Cell turgor: A critical factor for the proliferation of cyanobacteria at unfavorable salinity.

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.