Pathway(s) and distribution of photosystem II-cyclic electron flow

Photosystem II is a highly conserved protein complex found across all oxygenic phototrophs and the source of almost all biologically available electrons and atmospheric oxygen. This large, membrane-bound complex is at continuous risk of damage under direct, full sunlight, and those organisms that live under such conditions have developed photoprotective mechanisms to address this stressor. One such mechanism, photosystem II-cyclic electron flow (PSII-CEF), is a poorly understood pathway by which charge-separated electrons are recombined with holes after an extended separation and multiple electron transfer events. This mechanism is critical to the survival of the fastest-growing known eukaryotic phototrophs, playing a critical role in both native extremophiles and lab-adapted strains. However, the path electrons take during this recombinative process is not known. In the present review, we address the state of the field and the challenges facing implementation of PSII-CEF as a component of photosynthetic organism protection, survival, and improved growth.