Temporal fluctuation in the abundance of alginate-degrading bacteria in the gut of abalone Haliotis gigantea over 1 year

In this study, we identified and enumerated alginate-degrading bacteria in the gut of abalone over 1-year period. From a total of 360 colonies growing on agar medium enriched with alginate, 251 isolates (70%) had the ability to degrade alginate. In addition, a high number of viable alginate-degrading bacteria were detected throughout the survey period. Alginate-degrading bacteria were more abundant in the cold season relative to the summer season (10⁷ vs. 10⁴ CFU g^-1, respectively). Strong positive correlation was also observed between the number of alginate-degrading bacteria and feed intake (R = 0.854; P < 0.01). The identified alginate-degrading bacteria comprised of 35 species grouped into 11 genera including Algibacter, Formosa, Polarybacter, Tamlana, Tenacibaculum (CFB group), Roseobacter, Ruegeria, Silicibacter (α-proteobacteria), Agarivorans, Shewanella and Vibrio (γ-proteobacteria) respectively. More than 80% of the isolated alginate-degrading bacteria belonged to the genus Vibrio, showing high homology to Vibrio cyclotorophicus, Vibrio splendidus, Vibrio halioticoli and Vibrio neonatus. Based on the results, it was suggested that algal-polysaccharide (alginate) degrading bacteria (mainly Vibrio) commonly exist in the gut of abalone and may play an important role in the degradation and digestion of the host's feed.