Osmotic adjustment and organic solute accumulation in Chaetomorpha capillaris

Abstract

The physiological responses of the marine green alga Chaetomorpha capillaris were studied in hypersaline media in short-term (24 h) and longer-term (28 d) experiments. The principal inorganic solutes were K⁺ and Cl^-, which increased rapidly (within 6 h) in response to hypersaline treatment. Natural abundance ¹³C nuclear magnetic resonance spectroscopy showed glycine betaine to be the only major low-molecular-weight organic osmolyte in unstressed plants. However, on transfer to 200% sea-water or 300% sea-water, tissue sucrose levels increased within 6–12 h, while glycine betaine content remained unchanged. The intracellular levels of the principal organic solutes were substantially lower than the major inorganic ions, although the osmotic significance of organic solute accumulation may be increased by intracellular compartmentalization.

Changes in tissue K⁺, Cl^- and sucrose occurred over a similar time-scale to the observed increases in intracellular osmotic pressure in hypersaline media, measured by the technique of incipient plasmolysis. These solutes are likely to be involved in the initial phase of osmotic adjustment (osmoacclimation) in salt-stressed C. capillaris, while glycine betaine is not. Smaller changes in total α-amino acids and Na⁺ were also recorded.

Glycine betaine levels rose slowly in a hypersaline medium over a 14 d period, while plants incubated in a hyposaline medium showed a steady fall in tissue glycine betaine over 28 d. Thus, although glycine betaine may be involved in longer-term osmoacclimation in C. capillaris, changes in the intracellular level of this solute are unlikely to be sufficiently rapid to be of value during episodes of osmotic stress on the shore. However, the longer-term effects of salinity variation upon cellular glycine betaine content are consistent with data for several betaine-accumulating, salt-tolerant higher plants, where this methylated osmolyte may have a similar function.