| Abstract: | The dietary fibres of the seaweed Ulva sp (sea-lettuce) consist of water-soluble ulvan, alkali-soluble β(1,4)-D -glucuronan and β(1,4)-D -glucoxylan, and an insoluble α-cellulose containing xylose residues. They are poorly degraded by human colonic bacteria particularly when associated within the intact plant cell wall. In order to better understand this resistance to microbial attack, their organisation in the cell-wall has been investigated by light and electron microscopy after sequential chemical extractions. Their susceptibility to enzymatic degradation and their accessibility to bacteria and enzyme were also studied. Microscopic localisation in native and sequentially extracted Ulva sp demonstrated that ulvan is in all the cell-walls of the algae and particularly between the two cell layers constituting the thallus. Glucuronan is close to the cytoplasmic membrane facing the outside of the seaweed and between adjacent cells. The xylose and glucose containing polysaccharides form packed layers surrounding the cells. A model of the spatial distribution of the different polysaccharides within the algae is proposed. Ulvan and glucuronan did not limit the xyloglucan and α-cellulose degradation by an endo-xylanase in the whole seaweed and its insoluble dietary fibre but the α-cellulose was not affected by a cellulase. The cell-wall of Ulva sp was accessible to enzymes but poorly to bacteria as assessed from porosity measurements. These results established that the poor fermentation of sea-lettuce by human colonic flora is primarily due to the ubiquitous presence of the degradation-resistant ulvan in the cell wall of Ulva sp. ©1997 SCI |
