Study on the properties of the S3-state by mass spectrometry in the filamentous cyanobacterium Oscillatoria chalybea

In the present paper we analyzed the properties of the S₃-state in the filamentous cyanobacterium Oscillatoria chalybea by mass spectrometry. In this organism a substantial O₂-signal due to a single flash is observed even after extensive dark adaptation (20 min). This signal can be measured by mass spectrometry as well as amperometrically on an oxygen electrode and is not due to an interference of respiratory and photosynthetic electron transport in the prokaryotic membrane. The mass spectrometric analysis shows that, if S₃ is generated by two flashes in a medium containing only H₂¹⁶O, addition of H₂¹⁸O and subsequent firing of a third flash yields O₂ evolution labelled with ¹⁸O. It appears that the isotopic composition of the O₂ evolved corresponds to the isotopic composition of the water in the suspension. This experiment shows that water oxidation does not proceed via an oxygen precursor or water derivatives bound to the S₃-state. This conclusion has been reached shortly before ours by Radmer and Ollinger [15] in the reverse marker experiment. From our study with O. chalybea it appears that freshly generated S₃ can be distinguished from metastable S₃ by the mass spectrometric method. It looks as if in contrast to freshly generated S₃ metastable S₃ contained bound unexchangeable water or an oxidized water derivative.