How fast can Photosystem II split water? Kinetic performance at high and low frequencies

Molecular oxygen evolution from water is a universal signature of oxygenic photosynthesis. Detection of the presence, speed and efficiency of the enzymatic machinery that catalyzes this process in vivo has been limited. We describe a laser-based fast repetition rate fluorometer (FRRF) that allows highly accurate and rapid measurements of these properties via the kinetics of Chl-a variable fluorescence yield (Fv) in living cells and leaves at repetition rates up to 10 kHz. Application to the detection of quenching of Fv is described and compared to flash-induced O₂ yield data. Period-four oscillations in both Fv and O₂, caused by stimulation of primary charge recombination by the O₂ evolving complex (WOC) within Photosystem II (PS II), are directly compared. The first quantitative calculations of the enzymatic parameters of the Kok model (α - miss; β - double hit; S-state populations) are reported from Fv data over a 5 kHz range of flash frequencies that is 100-fold wider than previously examined. Comparison of a few examples of cyanobacteria, green algae and spinach reveals that Arthrospira m., a cyanobacterium that thrives in alkaline carbonate lakes, exhibits the fastest water-splitting rates ever observed thus farin vivo. In all oxygenic phototrophs examined thus far, an unprecedented large increase in the Kok α and β parameters occur at both high and low flash frequencies, which together with their strong correlation, indicates that PS II-WOC centers split water at remarkably lower efficiencies and possibly by different mechanisms at these extreme flash frequencies. Revisions to the classic Kok model are anticipated.