Nostoc of Peltigera canina when lichenized and isolated

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Abstract
The cyanobiont Nostoc sp. from the lichen P. canina was compared at the ultrastructural level when in its free-living and lichenized stages, as were certain of its physiological characters. The main results may be summarized as follows. The isolated cyanobiont shows typical Nostoc characters and resembles N. sphaericum and N. punctiforme. When lichenized, the vegetative cells are enlarged from 2.5 to 4.5 .mu.m in average diameter. The thylakoids increase in number; they are arranged in a reticular manner and spread throughout the cytoplasm. Phycobilisomes are found, and the phycobiliproteins show chromatic adaptation. In the symbiosis, the heterocyst frequency of Nostoc is considerably lower at the thallus margins than in more mature thalus parts, and the nitrogenase activity correlates with the heterocyst frequency. For the 1st time, Nostoc akinetes are reported in a lichen.