Ultrastructure and ontogeny of hair filaments in the subtidal saccate alga Hydroclathrus clathratus (Phaeophyta, Scytosiphonales)

Abstract

Hair filaments found in pits scattered over the surface of the saccate brown alga Hydroclathrus clathratus were examined ultrastructurally. Cells near the base of filaments are short and metabolically very active, containing many mitochondria, ribosomes, rough endoplasmic reticulum, and dictyosomes but no vacuoles. Numerous plasmodesmata connect cells of the filaments. The more distal cells elongate during development of the pits. As elongation begins, dictyosome activity dominates and vacuoles appear. Very elongate cells are filled almost exclusively by a vacuole with a narrow band of cytoplasm at the cell periphery. Chloroplasts are found only in basal cells of short filaments. Other cells near the filament base contain plastosomes, small spherical bodies derived from chloroplasts by a series of divisions. While morphologically distinct, plastosomes have retained many features of brown algal chloroplasts. They are bound by the chloroplast envelope and chloroplast endoplasmic reticulum and contain a ring genophore. The thylakoid system varies in that more than three thylakoids are usually loosely associated in the peripheral band and the interior bands are absent in more mature plastosomes.