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

Plectonema boryanum shows temporal separation of photosynthesis and nitroge n fixation under diazotrophic conditions. Low temperature fluorescence stud ies have shown that in vivo the nitrogen fixing and photosynthesizing cells are adapted to 'state 2' and 'state 1', respectively. During nitrogen fixa tion phycobilisomes seem to transfer excitation energy to photosystem I whe reas during oxygenic photosynthesis the energy is transferred to photosyste m II. The state 2 adapted N-phase cells failed to undergo transition to sta te 1 while P-phase cells exhibited state 1 to state 2 transition. The nitro gen fixing cells showed a decreased level of pshC transcript, lack of CP47 in thylakoid membrane, and presence of the F685 peak but absence of the F69 5 peak in 77 K fluorescence spectra. These results suggest that the metabol ic 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 pho toautotrophic growth under diazotrophic conditions. (C) 2000 Elsevier Scien ce B.V. All rights reserved.