ABSTRACT
We employed chlorophyll a fluorometry in order to measure the evolution of turgor threshold (intracellular osmolality) during the adaptation of two genetic transformants of the freshwater cyanobacterium Synechococcus sp. PCC7942 to unfavorable external salinity: PAMCOD cells which oxidize imported choline and accumulate approx. 0.06-0.08 M glycine betaine; and PAM cells which do not oxidize choline [Deshnium et al. (1995a) Plant Mol Biol 29: 897-909]. Turgor thresholds increased linearly (a) with the NaCl concentration in the culture, and (b) with the molar sucrose/chlorophyll a ratio in the cell. PAMCOD cells could proliferate in culture medium containing 0.4 M NaCl (external osmolality, 0.815 Osm kg(-1)), after a lag period, during which intracellular sucrose rose to 10 mol (mol Chl a)(-1), or more, and turgor threshold (cytoplasmic osmolality) exceeded 1 Osm kg(-1). At comparative conditions, PAM cells accumulated approx. half as much sucrose, and attained approx. half as high turgor thresholds as the PAMCOD cells, but they did not proliferate. These results indicate that glycine betaine improved the salinity tolerance of the PAMCOD cells synergistically, by means of two effects that implicate sucrose, the main organic osmolyte of Synechocccus: enhancement of sucrose biosynthesis, and/or alleviation of sucrose toxicity.
ABSTRACT
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 enlargement 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 enlarged 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 regulation on the distribution of phycobilisome (PBS) excitation 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 conditions of cyanobacterial cells. Thus, cyanobacteria may serve as quite convenient models of aquatic microorganisms in experimental studies directed toward the elucidation of perception mechanisms and defense mechanisms of water and solute stresses.
