Sorbitol and proline as intracellular osmotic solutes in the green alga Stichococcus bacillaris

A freshwater isolate of Stichococcus bacillaris Naeg. (strain UTEX 314) was evaluated for its ability to grow, photosynthesize, and osmoregulate over a wide range of salinity. The growth and photosynthetic measurements indicate that it is a euryhaline organism. Studies of the soluble organic metabolite pools showed that the steady-state levels of two solutes varied with salinity; sorbitol (a polyol) and proline (an amino acid). Intracellular proline levels increased from 0.002 to 0.28 M over the salinity range of 0 to40%c whereas the sorbitol level increased from 0.10 to 0.52 M. The level of total amino acids (excepting proline) remained relatively constant. No single amino acid of this group exceeded an intracellular concentration of 0.04 M. The changes in the concentrations of these solutes accounted for at least 75% of the required increase in intracellular osmolality in cells following adaptation to high salinity media. Sorbitol and proline are very soluble, nontoxic, and are efficient osmotic solutes. These properties make them ideal solutes for osmoregulation.