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The marine red alga Stictosiphonia hookeri was isolated from three locations in cold-temperate Chile and Argentina, and the comparative ecophysiology of the isolates was studied. The growth response in relation to various salinity, light and temperature conditions was investigated and photosynthesis-irradiance curves were determined. The effect of increasing salinity on the intracellular concentration of the sugar alcohol sorbitol, and the accumulation/degradation kinetics of this compound, were studied. All isolates are characterized as euryhaline, eurythermal and low-light-adapted organisms because of the growth and photosynthesis response patterns. With increasing salinities the macroalgae accumulated high concentrations of sorbitol, which acts as an organic osmolyte and compatible solute. However, this process requires more than 24 h, and hence is too slow to contribute to osmotic acclimation within a tidal cycle. The geographic distribution and the growth versus temperature data indicate infraspecific differences which are interpreted as a stage in the development of different (physiological) ecotypes.Key words: growth, osmotic acclimation, photosynthesis, salt stress, sorbitol, temperature ecotypes IntroductionThe marine red alga Stictosiphonia hookeri (Harvey) J. D. Hooker et Harvey is widely distributed in the SouthernHemisphere from temperate (about 30 ° S) to cold (about 55 ° S) waters (King & Puttock, 1989). It preferentially grows on shaded rocks in the upper littoral zone of sheltered to exposed coastlines and less frequently as an epiphyte on the pneumatophores and trunks of mangrove trees in south-eastern Australia and New Zealand (King & Puttock, 1989). In the latter habitat S. hookeri occurs intermixed with other red algae of the so-called Bostrychia-CaIoglossa association (Post, 1936), a group of algae which are considered characteristic of mangrove communities. In both habitats S. hookeri is subject to alternating emersion-immersion cycles because of the tides. Under these conditions it may be exposed to wide salinity and temperature fluctuations.The genus Stictosiphonia was segregated from the genus Bostrych& (Ceramiales) on the basis of some morphological features (King & Puttock, 1989). There are also biogeographic differences between these closely related genera. While Bostrychia species typically dominate the macroalgal flora in the eulittoral zone of tropical mangrove and temperature salt marsh communities, several Stictosiphonia species occur mainly in cold-temperate and subantarctic regions. This is particularly interesting since these algae are thought to originate from the tropical Indo-Pacific region (Post, 1968), but during evolution Stictosiphonia has occupied locations in much higher latitudes (30°S to 55°S) than has Bostrychia. This is well documented by the present geographic distribution of four of the six known Stictosiphonia species (including S. hookeri) (King & Puttock, 1989).Recent investigations of Bostrychia species consider the physiological and biochemical capabilit...