In holobionts, the digestive enzymes of the host do not allow a complete degradation of complex polysaccharides which suggests the importance of a digestive microbiota. Marine generalist herbivores such as the abalone face the challenge of feeding on the three marine macroalgal lineages, which are composed of different combinations of neutral, carboxylated or carboxylic-bearing and sulfated polysaccharides . Here, we determine the taxonomic composition and the seasonal structure of the digestive microbiota of the abalone holobiont, a marine gastropod grown in natural conditions on four monospecific diets over a year. This long-term study revealed that (i) the marine herbivore gastrointestinal tract offers a specific niche for a dominant core microbiota, constitutive of the abalone holobiont, (ii) microbiota composition and fluctuations are correlated to seasonal variations of contextual parameters, and that (iii) few specific microbial groups were associated with a monospecific macroalgal diet.