Quantifying the energy spillover between photosystems II and I in cyanobacterial thylakoid membranes and cells.

The spatial separation of photosystems I and II is thought to be essential for efficient photosynthesis by maintaining balanced flow of excitation energy between them. Unlike the thylakoid membranes of plant chloroplasts, cyanobacterial thylakoids do not form tightly appressed grana stacks that enforce strict lateral separation. The coexistence of the two photosystems provides a ground for spillover - excitation energy transfer from photosystem II to I. Spillover has been considered as a pathway of energy transfer from the phycobilisomes to photosystem I and may also play a role in state transitions as means to avoid overexcitation of photosystem II. Here we demonstrate a significant degree of energy spillover from photosystem II to photosystem I in reconstituted membranes and isolated thylakoid membranes of Thermosynechococcus (Thermostichus) vulcanus and Synechocystis sp. PCC 6803 by steady-state and time-resolved fluorescence spectroscopy. The quantum yield of spillover in these systems was determined to be up to 40%. Spillover was also found in intact cells but to a considerably lower degree (20%) than in isolated thylakoid membranes. The findings support a model of co-existence of laterally separated microdomains of PSI and PSII in the cyanobacterial cells as well as domains where the two photosystems are energetically connected. The methodology presented here can be applied to probe spillover in other photosynthetic organisms.