Uncoupling of photosystems during light dependent dinitrogen fixation by a non-heterocystous cyanobacterium Plectonema boryanum.

Plectonema boryanum grows under microaerobic nitrogen starvation conditions by temporal separation of the oxygen-sensitive nitrogen fixation and the oxygen-evolving photosynthesis. During the nitrogen-fixation phase, depression in light-dependent oxygen evolution results from alterations in the stare of QA/QB complex. The reaction centre in photosystem II is not oxidised efficiently. Light dependence of nitrogenase function under such conditions appears to be related to the coupling of photosystem I to endogenous electron donors, such as glycollate. This cyanobacterium provides a natural example of in vivo shift in coupling between the photosystems during nitrogen-fixing growth.