NITROGEN-FIXATION BY PLECTONEMA-BORYANUMA PHOTOSYSTEM-II INDEPENDENT COMPONENT

The non-heterocystous filamentous cyanobacterium Plectonema boryanum showed a several-fold decline in photosystem II dependent O-2 evolution during the diazotrophic phase of growth. A sharp fall in the amounts of light-harvesting pigments and an uncoupling between the electron transfer from the photosystem II complex to the quinone accepters may lead to the depression of light-dependent O-2 evolution during the diazotrophic phase. Nitrogen fixation as well as CO2 fixation required light, but were partly supported independently of photosystem II. A stimulation of photosystem I and a depression of photosystem II occurred during the diazotrophic phase of a nitrogen-fixing culture of P. boryanum.