A quantum picture of light-suppressed photosynthetic charge transfer

We propose a dynamic mechanism for the reversible regulation of photosynthesis in varying light environments. We employ a three-level quantum model to take into account the correlations between charge donors and charge acceptors immediately before photoexcitation, and show that under continuous illumination, the transfer efficiency of a single charge is inversely proportional to the intensity of light, which can be suppressed so severely that it becomes a limiting factor on linear electron transport. This result is used to derive a set of analytical expressions that characterize the light response curves of photosynthetic parameters, including that of gross photosynthetic rate which saturates in high light and has long been assumed to obey a Michaelis–Menten function. We discuss the implications of thermal fluctuation in the light source, and argue that at a given intensity of light, the quantum yields measured with an incandescent lamp may be higher than those measured with a laser, a manifestation of thermal fluctuation in lamp illumination. Our new picture helps understand the observed plastocyanin-dependent electron transport in photosystem I and provides a donor-side scheme for the onset of irreversible damage to photosystem II by visible light.