Ultrastructural observations on the vegetative filaments, motile cells and processes leading to zoospore differentiation in Chrysonephos lewisii are presented. Vegetative filaments are embedded in a mucilaginous envelope, preserved by Alcian blue fixation, which favours their aggregation and also the attachment of bacteria, diatoms and various types of debris. The filaments are provided with an external wall consisting of microfibrils positive to the PATAg test and ConA-colloidal gold labelling. Inside the filaments the vegetative cells differentiate into zoospores. This accounts for the ultrastructural similarity between vegetative cells and zoospores, although zoospores are typically provided with a flagellar apparatus. They share an external cell covering or theca, vacuolar or cytoplasmic ‘scale-like structures’, protruding stalked pyrenoids, scattered nucleoids of plastid DNA and absence of a photoreceptor-eyespot complex. The flagellar apparatus of zoospores is characterized by two distinct basal plates, the central pair of flagellar microtubules originating some distance above the transitional plate and basal body angle less than 90°. These results provide strong taxonomic evidence that this genus should be placed in the Sarcinochrysidales sensu stricto, rather than remaining in Chrysomeridales. The key role of the Golgi apparatus in the synthesis of different structural elements – depending upon the functional stages of the cells – and in the processes of exocytosis during zoosporogenesis is also discussed.

The fine structure of the flagellar apparatus of male gametes of Ectocarpus siliculosus has been studied in negatively stained whole-mount preparations of isolated flagellar apparatuses and by transmission electron microscopy of embedded whole cells and flagellar apparatuses. Reconstructions from serial sections revealed a single absolute configuration and new structural details of the flagellar apparatus. A set of 5 microtubular roots is associated with the 2 flagellar basal bodies: the major anterior root consisting of 9 microtubules originates at the anterior basal body and loops through the anterior part of the cell. Cytoplasmic microtubules originate from this root at the apex of the cell and splay out in a posterior direction through the cell, thus determining cell shape. A broad microtubular band composed of a 9-membered bypassing root and a 3-membered major posterior root runs across the basal bodies. Two additional flagellar roots, the minor anterior and minor posterior rootlets, consist of a single microtubule each. The proximal end of the posterior basal body is laterally connected to the anterior basal body by a cross-striated connective (deltoid band). The basal bodies are connected to the nucleus by a small rhizoplast, and several other fibrous structures are associated with the basal bodies. The results are discussed in relation to earlier observations on flagellar apparatus organization in brown algae.

The ultrastructure of male gametes of Ectocarpus siliculosus is described and discussed in relation to the information available on other brown algal zoids. In general, the fine structure and the organelle associations agree with those of other brown algal cells. In particular, the male gametes typically contain a single chloroplast with a pyrenoid and a prominent concave eyespot, into which the swelling of the posterior flagellum (the photoreceptor) fits. The continuity of the chloroplast endoplasmic reticulum and nuclear membrane, as well as the presence of a perinuclear Golgi consisting of several dictyosomes, is shown. The fine structure of microbodies, lipid bodies and different types of vesicles, including physodes and coated vesicles, is also discussed. Several specializations of cellular elements and their interactions are unique and have not been reported before. These features include the connection between the nuclear envelope, the Golgi and the flagellar bases mediated by fibres resembling intermediate filaments, and the organization of the Golgi as well as the association of mitochondria and a microtubular flagellar root.