Oxygen release may limit the rate of photosynthetic electron transport; the use of a weakly polarized oxygen cathode

The kinetics of photosynthetic oxygen evolution upon flash illumination were measured electrochemically at a much less negative cathode potential than used in conventional oxygen polarography, so that the electrode current was an essentially non-disturbing probe of the oxygen concentration. This method allows a good signal-to-noise ratio at sub-millisecond time resolution with stationary suspensions of photosynthetic material and avoids a strong inhibition of photosynthesis occurring under some conditions of measurement with a bare cathode. The results contradict the generally accepted notion that oxygen release promptly follows the 1.2 ms reduction of the oxygen-evolving complex after its four-step photooxidation. A much slower process has to take place before oxygen is detected and before a next cycle of four photoreactions in Photosystem II can be completed successfully: the next three photooxidations of the complex are unaffected, but the fourth is lost for oxygen evolution. This indicates that oxygen is released only slowly from the site of water oxidation, and that water oxidation is required to stabilize the fourth charge separation. Half-times of 30-130 ms were measured for oxygen release in different batches of Photosystem II membranes, chloroplasts and algae. In some conditions oxygen release may be a significant rate-limiting step in photosynthesis.