The Effects of Salinity upon Cellular Volume of the Marine Red Alga Porphyra purpurea (Roth) C.Ag.

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Abstract
Changes in cell volume of the marine red alga Porphyra purpurea have been investigated using photomicroscopic and radioisotopic techniques. There is an inverse relationship between cell volume and external salt content. The alga responds to changes in the water potential of its bathing medium by rapid swelling in hyposaline media and shrinkage in hypersaline conditions. Cells P. purpurea behave as osmometers in concentrated sea-waters, obeying the Boyle-Van't Hoff law. A non-osmotic volume, 20–25% of the total cell volume in sea-water, can be predicted from the linear plot of volume versus reciprocal pressure in concentrated sea-water media. In dilute sea-waters the presence of non-rigid cell walls serves to limit any increases in cell volume. The primary response to dilution stress is thus an increase in turgor. Cell volume is not returned to its original value following prolonged immersion in either hyposaline or hypersaline media, showing that the alga does not ‘osmoregulate’ sensu stricto.