Sodium Chloride-Induced Volume Changes of Freshwater Cyanobacterium Synechococcus sp. PCC 7942 Cells Can Be Probed by Chlorophyll a Fluorescence Kostas Stamatakis, Nectarios P. Ladas, Aikaterini Alygizaki-Zorba, and George C. Papageorgiou 1 Institute of Biology, National Research Center Demokritos, Athens, Greece 153 10 Received May 1, 1999, and in revised form June 24, 1999 Freshwater species of the cyanobacterial genus Synechococcus import NaCl passively, and export Na  actively, by means of primary and secondary extrusion mechanisms. As a result of the ion and water fluxes, cell volumes are enlarged. We show in this paper that the NaCl-induced volume enlarge- ment of Synechococcus sp. PCC 7942 cells is attended by a rapid (k  0.39 s 1 ) increase in chlorophyll (Chl) a fluorescence. The cell turgor threshold (measured by osmotic titration of Chl a fluorescence) was lower in the absence of NaCl (0.195 Osm kg 1 ) than in the presence of 0.4 M NaCl (0.248 Osm kg 1 ) indicating NaCl uptake by the cells. Turgor thresholds of cells suspended in NaCl-containing medium were en- larged further by protonophoric uncouplers, P-type ATPase inhibitors, and light starvation, conditions that are known to interfere with the active extrusion of Na  ions. Cell swelling exerts probably a regula- tion on the distribution of phycobilisome (PBS) ex- citation between photosystem II (fluorescent Chl a) and photosystem I (nonfluorescent Chl a), since it affects PBS-sensitized Chl a fluorescence, but not directly excited Chl a fluorescence. The dependence of the Chl a fluorescence of cyanobacteria on cell volumes allows probing of bioenergetic phenomena that are related to dynamic osmotic volume changes, transmembrane solute and water fluxes, plasma membrane permeabilities, and internal osmotic con- ditions of cyanobacterial cells. Thus, cyanobacteria may serve as quite convenient models of aquatic microorganisms in experimental studies directed to- ward the elucidation of perception mechanisms and defense mechanisms of water and solute stresses. © 1999 Academic Press Key Words: cyanobacteria; chlorophyll fluorescence; osmotic cell volume; turgor threshold; Synechococcus sp. PCC 7942. Cyanobacteria mobilize several defenses when they are challenged by unfavorable salinity. A cyanobacte- rium whose responses to salinity have been studied extensively is the freshwater species Synechococcus 6311 (Anacystis nidulans, Synechococcus 6301, and Synechococcus 6311) and the related strain Synecho- coccus sp. PCC 7942 (A. nidulans R-2 and S. leopolien- sis). Adaptation to salinity is initiated by the passive importation of NaCl (along the concentration gradient) and water (along the water potential gradient; Refs. 1– 4) and is followed by the active extrusion of Na + ions (against the concentration gradient) by means of pri- mary and secondary mechanisms (1, 5–15). A second- ary mechanism that attracted special attention is Na + ion extrusion, in exchange for H + imported along the pH gradient, by Na + /H + antiporters located in the plasma membrane (1, 6 –13). These initial events cause transient maxima (within 2–5 min) in the cytoplasmic Na + ion concentration and the osmotic cell volume (13). Longer-term adaptive responses comprise synthesis and accumulation of sucrose (16) and glycogen (17), as well as alterations in the lipid composition and the physical properties of plasma membranes (18, 19). Light-acclimated cyanobacteria (light state 1 cells) emit more intense chlorophyll (Chl) 2 a fluorescence 1 To whom correspondence should be addressed. Fax: +30 1 651 1767. E-mail: gcpap@mail.demokritos.gr. 2 Abbreviations used: Chl, chlorophyll; CCCP, carbonyl cyanide m-chlorophenylhydrazone; DCCD, N,N'-dicyclohexylcarbodiimide; DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; Hepes, N-2-hy- droxyethylpiperazine-N'-2-ethanesulfonic acid; NBD, 7-chloro-4-ni- trobenz-2-oxa-1,3-diazole; Osm IN and Osm OUT , inner (cytoplasm) and outer (medium) osmolality; PBS, phycobilisome; PS I and PS II, 240 0003-9861/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved. Archives of Biochemistry and Biophysics Vol. 370, No. 2, October 15, pp. 240 –249, 1999 Article ID abbi.1999.1366, available online at http://www.idealibrary.com on