| Summary: | Morphological, ultrastructural, and cytochemical characteristics of the red alga Plocamium cartilagineum from Antarctica have been studied and compared with those of the same species from the Mediterranean Sea. Distinct regions with possibly different functions were recognisable in the bushy thallus of Plocamium cartilagineum. Chloroplasts were concentrated in the outermost cell layer, while in the other cortex cells abundant floridean starch accumulated. No starch was found in the large medullary cells, characterised by the insertion in their walls of several large and lenticular pit connections devoid of plug caps and proteic in nature. Sulphated polysaccharides were distributed in the extracellular compartments of all the regions of the thallus, except in the surface layer. These acidic phycocolloids were particularly concentrated in the cell wall proper, which also exhibited a well organised fibrillar component probably consisting of cellulose. The distinctive feature of the Antarctic thalli, compared with the Mediterranean ones, was the lack, in the former, of recognisable phycobilisomes on the thylakoid surfaces. This peculiarity had already been noticed in the other Antarctic red alga Iridaea cordata, collected, like Plocamium cartilagineum, in summer and from ice free waters. The loss of phycobilisomes may be a defence mechanism activated by benthic shade-adapted red algae from Antarctica to protect the photosynthetic apparatus against damage from high light intensities.
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