We report here the complete sequence of the mitochondrial genome of the brown alga Laminaria digitata (Hudson) J.V. Lamouroux. L. digitata mtDNA is a circular molecule of 38,007 bp (64·9% A+T), encoding 63 genes and 3 ORFs and with only 6·7% of non-coding sequences. Based on gene content and order, its overall organization is very similar to that of the mitochondrial genome of Pylaiella littoralis, another brown alga belonging to a different sublineage of the Phaeophyceae. In particular, the two genomes share unusual features, which hence could be unique to brown algae among the heterokont lineage, namely the presence of a rn5 gene, a short nad11 gene, a cox2 gene with a large in-frame insertion and α-proteobacterial-like promoter sequences. On the other hand, L. digitata lacks the sequences which are thought to have been transmitted horizontally to the P. littoralis genome, that is, the group-II introns in the rnl and cox1 genes, and it features only traces of an ancestral T7-like RNA polymerase. Distance phylogenetic trees inferred from concatenated mitochondrial genes confirm that speciation of brown algae occurred recently compared to other heterokonts.

Adenocystis and Utriculidium share many morphological characters. They both possess a hollow, bladder-like thallus, made of a solid cortex and a fluid-filled medulla penetrated by a network of longitudinal filaments, a trichothallic apical meristem responsible for their multiaxial structure, and cryptostomata. There are two somatic phases in their life history: a haplostichous macrothallus and a polystichous microthallus. However, Adenocystis has unilocular sporangia associated with unicellular paraphyses resembling those of the Laminariales, whereas Utriculidium has plurilocular sporangia resembling those of the Scytosiphonales. Adenocystis has been classified either in the Laminariales or in different families of the Ectocarpales sensu lato, or suggested to be close to the Scytothamnales. Utriculidium has also been classified in different groups within the Ectocarpales sensu lato, depending on the authors. The two genera were either considered as closely related and even to be phases of the same complex life history, or placed in separate orders. The life histories obtained in culture have suggested that Adenocystis and Utriculidium are independent taxa. The taxonomic positions of the two genera Adenocystis and Utriculidium, which are currently unresolved, were explored in the present study, using phylogenetic analysis of combined partial LSU and SSU rDNA sequences. A data set of 20 species included Chorda, Chordaria, Dictyosiphon, Laminaria, Punctaria, Scytosiphon and Scytothamnus, the type genera of the families in which Adenocystis and/or Utriculidium were sometimes previously placed. Adenocystis and Utriculidium grouped together in all analyses within Ectocarpales sensu lato but failed to group with the ectocarpalean genera Chordaria, Dictyosiphon, Punctaria or Scytosiphon, as previously hypothesized. In fact they did not show any clear affinity with any other ectocarpalean taxa. On the basis of this result, and morphological data, we propose a new family, Adenocystaceae, to include these two genera. It is hypothesized that their current life cycles derive from an ancestral isomorphic life cycle.

Partial sequences of the large subunit (LSU) and small subunit (SSU) of the ribosomal DNA of fucalean algae (Phaeophyceae) from the northern and the southern hemispheres, as well as from other brown algal genera such as Durvillaea, were analysed and compared with available morphological data. Combined SSU+LSU sequence data appear to be suitable for inferring phylogenetic relationships within the Fucales. Results suggest that Durvillaea and Notheia should be included in the Fucales and that the paraphyletic Cystoseiraceae be merged into a large family, Sargassaceae Kützing emend. De Toni. The genus Axillariella, the taxonomic position of which was uncertain, clearly belongs to the Sargassaceae. At present, at least seven families are recognized in the order Fucales: Durvillaeaceae, Seirococcaceae, Notheiaceae, Sargassaceae, Himanthaliaceae, Hormosiraceae and Fucaceae. More investigations are needed to assess whether Xiphophora should be placed in its own family.

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.

The in vitro degradation of dietary fibre from three brown seaweeds (Himanthalia elongata, Laminaria digitata and Undaria pinnatiJda) was studied, using human faecal flora. Two sets of fibre were tested: (1) total algal fibres extracted from the whole algae, mainly composed of alginates, and (2) purified fibres (sulphated fucans, Na-alginates and laminarans) representative of those contained in the whole brown algae. Mannuronate, one algal component, was also investigated. Substrate disappearance and short- chain fatty acid (SCFA) production were monitored after 6, 12 and 24 h fermentation. Gas production was followed hourly during the first 9 h and then at 12 and 24 h. Sugarbeet fibre was used as a fermentation reference substrate. According to the fermentative indices used, most of each of the total algal fibres disappeared after 24 h (range 60–76 %) hut, unlike the reference substrate, they were not completely metabolized to SCFA (range 47–62 %). Among the purified algal fibres, disappearance of laminarans was approximately 90% and metabolism to SCFA was approximately 85% in close agreement with the fermentation pattern of reference fibres. Sulphated fucans were not degraded. Na- alginates exhibited a fermentation pattern quite similar to those of the whole algal fibres with a more pronounced discrepancy between disappearance and production of SCFA: disappearance was approximately 83 % but metabolism was only approximately 57 YO. Mannuronate was slowly fermented hut its metabolism corresponded to its disappearance from the fermentative medium. Thus, the characteristic fermentation pattern of the total fibres from the three brown algae investigated was attributed to the peculiar fermentation of alginates, and mannuronate was shown not to be directly involved.