Fig. 6
Proposed processes of algal polysaccharide degradation in the abalone digestive gland. For information, localization of the abalone digestive gland on an abalone without the shell (a). The schematic illustration recapitulates hypothetical processes of algal polysaccharide degradation in the case of a brown algal cell wall (b). The algal diet comes along with epiphytic bacteria, including (1) strictly aerobic bacteria which may die due to specific physiochemical conditions (low O2, low pH) and (2) facultative aerobic bacteria, which may act as primary degraders of complex algal polysaccharides. Primary degraders may (3) directly ferment polysaccharides to short-chain fatty acids (SCFA, e.g. Vibrio) or (4) transform polysaccharides into pyruvate or solubilized mono- and oligosaccharides (members of Flavobacteriia, Alpha-, and Gammaproteobacteria). These may then be (5) fermented by strictly or facultative anaerobic bacteria (e.g. Psychrilyobacter, Mycoplasma), which are probably localized in an anaerobic or microaerophilic part of the gland, such as the epithelial mucus. Resulting SCFA are then (6) assimilated by the host. Parts of the illustration are inspired from [6, 96]. Abalone picture: courtesy of Monique Ras