Osmoacclimation inBangia atropurpurea(Rhodophyta, Bangiales): the osmotic role of floridoside

Aspects of the osmotic physiology of freshwater-grown plants of the euryhaline alga Bangia atropurpurea (Roth) C. Ag. have been studied over the salinity range from freshwater to full-strength sea-water (35‰). Synthesis of floridoside [O-α-d-galactopyranosyl-(1 → 2)-glycerol], the major low molecular weight organic solute in B. atropurpurea, occurred upon transfer of plants from a freshwater-based medium to saline media, with a maximum level of 134 mmol kg (cell water)-1 being recorded for thalli incubated in 100% sea-water for 24 h. In contrast, photosynthetic activity was depressed in high-salt media. Differences in the kinetics of floridoside production and photosynthetic rate were also observed during transfer to 100% sea-water; floridoside showed a rapid increase, doubling in concentration within 30 min and reaching a new elevated steady state within 24 h while photosynthetic activity showed a transient decrease of approximately 75% upon transfer, followed by a recovery to 60% of the initial (freshwater) rate. Studies using 14C showed an increasing allocation of radiocarbon to floridoside with increasing external salt concentration, with a six-fold increase in the percentage of 14C incorporated into this heteroside between freshwater and sea-water. Overall, the data are consistent with the hypothesis that changes in floridoside concentration serve to compensate, in part at least, for changes in external osmolality.