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

Abstract

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).