponds to a 700-nm photon, representing the well-known “red limit” of oxygenic photosynthesis. Recently, some cyanobacteria containing the red-shifted pigment chlorophyll f have been shown to harvest photons up to 800 nm. To investigate the mechanism responsible for converting such low-energy photons, we applied steady-state and time-resolved spectroscopies to the chlorophyll-f-containing PSI and chlorophyll-a-only PSI of various cyanobacterial strains. Chlorophyll-f-containing PSI displays a less optimal energetic connectivity between its pigments. Nonetheless, it consistently traps long-wavelength excitations with a surprisingly high efficiency, which can only be achieved by lowering the energy required for photochemistry, i.e., by “breaking the red limit”. We propose that charge separation occurs via a low-energy charge-transfer state to reconcile this finding with the available structural data excluding the involvement of chlorophyll f in photochemistry.

光系统I（PSI）将光子转换为具有近100％量子效率的电子。它对光化学的最低能量要求对应于700 nm光子，代表了氧光合作用的众所周知的“红色极限”。最近，一些含有红移色素叶绿素f的蓝细菌已显示出可收获高达800 nm的光子。为了研究引起这种低能光子转化的机制，我们将稳态和时间分辨光谱学应用于各种蓝藻菌株的含叶绿素f的PSI和仅叶绿素a的PSI。叶绿素f含PSI的颜料之间的能量连接性欠佳。尽管如此，它始终以惊人的高效率捕获长波长激发，这只能通过降低光化学所需的能量，即“突破红色极限”来实现。我们建议通过低能电荷转移状态发生电荷分离，以使这一发现与除叶绿素f参与光化学以外的可用结构数据相一致。