Abstract
A comparative TEM study was carried out on the phaeophyte Petroderma maculiforme , collected from San Francisco Bay both as free-living thalli and as phycobiont of the lichen fungus Verrucaria tavaresiae . The free-living alga usually had thicker cell walls and substantially greater reserves in storage bodies than the lichenized form. Cell division patterns were altered in symbiosis, and formation of zoosporangia was not observed on the lichenized filaments. Both free-living and lichenized Petroderma showed an abundance of densely osmiophilic deposits associated with vesicles, which were interpreted as physodes; Golgi and ER appeared to be involved in the elaboration of their contents. Plastids of both free-living and lichenized forms possessed a large pyrenoid deeply penetrated by tubular invaginations of the plastidial membranes. This pyrenoid was often enfolded by lobes of the plastid as previously reported, but also commonly occurred as a broad-based protrusion from the surface of the plastid. Smaller, exserted pyrenoids with a very narrow base and no tubular invaginations were sometimes also observed on plastids of the lichenized alga, but were not seen on those of free-living thalli. The significance of this variation in pyrenoid type was not clear. The considerable variation in pyrenoid position and form in P . maculiforme suggests that the protruding, enfolded, and stellate pyrenoid positions known in brown algae are not profoundly different from each other developmentally.