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 (alpha - miss; beta - 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 alpha and beta 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.