Gut microbiome modulation and gastrointestinal digestibility in vitro of polysaccharide-enriched extracts and seaweeds from Ulva rigida and Gracilaria fisheri

•Ulva rigida and Gracilaria fisheri improve gut health in vitro differently.•Seaweed polysaccharides are not digestible by small intestinal enzymes.•Seaweeds and polysaccharides stimulate beneficial microbes and SCFAs output in vitro.•The red seaweed Gracilaria fisheri has more potential as a source of prebiotics.