| Summary: | The morphological and ultrastructural characteristics of the vegetative thallus have been studied in Iridaea cordata from Antarctica. The chemical composition of the iridescent cuticle and the distribution of sulphated and neutral polysaccharides in the extracellular regions have been analyzed by cytochemical tests. The ultrastructural organization and pigment composition of the photosynthetic apparatus have been compared in thalli of I. cordata collected during austral summer from ice-covered and ice-free sea waters. The analyses showed that, with respect to those growing under ice, the thalli growing at the higher irradiance in ice-free water had chloroplasts with a reduced thylakoid system and a lower content of both chlorophyll and carotenoids. They also contain phycobiliproteins, which, however, were not assembled in well structured phycobilisomes.
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