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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.

The in vitro degradation of dietary fibre from three brown seaweeds (Himanthalia elongata, Laminaria digitata and Undaria pinnatifida) 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%) but, 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%. Mannuronate was slowly fermented but 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.

Mabeau, S. and Kloareg, B. 1987. Isolation and analysis of the cell walls of brown algae: Fucus spiralis, F. ceranoides, F. vesiculosus, F. serratus, Bifurcaria bifurcata and Laminaria digitata.—i. exp. Bot. 38: 1573-1580. Cell walls were isolated from six marine brown algae, Fucus spiralis, F. ceranoides, F. vesiculosus, F. serratus, Bifurcaria bifurcata (Fucales, Phaeophyta) and Laminaria digitata (Laminariales, Phaeo phyta). Yields of isolated cell walls ranged from 35-45% of thallus dry weight. Walls were composed mainly of alginates and sulphated fucans, the proportions of which correlated with species zonation in the intertidal region. This result is consistent with the hypothesis that the sulphated fucans are associated with the adaptation ofmacroalgae to the intertidal environment. Comparing the chemical composition of isolated cell walls with that of whole plants, we conclude that alginic acid is mainly part of the fibrillar wall while a significant proportion of the sulphated fucans probably belongs to the intercellular space matrix. Since ascophyllan-like polysaccharides were more abundant in the fucan extracts from the isolated cell walls than from the whole plants, it is suggested that differences in the structure of fucans might be related to differences in their localization throughout the tissue. Key words—Ce 11 walls, Phaeophyta, sulphated fucans. Correspondence to: Centre d'Etudes Oceanologiques et de Biologie Marine, CNRS, LP 4601, Place