Fermentation of green alga sea-lettuce (Ulva sp) and metabolism of its sulphate by human colonic microbiota in a semi-continuous culture system.

The green alga, sea-lettuce (Ulva sp), could be considered as a new source of dietary fibre. Ulva, however, contains high levels of sulphate, part of which is chemically bound in soluble polymers (ulvan). The purpose of this study was to assess the fermentation characteristics and sulphate metabolism of Ulva and ulvan by human faecal bacteria fermentation system using a semi-continuous fermenter. Ulva and ulvan were poorly fermented, even after adaptation of the microbiota. Only 16.6% and 8.9% of Ulva and ulvan organic matter, respectively, were recovered as short chain fatty acids. Nevertheless, 40% of the sulphate in Ulva was dissimilated to sulphide by sulphate-reducing bacteria. Supplementation of Ulva with more fermentable polysaccharides, such as algal xylan and resistant starch, though decreasing the ammonia production originating from Ulva protein degradation, did not significantly reduce the sulphide levels. It is postulated that unless crude Ulva is desulphated, its daily consumption at a level of 20 g of dry product could stimulate colonic microbial sulphate reduction, which may have detrimental effects for the host.