The osmolality of the cell suspension regulates phycobilisome-to-photosystem I excitation transfers in cyanobacteria.

The chlorophyll a (Chla) fluorescence of cyanobacteria, which at physiological temperature originates from photosystem (PS) II holochromes, is suppressed in hyperosmotic suspension, and enhanced in hypo-osmotic suspension (G.C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4). We investigated the mechanism of this phenomenon by comparing Synechococcus sp. PCC 7942 cells that had been treated with N-ethylmaleimide (NEM) in order to inhibit electronic excitation transfers from phycobilisomes (PBS) to Chlas of PSI (A.N. Glazer, Y.M. Gindt, C.F. Chan, K. Sauer, Photosynth. Res. 40 (1994) 167-173) with untreated control cells. The NEM-treated cells were indistinguishable from the control cells with regard to PSII-dependent oxygen evolution, reduction of post-PSII oxidants, and osmotically induced volume changes, but differed in the following properties: (i) they could not photoreduce post-PSI electron acceptors; (ii) they diverted more PBS excitation to PSII; (iii) the rise of Chla fluorescence upon light acclimation of darkened (state 2) cells was smaller; and (iv) the Chla fluorescence of light-acclimated (state 1) cells was insensitive to the cell suspension osmolality. These properties suggest that osmolality regulates the core-mediated excitation coupling between PBS and PSI, possibly by influencing mutual orientation and/or distance between core holochromes (ApcE, ApcD) and PSI holochromes. Thus, in hyper-osmotic suspension, PBS deliver more excitation to PSI (hence less to PSII); in hypo-osmotic cell suspension they deliver less excitation to PSI (hence more to PSII).