Excitation energy transfer from phycobilisomes to photosystems: a phenomenon associated with the temporal separation of photosynthesis and nitrogen fixation in a cyanobacterium, Plectonema boryanum(1).

Plectonema boryanum shows temporal separation of photosynthesis and nitrogen fixation under diazotrophic conditions. Low temperature fluorescence studies have shown that in vivo the nitrogen fixing and photosynthesizing cells are adapted to 'state 2' and 'state 1', respectively. During nitrogen fixation phycobilisomes seem to transfer excitation energy to photosystem I whereas during oxygenic photosynthesis the energy is transferred to photosystem II. The state 2 adapted N-phase cells failed to undergo transition to state 1 while P-phase cells exhibited state 1 to state 2 transition. The nitrogen fixing cells showed a decreased level of psbC transcript, lack of CP47 in thylakoid membrane, and presence of the F685 peak but absence of the F695 peak in 77 K fluorescence spectra. These results suggest that the metabolic and molecular changes associated with nitrogen fixation may favor direct energy transfer from the phycobilisomes to photosystem I. This should help the organism to achieve low photosystem II and high photosystem I activity to set temporal separation of nitrogen fixation and photosynthesis for photoautotrophic growth under diazotrophic conditions.