In the marine environment, herbivores feed on macroalgae which live associated with an epiphytic microbiota. These play major roles in health and physiology of macroalgae , such as host adaptation to varying environmental conditions , bacteria-induced morphogenesis [9, 10], or via their antifouling activity . Reciprocally, algal biomass constitutes a large choice of carbon sources for heterotrophic bacteria, especially polysaccharides, which represent about 50% of the algal dry weight. Red, green, and brown algae each have a different polysaccharide composition and most of the macroalgal polysaccharides are absent from land plants [12, 13]. To feed on seaweeds, marine heterotrophic bacteria have evolved specific enzymes such as agarases, carrageenases, fucoidanases, or ulvan lyases [14, 15]. Therefore, the macroalgal microbiota is significantly enriched in algal polysaccharide-degrading bacteria (mainly Bacteroidetes and Gammaproteobacteria) in comparison to the water column . Notably, the macroalga-associated bacterial community differs between red, green, and brown algae [17–20].